Download Audiology Clinic V2

Instruction Manual
Audiology Clinic V2
By Parrot Software
Contents
Introduction
1
Using The Audiology Clinic, Version 2 ......................................................... 1
Screen Area ....................................................................................... 1
File Menu .......................................................................................... 1
Case Menu ......................................................................................... 2
Test Menu.......................................................................................... 2
Audiogram Menu ............................................................................... 4
Auto Test Menu ................................................................................. 4
Immittance Menu ............................................................................... 5
Options Menu .................................................................................... 6
Case Menu revisited........................................................................... 7
Controlling the Audiometer with the Keyboard .................................. 7
Summary ........................................................................................... 7
Chapter One
8
Audiologic Screening .................................................................................... 8
Procedure........................................................................................... 8
Additional Considerations.................................................................. 8
Practice .............................................................................................. 9
Technique .......................................................................................... 9
Summary ........................................................................................... 9
Chapter Two
10
Air And Bone Conduction Threshold Audiometry ........................................10
Auditory Thresholds .........................................................................11
Clinical Audiometry..........................................................................11
Earphones .........................................................................................11
Threshold Testing Procedure.............................................................11
Modified Hughson-Westlake Method................................................12
Automatic Testing.............................................................................12
Interpreting Automatic Tests.............................................................13
Response Chart .................................................................................13
Manual Testing .................................................................................13
The Audiogram.................................................................................14
Practice .............................................................................................14
Audiogram Interpretation ..................................................................14
Threshold Considerations..................................................................15
Instruction Manual Audiology Clinic V2
Contents  i
Maximum Intensity Limits................................................................15
Summary ..........................................................................................16
Chapter Three
16
Masking Air Conduction Thresholds ............................................................16
Crossover..........................................................................................16
Masking ............................................................................................17
Three Masking Rules ........................................................................17
Example Of Crossover ......................................................................17
Synopsis Of Rule One.......................................................................18
Masking Strategies............................................................................19
The Plateau Method ..........................................................................19
Automatic Testing.............................................................................20
Establishing The Plateau ...................................................................20
Interpretation Of Results ...................................................................22
Four Parts Of The Masking Curve....................................................22
Determining The Real Threshold ......................................................23
Overmasking.....................................................................................23
Definition Of A Plateau ....................................................................24
Limitations To Plateau Definitions....................................................25
Summary ..........................................................................................26
Chapter Four
26
Masking Bone Conduction Thresholds .........................................................26
Interaural Attenuation By Bone Conduction......................................26
Air-Bone Gap ...................................................................................27
Occlusion Effect ...............................................................................27
Air-Bone Gaps In Only One Ear .......................................................28
Practice .............................................................................................29
Bilateral Air-Bone Gaps....................................................................29
The Masking Dilemma......................................................................30
Summary ..........................................................................................31
Chapter Five
32
Re-examining Air Conduction Thresholds ....................................................32
Retesting Only One Ear ....................................................................32
Practice .............................................................................................33
Disparity Between Air And Bone Thresholds....................................33
Realistic Clinical Responses..............................................................34
Summary ..........................................................................................35
Chapter Six
35
Variability In Listener Responses .................................................................35
Practice .............................................................................................36
Summary ..........................................................................................36
ii  Contents
Instruction Manual Audiology Clinic V2
Chapter Seven
36
Aural Acoustic Immittance ...........................................................................36
Impedance Basics .............................................................................37
Resistance.........................................................................................37
Reactance..........................................................................................37
The Problem Of Timing ....................................................................37
Acoustic Admittance.........................................................................38
Static Admittance..............................................................................39
Summary ..........................................................................................40
Chapter Eight
40
Tympanometry .............................................................................................40
Tympanometry Procedure .................................................................41
Tympanometric Norms .....................................................................42
Tympanogram Classification.............................................................43
Tympanometric Screening For Middle Ear Disorders........................45
Summary ..........................................................................................45
Chapter Nine
45
Acoustic Reflex ............................................................................................45
Ipsilateral Stapedial Reflex ...............................................................46
Contralateral Stapedial Reflex...........................................................47
Normative Stapedial Reflex Behavior ...............................................47
Clinical Patterns................................................................................48
Summary ..........................................................................................55
Chapter Ten
55
Speech Audiometry ......................................................................................55
Speech Recognition Threshold ..........................................................55
Word recognition ..............................................................................56
Recorded vs. Live Voice Presentation ...............................................56
Descending Threshold Protocol.........................................................56
Calibration of the Speech Signal .......................................................57
Masking Speech Thresholds..............................................................57
Practice .............................................................................................58
Summary ..........................................................................................58
Index
Instruction Manual Audiology Clinic V2
59
Contents  iii
Introduction
Using The Audiology Clinic, Version 2
Did you ever hear of a computer program that wasn’t user-friendly and
easy to use? On the other hand, did you ever try one the first time that
was? The point is that most computer programs are not simple and
intuitive until after you have learned how to use them.
The objective of this chapter is to get you over the hurdle of learning a
new program so that you can use it readily to simulate the tests used in
audiology to assess hearing. It is assumed, incidentally, that you know
how to manipulate Windows programs. If not, it would be advisable to
get some help. You will find installation instructions at the end of the
book, just before the Index.
Screen Area
Version 2 of The Audiology Clinic automatically detects the screen
area of your computer monitor. The numbers that are displayed
represent how many pixels your screen can show horizontally and
vertically. The larger the set of numbers, the more “things” can be
shown on the screen. Three settings are checked for: 1) 640x480, the
setting used by Version 1, 2) 800x600, and 3) 1024x768. To simplify,
we will call the 1024x768 screen area high resolution, the 800x600
screen area medium resolution, and the 640x480 screen area low
resolution. You can learn the screen area of your computer by starting
The Audiology Clinic and clicking the Options menu. The last item on
that menu indicates the current screen area of your monitor. Ideally,
your computer should have high resolution because this permits most
Instruction Manual Audiology Clinic V2
Introduction  1
windows used in The Audiology Clinic to be displayed on the screen
simultaneously without any overlapping. Furthermore, if your
computer is set to high resolution, the program can be run on a portion
of the screen so that it looks exactly like Version 1 by clicking Options
and then Run in Window. This cannot be done with low or medium
resolution. Using Run in Window permits the remainder of the screen
to be used for other programs. It is possible to switch back and forth
between full-screen and partial screen by clicking Run in Window on
the Options menu. If a case is currently displayed, it must be closed
(on the menu click Case, then Close) before the display can be
changed. Remember that high resolution is the recommended setting
because more windows are visible at one time.
The screen area setting can be changed on most computers, so you
may want to experiment with the different settings to ascertain the one
that is best for you. If you are working in a lab, check with the lab
supervisor first. However, if you are using your own computer, the
screen area can be adjusted by clicking Start, Settings, and Control
Panel. Then double click the Display icon. In that window click the
Settings tab. Finally, drag the arrow in the Screen Area box to the
desired setting. Finally, click OK. If you have made a change, a second
window will appear, so click OK again.
The screen area setting can be changed on most computers, so you
may want to experiment with the different settings to ascertain the one
that is best for you. If you are working in a lab, check with the lab
supervisor first. However, if you are using your own computer, the
screen area can be adjusted by clicking Start, Settings, and Control
Panel. Then double click the Display icon. In that window click the
Settings tab. Finally, drag the arrow in the Screen Area box to the
desired setting. Finally, click OK. If you have made a change, a
second window will appear, so click OK again.
File Menu
After starting The Audiology Clinic, the first step is to open a data file.
Data files come in two varieties: the Standard data file and Extra data
files. The former contains all the cases discussed in this text/manual,
while the latter enable you to access additional cases that your
instructor may want you to test. Extra data files are not accessible from
the Lite edition of the program.
2  Introduction
Instruction Manual Audiology Clinic V2
Figure 1-1. File menu.
To open a data file, on the menu bar click File, and a submenu will
appear. Pick the type of file you want on this submenu (see Figure 11).
Case Menu
The Case menu now becomes available, so the next step is to select the
Case you want to test. On the menu bar click Case, followed by Select
Case on the submenu, as revealed in Figure 1-2.
Figure 1-2. Case menu
This action will open the Case Selector window (Figure 1-3). You
may type in the number of the case you wish to test if you know it.
Alternatively, you may click the down arrow to reveal a list of all
available case numbers, using the scroll bar to move among the
choices. Click the case desired. Finally, click OK.
Figure 1-3. Case selector.
Test Menu
Instruction Manual Audiology Clinic V2
Introduction  3
The last step is to select the type of test you want to conduct. The Test
item on the menu has now become enabled. Click Test to see the
submenu, which is shown in Figure 1-4.
Figure 1-4. Test menu.
If your immediate objective is to begin pure tone or speech testing,
then click Pure tone audiometry; however if you want to do
immittance first, then click Immittance. Selecting one type of testing
protocol does not prevent your performing the other test because you
may switch at any time. Just return to the Test menu and click on the
other name to change testing modes.
If you choose Pure tone audiometry, your screen will look like Figure
1-5 on a monitor that has high resolution. Notice that there are five
parts to the display, which are identified in the figure below.
Figure 1-5. Screen display for high resolution.
On the other hand, if you are using the program with low or medium
resolution or have selected the Run in Window option, then the screen
will appear like the representation shown in Figure 1-6. Notice that the
screen is divided into three major parts. On the left the listener is in the
upper window, and the audiometer is in the lower window. On the
right is the audiogram. Behind the audiogram is the immittance form,
which also reveals the case history.
4  Introduction
Instruction Manual Audiology Clinic V2
Figure 1-6. Screen for pure-tone audiometry.
On the other hand, if your choice on the Test menu was Immittance,
your screen will look like the picture below (Figure 1-7). You can see
that the immittance instrument is at the left and the immittance and
case history form at the right with the audiogram behind it.
Figure 1-7. Screen for immittance.
It is important to understand that to switch back and forth between
these two forms click on the title bar of the one that is underneath.
Refer to Figure 1-8. In this instance the audiogram is active, and the
immittance/history form is mostly hidden underneath. Only its title bar
is visible above the audiogram. To make the entire immittance/history
form visible, click on its title bar. The immittance/history form then
moves to the front, and the audiogram goes behind. To view the whole
audiogram again, click on its title bar.
Instruction Manual Audiology Clinic V2
Introduction  5
Figure 1-8. Audiogram and immittance title bars.
Remember that you can easily alternate testing modes between
audiometry and immittance. Simply click Test and then click the type
of test desired. For instance, if you are doing pure tone testing and you
want to switch to immittance, click Test and then click Immittance.
Audiogram Menu
Assume that from the Test menu you picked Pure tone audiometry.
This activates the Audiogram menu and affords the choices presented
in Figure 1-9 below. Most importantly, you can plot the audiogram of
the person represented by the current case. This grants you the
opportunity to check your results against the correct results.
Recognize, however, that it is possible for your instructor to revoke
this privilege.
Another choice on the Audiogram menu is Erase. This will delete all
pure tone thresholds and speech and immittance results. After an
audiogram is plotted, it may be printed by clicking Print. The final
choice on the Audiogram menu is Symbols. Clicking this opens a
window that reveals the explanation of the symbols used on the
audiogram.
Figure 1-9. Audiogram menu.
Auto Test Menu
Again assume that from the Test menu you selected Pure tone
audiometry. Observe that Auto Test has become available on the
menu. It offers five alternatives (Figure 1-10) A primary feature of The
Audiology Clinic is its capacity to find pure tone unmasked and
6  Introduction
Instruction Manual Audiology Clinic V2
masked thresholds automatically. The first item on the submenu,
Modified H-W, measures the unmasked pure tone threshold at the
current audiometer setting (Left or Right ear, Air or Bone conduction,
at the selected frequency). Clicking this item will display a new
window, the Response Chart. This chart will appear on top of the
Immittance instrument, if operating in high resolution mode (Figure 15) or on top of the Audiogram and Immittance windows at the right of
the screen, if operating in low or medium resolution mode or Run in
Window (Figure 1-6).
Figure 1-10. Auto test menu.
It should be emphasized that during auto testing using low resolution
or Run in Window mode there are three overlapping windows on the
right half of the screen. It is possible to view any one of them in its
entirety by clicking on its title bar. This action will bring that window
to the front and place it on top of the others.
The unmasked threshold can be obtained manually by repeatedly
clicking the Next button on the Response Chart or automatically by
setting the Speed to a value between 1 and 9 by clicking the arrows
above the Speed label. Setting 1 is the slowest and 9 is the fastest. The
process can be interrupted at any time or reset when completed by
clicking the Reset button. These controls are revealed in Figure 1-11.
Figure 1-11. Top part of Response Chart.
Instruction Manual Audiology Clinic V2
Introduction  7
The masked threshold can be acquired by choosing Standard masking
or Complete masking from the Auto Test menu. These procedures are
described in Chapter Three. The Complete masking operation reveals
the masking curve at all masking levels and is intended to illustrate
theoretical concepts useful for learning purposes. It is not the method
that would be used clinically. Normal clinical protocol is illustrated by
the Standard masking selection.
When you are performing tests or having the computer do automatic
Standard masking or Complete masking, the Crossover diagram option
becomes available (for bone conduction testing the non-test ear must
be occluded - see Chapter 4). Choosing this menu item opens a
window on the screen that depicts a schematic of the head showing the
amount of hearing loss in the conductive mechanism, in the
sensorineural mechanism, and the total loss for each ear. It also reveals
the signal and masking levels at each ear and any crossover that may
be occurring. Using this diagram permits thorough examination of the
variables involved in obtaining correct thresholds when masking. The
Crossover window is depicted in Figure 1.12.
Figure 1-12. Crossover diagram.
Lastly, it is possible to click Close auto test and remove the Response
or Masking chart and the Crossover diagram from the screen.
Immittance Menu
The Immittance menu item is accessible only when there is not an
active case. To close the current case, click the Case menu and then
click Close Case. Now Immittance can be chosen. Clicking this item
opens the submenu displayed in Figure 1-13 below.
8  Introduction
Instruction Manual Audiology Clinic V2
Figure 1-13. Immittance menu.
The Probe tip presents an animation of tympanometry in the normal
and pathological ear. The Ipsilateral reflex arc and the Contralateral
reflex arc demonstrate the neural pathways involved when eliciting
acoustic reflexes. Impedance opens a submenu that demonstrates
instances of the mathematics involved in calculating acoustic
impedance. Finally, Examples accesses illustrations referred to in
Chapter 9.
Options Menu
Last of all, the Options menu item provides nine alternatives. These
are shown in Figure 1-14 on the next page. All choices are visible in
this figure, although when the program is running, not all choices are
available at all times. These operations are described below.
Figure 1-14. Options menu.
Instruction Manual Audiology Clinic V2
Introduction  9
1.Transducer allows you to switch between standard circumaural
earphones and the newer insert earphones (unless your instructor has
authorized only one kind of earphone to be used). This choice must be
made after choosing Pure tone audiometry as the type of test but
before any further action is taken. Otherwise said, the kind of
earphones in use cannot be changed once a test is begun (by clicking
the mouse or pressing any key on the keyboard).
2.Audiometer lets you pick either a clinical or a portable audiometer
to test with. This item is only available when there is no active case.
Close the current case to obtain access to this option.
3.Listener enables you to opt to test a particular listener; otherwise
one of four listeners is selected randomly when a case number is
picked.
4.Non-test ear occluded prepares for the measurement of masked
bone conduction thresholds by placing a circumaural earphone on or
an insert earphone in the non-test ear (Chapter 4).
5.Calibrate speech is only available when Speech has been selected
as the mode on the audiometer. It opens a window with a pointer that
lets you calibrate the level of the speech signal prior to doing speech
audiometry (Chapter 10).
6.Sound on determines whether the program produces sound. If your
computer has a sound card, then the Sound on item has a check mark
before it, and the program produces sound. Sound output can be
defeated, however, by clicking this item to remove the check mark.
This may be preferable in a laboratory. If there is not a sound card in
the computer then this item is unchecked and cannot be selected, and
there is, of course, no sound output.
7.Show privileges informs you whether for the current case you can
view the correct audiogram, the immittance results, the speech results,
and the case history It also notifies you whether you can do the three
types of auto testing. These privileges can be controlled by your
instructor. When any of these privileges is denied, it will not be
available for selection on the menu.
8.Run in Window permits you to run the program on a reduced
portion of the entire screen, if your computer monitor has high
resolution as discussed above.
9.Screen area discloses the current screen area setting of your
computer monitor.
Case Menu revisited
10  Introduction
Instruction Manual Audiology Clinic V2
When the Case menu was presented above, two items were not
discussed: Record results and Save case(s). These two items are not
accessible from the Lite edition of the program. For evaluative
purposes your instructor may require you to test one or more cases and
submit the results to him or her. In order to record the results of your
testing procedures, click the Case menu followed by Record results
before selecting the case to be evaluated. After you have finished all
the tests you want to administer, click Case again, followed by Save
case(s), select the disk you want to save the case(s) on, and name the
file according to instructions given to you.
Controlling the Audiometer with the Keyboard
When performing pure tone audiometry (Chapters One through Six),
the audiometer can be controlled with the mouse or with the keyboard
(your preference). When using the keyboard, the function of the
relevant keys is shown in the table below. These keys are consistent
with the older Pure Tone Simulation program.
The audiometer window must be the active window to use the
keyboard to control the audiometer (i.e., the title bar that says
“Audiometer” must be blue). If the audiometer window is not the
active window, click anywhere on the window with the mouse to
activate it. Note: if the audiometer window cannot be activated, then
the program is in a mode, such as Auto Test, which does not permit the
audiometer to be activated. Exit that mode first, then click on the
audiometer window.
Function
Change Output: Left - Right
Change Mode: Air - Bone - Speech
Intensity - increase
Intensity - decrease
Frequency - increase
Frequency - decrease
Present signal
Plot threshold
Erase audiogram & immittance
Key
HOME
END
UP ARROW
DOWN ARROW
RIGHT ARROW
LEFT ARROW
SPACE BAR
P
E
Summary
Instruction Manual Audiology Clinic V2
Introduction  11
An overview of the operation of The Audiology Clinic has been
presented by explaining the menu choices. Practice manipulating the
features of the program until you become comfortable with it, and
refer back to this chapter when needed.
Chapter One
Audiologic Screening
In this chapter we will discuss the techniques used in audiologic
screening using the pure tone audiometer. The procedures to be used
are described by the Guidelines for Audiologic Screening published by
the American Speech-Language-Hearing Association (ASHA) in
1997. “The purpose of screening is to detect, among apparently
healthy persons, those individuals who demonstrate a greater
probability for having a disease or condition, so that they may be
referred for further evaluation” (ASHA, 1997, p. 6). These guidelines
are extensive, and only the specific details that relate to pure-tone
audiometry will be presented here. If you are administering a
screening program, it is strongly suggested that you read these
guidelines and become thoroughly familiar with their content.
Procedure
The recommended screening procedure differs somewhat depending
on the goal of the screening and the age group being screened. We will
concern ourselves with screening for hearing impairment in three age
groups: 1) 3-5 yrs., 2) 5-18 yrs., and 3) adults. In all cases the
frequencies to be used are: 1000, 2000, and 4000 Hz.
12  Chapter One
Instruction Manual Audiology Clinic V2
1.The first group, ages 3 - 5 yrs., is screened at 20 dB HL. Each
stimulus is to be presented at least twice at each frequency in each ear,
and the criterion for passing is to respond to at least 2/3 of the
presentations.
2.The second group comprises school-aged children between 5 yrs.
and 18 yrs. This group is also to be screened at 20 dB HL. To pass
they must respond to all the signals; otherwise they are to be
rescreened after repositioning the earphones and being reinstructed.
3.The third group is comprised of adults. They are to be screened at 25
dB HL and must hear all the signals to pass the screening.
Another group, 7 mo. - 2 yrs., is included in the guidelines, but
screening this group involves specialized techniques. Consequently,
the details will not be covered here.
The guidelines provide actions to be taken, if a listener fails the
screening. These differ depending on the age group. Of interest here is
simply whether the listener passes or fails the screening test.
Additional Considerations
Other details of the screening procedure will be mentioned only
briefly. As always in audiometry the instructions to the listener are of
the utmost importance. They should be as concise as possible without
causing the person being tested to be confused due to insufficient
information about the task expected of them.
Although multiple presentations of the signal can be delivered to the
listener, under no condition should the tone be presented repeatedly
until the listener responds. The criterion for the 3 - 5 yrs. group serves
as a good rule of thumb: present the tone at each frequency a
maximum of three times and consider two or more responses as
passing.
In addition, as with all audiometric testing, a sufficiently quiet acoustic
environment and a properly calibrated audiometer are of paramount
importance.
A checklist of the steps is provided in the box below.
Operational Checklist
Preparation:
1. Adjust the Intensity to 20 or 25 dB depending on the age group.
2. Confirm that the Frequency is 1000 Hz.
3. Set the Output to Right, the Mode to Air, and be sure the Masking is
off (0 dB).
Instruction Manual Audiology Clinic V2
Chapter One  13
Testing:
1. Present the signal for approximately one second and observe
whether the listener heard the signal.
2. Continue by offering a second, and if necessary a third, presentation;
note whether there was a response.
3. Decide whether the signal was heard on a majority of the
presentations.
4. Repeat at other frequencies in current ear and in other ear.
Practice
If you haven’t already read the Introduction to find out how to use The
Audiology Clinic, be sure to do that now. Screening audiometry is
typically done with a portable audiometer. The type of audiometer you
want to use must be selected before a case is chosen. Immediately after
starting the program click Options followed by Audiometer; finally
click Portable. (If there is a case currently being tested, click Case,
then Close case before going to the Options menu as previously
stated.)
Let’s begin with Case No. 1, so select that case. Convention dictates
that you always start at 1000 Hz in the right ear, unless you have
reason to do otherwise. In general, it is advisable to begin the test in
the ear that is known (or believed) to have the better hearing
sensitivity. The steps in the ASHA guidelines for screening are
summarized in the following panel. Let’s assume that this listener is an
adult; therefore the proper intensity to use is 25 dB. Now begin to
screen the hearing of this listener (i.e., test at 1000 Hz).
You will discover that the listener heard the 1000 Hz tone in the right
ear. Now change the frequency to 2000 Hz and present the tone; once
more the listener hears the tone. Continue by switching to 4000 Hz and
deliver the signal. At this point you should have presented three tones
and noted three responses.
Next switch to the opposite ear, the left ear, and continue the test.
Don't forget to reset the frequency to the starting setting: 1000 Hz.
Complete the test (i.e., present tones at 2000 and 4000 Hz). You have
now completed the screening text on the first listener. Note that he or
she passed the test because a hand was raised after each of the six
tones was presented.
To continue, choose Case on the Menu, followed by Select Case, and
pick Listener No. 2, another adult. Employ the same technique that
you have just used to test this listener.
The listener associated with Case 2, as you will discover, fails the
screening test because he or she did not respond to all three tones in
14  Chapter One
Instruction Manual Audiology Clinic V2
the left ear. There are a total of ten listeners to be screened (5 in the
Lite edition), so continue with Case 3. Assume that Case 3, 4, and 5
are ages 18 or younger, so set the intensity to 20 dB HL. Consider the
remainder to be adults, so reset the intensity to 25 dB HL.
Technique
There are two aspects of a clinician’s technique that are important to
consider at all times.
First, it is essential to present the signal to the listener with a duration
that is sufficiently long for the listener to perceive it. The length of a
tonal signal should be approximately one second. Durations shorter
than one second may not be perceived, while signals longer than one
second do not improve detection and are inefficient time-wise. The
listeners in The Audiology Clinic will not respond to very short stimuli
regardless of the intensity.
The second factor is the temporal pattern with which the signals are
presented. It is necessary to vary the interstimulus interval and not to
present the signals with a fixed, predictable pattern. In other words
vary the amount of time between one signal presentation and the next.
Otherwise, the listener will consciously or unconsciously anticipate the
next signal presentation and respond accordingly, since reacting to
very soft signals often involves making a guess.
Summary
This chapter has described pure tone screening tests for different age
groups using procedures defined in the ASHA guidelines. In the next
chapter the topic of air conduction threshold tests will be explained.
Instruction Manual Audiology Clinic V2
Chapter One  15
Chapter Two
Air And Bone Conduction Threshold Audiometry
The purpose of this chapter is to master the technique of obtaining
hearing thresholds by both air conduction and bone conduction.
Furthermore, you will learn to use several new features of The
Audiology Clinic.
Let us begin by quickly reviewing the reasons for measuring hearing
thresholds by both air and bone conduction. The objectives are
primarily to quantify the individual's sensitivity at each frequency and
secondarily to specify the locus of the hearing impairment. Testing
performed with the earphones directs sound waves into the ear canal
toward the middle ear, thus the term air conduction; e.g., the sound
waves are conducted through the air to the eardrum. In contrast, testing
done with the bone conduction vibrator placed behind the pinna (or,
much less frequently, on the forehead) vibrates the temporal bone and
stimulates the cochlea directly; this testing mode is called bone
conduction.
Signals presented by air conduction (AC) must pass through the entire
auditory mechanism: first the outer ear, then the middle ear, next the
inner ear, and finally along the auditory nerve. Air conduction testing,
therefore, indicates how much hearing loss an individual has. This
total loss may be comprised of abnormalities in any or all of the four
sections of the auditory mechanism. On the other hand, signals
introduced through the bone conduction (BC) vibrator bypass the outer
and middle ears and directly quantify the amount of the sensorineural
deficit; so the bone conduction thresholds unambiguously reveal the
amount of loss in sensitivity due to impairment of the inner ear and/or
the auditory nerve.
Together the air conduction threshold and the bone conduction
threshold determine what kind of hearing impairment a person has.
The establishment of the kind of hearing loss is straightforward. As
already stated, the bone conduction thresholds directly indicate the
16  Chapter Two
Instruction Manual Audiology Clinic V2
amount of sensorineural loss (inner ear and/or auditory nerve).
Ascertaining the amount of conductive loss (outer and/or middle ear)
necessitates subtracting the bone conduction thresholds (sensorineural
loss) from the air conduction thresholds (total loss).
To illustrate these ideas, let us consider three different people.
1.Assume that person "A" has air and bone conduction thresholds of
50 dB HL in both ears at all frequencies. Since the air and bone
conduction thresholds are equivalent, this person has a sensorineural
deficit. There is no conductive involvement because AC (50) - BC (50)
= 0.
2.Now let’s assume that person "B" has the following thresholds in
both ears at all frequencies: air conduction = 50 dB HL and bone
conduction = 0 dB HL. Clearly, this individual has no sensorineural
involvement because there is no reduction in sensitivity for bone
conduction stimuli; however he or she does have a 50 dB conductive
loss because AC (50) - BC (0) = 50.
3.Lastly, let’s suppose that person "C" has air conduction thresholds
equal to 50 dB HL and bone conduction thresholds equal to 30 dB HL.
This individual has a mixed hearing loss, which is to say that part of
the loss is conductive and part is sensorineural. The magnitude of the
conductive component is 20 dB, as AC (50) - BC (30) = 20, while the
degree of the sensorineural component is 30 dB, because the bone
conduction thresholds were obtained at 30 dB HL.
Be sure to avoid confusing the terms: air conduction and conductive.
Air conduction refers to a type of testing, namely presenting stimuli
through earphones, while the word conductive implies a kind of
hearing impairment, that is one involving the outer and/or the middle
ear. It is not advisable to refer to an "air conduction loss"; instead refer
to a "conductive loss" (vs. a "sensorineural loss").
Auditory Thresholds
As was the case with Audiologic Screening, discussed in Chapter 1,
guidelines for pure-tone threshold audiometry have been published by
ASHA (ASHA, 1978). You are urged to become thoroughly familiar
with them.
There has been much discussion in the scientific literature, especially
in sensory psychology, regarding the definition of auditory
"threshold". In audiometry we are primarily concerned with the
measurement of absolute thresholds (Humes, 1994), or the softest
intensity an individual can hear 50% or more of the time. We shall use
an operational definition that has been published by ASHA to quantify
this threshold. Simply stated, a threshold is the lowest intensity tone
Instruction Manual Audiology Clinic V2
Chapter Two  17
that can be heard on three (usually nonconsecutive) presentations. The
thresholds for tones across the frequency range from 125 to 8000 Hz
for a listener with perfectly normal hearing would be 0 dB HL. 0 dB
HL thresholds are analogous to the commonly used term 20/20, when
referring to vision.
At the opposite end of the intensity continuum is the threshold of
feeling. The extremely loud sounds that elicit this sensation are very
unpleasant to the listener, and the audiometer cannot deliver such
excessively intense stimuli to most people. The maximum output of
most portable and clinical audiometers through the earphones is 110
dB HL in the mid-frequencies (500-6000 Hz). Tones of this intensity
usually will not elicit the sensation of feeling, but they will be
uncomfortably loud for individuals with recruitment. Consequently,
the clinician must always be cautious when presenting stimuli at or
near maximum intensity so as not to cause distress to the listener.
Clinical Audiometry
Clinical audiometry involves both air conduction and bone conduction
testing. The frequencies used for air conduction measurements include
the octave frequencies 125-8000 Hz, in other words, 125, 250, 500,
1000, 2000, 4000, and 8000 Hz. The intraoctave frequencies, 750,
1500, 3000, and 6000 Hz, need only be tested when there is a 20 dB or
greater difference between any adjacent octave frequencies. For
example, according to this provision, if you got a threshold of 40 dB
HL in the right ear at 4000 Hz and a threshold of 70 dB HL in the
same ear at 8000 Hz, then you should subsequently obtain a threshold
in that ear at 6000 Hz because the difference in the thresholds at the
adjacent octave frequencies is 30 dB. The lowest frequency, 125 Hz,
need be tested only when there is a low frequency hearing loss.
Because the vast majority of hearing impairments are high frequency,
it is usually unnecessary to test at 125 Hz.
Bone conduction thresholds are typically measured at the octave
intervals between 250 and 4000 Hz, that is at 250, 500, 1000, 2000,
and 4000 Hz. But bone conduction thresholds may be assessed at 750,
1500, and 3000 Hz as well, if the difference between the thresholds at
adjacent octave frequencies is 20 dB or greater.
Earphones
There are two types of earphones commonly used in clinical
audiometry. One kind is the older, larger earphone with a rubber
cushion that makes contact with and surrounds the pinna. Such
18  Chapter Two
Instruction Manual Audiology Clinic V2
earphones are referred to by several terms. The one we will use is
circumaural earphones. In contrast, the smaller, newer type is called
the insert earphones. This name is somewhat of a misnomer because it
suggests that the earphone is inserted into the ear canal, which it is not.
Rather the earphone is external to the ear, from a few millimeters to
several centimeters depending on the brand. In either case a narrow
tube leads from the actual earphone to an earplug which is inserted
into the ear canal to deliver the sound. Insert earphones offer several
advantages including ease in fitting and placement because of their
small size and light weight, avoidance of closure of the ear canals
(called collapsed canals), and most importantly increased interaural
attenuation. This last factor is very significant and will be explained in
the next chapter.
The Audiology Clinic will permit the use of either type of earphone
but defaults to the standard, circumaural earphones for the cases
described in this text/manual. The results obtained, of course, will be
identical except in a few very difficult cases. The intricacies of these
cases will be the topics of future chapters. Also in the next chapter you
will learn how to select between the two kinds of earphones with this
simulation.
Threshold Testing Procedure
The ASHA guidelines describe two phases to obtaining each
threshold: familiarization of the signal and measurement of the
threshold. The motive for this two-stage method is that the pure-tones
used for determining hearing thresholds are not common sounds in
many people's lives. Furthermore, ascertaining a threshold involves, by
definition, the use of a stimulus of very low intensity. Therefore, it is
of foremost importance to familiarize the listener with the nature of the
signal before presenting it at low levels just below and just above
threshold.
For the familiarization part of the procedure, the tone is presented
initially at 30 dB HL. The choice of this intensity presumes a listener
with normal or near normal sensitivity. Because many individuals
receiving hearing tests will not have normal thresholds, the following
steps are further reported by the guidelines. If there is not a response at
30 dB HL, then increase the intensity to 50 dB HL. If there is still no
response, then continue to increase the intensity in 10 dB steps, until
there is a response to the tone. The first response to the tone concludes
the familiarization phase of the threshold determination. If the
maximum output of the audiometer at the frequency being tested is
reached, then present the tone three times at this maximum level. If the
listener responds to fewer than half of these presentations (i.e., none or
Instruction Manual Audiology Clinic V2
Chapter Two  19
one), the threshold is unmeasurable at the current frequency. On the
other hand, if the listener hears more than half of the tones (i.e., two or
all three), then his or her threshold is this maximum intensity.
Usually the listener will respond at a level less than the maximum
intensity, so next the threshold measurement phase of the test
commences. Actually, we would label an additional phase called a
transition step, as it leads to the measurement of the threshold which
begins after the listener is no longer able to hear the signal. In the
transition phase, the level of the tone is decreased by 10 dB and
presented to the listener. If there is a response, the tone is again
decreased by another 10 dB and the signal introduced. This process is
repeated until an intensity is reached at which the listener does not
respond. If the listener responds to the tone at the lowest intensity the
audiometer can produce, which is -10 dB HL, then the tone is again
presented at that level. Finally, a third presentation is made. If the
listener has responded either two or three times in a row to the -10 dB
HL signal, then his or her threshold at the frequency being tested has
been obtained at -10 dB HL. Moreover, this person has better-thannormal hearing at that frequency.
The more normal situation is for the listener to stop responding to the
tones before -10 dB HL is reached. After the first non-response, the
actual threshold search (or what the ASHA document calls
measurement) begins. The intensity of the tone is increased by 5 dB
and presented to the listener. If he or she hears it, a note is made of this
level. Initially, you should make a notation (i.e., write it down) of this
intensity; after much practice and experience you will learn to make
only a mental note of this level. If the listener does not respond to the 5
dB increase in intensity, again raise the signal by 5 dB and present it.
Keep increasing the tone in 5 dB steps until the listener hears it.
As soon as you obtain a positive response, repeat the above procedure:
that is, decrease the intensity of the tone in 10 dB steps until you get a
non-response, then increase the tone in 5 dB steps until you do get a
response. Each time you get a response after increasing the intensity of
the tone, record the level. Threshold is reached as soon as the listener
has responded to the tone at the same intensity three times. This
procedure is indeed cumbersome to describe in words, but fortunately
it lends itself to graphical representation with great ease. Thus, we will
very shortly view this process using The Audiology Clinic.
To summarize the threshold-measuring process, first increase the
intensity of the tone until there is a response, then decrease the level of
the tone until there is not a response, then alternately increase and
decrease the intensity of the tone until three responses are recorded.
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Modified Hughson-Westlake Method
The process just portrayed is widely known as the modified HughsonWestlake procedure, and it was discussed in detail by Carhart and
Jerger (1959). This method of measuring thresholds is known in
sensory psychology as an "ascending" technique, as responses are
recorded only when the signal is being increased in intensity. Unlike
strictly ascending methods, however, each different frequency is first
presented at a supra-threshold level to familiarize the listener with
what the stimulus sounds like.
Obtaining thresholds quickly and reliably using the modified
Hughson-Westlake technique is the quintessence of pure-tone
audiometry. The adept clinician must be able to execute this procedure
with great efficiency and expertise.
Both air and bone conduction thresholds are obtained using the same
procedure. Ordinarily, air conduction testing is done first in both ears.
Then the earphones are removed, and the bone conduction vibrator is
positioned behind one pinna (and one earphone may be replaced in or
on the opposite ear as we shall see in Chapter 4). Bone conduction
thresholds are obtained for that ear at all frequencies, and finally, after
the bone conduction vibrator is placed behind the other pinna, bone
conduction testing takes place in the opposite ear.
Automatic Testing
Before attempting to measure some thresholds yourself, let’s watch
The Audiology Clinic obtain some. This is done by selecting Modified
H-W from the Auto test menu as described in the Introduction. The
case we want to examine is Case 11, so select that case now.
Furthermore, let’s use the clinical audiometer from now on. A
summary of the steps appears below.
Setup checklist:
1. File: Open - Standard data file
2. Options: Audiometer, Clinical
3. Case: Select case (choose Case 11)
4. Test: Pure-tone audiometry (be sure that the Masking is
set to 0 dB)
5. Auto test: Modified H-W
6. Speed: manual
Action:
1. Click Next repeatedly until the threshold is obtained;
watch the explanation in the green box at the bottom
Instruction Manual Audiology Clinic V2
Chapter Two  21
Interpreting Automatic Tests
Notice that the threshold measuring process was graphed on the
Response chart. This chart characterizes the intensities presented and
the results of each presentation: an "R" means that the listener
responded, and an "N" shows that the listener did not respond. The
recurrent presenting of the tone and recording of the response on the
graph will continue until the threshold is measured, or until it is found
that the listener cannot hear this frequency even at the maximum
intensity.
The test proceeded in accordance with the rules outlined previously.
There are several factors to observe. First the intensity of the tone was
adjusted to 30 dB HL. You could observe the numbers change in the
intensity window of the audiometer. The dialog at the bottom of the
Response Chart explained each step. When the level of the tone
reached 30 dB HL, the tone was sounded. The listener, who heard the
signal, responded by raising his or her hand. Afterwards, the level of
the tone was decreased by 10 dB HL, and presented again. The tone
continued to be lowered in 10 dB steps until there was not a response
on the part of the listener (at 0 dB). This was because the signal was
too soft to hear, in other words, below his or her threshold. Next, the
intensity of the tone was increased by 5 dB and presented; the listener
heard it and raised his or her hand. And so the test continued until the
listener heard the tone at the same level three times. The intensity
representing threshold is 5 dB HL.
Click Reset and repeat the process until you are able to follow all the
steps in the modified Hughson-Westlake procedure. If you wish,
change the “Speed” to a number between “1” and “9” to have the
entire process completed automatically.
Response Chart
When interpreting the response graph after a threshold has been
automatically obtained by The Audiology Clinic, keep in mind the
three phases of the threshold-measuring procedure that have been
defined. First, familiarization (increase the intensity until audible);
second, transition (reduce intensity to below audibility); and third,
measurement (increase and decrease intensity until the tone is heard
three times). As already stated, initially you should keep a written
record of the listener's responses. Later, you will be able to keep track
of the responses mentally.
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Unless contraindicated by the case history information (the object is to
test the ear having the better hearing first), air conduction testing
generally begins in the right ear at 1000 Hz. Such was the case in the
automatic sequence just witnessed.
To experience a different threshold being measured, change the output
of the audiometer to Left by clicking the Left button on the Output of
the audiometer or by pressing the Home key. Again perform an
automatic test by executing all the steps outlined above. Watch the
screen. You will discover that the threshold in the left ear is 10 dB HL,
which reflects slightly reduced sensitivity, but hearing that is still
considered to be within the normal range (as will be explained later in
this chapter).
Manual Testing
Now that you have viewed The Audiology Clinic while it measured
hearing thresholds, it is time to try it yourself. A checklist of the steps
to complete before starting the test is presented in the box on the next
page.
Next obtain the air conduction thresholds at the remaining frequencies.
Recall that if there are no differences between any two octave
frequencies of more than 20 dB, you need not test at the intraoctave
frequencies; i.e., 750, 1500, 3000, and 6000 Hz. The usual order is to
follow 1000 Hz by 2000 Hz and then proceed to 4000 and 8000 Hz.
After that 1000 Hz should be retested to verify reliability. Lastly,
measure the thresholds at 250 and 500 Hz.
IMPORTANT! You must use a bracketing threshold technique like the
modified Hughson-Westlake procedure in order to plot your
thresholds.
Proceed by obtaining all the air conduction thresholds for the right ear
followed by the left ear. You can verify your results by having The
Audiology Clinic show the correct results. If you are using high
resolution, click Audiogram followed by Show results. For low or
medium resolution or Run in Window the Response Chart (if visible)
mostly covers the Audiogram, so click on the title bar of the
Audiogram, which will enable the Audiogram menu. Now click
Audiogram followed by Show results. An alternative procedure would
have been to remove the Response Chart, thus returning the
Audiogram to its position on top. This is done by clicking Auto test,
then Close auto test. Note: as indicated in Chapter 1, your instructor
can revoke your privilege to view any or all of these results.
Instruction Manual Audiology Clinic V2
Chapter Two  23
Operational Checklist
1. Adjust the audiometer to the initial settings. This can be done by clicking the controls
on the audiometer with the mouse or by using the keyboard as indicated in the last
chapter.
2. Confirm that the Frequency is 1000 Hz., the Intensity is 30 dB HL, the Output is set
to the better ear, or to the Right ear if the better ear is unknown or hearing is
believed to be equivalent.
3. Present the signal for about one second by clicking the Present Signal button or
pressing the Space bar on the keyboard.
4. Observe whether the Listener responds. A response is indicated by a hand-raise and
the Response light at the top of the audiometer glows red.
5. Adjust the intensity and go to Step 3.
Normally, all frequencies are tested by air conduction in both ears
before removing the earphones and placing the bone conduction
vibrator, so next measure the bone conduction thresholds. Reset the ear
to Right, as the right ear is usually tested first unless contraindicated.
Finally, set the output to Left and get the left bone conduction
thresholds.
The Audiogram
The thresholds obtained from threshold audiometry in the clinic are
recorded one at a time on the audiogram. To do this, you can use the
audiogram utilized at your clinic or office, or you can employ the
audiogram feature of The Audiology Clinic.
To use the on-screen audiogram, first find the correct threshold using
the modified Hughson-Westlake technique. Then click on the plot
symbol (shown below) on the audiometer or press P on the keyboard.
"P" stands for "plot", and your just-measured threshold will be plotted
on the audiogram, using the correct symbol for the ear you are testing.
Practice
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Ten cases are affiliated with this chapter (5 in the Lite edition), each
displaying a different configuration of hearing. Practice by measuring
the air conduction and bone conduction thresholds of each of the
remaining cases (Cases 12 - 20). The correct results for all of the
audiograms can be found by plotting the audiogram (Click Audiogram,
then Show results). Of course these audiograms should only be
examined after you have acquired your own results. For additional
practice return to Chapter 1 and find the thresholds for Cases 1 - 10.
Audiogram Interpretation
There are two aspects to the specification of every hearing loss: 1)
what kind of loss, and 2) how much loss. The first of these was
discussed at the beginning of this chapter. The second consideration,
how much loss, is determined from the air conduction thresholds,
which directly reflect the total impairment. Many audiologists describe
categories of hearing, such as: hearing within normal limits, slight
loss, mild loss, moderate loss, moderately-severe loss, severe loss,
profound loss, and no measurable hearing. These categories, listed in
Table 2-1 below, are discussed in an article by Goodman (1965) and
were modified by Clarke (1981). For instance, if an individual had
thresholds of 35, 40, and 50 dB HL at 500, 1000, and 2000 Hz
respectively in the left ear, then the average threshold across these
three frequencies would be 42 dB HL. According to Table 2-1, this
person's loss would be defined as "moderate".
Using these same words to describe every individual's hearing
sensitivity may an oversimplification as different people with the same
numeric thresholds have very different handicaps resulting from their
hearing losses. A parallel situation might be to report one's visual
acuity as a "little" nearsighted. As a result using a phrase, like "severe
loss", must always be considered carefully. Nevertheless, many
clinicians prefer to use a single word to summarize hearing test results,
rather than merely reporting a set of numbers.
Table 2-1. Descriptive terms for hearing loss categories and the associated range of
thresholds (after Goodman, 1965, and Clarke, 1981).
Descriptive Term
Normal Limits
Slight Loss
Mild Loss
Moderate Loss
Moderately-Severe Loss
Severe Loss
Instruction Manual Audiology Clinic V2
Average of Hearing Thresholds
at 500, 1000, and 2000 Hz
-10 to 15 dB
16 to 25 dB
26 to 40 dB
41 to 55 dB
56 to 70 dB
71 to 90 dB
Chapter Two  25
Profound Loss
91 to 110 dB
Threshold Considerations
Simulation is useful for many purposes, but seldom includes all the
conditions that can occur in real life. Relating this to obtaining
auditory thresholds, there are several factors that you should be aware
of as influencing the validity of the test.
Instructions. The listener should be directed to respond to the faintest
sounds detectable; furthermore, he or she should be told to respond
immediately to each signal heard by raising a hand and/or pressing the
signal button and to cease the response (i.e., lower the hand/release the
button) as soon as the tone has ended.
Transducer Placement. The proper positioning of the earphones and
bone conduction vibrator is critical to a successful test. Especial care
must be taken to prevent the circumaural earphones from collapsing
the ear canals, and the vibrator should not touch the pinna.
These and other significant considerations relating to the finding of
pure-tone thresholds are discussed in considerable detail by Yantis
(1994).
Maximum Intensity Limits
As you have seen, there are two types of audiometers available for use
with The Audiology Clinic. The default represents a clinical
audiometer. This type of instrument is permanently installed and
generally contains numerous features that vary among manufacturers
but permit an extensive range of tests to be performed, many of which
are not incorporated into this simulation.
The other instrument is a portable audiometer. As the name suggests
this kind of instrument can easily be transported such as from school to
school, for example. The portable audiometer has fewer features than
the clinical audiometer.
The portable audiometer in The Audiology Clinic cannot do speech
tests. In addition, it incorporates an undesirable feature often found in
portable audiometers. To change audiometers, there cannot be a case
in progress, so terminate the current case by clicking Case, followed
by Close Case. Now click Options, followed by Audiometer and click
Portable. Finally, reselect a case to test. Lastly click Test, then Puretone audiometry.
26  Chapter Two
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Look at the audiometer. A noticeable difference is the little box
labeled "Max dB" in the middle of the audiometer. Due to the
differential sensitivity of the human ear to sounds of varying
frequencies, audiometers are incapable of supplying equivalent
maximum intensities (dB HL) at all frequencies. The reason is that the
decibel scale used on audiometers (that is, dB HL) represents different
physical magnitudes across frequency. For instance, 100 dB HL at 125
Hz is a greater physical intensity than 100 dB HL at 1000 Hz. The net
result is that audiometers are incapable of providing the same
maximum intensity at the frequency extremes (125 and 8000 Hz) as
they are in the mid frequency range. Even more importantly, the bone
conduction vibrator cannot yield nearly as great a maximum output as
the earphone. In fact, at 125, 6000, and 8000 Hz, the bone conduction
vibrator does not generate any sound at all. Consequently, many of the
simpler audiometers display a notation of what the maximum intensity
is at each frequency for both air conduction and bone conduction
signals. The number under the "A" stands for the maximum output by
air conduction, and the value beneath the "B" represents the maximum
intensity by bone conduction for the frequency displayed in the
frequency window. Rotate through the frequency range for both air
conduction and bone conduction At 125, 6000, and 8000 Hz a dash (--)
can be seen for bone conduction; this means that bone conduction
cannot be tested at these three frequencies.
The clinical audiometer simulated by The Audiology Clinic will not
permit the intensity to be increased past the maximum intensity;
however, the portable audiometer will. Thus, it is the clinician’s
responsibility to ensure that the intensity is not increased past the
maximum level. Exceeding the limits results in varying effects and
unreliable results with portables.
Many older audiometers of all types do not incorporate a warning
feature to indicate when the maximum intensity limit has been
exceeded. To clarify, the intensity dial can always be adjusted to any
intensity setting, including the maximum of 110 dB HL, regardless of
whether the audiometer is capable of producing a sound of that
intensity at the selected frequency. In fact, some audiometers actually
have been known to attenuate the signal if the intensity dial is
advanced beyond the maximum limit at a particular frequency.
Therefore, on audiometers that do not automatically restrict illogical
intensity settings, it is the clinician's responsibility to check the
intensity limit, which is always displayed somewhere on the face of
the audiometer, and increase the tone only as far as the maximum
intensity limit and never beyond it. Furthermore, remember not to
attempt testing bone conduction at 125, 6000, or 8000 Hz.
Instruction Manual Audiology Clinic V2
Chapter Two  27
Summary
This chapter has been devoted to obtaining air conduction thresholds
using the procedure described by ASHA. In particular, the modified
Hughson-Westlake protocol was described as the appropriate method
for varying the intensity of the stimulus when measuring a particular
threshold. Unfortunately, the responses the listener gives us are not
always representative of the actual, or organic, threshold because of a
phenomenon called crossover. Due to crossover, we must "mask" or
retest some thresholds. This is the topic of Chapter 3.
Chapter Three
Masking Air Conduction Thresholds
The focus of the current chapter will be on masking. This process
involves retesting a threshold to confirm whether it is the actual
threshold while directing a noise into the opposite ear to prevent or
"mask" it from detecting the signal. The original threshold may be
incorrect due to the phenomenon of crossover.
Crossover
Crossover is the situation wherein a signal presented to one ear is
actually perceived at the opposite ear. For example, if a signal
introduced through the right earphone were really heard in the left ear,
then crossover has occurred. As a result an erroneous threshold will be
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recorded for the right ear. Crossover occurs when relatively intense
stimuli (about 40 dB or greater) are presented to an ear. Two factors
contribute to crossover. First, the sound waves may leak from under
the earphone cushion or radiate from a bone conduction vibrator and
travel around the outside of the head to the other ear. Second, the
sizable vibrations caused by comparatively intense sounds may vibrate
the entire skull and thereby stimulate the contralateral cochlea because
this organ is encased within the temporal bone. In either instance the
listener may respond to the tone because it was perceived at the nontest ear rather than at the test ear.
The resistance of the skull to vibration as a whole and thus from
transmission of a sound from one ear to the other through the bony
structures is called interaural attenuation. That is, a tone becomes
significantly weaker as it traverses the bones of the head from one side
to the other. The amount of interaural attenuation experienced when a
tone is presented by air conduction depends on the type of earphone
that generates the sound. With the older circumaural earphones
interaural attenuation is almost always 40 dB or more. Actually,
interaural attenuation varies as a function of frequency. Although it
may be as little as 40 dB at the low frequencies, it increases to 50 dB
or more at the high frequencies. On the other hand with the newer
insert earphones interaural attenuation is usually at least 60 dB,
although again it differs depending on the frequency and the manner of
insertion of the earplugs (Killion, Wilber, and Gudmundsen, 1985;
Munro and Agnew, 1999). In all cases interaural attenuation will vary
among individuals. To promote consistency in this simulation,
however, interaural attenuation will remain fixed at 40 dB for
circumaural earphones and at 60 dB for insert earphones throughout all
of the following lessons. Be aware, nevertheless, that your instructor
may prepare cases for you to test that have different values for
interaural attenuation than 40 dB or 60 dB.
Crossover also occurs when testing by bone conduction as will be
discussed in Chapter 4.
Masking
The purpose of masking is to eliminate the possibility of an incorrect
threshold due to crossover. The concern is that the signal will be
perceived at the listener's other ear, not the one presently being tested.
The process of masking consists of retesting any thresholds that are
regarded as suspect, after obtaining the original, unmasked thresholds
(as in the previous chapter). A noise stimulus is introduced into the ear
opposite the one under test, to prevent, or "mask out", its participation.
Instruction Manual Audiology Clinic V2
Chapter Three  29
Then the threshold is redetermined in the test ear. The conditions that
create doubt about the validity of an unmasked threshold will be
discussed momentarily.
Incidentally, a frequent comment on the part of beginning students in
audiology is that surely the listener can differentiate which ear he or
she hears the signal in. Lateralization of the tone should not be used as
a criterion for determining the cochlea in which perception has
occurred. Although some individuals are able to make this distinction
reliably, many of the listeners tested by audiometry are either too
young, too old, or too handicapped to make this often subtle
discrimination. Accordingly, it is not recommended that a listener be
instructed to raise the left hand when hearing the tone in the left ear or
the right hand for the right ear as the correspondence between ears and
hands is just too undependable.
The common usage of the term "mask" can be confusing and
misleading. By way of illustration, if the left ear is under test by air
conduction and the thresholds are suspect, then the convention is to
say, "Mask left air conduction". This terminology means to retest the
left ear while simultaneously applying the masking signal to the right
ear. Understand, then, that the term "mask" as used in audiology refers
to a process: namely, retest the ear currently being examined and
present masking noise to the opposite ear. Likewise, "mask right air
conduction" would signify that the right air conduction threshold is to
be reevaluated while masking noise is delivered through the left
earphone to the left ear.
Three Masking Rules
Now that we have an understanding of what masking is and why it is
used, let us consider when it must be done. Three rules will be
introduced to indicate when masking must be performed. Each rule
will be discussed in separate chapters, beginning with the first rule in
this chapter.
MASKING RULE NO. 1
An air conduction threshold at a given frequency in a given ear must
be masked whenever it is 40 dB or more poorer than the air
conduction threshold at the same frequency in the other ear when
using circumaural earphones. It must be masked whenever it is 60
db or more poorer when using insert earphones.
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Which ear is suspect? Which ear gets retested? It is always the ear
with the poorer threshold that must be reexamined.
Upon retesting, the threshold in question can either remain the same
or become poorer, thus reflecting an even greater loss than that
originally measured.
Example Of Crossover
Let’s use a diagram to clarify the masking process. Refer to Figure 31. In this schematized head, two boxes are used to represent the
auditory mechanisms. The two boxes are further subdivided into two
parts each. The lateral (outer) boxes stand for the conductive portion of
the ear, that is the outer and middle ears. The medial (inner) boxes
designate the sensorineural structures (inner ear and auditory nerve).
The number in each box reveals the actual, organic hearing loss
attributed to that part of the auditory apparatus. To reiterate, these
values show the actual loss, not the measured thresholds obtained from
testing, which can be erroneous due to crossover. The numbers below
the boxes show the total hearing impairment, that is the sum of the
conductive loss (the lateral box) and the sensorineural loss (the medial
box).
Instruction Manual Audiology Clinic V2
Chapter Three  31
Figure 3-1. Schematic representation of the conductive and
sensorineural components of a hearing loss.
To illustrate the undesirable effects of crossover, let us assume as
shown in Figure 3-1 the instance in which there is a profound hearing
loss in the left ear; this situation is noted by the fact that the left ear's
sensorineural mechanism shows a threshold denoted as 110 dB HL,
meaning that there is a response only at the maximum intensity (in the
mid-frequencies, such at 1000 Hz). The conductive part of the left ear,
however, is normal; i.e., 0 dB HL. Furthermore, the right ear is
perfectly normal (i.e., 0 dB HL thresholds by both air conduction and
bone conduction), thus all 0s are shown for the right ear.
Now that we know beforehand the real thresholds, we can predict what
will happen when this hypothetical person is tested. When the right ear
is assessed by air conduction, the results will divulge thresholds at 0
dB HL. On the other hand, when the left ear is tested, thresholds will
be recorded at about 40 dB HL when using circumaural earphones and
at about 60 dB HL when using insert earphones. These results are, of
course, incorrect as the listener has a profound loss on the left side.
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Why is there such a surprising discrepancy? It occurs because of
crossover. Remember that for air-conducted signals the skull provides
an interaural attenuation of approximately 40 dB for circumaural
earphones and about 60 dB for insert earphones; therefore, when the
tones delivered through the left earphone reach or exceed these levels,
they cross the skull and/or leak from beneath the earphone cushion and
are heard at the right ear. Specifically, in our example a 40 dB tone
from the left circumaural earphone will create an intensity of 0 dB at
the right cochlea, enough to be perceived since the right inner
ear/auditory nerve (medial box) is depicted as having a threshold of 0
dB HL (bear in mind that 0 dB HL does not mean no sound; it is just a
position along the sound intensity continuum, just as 0 degrees
Fahrenheit is a point along the temperature continuum). If an insert
earphone is being used, then an intensity of 60 dB in the left earphone
will create an intensity of 0 dB at the right cochlea.
To continue the illustration of crossover, if the earphone is of the
circumaural type and a 50 dB HL tone were sent to the left ear, 10 dB
would cross over to the right ear: 50 dB tone - 40 dB interaural
attenuation = 10 dB crossover. For that matter, the same situation
pertains in reverse; if a 110 dB tone were channeled into the right ear,
it would cross over to the left ear as follows: 110 dB HL tone - 40 dB
interaural attenuation = 70 dB crossover. Naturally, this crossed-over
stimulus would not be perceived because we know a priori in this case
that the listener has a 110 dB hearing loss in the left ear; nevertheless
the tone has crossed the head to the contralateral side and is causing
vibrations throughout the cochlea even though such vibrations are not
transduced into nerve impulses. In contrast, if insert earphones are in
use and a 70 dB tone were delivered to the left ear, 10 dB would cross
over to the right ear: 70 dB tone – 60 dB interaural attenuation = 10 dB
crossover.
Synopsis Of Rule One
To recap, if circumaural earphones are being used, whenever the air
conduction threshold in one ear is 40 dB or more poorer than the air
conduction threshold at the same frequency in the other ear, the poorer
threshold must be masked. That is, the poorer threshold must be
retested to determine whether it is the actual organic threshold, or
whether the real threshold is even poorer than the level originally
obtained. In reality, the true threshold could be anywhere between the
one first determined (the unmasked threshold) and the maximum
intensity limits of the audiometer at the frequency being examined.
There could even be no measurable response if the loss were greater
than 110 dB HL. Remember that the crucial intensity of 40 dB is
raised to 60 dB when using insert earphones.
Instruction Manual Audiology Clinic V2
Chapter Three  33
Masking Strategies
Once it is determined that a threshold must be masked, or retested, the
next step is to decide how much masking noise is necessary to prevent
participation by the non-test ear. Numerous methods have been
advocated for deciding the intensity of the masking stimulus. They can
be reduced, nevertheless, to two fundamental strategies.
The first method is calculable; it involves estimating the amount of the
masking stimulus required to preclude the contralateral ear from any
possible perception of the tonal stimulus directed to the test ear. This
technique requires computing a value influenced by numerous
variables. Although only one level of masking noise need be applied,
still the factors contributing to its calculation are subject to dispute;
additionally, it takes valuable time to perform the calculation prior to
testing each frequency.
The second method is empirical. It requires the use of several masking
intensities, but there need be no complex, beforehand computation of
the levels to use. Furthermore, this latter strategy absolutely identifies
the correct threshold, if a simple graphing method is utilized (or
merely visualized mentally after sufficient experience). Because of its
simplicity and validity the second rationale, called the plateau method,
is currently used by more than 3/4 of practicing audiologists (Martin
and Sides, 1985).
A thorough discussion of various masking strategies is presented by
Goldstein and Newman (1994).
The Plateau Method
The so-called plateau method of masking was first discussed by Hood
(1960) in an article well worth reading. The primary decision that the
clinician must make is the initial level at which to set the masking
noise; nevertheless, after the starting intensity is decided, the
remainder of the technique is straightforward.
The recommended beginning level is the air conduction threshold of
the non-rest ear. This fact is predicated upon the critical assumption
that the masking dial represents effective masking; in other words that
a tone of x dB will be just at threshold with a noise of x dB, and if the
noise were increased by 1 dB more, the tone would be masked out (be
inaudible). Most modern clinical audiometers have a masking dial that
is calibrated in effective masking. This is sometimes not true for
portable audiometers.
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In the ensuing example it will be assumed that circumaural earphones
are being used. The same strategy pertains if insert earphones are used,
but the number that represents interaural attenuation changes from 40
dB to 60 dB. For this reason each value that applies if using insert
earphones is shown in parentheses after the corresponding value for
circumaural earphones.
To examine the masking process, suppose that we obtain an unmasked
air conduction threshold of 60 (80) dB in the right car and 20 dB in the
left ear for a pure-tone at 1000 Hz. The right ear must be masked
(retested) according to the first rule of masking because the response at
60 (80) dB is 40 (60) dB greater than that in the left ear. Consequently,
this response may have been due to stimulation of the left cochlea. The
initial intensity of the masking noise that goes to the left ear should be
20 dB, the level of the threshold in that ear. Introducing noise at a
lesser intensity, cannot be perceived by the listener; while starting with
masking at a suprathreshold level may, in some instances, lead to an
erroneous result. This point will be illustrated later.
Once the initial masking intensity is applied, then a simple strategy
prevails. The tone is again introduced into the test ear at the same level
as the original, unmasked threshold. Referring to our current example,
the intensity would be 60 (80) dB HL. If the listener responds, the
masking noise is increased by 5 dB. On the other hand, if the listener
does not respond, then the pure-tone signal is raised by 5 dB in the test
car and presented to the listener again. When obtaining a masked
threshold, unlike the situation when measuring an unmasked threshold,
the convention is to present the tone only once, unless, of course, there
is a legitimate reason to presume that the listener was distracted,
inattentive, etc. Note especially that the intensity of the tone is not
alternately decreased and increased. The interchange between
increasing the level of the tone and increasing the level of the noise
continues in 5 dB increments until either a plateau is reached, or
overmasking is a risk, or the masking signal cannot be increased any
further. Distinguishing among these alternatives is critical and will be
discussed presently.
It is necessary to digress momentarily to argue for the use of 5 dB
increments when searching for a masked threshold. Some clinicians
use 10 dB increments, maintaining that such a strategy is quicker but
yields equally accurate results. It is conceivable, nevertheless, that the
correct threshold may not be found when using 10 dB changes in the
masking noise. Ultimately, there may be numerous shortcuts that you
employ; but for the present time we will endeavor to support the most
complete and accurate technique for instructional purposes; e.g., 5 dB
increments.
If a case is currently active on the screen, click Case and then Close
Case. Next click Case again, followed by Select case. Pick Case 21
Instruction Manual Audiology Clinic V2
Chapter Three  35
and use the default (circumaural) earphones. Click Test and Pure tone
audiometry. At 1000 Hz manually obtain right and left air conduction
thresholds. You will find a right air conduction threshold of 15 dB HL
and a left air conduction threshold of 55 dB HL. Plot them on the
audiogram so that you do not forget them.
According to the first rule of masking, stated above, the result in the
left ear satisfies the criterion requiring masking; that is, the threshold
in the left ear is suspect, as it is 40 dB poorer than the threshold in the
right ear. Thus, the left threshold could be the result of crossover with
the sound having stimulated the right cochlea. On the other hand 55
dB HL could be the correct, organic threshold (the listener could have
actually perceived the sound at the left cochlea when responding at 55
dB HL to the tone. To clarify which situation pertains, this threshold
must be retested, or masked.
Now repeat this case using insert earphones instead. Close the current
case and reselect Case 21 again. Click Test and Pure tone audiometry.
Before doing anything else, click Options, Transducers, and Insert.
Notice that the earphones on the listener change from circumaural to
insert. Now proceed as before. This time the left unmasked air
conduction threshold will be 75 dB HL due to the greater interaural
attenuation provided by the insert earphones.
Automatic Testing
Before you endeavor to obtain your first masked threshold, allow The
Audiology Clinic to demonstrate the process to you. This simulator is
capable of automatically performing the masking procedure and
displaying the results by plotting a masking curve, a graph that has a
horizontal axis labeled "masking level" (noise) and a vertical axis
labeled "hearing level" (tone).
Make sure that the output of the audiometer is set to the Left ear,
which is the ear to be retested. Also make certain that the Frequency =
1000 Hz and Mode = Air and that the listener is wearing circumaural
earphones. With our simulated audiometer, as is the case with most
actual audiometers, the masking stimulus is automatically channeled to
the non-test ear (i.e., the ear opposite the one designated in the output
box on the audiometer), in this instance the right ear.
To invoke automatic testing, click Auto test, then click Standard
masking. Repeatedly click Next and watch the screen. Observe the
narrative description that appears in the box at the bottom of the
masking chart.
If the points plotted were connected, they would look like the solid
line at the left side of in Figure 3-2. The significance of the shape of
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this curve will be explained in detail soon. Remember that this process
can also be executed automatically by selecting a speed between 1 and
9. If the arrow buttons to the upper right of the Speed label are not
enabled, click Reset. Then these buttons can be clicked to vary the
speed.
Establishing The Plateau
This illustration assumes 40 dB of interaural attenuation (i.e., that
circumarual earphones are in use). Now plot your own masking curve
on the same ear at the same frequency. Begin by erasing the existing
masking curve by clicking Reset. Next close the Masking curve
window: click Auto test, then Close auto test.
Figure 3-2. Masking curve that reveals the plateau.
To execute the plateau masking procedure, first adjust the intensity of
the masking signal to the level of the threshold in the non-test ear.
Since the threshold in the right car was 15 dB, set the masking
intensity to 15 dB.
It should be understood that the masking procedure will work correctly
even if the masking level is begun at 5 dB, in other words, at the
lowest masking intensity. Because this level is below the air
conduction threshold of the non-test ear, no masking can possibly
occur. To repeat, beginning the masking process at the lowest masking
level will not cause any errors, it will just take longer to establish the
correct threshold. Therefore, to maximize efficiency, always initially
Instruction Manual Audiology Clinic V2
Chapter Three  37
set the masking stimulus to the level of the non-test ear's threshold
(unless otherwise instructed to demonstrate a particular point).
Your previously plotted unmasked Left air conduction threshold
should still be at 55 dB on the audiogram. To proceed, with the
masking intensity at 15 dB, adjust the test tone to the original
threshold (55 dB HL); then present the tone. Watch the screen: The
listener responds, and the first point is plotted on the masking curve.
Next follow the steps listed in the box below and confirm the results
indicated.
1.
2.
3.
4.
5.
6.
7.
8.
9.
10
11
12
Adjustment
Increase noise to 20 dB
Increase signal to 60 dB
Increase noise to 25 dB
Increase signal to 65 dB
Increase noise to 30 dB
Increase signal to 70 dB
Increase noise to 35 dB
Increase signal to 75 dB
Increase noise to 40 dB
Increase signal to 80 dB
Increase noise to 45 dB
Increase noise to 50 dB
Action
Present signal
Present signal
Present signal
Present signal
Present signal
Present signal
Present signal
Present signal
Present signal
Present signal
Present signal
Present signal
Result
No response
Response
No response
Response
No response
Response
No response
Response
No response
Response
Response
Response
Note that the listener responded at the same tonal intensity while the
noise level was increased twice beyond 40 dB. In other words the
listener responded three times to the same level of the tone, namely 80
dB HL (steps 10 - 12). The important part of this lesson occurred when
the magnitude of the tone reached 80 dB HL. At this point further
increases in the masking noise beyond 80 dB did not result in any
change in the level of the signal necessary to elicit a response.
Reexamine Figure 3-2; the above steps formed the solid part of that
curve.
Now continue increasing the magnitude of the masking stimulus in 5
dB increments to its maximum: 110 dB. Present the tone after each
advance in the level of the noise. Do this as an experiment to develop
the masking curve that resembles Figure 3-2. The listener continues
responding to the tone at 80 dB HL, thus creating the dotted portion of
the curve in Figure 3-2. Observe the masking curve: it has leveled off
or reached a plateau, thus the name of the masking procedure: the
plateau method. You have found the plateau, which denotes the real
threshold of the ear under test. To reiterate, when further increases in
the masking level do not demand additional increases in the signal
level for the listener to respond, then the actual threshold has been
measured.
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In this example the masked threshold (80 dB HL) was 25 dB poorer
than the unmasked threshold (55 dB HL), leading us to conclude that
the original, unmasked threshold was indeed the result of crossover.
The masking curve you generated should look exactly like that drawn
by the computer using the automatic testing mode described above.
When you were instructed previously to view the automatic masking
procedure, you could select either the Standard masking or the
Complete masking curve. You were instructed to choose the standard
method, which duplicates normal clinical procedures. The other
option, the complete curve, results in the masking noise being
increased in 5 dB steps from 5 dB all the way to 110 dB. Understand
that the latter choice, Complete curve, does not replicate clinical
methodology, but rather is available to illustrate what happens at
masking levels other than those generally used, in other words to draw
the masking curve that would result from the utilization of all masking
levels for use as a learning tool. There are occasions where the
complete masking curve represents important points about the masking
process. To verify the results you just obtained, click Auto test,
Complete curve and set Speed to 9 (click the right-facing arrow). Click
Start and watch the plotting of the curve you just obtained, which is
very similar to Figure 3-2.
Now complete the manual testing of this listener. First click Reset,
Auto test, Close auto test. Since clearly the right ear is the ear with the
better hearing thresholds, switch the output to the right ear and
measure the thresholds at the other frequencies. Afterwards return to
the left ear and obtain the remaining thresholds in that ear. Each time
that a threshold is found at a different frequency, the masking rule is
applied; and if need be, the masked threshold is measured before
advancing to the next frequency. It is important to plot each threshold
on the audiogram as it is obtained. Notice that the symbol for a masked
threshold is plotted.
You will discover that every frequency in the left ear when tested by
air conduction requires masking. If you want confirmation of your
results, return to the automatic masking demonstration discussed
previously. Furthermore, you may use the automatically-drawn curves
to check the accuracy of the curves that you derive.
Switch to insert earphones and repeat the above illustration. Of course
because of the greater interaural attenuation afforded by this type of
earphone, the initial unmasked left air conduction threshold at 1000 Hz
will be 75 dB, so this value will be used as the initial intensity.
Accordingly, the correct masked threshold will be derived much more
quickly, as there will only be a 5 dB shift between the original
unmasked threshold and the final masked threshold.
Instruction Manual Audiology Clinic V2
Chapter Three  39
To see the correct, completed audiogram, click Audiogram and click
Show results.
Interpretation Of Results
The two questions always to be answered when doing audiometry are:
1) how much loss, and 2) what kind of loss. In the example just
completed only the first issue can be addressed. The amount of loss
expressed numerically is 80 to 85 dB HL in the left ear and 10 to 15
dB HL in the right ear. Using the categories specified in Chapter 2,
these thresholds would be described as a severe loss in the left ear and
hearing within normal limits in the right ear. It is impossible to
stipulate the kind of loss because both air conduction and bone
conduction thresholds are required to make this determination. It is
worthwhile considering only the air conduction results, nevertheless,
because the air conduction thresholds do quantify the overall amount
of the hearing deficit.
Four Parts Of The Masking Curve
The yet unanswered question is, “how much masking is necessary to
get the correct threshold?” The answer to this issue can be found by
examining the masking curve, specifically the plateau: as soon as the
plateau can be defined, the true threshold has been measured. A
masking curve can theoretically have four (4) parts. These divisions
will be found when you test Case 22 with circumaural earphones.
With Case 22 you will discover that with circumaural earphones the
right ear has unmasked air conduction thresholds of 45 to 55 dB HL
across the frequency range from 250 to 8000 Hz. Furthermore, the
thresholds in the left ear are 5 to 15 dB HL, or precisely 40 dB better
than the corresponding thresholds in the right ear. Recall Masking
Rule No. 1: all thresholds in the right ear, the ear with the poorer
hearing, are in doubt and must be masked.
Begin by obtaining the unmasked thresholds in the right and left ears
at 1000 Hz; they are 50 and 10 dB HL, respectively. Next apply
masking where required. Realize that the right air conduction threshold
(50 dB HL) is equivocal, for it is 40 dB poorer than the corresponding
threshold in the other ear. Either acquire the masked thresholds
manually, if you are comfortable with the masking protocol, or have
The Audiology Clinic draw the curve. If you use Auto test, select
Complete curve as we want to plot the entire masking curve. If you
choose to plot the curve yourself, set the masking noise initially to 5
dB and then increase in 5 dB increments. Understand that this is not
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normal clinical practice but it being employed here to demonstrate that
there can be four possible components to a masking curve.
The resulting curve will resemble that shown in Figure 3-3. Inspect the
four (4) parts of this masking curve. Proceeding from left to right, the
first part is a horizontal line that represents undermasking. We know it
is the undermasking portion of the curve because the masking level
remains lower than the threshold for that ear. If we follow standard
procedure and initially set the masking intensity to the threshold of the
non-test ear, this portion will not appear on our graph. In this example
we purposely started masking below the threshold of the non-test ear
so we could see this part of the curve. It must be emphasized that an
incorrect result will not materialize if masking is begun at 5 dB, but it
is a waste of time to do so except for demonstration purposes.
The second part of the curve is called the shifting segment. The
questionable threshold is being shifted from its crossed-over
(unmasked) value to its correct (masked) value.
The third division of the curve is the plateau and denotes the true
threshold. By extending the plateau leftward until it intersects the
vertical axis of the graph, we can observe the dB value of the actual
threshold (60 dB HL).
Figure 3-3. The four parts of the masking curve.
The fourth and final element of the masking curve is the overmasking
section. At this point so much noise has been delivered to the non-test
ear that it has crossed over and has masked the test ear. The dominant
part of the masking curve depicted in Figure 3-3 is the plateau. Be
Instruction Manual Audiology Clinic V2
Chapter Three  41
advised that many masking curves will have a much narrower plateau,
or perhaps none at all. Additionally, not all hearing configurations will
lead to the development of a masking curve having all four parts, as
we shall see with future examples. One such example has already been
presented: review Figure 3-2, as that masking curve had no
overmasking component.
When this case is repeated using insert earphones, there is no
crossover, that is the original unmasked air conduction thresholds in
the right ear (55-65 dB) are the correct thresholds because there is not
a 60 dB or greater difference between the air conduction thresholds in
the two ears. There was no crossover due to the increased interaural
attenuation offered by insert earphones.
We should point out in closing that our nomenclature, that of labeling
four parts to the masking curve, is different than that used in some
texts.
Determining The Real Threshold
Establishing the exact masked threshold is straightforward: it is
indicated by the plateau on the masking graph. The only possible
confusion is whether a horizontal segment represents the
undermasking part or the plateau of the graph. (Recall from Figure 3-3
that there were two horizontal sections on that graph). The section
representing undermasking will not ordinarily be obtained; it was
developed in the previous example solely for illustrative purposes.
Good clinical practice dictates that the beginning masking level be
equal to the air conduction threshold in the non-test ear. Thus the
possibility that the horizontal part of the curve is due to undermasking
is impossible if masking were begun at the proper level. This being the
case, the only horizontal segment generated when using standard
clinical procedures is the plateau.
Now complete Case 22 using both circumaural and insert earphones by
finding the masked thresholds for the remaining frequencies in the
right ear. They are similar, but not identical, to the result at l000 Hz.
Clearly with one type of earphone masking is necessary but not with
the other.
Overmasking
For the most part the danger of overmasking arises when masking
bone conduction thresholds; this is the topic of the next chapter. As an
introduction to this situation, however, there are a few instances
wherein overmasking can happen with air conduction testing. Inspect
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the situation portrayed by the audiogram segment at the right side of
Figure 3-4. According to Masking Rule No. 1, the right ear must be
masked when using circumaural earphones. Hence, noise is applied to
the left ear. To simulate this using The Audiology Clinic, select Case
23 with circumaural earphones. Be sure the frequency is set to 1000
Hz. First find the unmasked air conduction thresholds. They are: 40 dB
HL in the right ear and 0 dB HL in the left ear. The former ear,
therefore, requires masking. Since the left air conduction threshold is 0
dB HL, the initial masking intensity in this ear will be 0 dB, while the
beginning intensity of the test signal will be 40 dB HL, the level of the
unmasked threshold.
Again plot the masked threshold automatically (click Auto test,
Complete curve); then click Next. Pay attention to the fact that as you
deliver tones to the right ear and increase the noise in the left ear, a
plateau is found at 40 dB (i.e., the level of the unmasked threshold.
Notice that as you proceed, overmasking occurs, and a warning
message is displayed in the text dialog box. Unlike the previous
examples, there is no shifting part on this masking curve. Such is the
case when the original unmasked threshold is the real organic
threshold. The plateau is realized as soon as the initial masking
intensity is introduced because the unmasked threshold is the true
threshold.
As the intensity of the masking sound increases, the listener continues
to respond until it reaches a value of 85 dB. At that point the listener
no longer responds. Accordingly, the intensity of the tone is increased:
to 45 dB HL. Next the masking level is increased to 90 dB, and the
tone is subsequently raised to 50 dB HL in order to elicit a response.
What happens to the masking curve at this high masking intensity?
The threshold of the tone is shifted to 50 dB HL. This threshold is
incorrect; as can be seen by inspecting the head in Figure 3-4, the
proper threshold is 40 dB HL. The 10 dB shift of the threshold in the
right ear is the result of overmasking. Overmasking is the situation
when so much masking noise is delivered to the non-test ear that it
crosses the head to the cochlea of the test ear and masks out the signal
being delivered there.
Why overmasking occurred can be understood by considering Figure
3-4 once more. What will the amount of crossover be when a 90 dB
masking noise is sent into the left ear? As always, the intensity of the
crossed-over signal will be the applied signal (90 dB) minus the
interaural attenuation (40 dB), or 90 - 40 = 50 dB. The intensity of the
masking noise in the test (right) ear will be 50 dB; this will shift the
true threshold by 10 dB because the masking noise will have crossed
over into the test ear and prevented it from responding to a 40 dB tone
any longer.
Instruction Manual Audiology Clinic V2
Chapter Three  43
Figure 3-4. Configuration in which overmasking can occur with air
conduction testing.
Now repeat the above process again, and this time display the
Crossover diagram on the screen. Click Reset on the Masking Chart, if
necessary, then click Auto test followed by Crossover diagram. (This
diagram represents both circumaural and insert earphones the same
way--as a gray rectangle.) The process described above can be seen
dynamically as the masking process occurs. Proceed slowly and study
carefully the signal and noise levels and the resulting crossover after
each change in the signal level and the masking level. Figure 3-5
shows a snapshot of the Crossover diagram at the point that
overmasking begins to occur. As discussed above this is when the
masking level reaches 85 dB. As can be seen 45 dB of masking crosses
over to the non-test ear, and this shifts the threshold in that ear from 40
to 45 dB. Thus, the threshold of the test ear has been changed.
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Figure 3-5. Crossover diagram.
Overmasking is a very serious circumstance and will be studied
thoroughly in the next chapter when we discuss the masking of bone
conduction thresholds.
Close auto test and complete this audiogram by obtaining masked
thresholds for right air conduction at the other frequencies. After each
threshold has been masked, remember to reduce the masking intensity
to 0 dB before switching to the next frequency.
Definition Of A Plateau
The emphasis thus far has been to develop a complete masking curve,
that is, to increase the masking signal to the maximum intensity. This
has been done to familiarize you with all the component parts of a
masking curve. Needless to say, this is not the procedure followed in
actual clinical testing. The masking signal need only be increased far
enough to define the plateau; moreover the plateau is defined when
three points have been found on it. Reexamine Figure 3-2; the masking
procedure is complete when the masking level reaches 50 dB, that is
the part of the curve represented by the horizontal section of the solid
line. Otherwise said, the masked threshold has been determined when
responses are elicited to masking levels of three different intensities,
while the level of the tone remains constant. The Audiology Clinic
comprehends when a plateau has been defined. To experience this,
remask some of the thresholds in Cases 21, 22, and 23 using the Auto
test with the Standard curve selected. This causes the computer to use
the standard clinical procedure, so the masking process stops as soon
as the plateau has been adequately defined. Use both circumaural and
insert earphones; you will observe that the need to mask and the
subsequent shift of an unmasked threshold occurs less often when
using insert earphones.
Limitations To Plateau Definitions
Another way to view the plateau is to consider it as being the segment
of the masking curve that connects the shifting and overmasking
portions (Figure 3-3). Some hearing configurations can cause these
two parts of the curve to intersect, that is, they are plotted as one
continuous line (their slopes are identical). In other words, there is no
plateau because overmasking starts immediately after the threshold has
Instruction Manual Audiology Clinic V2
Chapter Three  45
been shifted. Examined from still another angle, identification of the
plateau necessitates distinguishing between the shifting and
overmasking parts of the masking curve, which both extend toward the
upper right of the graph. In addition to being nonexistent it is also
possible that the plateau could exist but be very narrow, only 5 dB
wide in fact.
A plateau could theoretically be defined as the situation in which a 5
dB increase in masking intensity does not shift the threshold of the test
ear any further, that is, the threshold for the tone remains the same
after the last increase in masking level. To see this idea portrayed,
refer to Figure 3-6. Assuming that right air conduction is being
masked, note that with 30 dB of masking introduced into the non-test
ear the threshold has been shifted to 70 dB HL (the left side of the
plateau); furthermore when the masking level is increased to 35 dB
there is no change in the threshold; in other words the graph proceeds
horizontally: this is the plateau. In this hypothetical case the next
increase in masking intensity (to 40 dB) leads to overmasking. Thus,
to be heard, the signal must be raised to 75 dB. This is an extremely
narrow plateau. Many clinicians require at least three points on the
plateau, rather than the two points just demonstrated, to feel secure in
defining the actual threshold. The reason for this seemingly
conservative attitude will become evident later on when we consider
variability in listeners' responses.
To summarize for the present, real listeners are often not as reliable in
their responses as The Audiology Clinic has been representing. The
reality is that many listeners, if tested and retested, will have
thresholds that vary by ± 5 dB (or even ± 10 dB). These
inconsistencies dictate that the plateau be unquestionably defined, a
requirement that can be realized by requiring three, or even four,
points on the plateau.
Recognize that a narrow plateau may evade detection if a 10 dB
increment in signal and noise intensity were to be used. This reality
provides the rationale for recommending only 5 dB changes in both
the signal and the masking noise.
To gain experience with narrow plateaus, choose Case 24 with
circumaural earphones. Use the following setup: Output: Left;
Frequency: 250 Hz, Auto test, Complete masking, Speed = 9. Note the
width of the plateau. Continue by testing at 500, 1000, 2000, 4000, and
8000 Hz. This example illustrates the fact that a demonstrable plateau
may exist at some frequencies but not at all frequencies. Verify that
the plateau narrows, as in Figure 3-6, and then disappears completely.
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Figure 3-6. Masking curve with a vary narrow plateau.
Although three points can be considered as adequately defining the
plateau, at this stage experiment by increasing the masking noise to its
maximum level. Although this is not an acceptable clinical method
because it requires excessive time and unnecessarily loud stimuli in the
listener's ear, the practice in observing complete masking curves will
be most worthwhile as a learning experience.
Complete Cases 25 through 30 (not available in the Lite edition)
before advancing to the next chapter. Furthermore, review any
concepts you are unsure of by testing a particular case over again. Of
course, not all of the audiograms will require masking at every
frequency. Bear in mind that the decision to mask is a frequency by
frequency judgment. Remember, too, that currently we are concerned
only with the masking of air conduction thresholds according to
Masking Rule No. 1. In reality some bone conduction thresholds will
end up being incorrect, but that situation cannot be corrected until the
next chapter.
Summary
Chapter 3 has introduced the need to mask air conduction thresholds
and described the first masking rule. The air conduction threshold at a
particular frequency in one ear must be masked whenever it is 40 dB
or more poorer than the air conduction threshold in the other ear for
circumaural earhones or 60 dB or more poorer for insert earphones.
Instruction Manual Audiology Clinic V2
Chapter Three  47
The correct threshold is signified by the plateau, the plateau being the
condition in which the signal is heard at the same level after the
masker is increased at least 10 dB (two 5 dB increments). In the
following chapter consideration will be given to the masking of bone
conduction thresholds, which unfortunately must be retested far more
frequently than air conduction thresholds. Luckily, the technique is
exactly the same.
Chapter Four
Masking Bone Conduction Thresholds
Although air conduction thresholds have to be masked when there is
an asymmetrical loss with differences between ears of 40 dB or
greater, bone conduction thresholds have to be masked much more
frequently. Masking when testing bone conduction is the topic of this
chapter.
Interaural Attenuation By Bone Conduction
The reason for the necessity to mask bone conduction thresholds so
often is due to the fact that the interaural attenuation afforded by the
head for a signal presented through the bone conduction vibrator is
essentially nil. In fact, some authorities indicate that there is no
interaural attenuation whatsoever for bone conducted tones, while
others attribute 10 dB as a realistic amount of interaural attenuation.
From a clinical standpoint the magnitude of the actual interaural
attenuation is chiefly a moot point because additional factors such as
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placement of the bone conduction vibrator on the mastoid, variability
in a listener's response between tonal presentations, etc. confound the
exact measurement of interaural attenuation.
Another way of looking at this situation is to realize that pragmatically
a tone presented from a bone conduction vibrator placed on one
mastoid (either the left or the right) stimulates both cochleas
essentially equally. As an example, suppose that a 30 dB HL sound is
delivered from the bone vibrator, which is placed behind the right ear.
The intensity of the tone at the left cochlea will also be 30 dB because
30 dB - 0 dB "realistic" interaural attenuation = 30 dB crossover.
In The Audiology Clinic the interaural attenuation by bone conduction
will be considered to be 0 dB. While this may be a conservative value,
nevertheless guessing which bone conduction threshold has crossed
over based on slight differences in unmasked bone conduction
thresholds is totally unreliable due to the reasons outlined above. The
net result is that it is frequently essential, when testing by bone
conduction, to apply masking to the opposite ear to preclude its
participation in the measurement of the threshold because the tonal
stimulus so readily crosses the head to the non-test ear.
Air-Bone Gap
Before proceeding to define the second rule of masking, a new term
must be explained. An air-bone gap is defined as the difference
between the air conduction and the bone conduction thresholds in a
given ear at a particular frequency. It must be realized that this
difference can only exist in one direction: the air conduction threshold
can be poorer than the bone conduction threshold, but the reverse
situation is impossible. An example of an air-bone gap is depicted in
Figure 4-1. The magnitude of this air-bone gap is 40 dB. Remember
that bone conduction reflects only the condition of the sensorineural
Instruction Manual Audiology Clinic V2
Chapter Four  49
Figure 4-1. Example of an air-bone gap.
mechanism, while air conduction tests the conductive pathway plus the
sensorineural portion; therefore it would be inconceivable for a part
(bone conduction) to be greater than the whole (air conduction)
resulting in a bone conduction threshold that was poorer than the
corresponding air conduction threshold.
In reality bone conduction thresholds are sometimes measured
clinically that are slightly poorer (5 dB, possibly even 10 dB) than the
corresponding air conduction threshold (Barry, 1994). This seemingly
impossible result can be due to any or all of the following: calibration
errors, placement of the earphone and/or bone conduction vibrator, or
inattention by the listener. Skilled clinicians generally disregard an
occasional occurrence of this situation.
We are now prepared to learn the second rule of masking.
MASKING RULE NO. 2
A bone conduction threshold at a given frequency in a given ear
must be masked whenever there is an air-bone gap.
As was the case with air conduction thresholds in the last chapter, a
bone conduction threshold, when masked (retested), can remain the
same as the unmasked threshold, or it can become poorer, thus
reflecting a greater degree of hearing loss. The masked threshold can
never become better. In a given ear there may be air-bone gaps at just
one frequency, at several frequencies, or at all frequencies. In addition,
depending on the configuration of the air conduction thresholds, there
may be air-bone gaps in only one ear or in both ears. Regardless of the
number of air-bone gaps, each one must be masked. The one exception
to this rule will be explained shortly.
Occlusion Effect
There is one additional issue that must be addressed when masking
bone conduction thresholds. To perform the masking task, an earphone
must be placed over the contralateral ear in order to deliver the
masking noise. Covering the ear canal will, with some hearing loss
configurations, enhance the signal strength in the cochlea at some
frequencies. This situation is known as the occlusion effect. Its
dominance is seen when the non-test ear has normal hearing or a
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sensorineural loss, but not a conductive loss. Its presence is noted at
the low and mid frequencies up to and including 1000 Hz.
The existence of the occlusion effect means that slightly different bone
conduction thresholds will be obtained when the non-test ear does not
have an earphone covering it (known as the unoccluded condition),
compared to when the non-test ear is prepared to be masked and has an
earphone covering it (known as the occluded condition).
Some audiologists prefer to obtain right and left bone conduction
thresholds unoccluded, so the only transducer on the head is the bone
conduction vibrator (no earphones). Then they proceed to mask bone
conduction later, if needed. Other clinicians recognize the frequent
responsibility to mask bone conduction and configure the transducers
to allow masking at the outset. To mask a bone conduction threshold
when utilizing circumaural earphones, the bone conduction vibrator is
placed behind the test ear, and the earphone is positioned over the nontest ear. The other earphone is not placed over the pinna of the test ear
(the ear with the bone conduction vibrator). Rather it is placed against
the head between the pinna and the eye. Thus, if the test ear must be
masked, it can be done without having to return to the listener to place
the earphones.
The Audiology Clinic simulates both the unoccluded and occluded
situations. The default state is for the non-test ear to be unoccluded. In
order to mask, you must “place” the earphones. To change to the
occluded condition, click Options, then Non-test ear occluded. Don’t
forget: the default condition is unoccluded. Moreover, every time you
change ears on the audiometer or select a new case, the simulation
returns to unoccluded.
As a result of the occlusion effect, the initial masking intensity at 250,
500, and 1000 Hz must be somewhat greater than normal. Table 4-1
shows suggested amounts by which the initial masking intensity
should be increased to overcome the occlusion effect (Goldstein and
Newman, 1985). To illustrate, if the air conduction threshold of the
non-test ear were 30 dB HL at 500 Hz, then the appropriate intensity
with which to commence masking would be 45 dB: the threshold plus
the allowance for the occlusion effect at that frequency.
Although it is clear when the non-test ear has normal hearing, it may
be indeterminate whether a loss in that ear is conductive or
sensorineural, as the non-test ear may also have to be masked. Other
test results, like immittance and/or case history information, may
resolve this quandary. In the case studies that follow the occlusion
effect will be evident; you should attempt to account for it by elevating
the initial masking intensity.
Table 4-1. Magnitude of the occlusion effect at three frequencies (after
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Chapter Four  51
Goldstein and Newman, 1994).
Frequency
250 Hz
500 Hz
1000 Hz
Magnitude of the Occlusion Effect
15 dB
15 dB
10 dB
Air-Bone Gaps In Only One Ear
Air-bone gaps are more easily masked when they are present in just
one ear rather than in both ears. With a unilateral air-bone gap
overmasking will not be a problem, and a plateau can easily be defined
that reflects the true threshold by bone conduction for the ear under
test. Let us suppose a right air conduction threshold of 35 dB HL and
an unmasked right bone conduction threshold of 0 dB HL, thus
reflecting an air-bone gap of 35 dB. Further, suppose that there is no
air-bone gap in the left ear. Such a configuration is presented in Figure
4-2 (top). It is evident by recalling the second rule of masking that the
right bone conduction threshold must be masked (due to the air-bone
gap). Consequently, at this point in time it is totally incorrect to
describe the right ear as having a conductive loss. Why? Because the
real right bone conduction threshold is unknown until after it has been
masked.
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Figure 4-2. Unilateral air-bone gap (top) and three possible results
after masking (bottom).
There are three possibilities after masking, and these are illustrated in
the lower section of Figure 4-2. Focus on the right bone thresholds
( [ ).
1.The masked bone conduction threshold can remain the same as it
was before being masked. In this instance the test (right) ear has a
conductive loss (Fig. 4-2a).
2.The masked bone conduction threshold can shift until it is equal to
the air conduction threshold at the frequency under test, thus
designating a sensorineural loss (Fig. 4-2b). Therefore, the original,
unmasked results, which showed normal bone conduction, were in
error. The initial air-bone gap was due to crossover, so the test ear did
not actually have a conductive loss as suggested by the unmasked
thresholds.
3.The masked bone conduction threshold can become poorer than the
unmasked threshold but not as great as the air conduction threshold
(Fig.4-2c). This instance denotes a mixed loss with a conductive
component of 15 dB (the difference between the air and the bone
thresholds) and a sensorineural component of 20 dB (the amount of
loss by bone conduction).
Practice
Cases 31 and 32 will furnish the opportunity to mask audiograms of
the type depicted in Figure 4-2, that is unilateral air-bone gaps. Since
they portray the simplest examples of masking by bone conduction,
they should be completed before progressing on to the next section.
Don't forget the technique we advocate is that the initial masking
intensity be equal to the air conduction threshold in the non-test ear
with allowance for the occlusion effect where appropriate (Table 4-1).
Also 5 dB increases in both tone and masking noise should be used.
For these audiograms the masking process is completed when a
plateau has been defined. For a more thorough understanding of the
masking curve, you may prefer to begin masking with 5 dB of noise
and alternate increases in tone and noise levels until you reach 110 dB
of masking; i.e., a complete masking curve. Many examples will not
reveal a masking curve with all four parts; nevertheless, the plateau
will be readily identifiable. Such experimentation is acceptable on the
simulator but not for clinical use due to the unreasonable duration of
the test and excessive sound levels stimulating the listener's ears.
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Chapter Four  53
Bilateral Air-Bone Gaps
An especially complex masking predicament can arise when there are
air-bone gaps in both ears; and in general the greater the air-bone gaps,
the more difficult it becomes to derive the real thresholds. As an
introduction to bilateral air-bone gaps examine the loss symbolized by
the partial audiogram displayed in Figure 4-3 (top). This configuration
represents a particularly interesting situation. Three distinct
possibilities exist regarding the actual bone conduction thresholds.
1.Both true bone conduction thresholds could be the same as the
unmasked thresholds shown on the audiogram; thus when masked,
they would portray the final effect presented in segment (a) at the
bottom of Figure 4-3, which specifies bilateral conductive
involvement.
2.Or the right bone conduction threshold could really be poorer than
shown in the upper panel as the result of crossover. For purposes of
simplification let us assume that the right ear has a sensorineural (as
opposed to a mixed) loss, causing the actual right bone conduction
threshold, when masked, to be equal to the right air conduction
threshold, as shown in part (b). This outcome reveals a conductive loss
in the left ear, and a sensorineural loss in the right ear.
3.Lastly, the left bone conduction threshold could in fact be poorer and
equal to the left air conduction threshold, as indicated in panel (c),
thereby presenting a conductive loss in the right ear, and a
sensorineural loss in the left ear.
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Figure 4-3. Bilateral air-bone gap (top) and three possible and one impossible result after
masking (bottom).
Remember that in alternatives 2 and 3 above the real bone conduction
threshold could have been anywhere between 0 dB HL and the air
conduction threshold (30 dB HL), thus Figure 4-3. Bilateral air-bone
gap (top) and three possible and one impossible result after masking
(bottom).
designating varying degrees of mixed loss. For simplicity, only two
possibilities have chosen as illustrations: a "pure" conductive loss and
a "pure" sensorineural loss.
It is essential to understand that the configuration revealed in section
(d) in the lower part of Figure 4-3 is impossible to derive from the
unmasked audiogram presented in the top part of the figure. One
cochlea (if not both) heard the tone at 0 dB HL. Thus, the significance
of the unmasked audiogram is that a conductive loss does exist in at
least one ear. Figures 4-3(a) through 4-3(c) have demonstrated that the
conductive loss may be in the left ear, or it may be in the right ear, or it
may be in both ears. But there is no way that the masked audiogram
can show a sensorineural loss in both ears as in Fig. 4-3(d).
It needs to be underscored, therefore, that even the unmasked
audiogram with bilateral air-bone gaps reveals significant information:
Instruction Manual Audiology Clinic V2
Chapter Four  55
because there must be a conductive loss in at least one ear (if not
both), a medical referral is often appropriate. Realize, furthermore, that
it is unnecessary to mask both ears, if, after masking one ear, that
threshold has shifted (shows a greater loss). On the other hand, if the
threshold of the first ear to be masked does not change, then masking
the other bone conduction threshold is required.
Which bone conduction threshold should be masked first? The one
with the suspected mixed or sensorineural loss, as that threshold will
shift. If it does, you do not have to mask the opposite threshold;
remember only one threshold can shift (become poorer). Valuable
assistance in determining which ear is likely to have a conductive (as
opposed to a sensorineural) component can be attained from
immittance tests, if given before pure-tone audiometry, and from case
history information.
To repeat, left bone (Fig. 4-3b) and right bone (Fig 4-3c) do not have
to be masked according to Masking Rule No. 2. These cases
demonstrate the exception to the rule, and comprehending this fact
eliminates unnecessary masking and saves much valuable time for the
knowledgeable clinician.
The opportunity to acquire experience with bilateral air-bone gaps is
afforded by Cases 33 and 34. Test these simulated listeners before
progressing to a much more difficult configuration. Especial care must
be devoted to the masking of bilateral air-bone gaps. If the gaps are not
too large, as in these three audiograms, plateaus on the masking curve
can be described, thus accurately defining the threshold. Beware,
nevertheless, that the plateaus are narrow with circumaural earphones,
so care must be taken to follow the masking protocol precisely. Again,
plot the entire masking curve, if you want to identify the various parts
of the masking function.
The Masking Dilemma
The anathema of every audiologist is the masking dilemma. This
predicament derives from large, bilateral air-bone gaps of
approximately 40 dB in magnitude when using circumaural earphones
and 60 dB in magnitude when using insert earphones. To demonstrate
such a situation, look at Figure 4-4. This diagram of the head indicates
the real thresholds, while the fragment of an audiogram discloses the
unmasked thresholds, which are unreliable until after they are masked
due to the possibility of crossover confirmed by the bilateral air-bone
gaps. Although the audiogram reveals a left air conduction threshold
of 40 dB and a left bone conduction threshold of 0 dB, an examination
of the left ear within the head identifies the true thresholds of 80 dB
for air conduction and 60 dB for bone conduction. Recall that the total
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loss (the air conduction threshold) is shown below the boxes (80 dB).
The discrepancy between the unmasked air and bone conduction
thresholds and the authentic thresholds disclosed within the head can
be explained by crossover. The left bone conduction threshold that is
displayed on the audiogram resulted from the tone's being heard in the
right cochlea; the same circumstance happened with the left air
conduction threshold (this latter information creates a paradox that
cannot be resolved until the next chapter). By inspecting the
audiogram, we acknowledge that the bone conduction thresholds must
be masked due to the bilateral air-bone gaps.
Let us assume that we are using circumaural earphones and that we
decide to retest right bone conduction first, thus directing the masking
noise into the left ear. Inspection of the head reveals that this was the
wrong choice, as the right unmasked bone conduction threshold is the
actual threshold (0 dB HL) and consequently does not need to be
masked. Realize, however, that the clinician often has only the
audiogram to go by and certainly not the internal anatomy of the
listener's head, and the audiogram reveals an air-bone gap on the right
side (as well as the left side).
The unmasked audiogram informs us that we must start masking with
a 40 dB noise, because that is the level of the unmasked left air
conduction threshold (consider the situation at 2000 Hz in order to
disregard the occlusion effect and thus simplify this case). Be sure to
observe that 40 dB of noise will cause no masking in the non-test (left)
ear because the bone conduction sensitivity of the left ear is really 60
dB as revealed by the diagram of the head. Nevertheless, what does
transpire as soon as we apply the noise at 40 dB? It crosses over to the
right cochlea: 40 dB masking - 40 dB interaural attenuation
Instruction Manual Audiology Clinic V2
Chapter Four  57
Figure 4-4. Example of unmasked thresholds that will lead to a
masking dilemma.
= 0 dB crossover. A 0 dB noise, however, will not mask out a 0 dB HL
tone, so the listener responds. Since there will be a response, our
masking protocol dictates that we increase the noise to 45 dB in our
pursuit to define the plateau. This time 5 dB of noise crosses over (45
dB noise - 40 dB interaural attenuation = 5 dB crossover), and this 5
dB noise will mask out the 0 dB HL right bone threshold. As a result,
the level of the tone must be increased to 5 dB. Alternately, the levels
of the tone and masking noise are increased, attempting to find the
intensity of the tone that can be heard with at least three increases in
masking level (i.e., the plateau). But it never takes place. Refer again
to Figure 3-3; in our present example the shifting and overmasking
parts of the masking curve are united with no plateau between them.
(Restart the test with 5 dB of masking in order to the plot the complete
masking curve and note that a plateau never emerges.) This example
demonstrates a case in which the masked threshold cannot be
measured: thus a masking dilemma.
Suppose, instead, that we had decided to mask (retest) the left bone
conduction threshold first. The same unfortunate situation results. As
we apply greater and greater amounts of masking, a plateau never
materializes. This could be predicted by looking at the head in Figure
4-4. The original masking intensity must be 40 dB, the degree of the
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unmasked air conduction threshold. As the level of the masking
stimulus is increased above 40 dB, it overcomes the conductive
component in the right ear, reaches the right cochlea, and begins to
shift the crossed-over left bone conduction threshold toward its true
value of 60 dB HL. Specifically, 50 dB of noise will shift the threshold
to 10 dB HL; 60 dB of noise will shift it to 20 dB HL, etc. Continuing
along the same line, 100 dB of noise will finally shift the left bone
conduction threshold to its real value, 60 dB, which is the first point on
the plateau. But greater masking levels will cross over and mask out
the test tone (e.g., 110 dB masking - 40 dB interaural attenuation = 70
dB crossover, and the actual left bone conduction threshold of 60 dB
HL will be shifted to 70 dB HL, an overmasked condition). The final
result is a masking dilemma when retesting the left bone conduction
threshold, just as was the case when masking the right bone
conduction threshold.
You can discover this bilateral masking dilemma for yourself by
testing Case 35 with circumaural earphones. This is a very important
example; explore it thoroughly. Be sure to derive a complete masking
curve for both left and right bone conduction. Carefully examine the
shape of the curve. There is not a plateau; therefore, the true threshold
cannot have been measured. Notice that plotting the standard curve
using auto test does not reveal the correct masked threshold. For the
right ear, you will observe that the dots change from green to red as
the signal level advances from 60 to 70 dB to denote overmasking at
this level, as pointed out in the previous paragraph.
The details contributing to the masking dilemma just explained can be
seen by viewing the Crossover diagram available within Auto test.
Incidentally, if you have The Audiology Clinic mask right bone
conduction, you will discover that it shows the correct thresholds,
unobtainable by you due to the masking dilemma. The computer
calculates the thresholds mathematically, not based on the realities of
human anatomy.
What does the audiologist do when realizing there is a masking
dilemma? Remember foremost that there is a conductive loss in at
least one ear: that’s valuable information. Also for some individuals
interaural attenuation may be greater than 40 dB for air-conducted
stimuli particularly at the higher frequencies, which will make
establishing a plateau possible. Furthermore, pure-tone audiometry is
just one test, and other information from the case history and
immittance tests must be taken into account. Clinical decisions are
based on a test battery, not just one test in isolation.
There is something else that the audiologist can do to avoid the
masking dilemma. Recall from Chapter 2 that we said that the greatest
advantage to be gained by the use of insert earphones is increased
interaural attenuation. Retest case 35 using insert earphones. This time
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Chapter Four  59
you will discover that the masked bone conduction thresholds can be
obtained in the left ear. Even though overmasking still occurs when
trying to mask right bone conduction, the astute clinician can deduce
that the correct bone conduction thresholds are 0 dB in the right ear
and 60 dB in the left ear. This conclusion can be reached by process
of elimination. Recall the unmasked bone conduction thresholds of 0
dB. Since the correct left bone threshold is 60 dB that leaves only right
bone to have responded at 0 dB. This case illustrates the very situation
in which insert earphones make a real difference in the results that can
be obtained.
Finally, complete the examples in this chapter by determining the
thresholds for Cases 36 through 40 (not available in the Lite edition),
which will offer additional cases like those illustrated by Figures 4-2
and 4-3. Distinguish the subtle difference between right bone results at
2000 and 4000 Hz in Case 37 with circumaural earphones. Realize in
one instance you can determine three points on the plateau and thus a
valid masked threshold, whereas in the other instance you can find
only two points, which makes the validity of the threshold
questionable. In such cases the results at other frequencies, and other
information such as immittance results and case history, are of the
utmost importance in leading you to the correct clinical interpretation.
To gain additional practice, return to Chapter 3 and mask the bone
conduction thresholds of those audiograms whenever necessary.
Summary
This chapter has discussed the requirements for masking bone
conduction thresholds as represented by the second rule of masking.
All bone conduction thresholds must be retested when an air-bone gap
is present except in one circumstance. This exception exists when
there are bilateral air-bone gaps, a condition which dictates that both
bone conduction thresholds are initially suspect. If, however, after
retesting one of these bone conduction thresholds, it becomes poorer,
then the other unmasked threshold was a true threshold and need not
be reevaluated. In the next chapter we will examine an instance in
which an air conduction threshold may have to be masked due to the
results of masked bone conduction.
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Chapter Five
Re-examining Air Conduction Thresholds
When the necessity to mask was first discussed in Chapter 3, only the
air conduction thresholds were considered. Masking was predicated
upon a 40 dB or greater difference between the left and right
unmasked air conduction thresholds at any frequency for circumaural
earphones and a 60 dB or greater difference for insert earphones. The
fact of the matter is, however, that a crossed-over signal is transduced
or "heard" by the cochlea; and the sensitivity of the cochlea is directly
reflected by the bone conduction, not the air conduction, threshold. As
a result the need to mask a given air conduction threshold must be
based upon the opposite bone conduction threshold, not the opposite
air conduction threshold.
Why, then, was this not revealed earlier? Several reasons support the
current organization. The first rule of masking is quite valid, and in
many instances a 40 to 60 dB difference between air conduction
thresholds does materialize when left and right air conduction
thresholds are measured (recall that both air conduction thresholds are
obtained before any bone conduction thresholds are acquired). In this
situation, it is necessary when using circumaural earphones to retest
immediately all air conduction thresholds that are 40 dB or poorer than
the opposite threshold without ever removing the earphones and
placing the bone conduction vibrator. The crucial difference would be
60 dB for insert earphones. Furthermore, there may be some instances
in which only air conduction testing can be done due to time,
equipment, or conditions of the testing environment, so it is important
to realize the need to mask based on the air conduction results alone.
Finally, most complex processes are easier to learn when subdivided
into several segments; presumably this is true of masking.
With this background, the third rule of masking is defined in the
following box.
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Chapter Five  61
MASKING RULE NO. 3
An air conduction threshold at a given frequency in a given ear must
be masked whenever it is 40 dB or more poorer than the bone
conduction threshold in the opposite ear when using circumaural
earphones. It must be masked whenever it is 60 db or more poorer
when using insert earphones.
It is assumed that the bone conduction threshold in the opposite ear has
already been masked, if necessary. To clarify the order of testing,
consider the following sequence of events. First, air conduction
thresholds are acquired in both ears, masking in accordance with the
first rule, if required. Second, bone conduction thresholds are acquired
in both ears, masking, if required, by the second rule. At this point the
air and bone conduction thresholds on the audiogram can be examined
and the third rule of masking applied. Each air conduction threshold is
compared to the opposite bone conduction threshold to reveal the
necessity of retesting. If needed, the bone conduction vibrator is then
removed, and the earphones once again placed over the ears. It is
important to emphasize that there is no way to recognize that further
air conduction testing is mandatory until after the bone conduction
testing is completed, including any masking of bone thresholds that
may be needed.
Retesting Only One Ear
A significant aspect of the third rule of masking is that it pertains only
to one ear, just as was the situation with the first rule of masking. An
example will help to clarify the point being made. Let’s assume that
we are using circumaural earphones. Inspect the portion of an
audiogram in Figure 5-1(a). Look first at the air conduction thresholds:
they do not qualify for masking according to the first rule because
there is not a 40 dB or greater disparity between them. Then realize
that both bone conduction thresholds may need to be masked due to
the second rule (remember that if the first bone threshold to be retested
becomes poorer, the other threshold was originally correct and need
not be masked). Let us assume that after masking bone conduction, the
audiogram appears as in Figure 5-1(b). Lastly, apply the third rule of
masking: the right air conduction threshold is 40 dB below the left
bone conduction threshold; therefore, the right air conduction
threshold must be retested. If the initial response at 40 dB HL was due
to crossover, then the real right air conduction threshold will be
poorer; or, if the recorded threshold was really heard in the right
cochlea, then the masked threshold will remain the same. Recognize
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that the crucial difference between right air conduction and left bone
conduction would be 60 dB for insert earphones.
Figure 5-1. Configuration (a) in which bone conduction must be
masked. After bone conduction has been masked (b), right air
conduction must be masked.
Because the third masking rule is applied after masked bone
conduction results are finalized, you should realize that this rule
should now be applied to the audiograms in Chapters 3 and 4. Some of
those air conduction thresholds (e.g., Case 23) will require further
testing according to this third rule, so it would be beneficial to
reexamine your results and complete those tests where required.
Practice
The masking procedure itself is exactly the same when applying the
third rule of masking as it was for the other two rules. The object is to
find the plateau, if there is one, because the plateau reveals the true
threshold. But, before reaching the section of the test where the third
rule can be executed, it is first necessary to find the air conduction
thresholds bilaterally (masking by the first rule, if warranted), then
measure the bone conduction thresholds in both ears (masking by the
second rule, if required), and finally inspect the results to determine
whether any of the air conduction thresholds must be retested by the
third rule. Proceed with your practice by selecting Case 41. After that
examine Cases 42 and 43. These three cases will reveal instances in
which there are thresholds that have crossed over and other cases in
which the original thresholds are correct.
Disparity Between Air And Bone Thresholds
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Chapter Five  63
Up to this point there has generally been an exact association between
the air and bone conduction thresholds when the audiogram
represented normal hearing or a sensorineural loss. An example of this
is shown in Figure 5-2, wherein the right ear has both air and bone
conduction thresholds of 0 dB HL, and the left ear has equal air and
bone thresholds of 30 dB HL, signifying a mild sensorineural loss.
This equivalence was fabricated to permit easier learning of the
different kinds of hearing losses; i.e., conductive, mixed, and
sensorineural.
Figure 5-2. Equivalent air and bone thresholds.
Such precise correspondence between air and bone conduction
thresholds is not commonplace in the clinical situation. Rather there
will frequently be slight differences between the respective air and
bone conduction thresholds at nearly every frequency for both normal
hearing and sensorineural losses. A typical example is presented in
Figure 5-3, which depicts a listener with hearing within the normal
limits in the lower frequencies, and a mild sensorineural loss in the
higher frequencies. Such an audiogram is typical of may individuals in
the fifth or sixth generation of life.
Figure 5-3. Typical audiogram revealing intermingled air and bone
conduction thresholds.
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Notice that the air and bone conduction thresholds intermingle, with a
bone conduction threshold occasionally being slightly better or even
somewhat poorer than the corresponding air conduction threshold. It
was stressed in Chapter 4 that a bone conduction threshold can never
be poorer than the respective air conduction threshold. But audiometry
is somewhat imprecise due to a multitude of factors including
calibration of the pure-tone signals, accuracy of the tester in following
the exact modified Hughson-Westlake protocol, and several aspects of
the listener's behavior, including alertness, motivation, cooperation,
internal physiological noises (stomach, arteries, etc.). The consequence
is that a bone conduction threshold is occasionally measured as being
5 dB or even 10 dB poorer than the corresponding air conduction
threshold at the same frequency. Similarly, a bone conduction
threshold can be realized as 5 or 10 dB better than the corresponding
air conduction threshold and not be a true, though minimal, conductive
loss.
The frequent inequality of thresholds has an important implication for
masking with regard to Masking Rule No. 2. Inspect the bone
thresholds in Figure 5-4a. They differ by 5 dB with the left ear
appearing to have the better sensitivity, but they should be viewed as
equivalent due to the multiple factors that determine the accuracy of a
threshold. Let’s assume that the clinician decided to mask left bone
first. The result is indicated in Figure 5-4b: left bone did not shift after
masking, so according to Masking Rule No. 2, right bone now must be
masked. Indeed, after masking, right bone did shift as demonstrated by
Figure 5-4c. Remember that differences as small as 5 dB or even 10
dB cannot be regarded as significant differences and do not necessarily
represent better or poorer hearing sensitivity.
(a)
0
10
20
30
40
50
(b)
0
10
20
30
40
50
(c)
0
10
20
30
40
50
Figure 5-4. Masking Rule No. 2 and unequal unmasked bone
conduction thresholds.
Instruction Manual Audiology Clinic V2
Chapter Five  65
Realistic Clinical Responses
By now it is expected that you can find thresholds with considerable
ease and readily apply the three rules of masking. Therefore, the
remainder of the practice audiograms in this chapter will reveal results
more representative of actual everyday clinical experience; i.e., the air
and bone thresholds will not always match up exactly for normal
hearing and sensorineural losses. This realization will necessitate that
you inspect the entire audiogram to determine overall the kind of loss
that is present, rather than considering each frequency individually.
Like much of life, the audiogram reveals many "gray" areas. Only
extensive real-world experience soothes the qualms that arise when not
all of the air and bone conduction thresholds are equal as in the
"textbook" cases presented heretofore. Select the remaining cases in
this chapter, cases 44 and 45, to assess hearing losses chosen from
clinical files.
Summary
Chapter 5 has addressed the necessity of reexamining air conduction
thresholds after masked bone conduction thresholds have been
realized. In cases where the air conduction thresholds in one ear differ
by 40 dB or more from the masked bone conduction thresholds in the
contralateral ear, they must be retested. Chapter 6 will project you
farther into the "real world" by introducing variability in the listener's
responses to the tone.
Chapter Six
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Variability In Listener Responses
All of the topics have been covered that are prerequisite to obtaining
correct pure-tone thresholds. These include the modified HughsonWestlake threshold procedure and three rules of masking which
account for every type of configuration. Only one situation has not
been explored: variability in the listener's responses. To facilitate rapid
learning of the basic techniques used to determine pure-tone
thresholds, the listener has been, in every case, totally consistent.
Complete reliability is not realistic; rather in the actual clinic there will
be many instances wherein the listener behaves unreliably. For
instance, assume that when testing is first begun, the right air
conduction threshold is obtained at 1000 Hz and found to be 35 dB
HL. The ASHA protocol dictates that this threshold be remeasured
after the other thresholds in that ear are recorded before proceeding to
test the left ear. This being accomplished, the retested threshold is
found to be 45 dB HL, a 10 dB difference. This confirms one instance
of variability.
Now consider a different situation. It is most easily expressed in
graphical form, so refer to Figure 6-1. Notice that when the intensity of
the tone was increased in the threshold measurement stage of the
procedure (beginning with presentation 3), the listener responded at
three different levels that are circled: 25 dB, 35 dB, and 30 dB. What
is the threshold? It has not been determined yet, as there have not been
three responses at the same level while increasing the intensity from
below threshold to above threshold. In fact there have not even been
two responses at the same level; each response has been at a different
intensity. So the threshold determination must progress until three
responses at the same intensity are recorded. This example
demonstrates the kind of variability which some listeners exhibit.
Needless to say, such behavior is very frustrating to the clinician.
1
2
3
4
Presentation
5 6 7 8
9 10 11
0
10
dB 20
HL
30
40
N
N
R
R
N
N
N
N
R
R
Figure 6-1. Graph of inconsistent responses.
There are many reasons for variable responses. Some have to do with
the current physiological state of the listener. He or she can be
Instruction Manual Audiology Clinic V2
Chapter Six  67
momentarily distracted by a pain or an itch. He or she can become
drowsy, or too hot or too cold. He or she can have interfering tinnitus.
He or she can purposely be attempting to falsify the true threshold.
There are a plethora of causes, many of which can never be explained.
The occurrence of variable responses makes the clinician's task
considerably more difficult. Often persistence will lead to satisfying
the threshold criteria, although sometimes clinical intuition must be
invoked when three responses at the same intensity cannot be found.
Results may be obtained that require a special message to be added to
the audiogram; phrases such as "reliability: fair" or even "reliability:
poor" may be included under the "comments" section.
The nuisance of unstable responses becomes increasingly magnified
when attempting to obtain masked thresholds. Bear in mind that the
tone is ordinarily presented only once at each level during the masking
process. Additionally, the plateau representing the actual threshold
may be only 5 or 10 dB wide, thus variable responses can entirely
obscure your observation of a plateau.
Practice
Because fluctuating responses are very much a part of everyday
audiometry, they have been included as part of The Audiology Clinic.
The audiograms for this chapter, which includes Cases 51 - 55, will
supply ample opportunity to sample the "real world" in terms of
variable responses by the listener. These responses will change over a
range of 10 dB, and the variation from the exact threshold for a given
audiogram will take place about 50% of the time on the average. The
percentage is selected randomly by the computer. Thus, the same
audiogram may represent different behavior each time it is tested; that
is, one time it may display minimal, or even no, variability and the
next time extreme inconsistency. Practice, using all of the skills you
have acquired from studying the previous chapters. These audiograms
are guaranteed to be challenging!
Summary
This chapter completes your training in pure-tone audiometry.
Simulation offers the safe environment in which the student or
clinician can experiment, repeat, and practice without worry about the
listener's reaction to his or her mistakes. Therefore, redetermine the
many audiograms over and over again until such time as you can
record the correct results in no more than 15 minutes for even the most
complex configuration. With the ability to apply the rules and
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Instruction Manual Audiology Clinic V2
procedures presented in the previous chapters, you should have no
difficulty when testing a live listener.
Chapter Seven
Aural Acoustic Immittance
Use of the immittance meter allows one to contribute substantially to
the diagnosis of middle ear disease, separate cochlear from
retrocochlear dysfunction, and even assist in the prediction of hearing
sensitivity. The immittance test battery, completed in less than 10
minutes, can provide the information to make all these clinical
discriminations as well as provide a wealth of related information.
The auditory system, being an exquisite sound analyzer, is not without
a good bit of complexity in the manner in which its dysfunction is
revealed. Thus, while data from the immittance test battery provide a
quick and reliable means of assessing the entire anatomical chain of
structures in the ear, the task of arriving at a defensible conclusion
from the information obtained requires the integration of a
considerable breadth of knowledge.
With this in mind, it is the primary goal of this text and software to
present the essential components of electroacoustic immittance and
through the presentation of case studies to teach the integration of
clinical findings into comprehensive, defensible conclusions about the
state of the auditory system.
Impedance Basics
The movement of mechanical systems like the middle ear is governed
by physical properties common to all vibrating systems. Whether
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Chapter Seven  69
mechanical, electrical or acoustic, a system's impedance describes the
degree of opposition to the flow of energy through that system. In the
ear, we are interested in the acoustic impedance produced by the
middle ear and cochlear structures. The acoustic impedance of the
middle ear is comprised of two elements, resistance and reactance.
Resistance
The friction created by moving structures that are coupled together
produces an energy dissipation termed resistance. While the middle ear
system is an efficient sound transducer, it is not perfect. The manner in
which the ossicular structures are suspended in the middle ear along
with the interaction between the inner ear fluids and the basilar
membrane creates a resistance. This resistance may then be thought of
as the byproduct of the friction created by the conversion of acoustical
to mechanical vibration in the middle ear and by the conversion of
hydro-mechanical to electrical energy in the cochlea.
Reactance
The second component of middle ear impedance is quite unlike
resistance. Whereas energy is dissipated and lost through resistance, it
is stored through reactance. Systems that vibrate have a certain mass
and a certain stiffness, and it is not too difficult to visualize the middle
ear structures rocking back and forth under the influence of sound
energy moving the tympanic membrane. That element of impedance
related to the middle ear structures reacting to movement is termed
reactance.
The mass and stiffness of the structures each oppose separately the
positional change brought about by sound energy. Thus, reactance is
made up of two components: mass reactance and stiffness reactance.
Mass reactance is a product of the mass and resulting inertia of the
ossicular chain. Stiffness reactance is generated by the tympanic
membrane, and the oval and round window membranes. See Zwislocki
(1976) for a review of the resistance and reactance components of
middle ear impedance.
The Problem Of Timing
When sound waves set the middle ear structures in motion, the
characteristics of mass and stiffness reactance will dictate how the
structures move. For instance, if the tympanic membrane (TM) and
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ossicular chain have a high degree of stiffness, the condensation cycle
of the sound wave will displace the TM in proportion to the sound
pressure exerted, and the TM's motion will follow closely the pressure
cycle of the sound wave. In contrast, in a middle ear that is dominated
by mass, the inertia of the structures prevents them from moving in
unison with the pressure cycle of the sound wave. This is not unlike
what we experience when attempting to move a heavy object. The
inertia of a piano (even on coasters) prevents the initial force of our
pushing from immediately creating movement, unlike the push and
subsequent movement of a much lighter object where there is little
time lag between the applied force and subsequent motion.
The model describing how resistance and reactance interact must
account for the fact that the middle ear system responds differently
when it is dominated by stiffness as opposed to when the mass
components dictate its movement. The stiffness and mass components
of impedance affect the time lag between the sound pressure wave
impinging on the tympanic membrane and its subsequent movement in
a manner that allows us to consider them as opposites. They are not
opposite forces, but the sound wave cycle and the movement cycle of
the structure that the sound wave influences are out-of-step with one
another in mass and stiffness dominated systems. As such, their
relationship lends itself to depiction in a phasor diagram where
stiffness reactance and mass reactance are 180o out of phase with one
another and the terms are labeled:
RA = Acoustic Resistance
XA = Mass Reactance
-XA = Stiffness Reactance
To observe how the impedance elements are diagrammed, click
Immittance. From the submenu displayed, click Impedance and select
Example 1. Now, click the Next in sequence to observe the impedance
elements diagrammed.
This example assumed that we measured in arbitrary units the
following values for reactance and resistance:
XA = 1.0
-XA = -5.0
RA = 4.0
The net reactance of -4.0 is the arithmetic sum of the positive mass
reactance (1.0) and the negative stiffness reactance (-5.0). The
Instruction Manual Audiology Clinic V2
Chapter Seven  71
impedance value is the vector quantity derived by forming the
hypotenuse of the right triangle, and ZA = Acoustic Impedance is 5.7.
Note that the phasor points to the "stiffness dominated" quadrant (i.e.,
lower half of screen). This does not mean, however, that the ear did
not have any mass reactance. Rather, this ear had more stiffness than
mass. From geometry, recall the similarity in finding the impedance
value to that of solving for the hypotenuse of a right triangle when the
length of the two sides is known. The relationship between XA and XA (both XA terms are summed to a single value) and RA can thus be
expressed as:
That is, the absolute impedance equals the square root of the sums of
resistance and reactance squared. Impedance is expressed in acoustic
ohms, a unit of measurement that conveys the degree of opposition to
the flow of sound. We use the notation |ZA| rather than ZA to imply
that the above formula specifies the magnitude of impedance only.
For, without also specifying the fraction of time between sound energy
impacting the middle ear and its resultant motion, any number of
combinations of resistance and reactance could yield the same absolute
impedance.
Now click Immittance, Impedance, and Example 2. Observe that the
resulting impedance value is the same as that in Example 1 even
though the individual values of the components were different. Note
that both systems have the same absolute impedance, yet Example 1 is
a system dominated by stiffness; whereas Example 2 is a system
dominated by mass.
In summary, impedance is the vector quantity of resistance and
reactance that is described completely when phase information is
provided. See Lilly (1972) for a thorough explanation.
The terminology used to specify completely an ear's impedance
includes the frequency of the sound used to stimulate the ear as well as
the phase angle. In so-called "polar" notation:
ZTM = 1530 \ -74.1°\ 250 Hz
is read, "impedance at the TM (tympanic membrane) equals 1530
acoustic ohms at negative 74.1 degrees for 250 Hz. Clearly, this
impedance value is considerably larger than the abnormally small
values used to illustrate the principles of calculating impedance in the
computer simulation above.
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You may experiment with different values of resistance and reactance
by doing the following:
Click Immittance, Impedance, and Custom values. Then follow the
instructions to enter your own values.
Acoustic Admittance
Measuring the impedance of the middle ear is not without some
technical difficulties. It turns out that it is easier to measure the flow of
acoustic energy through a system than to measure its impedance. See
Van Camp and Creten (1976) and Newman and Fanger (1973) for a
complete discussion. Unlike impedance, admittance is an expression of
the ease of energy transfer through the middle ear. Whereas the
symbol for acoustic impedance is ZA, the symbol for acoustic
admittance is YA. Impedance and admittance are reciprocals such that:
The impedance components of reactance and resistance have
counterparts in admittance termed susceptance and conductance. Their
relationship is seen below.
ACOUSTIC IMPEDANCE |ZA|
•Resistance (RA)
•Mass Reactance (XA)
•Stiffness Reactance (-XA)
ACOUSTIC ADMITTANCE |YA|
•Conductance (GA)
•Negative Susceptance (-BA)
•Positive Susceptance (BA)
| Z A|= ( R A )2 + ( X A )2
|Y A |= ( G A ) + ( B A )
2
2
Since impedance and admittance are reciprocals, the ear with otitis
media, for instance, will exhibit a high impedance to the transmission
of sound energy and a low admittance to the transmission of the same
sound energy.
To review, the impedance of the middle ear governs its capacity to
move in response to the sound waves impinging on the tympanic
membrane. Since measuring the impedance of the middle ear poses
technical difficulties, it is more feasible to measure the admittance of
the ear. These two measures, impedance and admittance share a
reciprocal relationship. Thus, an ear with otitis media would have a
higher than normal acoustic impedance (ZA) while the same ear would
have a reduced acoustic admittance (YA).
Instruction Manual Audiology Clinic V2
Chapter Seven  73
Static Admittance
The admittance of the ear is recorded in one of two measurement
states: 1) while air pressure against the eardrum is systematically
changed (tympanometry) and 2) while the eardrum is at rest (static
admittance).
While it is confusing to navigate through the impedance and
admittance nomenclature, it is important to grasp how the two
concepts are related as admittance concepts are used more frequently
in the audiology literature.
With low frequency stimulation of the ear the mass (-BA) and resistive
(GA) elements of admittance contribute little to the overall admittance
of the middle ear system. Rather, the stiffness component, positive
susceptance (BA), derived from the tympanic, oval and round window
membranes, predominates when stimulated by low frequency sounds.
Note that this is true whether one is measuring in admittance or
impedance units. Stated another way, at low frequency stimulation the
ear is dominated by the stiffness reactance component which in
admittance terminology is the positive susceptance (BA). Because the
stiffness component predominates at low frequencies it is said to be
"stiffness dominated."
Describing the stiffness of the ear is made simple by equating it to the
compliance of a volume of air that is just as compliant. The logic of
equating the ear's compliance to an equivalent volume of air is
straightforward. An enclosed cavity of air has a certain compliance or
springiness to it. For instance, when attempting to inflate an air
mattress, one notices it is easier to inflate when there is little air in the
mattress than when nearing the end of the process and the mattress is
close to full inflation. This is because the enclosed volume of air has a
springiness or compliance that is dependent upon its volume. An
unfilled air space having a large volume (such as a very large, deflated
balloon) is highly compliant when inflation begins. Near full inflation
there is very little space left for air to fill, and the compliance is
reduced considerably. Since cavity size determines compliance, we
may equate a structure's compliance to that of an equivalent volume of
air.
Whether the immittance manufacturer describes their admittancemeasuring instrument as an impedance bridge, a middle ear screener,
or an otoadmittance meter, the admittance of the ear is usually equated
to the compliance of an equivalent volume of air, and expressed in
cubic centimeters (cm3) or milliliters (ml).
Aural acoustic immittance is the term used to denote the impedance
and/or admittance as it is measured by commercially available
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instruments. Refer to the ANSI standard (ANSI S3.39-1987) for a
description of the specifications for aural acoustic immittance
instruments.
Observe the following patient data:
Right:
Left:
YTM = 0.85 ml
YTM = 0.40 ml
@ 226 Hz
@ 226 Hz
We may conclude:
1.The admittance (YTM) at the right ear TM is 0.85 ml when
measured with a 226 Hz tone.
2.The admittance (YTM) at the left TM is 0.40 ml as measured with
the same frequency.
3.The right ear is more compliant than the left (because its admittance
value is greater) or said another way, the left ear has more stiffness.
4.The right middle ear has the same overall admittance as an enclosed
cavity of air of 0.85 ml. But, the external canal is not 0.85 ml in
volume, nor are any of the cavities or structures 0.85 ml. The right
middle ear system does, however, have the same overall admittance
that a cavity of air having a volume of 0.85 ml has.
Recall that acoustic admittance as expressed in equivalent volume is
appropriate only when the ear is stimulated with a low frequency tone.
This is because at low frequencies the mass and resistive components
are negligible in normal middle ears and the stiffness component (BA)
is very close to the overall admittance (YTM). To illustrate, the left ear
of one of the authors was measured in sequence first for YTM @ 226
Hz (the overall admittance being expressed in equivalent ml) then only
for acoustic susceptance (BA) @ 226 Hz (expressed in mmho) with
the following results: YTM=1.1 ml, BA=1.0 mmho. Thus, YTM @
BA for low frequency sounds.
The mmho (pronounced milli-mow) is the unit of admittance
measurement and for practical purposes is used when the admittance
components cannot be expressed in equivalent milliliters. This occurs
at higher frequencies where the mass and resistive components play a
greater role in the overall middle ear response.
At middle ear resonance, approximately 800-2000 Hz (Hunter and
Margolis, 1992), the mass and stiffness components are equivalent.
Above the resonance frequency the system's overall admittance is
controlled more by the mass component. Because of the additional
electronics required to measure the other admittance components and
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Chapter Seven  75
the high degree of clinical sensitivity provided alone by the 226 Hz
probe tone, some immittance manufacturers do not include the
instrumentation to measure the mass and resistive components
separately. However, some middle ear pathologies clearly are better
detected with higher frequency probe tones. When the higher
frequency probe tones are used, the mass and resistive components of
the total admittance are measured individually. For higher frequency
probe tones, such as 678 Hz or 1000 Hz, the mmho is the unit of
measurement. One may see notation such as: YA(678 Hz) = 5.62
mmho; that is, the admittance measured with a 678 Hz probe tone is
5.62 mmho (see the table below). Commercially available software has
allowed for the measurement of immittance components from 250 to
2000 Hz as well as determining the ear's resonant frequency. While
beyond the scope of this program, multi-frequency tympanometry will
be a valuable clinical procedure in further explaining the dynamics of
middle ear dysfunction. Hunter and Margolis (1992) provide an
excellent tutorial on multi-frequency tympanometry.
The Relationship between Test Frequency and Measurement Unit
Low Frequencies (220, 226 Hz)
High Frequencies (678, 1000 Hz)
• Overall admittance (YA or YTM) expressed
in equivalent volume (ml)
• Overall admittance (YA) in mmho
• Susceptance (BA) in mmho
• Susceptance (BA) in mmho
• Conductance (GA) in mmho
Summary
This chapter has introduced the physical concepts essential to
understanding the measurement units in the electro-acoustic
assessment of the middle ear structures. In the next chapter you will
begin using these concepts in the procedure known as tympanometry.
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Chapter Eight
Tympanometry
The basic immittance test battery consists of two tests along with the
subtests associated with them. The two tests are:
• Tympanometry
• Acoustic Reflex
Tympanometry is the measurement of the change in the admittance of
the middle ear while a positive to negative air pressure sweep is
introduced into the sealed ear canal. The admittance of the middle ear
is deduced by measuring the ear's effect on a "probe tone" which is
simultaneously presented into the canal.
Tympanometry Procedure
The patient is seated while examination of the pinna and external canal
structures is performed. Otoscopy of the canal and TM is carried out
with the intention not only of assisting in the interpretation of the
immittance findings but more importantly, to determine if a condition
exists that would preclude performing the immittance tests. Such
conditions include but are not limited to

foreign objects in the canal

drainage or open sores in the canal

whenever probe tip insertion is painful

recent TM or middle ear surgery
Assuming the immittance instrument is in the tympanometry mode and
the probe tone frequency has been selected, one may next insert the
probe. A common technique is to insert the probe tip into the ear canal
with one hand while the other hand gently pulls the pinna laterally and
back with a motion similar to that used in otoscopic examination. This
procedure is an attempt to slightly straighten the cartilagenous portion
Instruction Manual Audiology Clinic V2
Chapter Eight  77
of the canal to assist in probe tip insertion. Upon probe tip insertion the
air pressure sweep is commenced, usually by pressing a button, and
the tympanogram is plotted.
To observe the simulation of a tympanogram being plotted, click Case
and Select Case. Choose Case 2. Now click Test, then Immittance.
Click the Plot button on the lavender-shaded immittance instrument
and observe the tympanogram being plotted from positive to negative
pressure. You will have plotted the Right Tympanogram. To close the
demonstration, click Case on the main menu and Close Case from the
drop- down menu.
Since it is the delivered air pressure changes in the sealed ear canal
that produce the dramatic effects on sound transmission through the
middle ear and generate these fascinating tympanograms, the
measurement of air pressure is of substantial interest to us. Air
pressure measurement in tympanometry is notably different than other
expressions of air pressure. That air pressure is expressed in mmH2O
is an extension of the convention whereby we express atmospheric
pressure with a mercurial barometer by observing the height of a
column of mercury that any given air pressure will support. In
tympanometry, however, we use the term relative air pressure
implying that the air pressure measured in the ear canal is the same as
atmospheric pressure in the locale where the measurement is taking
place. A +100 mmH2O ear canal pressure means the pressure is 100
mm greater than the ambient pressure. An ear canal pressure of -150
mmH2O means the pressure is 150 mm less than atmospheric
pressure. The decaPascal (daPa) has replaced mmH2O as the unit of
measure for air pressure. You may recall from studying the decibel
that the Pascal (Pa) is the MKS (meter/kilogram/second) unit of
pressure. Since 1.02 mmH2O is equivalent to 10 Pa or 1 daPa, we
often interchange the air pressure units expressed in mmH2O with
decaPascals. Thus, 50 mmH2O @ 50 daPa.
Take note of the X (horizontal) and Y (vertical) axes on the screen.
The X axis represents relative air pressure changes expressed in
mmH20 or decaPascals (daPa). The air pressure began at +200 daPa,
swept negatively through 0 daPa (atmospheric pressure), and ended at
-400 daPa. The shape of the tympanogram may vary slightly as the
rate and/or direction of the air pressure change is altered (Shanks and
Wilson, 1986). The pressure extremes (+200 and -400 daPa) are used
to place the tympanic membrane (TM) into an immobile position.
Consider how tight and stiff the skin of a balloon is when it is fully
inflated. Having the TM rigid effectively decouples (for measurement
purposes) the middle ear from the ear canal which has its own
impedance characteristics. When the TM is made maximally stiff and
the admittance is measured with a low frequency probe tone, the
admittance of the volume of air in the ear canal alone is estimated.
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This is necessary since we ultimately want to know the admittance of
the middle ear beginning at the TM, not at the end of the probe tip
assembly, which would encompass a measurement of the column of
air in the ear canal as well. Modern immittance meters perform the
calculations that separate the admittance effects of the ear canal from
the intended measurement of the middle ear.
The Y axis represents the acoustic admittance expressed in milliliters
(ml) of equivalent volume. Note that as the air pressure is reduced,
admittance increases, then is maximized at 0 daPa, and again declines
as negative air pressure stiffens the TM. The static admittance (YTM)
is calculated by subtracting the peak admittance from the admittance
observed at the ±200 daPa points. In this example the admittance of
the ear is 1.1 ml. Note that the resulting trace, the tympanogram, is a
measure of admittance as a function of changing air pressure.
Within the probe tip assembly are three essential components used in
the measurement of middle ear immittance; a loudspeaker, a
microphone, and an air pump with a pressure transducer.
To see an animated demonstration of the probe tip mechanism, click
Immittance on the Main menu. Click Probe tip and observe the probe
mechanism diagram. Click the Normal and Pathological buttons to
observe the influence of eardrum stiffness on the measured sound
pressure in the sealed ear canal during tympanometry.
The point of the demonstration is to compare the size of the sound
waves traversing the eardrum into the middle ear cavity vs. the size of
the sound waves being reflected off the eardrum and back to the
microphone at ambient pressure.
It is instructive to stop and analyze what is actually taking place within
the sealed ear canal as the tympanogram is being generated. First, it is
essential that there be an air-tight (hermetic) seal of the probe tip
within the confines of the external auditory canal. The sealing of the
canal insures that the air pressure extremes targeted by the pneumatic
pump may be realized.
Once the +200 daPa air pressure is achieved, either manually by the
clinician or automatically by the instrument, the loudspeaker
transduces a 226 Hz pure tone into the ear canal. With the introduction
of +200 daPa air pressure into the sealed ear canal, the eardrum
becomes rigidly fixed in place. The rigid eardrum absorbs little (i.e.,
reflects most) of the probe tone, and for measurement purposes,
essentially separates the canal from the middle ear. The microphone in
the probe assembly measures the amount of sound pressure transmitted
by the loudspeaker that is reflected off the eardrum. Furthermore, this
sound pressure changes when the air pressure is varied from positive
to negative pressure. As the air pressure is gradually reduced and the
eardrum becomes more mobile, the sound pressure in the canal is
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reduced as well, since a less rigid eardrum will transfer more energy
through the middle ear than when rigid. As the air pressure approaches
atmospheric pressure (0 daPa) the eardrum's mobility is maximized.
This is because when the air pressure is equal on both sides of the
eardrum (the normal situation); maximum sound transfer occurs
creating a "peak" on the tympanogram. As air pressure varies from
positive 200 daPa through 0 daPa, the intensity of the reflected probe
tone decreases as TM rigidity is reduced. As air pressure moves
through 0 daPa toward -400 daPa, the intensity of the reflected probe
tone increases within the sealed canal as the TM is once again made
rigid. So, changes in the reflected sound pressure of the probe tone
indicate the amount of sound energy being transferred from the ear
canal into the middle ear. The immittance instrument converts these
probe tone sound level changes to a measure of one of several
"immittance quantities" that may include susceptance (BA) expressed
in acoustic millimhos, or compliance or admittance expressed as an
equivalent volume, or even in arbitrary units.
To review:
1.Air pressure is varied in the sealed ear canal.
2.The intensity of the reflected probe tone in the sealed canal changes
as the air pressure influences eardrum mobility.
3.When the air pressure in the external canal matches that of the
middle ear cavity, maximum sound energy transfer occurs, creating a
tympanometric peak.
4.The air pressure where the peak occurs is an approximation of the air
pressure in the middle ear cavity.
5.The amount of eardrum mobility observed at the height of the peak
is expressed as the ear's admittance.
Tympanometric Norms
The following norms were suggested by Margolis and Heller (1987)
and presented in the ASHA guidelines for the screening of middle ear
disorders (ASHA, 1990). The norms represent a 90% range of
observed immittance values. The normative values for children are
from 3-5 years.
1. EAR CANAL VOLUME: Children 0.4 - 1.0 cm3, mean 0.7 cm3;
Adults 0.6 - 1.5 cm3, mean 1.1 cm3. The norms are expressed in
equivalent cubic centimeters, (cm3 ) ,and denoted by Vec.
Ear canal volume is a significant measure in that it can help eliminate
the possibility of measurement artifact. For instance, an ear with a
perforation of the TM may show a tympanogram of the same shape as
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an ear having otitis media with effusion. One of the ways these
pathological conditions are distinguished is with the canal volume
measurement. When the TM is not intact, the probe tone circulates
throughout the middle ear cavity as well as in the external canal. This
will yield a volume measurement much greater than the 1.5 cm3 upper
limit. The actual volume will depend upon the state of the middle ear
mucosa. The canal volume measure can also aid in the detection of
measurement artifact, especially in young children where a probe tip
improperly positioned against the canal wall (easily done with young
children) may produce a spurious tympanogram. The manner in which
the canal volume measure assists in discriminating among several
artifactual and pathological states will be presented in the Case Studies
that follow.
2. PRESSURE PEAK: -150 to +50 daPa: The middle ear cavity may
be thought of as a gas chamber capable of a range of internal static
pressures. That the normal range of pressures includes both positive
and negative values is recognition that the Eustachian tube is not
perfect in its role of sustaining atmospheric pressure within the middle
ear cavity, nor is middle ear pressure solely determined by Eustachian
tube dynamics (Sadé and Luntz, 1990). Recall that the middle ear air
pressure is denoted on the tympanogram by the place of the
tympanometric peak on the X axis. Pressure values outside the normal
range may signal the onset of conditions likely to result in otitis media.
The clinical significance of pressure values outside the normal range is
dependent upon other tympanometric factors such as the static
admittance measure and the acoustic reflex test battery.
It is a common mistake to over-interpret the tympanometric pressure
peak since it may be transient, and its significance depends largely on
the case history, otoscopic inspection, and other test data. The range of
suggested normative values (-150 to +50 daPa) used in this program
does not imply a cut-off for normal vs. pathological states. Rather, it is
a guide based on clinical experience of the range of peak pressure
values typically observed in healthy middle ears, and will be used to
classify tympanograms to aid in their discussion.
3. STATIC ADMITTANCE: Children 0.2 to 0.9 ml, mean 0.5 ml;
Adults 0.3 to 1.4 ml, mean 0.8 ml. Static admittance is the
quantification of middle ear admittance, i.e., the height of the
tympanogram (YTM). With experience, many clinicians find the shape
of the tympanogram more useful than the admittance quantity per se.
Recall that static admittance when measured with a 226 Hz probe tone
may be expressed in equivalent volume, and as such equates the
immittance of the ear to that of an equivalent volume of air.
4. GRADIENT AND TYMPANOMETRIC WIDTH: The visual
inspection of the tympanogram yields much information. One of the
finer points of tympanogram interpretation has to do with its shape
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Chapter Eight  81
near the pressure peak, which the gradient measure quantifies (Brooks,
1969). The point of interest is the region + 50 daPa on either side of
the pressure peak. The calculation of the gradient yields the ratio of the
admittance around the tympanometric peak to the overall admittance
of the ear. Typically, tympanograms characterized as rounded or
shallow will yield gradients less than or equal to 0.2. A gradient
greater than 0.2 is considered normal. Tympanometric width (TW) is
likewise a measure conveying the “roundedness” of the tympanogram.
TW is the amount of air pressure distance that a defined region near
the peak of the tympanogram encompasses along the abscissa of the
tympanogram. For instance, a rounded tympanogram may have a TW
of 200 daPa, while a normal tympanogram may have a TW of 60 daPa.
Normative values for TW relate more to referral criterion for screening
of otitis media with effusion and as such will be discussed in the
Immittance Screening section found at the end of this chapter.
Tympanograms displaying comparative tympanometric widths are
seen in Figs 8-1a and 8-1b. Both the gradient and the TW are
tympanometric measures calculated by the immittance instrument.
Both of these measures of tympanometric width has been shown to be
associated with identification of middle ear effusion (Roush et al,
1995).
Figure 8-1a. Tympanometric width of 130 daPa
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Figure 8-1b. Tympanometric width of 60 daPa
Tympanogram Classification
The following tympanograms exemplify the classification system that
audiologists have adopted for common tympanometric forms as
conceived by Liden (1969) and Jerger (1970). Remember, the
classification of a tympanogram is not used as the definitive means of
contributing to the diagnosis of a middle ear condition.
1. TYPE A TYMPANOGRAM (see Fig 8-2)
The type A tympanogram is the normal tympanogram and is
characterized by: 1. Normal static admittance (as described by the
height on the Y axis), and 2. Normal peak pressure (as described by
the position on the X axis). Remember that the tympanogram reflects
the status of the middle ear only. A patient with profound
sensorineural loss will present with a type A tympanogram if the
middle ear is normal. The type A tympanogram verifies normal TM
mobility and normal ventilatory capacity of the Eustachian tube. Some
cases of stapes fixation present the type A tympanogram.
Fig 8-2 Type A tympanogram
2. TYPE B TYMPANOGRAM (see Fig 8-3)
The type B tympanogram has either a flat shape or a slightly rounded
appearance. This results from a maximally stiffened TM usually due to
middle ear effusion or its residual effects, and/or negligible air space in
the middle ear cavity.
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Chapter Eight  83
Fig 8-3 Type B tympanogram
3. TYPE C TYMPANOGRAM (see Fig 8-4)
The type C tympanogram is characterized by excessive negative
middle ear pressure as demonstrated by the peak of the tympanogram
occurring at a value more negative than –
Fig 8-4 Type C tympanogram
150 daPa. The static admittance, YTM,(reflected by the height of the
tympanogram) may not
necessarily be affected by the negative pressure.
4. TYPE AS TYMPANOGRAM (see Fig 8-5)
The AS tympanogram is characterized by normal middle ear pressure
(remember that normal may include a range of pressure values from 150 to +50 daPa) but abnormally reduced admittance, YTM, (height).
This tympanogram is observed in an ear with normal ventilatory
capacity but an overall stiffened vibratory mechanism. Ossicular
fixation and thickened TMs from chronic (but currently resolved) otitis
media yield the AS tympanogram.
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Fig 8-5 Type AS tympanogram
5. TYPE AD TYMPANOGRAM (see Fig 8-6)
The AD tympanogram is characterized by: 1) normal middle ear
pressure, and 2) excessive static admittance. Observe that the Y axis
has been rescaled. Accordingly, this tympanogram extends to nearly 6
ml. Scarred tympanic membranes as well as middle ears with ossicular
discontinuity will yield this tympanogram form.
Fig 8-6 Type AD tympanogram
Tympanometric Screening For Middle Ear
Disorders
While there is not a routine in this program to simulate tympanometric
screening for middle ear disorders, the subject is of considerable
interest to those who work with school-aged children. Screening for
middle ear disorders (ASHA 1979, ASHA 1990) has evolved to the
current state (ASHA 1997) in part, due to the recognition of
troublesome medical over-referral rates of children using the previous
screening guidelines. Because tympanometric peak pressure is quite
variable, and negative middle ear pressure alone is a poor predictor of
the onset of middle ear effusion, the present guidelines (ASHA 1997),
focus on the tympanogram to determine the necessity for medical
referral. The recommended referral criteria are: Ytm<0.3mmho or TW
> 200 daPa. The 1997 guidelines also benefit from previously lacking
specificity and sensitivity data provided by studies such as Nozza,
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Chapter Eight  85
1995; and Roush et al., 1995. You are encouraged to refer to the entire
document (ASHA 1997) prior to undertaking screening for middle ear
disorders.
Summary
This chapter has described the fundamentals of single probe-tone
tympanometry and introduced you to the basics of tympanogram
interpretation. Acoustic reflex data adds a significant refinement to
the diagnostic process. In the following chapter you will have a
chance to integrate audiometric and immittance information in a caseby-case format.
Chapter Nine
Acoustic Reflex
The reflexive contraction of the stapedius muscle upon presentation of
a sufficiently intense sound can provide diagnostic information about
the state of the middle ear, the cochlea, the VIIIth (auditory) cranial
nerve, auditory brainstem pathways, and the VIIth (facial) cranial
nerve. That the sound can be presented only to one ear yet produce a
contraction in both ears has led to confusion regarding the reporting of
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reflex thresholds. That is, a physiologic reaction to sound presented to,
say, the left ear can be monitored in the right ear as well as in the left
ear! Which ear then is being tested? It is conventional practice to
specify acoustic reflex thresholds for the ear that is stimulated. A
review of the neural pathway responsible for the reflex will help to
clarify this issue and facilitate an understanding of the clinical
significance of the test.
Ipsilateral Stapedial Reflex
It is instructive to observe the reflex's ipsilateral pathway, the neural
pathway responsible for a reflex contraction occurring in the same ear
that received the reflex eliciting sound. This can be accomplished in
the clinical setting because the reflex-eliciting sound stimulus can be
presented to each ear individually, whereas in everyday life it is more
likely that both ears would be in the sound field that elicits the reflex.
To observe an animation of the ipsilateral reflex arc click Immittance
on the menu. Next click Ipsilateral reflex arc and proceed as indicated.
Upon sound stimulation, the sensory route begins with neural
discharges from the cochlea, in this case the right cochlea, through the
auditory nerve to the first complex in the central auditory nervous
system, the cochlear nucleus. Describing the neural action in its
simplest form, when the neural discharges received at the right
cochlear nucleus reach a threshold value, a pathway connecting the
cochlear nucleus to the right motor nucleus of the facial nerve is
activated, resulting in a contraction of the right stapedius muscle.
Note, that not animated is the fact that when right cochlear nucleus is
activated, the superior olivary complex on the opposite, left side of the
brainstem is stimulated, initiating a reflex contraction of the left
stapedius muscle as well. Thus, the ipsilateral stapedial reflex does not
truly exist in the natural state because both left and right stapedial
muscles contract. The term does however have significant clinical
relevance as will be discussed below. Whether the tensor tympani
muscle is activated in concert with the stapedius muscle remains
unresolved. Borg (1973, 1977) claims that in humans the tensor
tympani does not contract in response to acoustic stimulation. Not
shown in the animations are the other neural pathways that
demonstrate that the acoustic "reflex" is not entirely reflexive in that
higher auditory centers may influence the occurrence and threshold of
a contraction. Influences such as immediately precedent stimulation
and anticipation of a loud sound are examples of such mediating
factors. To observe an ipsilateral acoustic reflex, the probe tip is
hermetically sealed in the ear to be tested, and the immittance meter is
changed from the tympanometric mode to the reflex testing mode. The
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Chapter Nine  87
results from tympanometry just obtained in the normal testing
sequence are used in the reflex test in that the observance of the reflex
is enhanced by presenting sound to the ear at an external canal
pressure equal to middle ear pressure. Thus, if middle ear air pressure
is -110 daPa, for reflex testing the external canal air pressure should be
set at -110 daPa. An impedance mismatch occurs when there exists an
air pressure differential between the external and middle ear air spaces
which can inhibit the vibratory motion of the TM and ossicular
structures. When testing reflexes, we want to maximize both the
transmission of sound through the ear as well as our ability to detect
minute changes to the ear's admittance caused by the resulting
contraction of the stapedius muscle. Some electroacoustic immittance
meters automatically set the reflex test mode's external canal air
pressure to match that of the middle ear pressure just obtained from
tympanometry. Note: In this computer simulation the appropriate air
pressure for reflex testing is set automatically.
Physiologically, sounds of sufficient intensity can elicit the acoustic
reflex. Whether the reflex is governed by sound intensity or its
loudness is open to debate (Margolis and Popelka, 1975). Whatever its
triggering mechanism, the subsequent contraction of the stapedius
muscle reduces the admittance of the middle ear thereby inhibiting the
motion of the TM. The immittance instrument continually monitors the
sound pressure of the probe tone in the external canal, so that any
change in middle ear admittance will change the probe tone's
transmission through the ear and hence its measured sound pressure in
the sealed ear canal.
To review the ipsilateral acoustic reflex:
1.The probe tip ear is the “test ear”.
2.A momentary (1-2 sec.) reflex eliciting sound is presented into the
probe tip ear.
3.An acoustic reflex stiffens the ossicular chain and decreases acoustic
admittance.
4.The probe tip microphone that has been monitoring the sound
pressure of the probe tone in the ear canal records a sudden increase in
sound pressure coincident with the decreased admittance of the middle
ear. (Proportionally more of the sound wave is reflected off the TM
and picked up by the microphone).
5.The nearly simultaneous reflex sound presentation and subsequent
change in acoustic admittance is noted on a graphic display.
To perform a threshold search for an ipsilateral stapedial reflex follow
this procedure. (Note: If the simulation is not set up to test Immittance
on Case 2 as on p. 58, click Case, Select Case, and choose Case 2.
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Now click Test, then Immittance. Replot the tympanogram by clicking
Plot.) Notice that under Stimulus Ear, Ipsilateral is selected and that
under Probe Ear Right is selected. Under Mode click Reflex thresh.,
then click Plot and observe the trace for a right ipsilateral reflex at
500 Hz. The initial intensity is 80 dB. This produces a deflection of
0.05 ml, which is above threshold. Next reduce the intensity to 75 dB
and click Plot; this results in a deflection of 0.02 ml which is the
criterion we will adopt for threshold. Finally, reduce the intensity to
70 dB and click Plot; there is no deflection, so this intensity is below
threshold. Accordingly, the ipsilateral threshold in the right ear at 500
Hz is 75 dB. Repeat this process by changing the frequency to 1000
Hz, 2000 Hz and finally to BBN (broad-band noise).
Contralateral Stapedial Reflex
While the presentation of a loud sound to only one ear may elicit a
reflex contraction,
the reflex occurs in both ears. The bilateral nature of the stapedial
reflex is best understood by observing its bilateral neural pathways.
To see an animated demonstration of the contralateral reflex auditory
pathway click Immittance. From the drop-down menu click
Contralateral reflex arc and proceed as instructed.
For contralateral reflex testing, the ear under test, in this case the left
ear, receives the reflex eliciting sound, typically a pure tone, and the
probe tip in the right ear monitors the occurrence of a reflex
contraction.
In the left ear, if the structures are intact and a tone is sufficiently
intense, the neural output from the cochlear nucleus is sent to the
ipsilateral and contralateral superior olivary complexes. From the left
and right superior olives the respective motor neurons of the facial
nerve are activated, resulting in a bilateral reflex contraction. As with
the ipsilateral reflex animation, only one stapedial muscle is shown to
contract. It is important to note that the right stapedius muscle does not
contract because the left stapedius contracted. Rather, the neural signal
for stapedial contraction was sent to both ears regardless of which ear
received the sound. Left stapedial contraction was not a prerequisite
for right stapedial activation.
To perform a threshold search for a contralateral stapedial reflex
follow this procedure. (Note: If the simulation is not set up to test
Immittance on Case 2 as on p. 58, click Case, Select Case, and choose
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Chapter Nine  89
Case 2. Now click Test, then Immittance. Replot the tympanogram by
clicking Plot.) Notice that under Probe Ear, Right is selected. Under
Stimulus Ear, click Contralateral. Under Mode click Reflex thresh.,
then click Plot and observe the trace for a left contralateral reflex
threshold at 500 Hz at 80 dB. The deflection is 0.02 ml, which signifies
a threshold. To confirm the validity of this response, repeat with an
intensity of 75 dB (which yields no deflection—below threshold) and
an intensity of 85 dB (which yields a deflection of 0.05 ml—above
threshold). Note that the left ear was presented with the reflex-eliciting
tone, while the presence of a reflex response was monitored in the
right ear. Repeat the process at 1000 and 2000 Hz and for BBN.
Finally click Tympanogram followed by Left ear and repeat the entire
process for that ear.
Given that we can measure both the ipsilateral and contralateral
reflexes separately and given that we have a rudimentary knowledge of
the neural architecture controlling the reflex, we can make some rather
bold conclusions regarding the structural integrity of the peripheral
and brainstem auditory pathways.
Before we consider how auditory abnormalities influence the stapedial
reflex, we should review the signal parameters under which the reflex
functions normally.
Normative Stapedial Reflex Behavior
1. PURE TONES. For normal hearing subjects the acoustic reflex
(ipsilateral and contralateral) is elicited by pure tones between 70 to 95
dB SL (Peterson & Liden, 1972). For the normal hearing patient this is
70 to 95 dB HL as well. The SL-HL distinction is important as we
shall learn in discussing reflex thresholds for auditory disorders.
2. NOISE. The acoustic reflex is elicited by broadband signals, such as
noises, at lower sound intensities than by pure tones (Peterson &
Liden, 1972). The acoustic reflex threshold for white noise occurs
about 10-15 dB below that observed for pure tones, and is related to
the cochlea's summation of loudness for broadband signals.
3. TIME. The acoustic reflex follows the temporal (on-off)
characteristics of a signal rather well so that an intermittent sound
would produce a well-matched intermittent acoustic reflex. And as
steady state sounds experience adaptation so, too, does the reflex. For
reflex eliciting sounds above 1000 Hz, the reflex response declines in
magnitude prior to 10 seconds of signal duration. A more rapid decay
of the response, especially for sounds at and below 1000 Hz, is
associated with acoustic neuromas. The test for this pathologic
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condition is termed "acoustic reflex decay." (Consult Stach and Jerger,
1991, for a literature review).
4. THRESHOLD TECHNIQUE. Reflex thresholds may be obtained in
an ascending or descending manner, usually in 2 or 5 dB increments.
While the threshold searching method may vary, one should employ
strict adherence to a threshold criterion. A 0.02 ml decrease in
admittance has been recommended by several immittance
manufacturers as the minimal reflex response upon which to base the
presence of an acoustic reflex.
Clinical Patterns
Ipsilateral Acoustic Reflex in Sensorineural Hearing Loss
For the purposes of the present discussion it is assumed that all the
structures in the anatomical and neurological chain of the ipsilateral
reflex arc are normal except for cochlear hearing loss. When the reflex
eliciting signal of sufficient intensity is presented to the ear with the
probe tip, an ipsilateral reflex is observed. Remember, the probe tip
provides a means of both presenting a reflex eliciting signal and
monitoring any admittance change that may result from the reflex. The
reflex will occur in both ears, but for ipsilateral testing we record
reflex thresholds only for the ear with the probe tip.
Recall that in the normal hearing ear the acoustic reflex threshold
occurs between 70-95 dB SL. In sensorineural ears however, the
sensation level at which the reflex occurs is diminished as hearing loss
increases. The audiograms on the next page illustrate this
phenomenon.
Note that the hearing levels at which reflex thresholds were obtained
were approximately the same for normal and sensorineural ears (85
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Chapter Nine  91
dB). Therefore, the sensation level for reflex threshold decreases as a
function of increasing hearing loss. The audiometric and reflex
threshold data from the right audiogram are shown in the table below.
Air conduction HL
Reflex HL
Reflex SL
500 Hz
0
85
85
1000 Hz
35
85
50
2000 Hz
60
85
25
4000 Hz
60
85
25
The rule is that reflex sensation level decreases with greater
sensorineural hearing loss. This is the distinguishing reflex
characteristic of sensorineural hearing loss, that in spite of the hearing
loss an acoustic reflex may be observed for even severe hearing loss
and may be elicited as low as 25 dB above the pure tone threshold (25
dB SL). What this means in practical terms is that if the output limit of
the reflex eliciting tone is 110 dB HL, the maximum loss likely to
produce a reflex is roughly 85 dB HL (110 minus the lowest likely SL
at which the reflex is observed). In clinical experience, one seldom
tests beyond 110 dB HL, so the upper hearing loss where a reflex may
still be elicited is approximately 85 dB HL.
It is important to remember that when testing the ipsilateral reflex, the
state of the contralateral ear has no consequence on the outcome of the
test ear, and no attempt is made to infer anything other than from the
ear under test. As we shall see later with the contralateral reflex test,
the state of the non-test ear is of paramount importance in interpreting
information obtained from the test ear.
It is often the case that the immittance test battery is the first and
primary set of tests administered to a patient. The results then guide
our subsequent diagnostic regimen. If this is the case, as one records
reflex thresholds one will not be able to assess the reflex sensation
level since the pure tone air conduction thresholds have yet to be
obtained. This is not a drawback, however, as we ultimately rely on
diagnostic patterns, the sum of which unfold as we progress through a
battery of tests.
When testing reflexes, caution should be exercised not to exceed a
patient's comfort level for loudness or to present sound intensities
capable of sustaining inner ear damage. A 500 Hz tone at 115 dB HL
produces approximately 126 dB SPL, a potentially injurious level of
sound even at short durations. There are occasions when one is only
assessing the capacity of the auditory system to produce an acoustic
reflex rather than to specify the reflex threshold per se. In such
instances a broad band noise stimulus will elicit the reflex at
considerably reduced sound levels, eliminating the necessity of
excessive sound level presentation. Furthermore, an uncomfortably
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loud sound may cause the activation of the facial nerve (and ultimately
the stapedial branch of the facial nerve) through a "wince," that while
appearing to be an acoustic reflex is instead a pure artifact of facial
nerve activation.
The following is a summary of sensorineural hearing loss and the
acoustic reflex:
1.Reflex thresholds are approximately the same for the sensorineural
and the normal ear.
2.Reflex sensation level (SL) is the air conduction (AC) threshold (in
dB HL) minus the reflex threshold (also in dB HL). For example:
Reflex Threshold=85 dB HL
AC Threshold=10 dB HL
Reflex SL=75 dB
3.Since reflex thresholds occur at the same Hearing Level (that is, the
same place on the audiogram) for both normal and sensorineural ears,
the reflex occurs at a reduced Sensation Level in the sensorineural ear.
For example:
Normal:
Reflex Threshold = 85 dB HL
AC Threshold = 5 dB HL
Reflex SL = 80 dB SL
Sensorineural:
Reflex Threshold = 85 dB HL
AC Threshold = 30 dB HL
Reflex SL = 80 dB SL
4.An ipsilateral reflex testing the probe tip delivers both the probe tone
and the reflex eliciting sound. The ear with the probe tip is the ear for
which reflex thresholds are recorded.
Contralateral Acoustic Reflex In Sensorineural Hearing
Loss
In contralateral reflex testing the ear that is stimulated by the reflexeliciting signal is the "test ear," and the contralateral ear has in it the
probe tip which monitors the presence of a stapedial reflex contraction.
Note, that the ear that is stimulated to have a reflex elicited is not the
ear where immittance changes will be observed, but by eliciting the
reflex in the right ear and monitoring its presence in the left, we can
make inferences about the integrity of the entire contralateral reflex
arc.
Considering hearing loss of cochlear origin only, with normal
contralateral neural pathways, what clinical patterns might be expected
with sensory loss in the test ear? The first consideration is that the
cochlear status of the probe ear is of no consequence. Refer again to
the animated contralateral reflex arc diagram to verify that if the reflex
is elicited in the left, whether the right cochlea is functional at all has
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Chapter Nine  93
no bearing on the right facial nerve producing a reflex contraction of
the right stapedial muscle. Because of this, the relationship between
sensorineural hearing loss (SNHL) and reflex sensation level is the
same for ipsilateral and contralateral testing. If the right test ear has a
cochlear loss we can expect that the reflex will be elicited at reduced
sensation levels re the right ear. After all, it is the right cochlea that is
responding to sound.
You may wonder of what use is the contralateral reflex test if the
reflex thresholds are the same for ipsilateral and contralateral
stimulation? Since ipsilateral and contralateral reflexes share the same
sensation level relationship to pure tone thresholds, the power of
comparing ipsilateral and contralateral reflex thresholds does not relate
to any sensation level differences between the two modes of reflex
stimulation. Rather, the power of the contralateral test is that many
other structures have to be normal for the reflex to be observed.
Moreover, by comparing the presence of the reflex as elicited
ipsilaterally and contralaterally and by right and left ears we can
deduce information about the integrity of important auditory and
brainstem structures that cannot even be acquired by today's most
sophisticated imaging technology.
A 2x2 matrix (Figure 9-1) is useful in helping to see the significance
of clinical reflex response patterns (see diagram below). Note very
carefully what is implied by the terminology of the 2x2. "Right Ipsi"
means that the reflex-eliciting stimulus is delivered to the right ear and
that the probe is also monitoring the response in the right ear. In
contrast, "Right Contra" means that the reflex-eliciting stimulus is
delivered to the right ear, while the probe is monitoring the response in
the left ear. Also realize the implications for measurement both in the
clinic and with this simulation: the probe is physically placed in one
ear at a time. If the probe is in the right ear, then right ipsilateral and
left contralateral reflex thresholds can be obtained, before moving the
probe to the left ear. Thus, referring to the 2x2 figure above, the
shaded conditions would be obtained with the probe placed in the right
ear, while the unshaded conditions would be obtained with the probe
placed in the left ear.
Assuming all other auditory and neural structures are normal, consider
the clinical findings shown in the 2x2 matrix. Note that the decibel
values in all the 2x2 matrices in the text
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Figure 9-1. 2x2 matrix presenting reflex thresholds
and in the simulation program are expressed in Hearing Level (dB
HL).
We conclude from these reflex thresholds:
1.For both ears reflex thresholds occur at normal Hearing Levels.
2.Since reflex thresholds occur at the same Hearing Levels in both the
normal and sensorineural, ear we cannot conclude "normal hearing"
from these findings alone.
3.Each ear may have as much as a 60 dB sensorineural hearing loss
(85-25) since the acoustic reflex can occur as little as 25 dB above
threshold in the cochlear-impaired ear.
Note that the 2x2 matrix does not specify the frequency at which the
reflex thresholds were derived. While there is a frequency effect for
acoustic reflex thresholds (some normal hearing individuals have an
absent or elevated 4000 Hz reflex threshold), none of the following
examples has a frequency effect that would compromise the
interpretation.
For all subsequent 2x2 reflex matrices it is assumed that the reflex
thresholds were elicited by a 1000 Hz tone.
Fourteen (14) examples will now be presented. To see the necessary
data, click Immittance and then click Examples. Note that in each
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Chapter Nine  95
frame of the 2x2 reflex matrix, when the mouse pointer is in any given
frame, a probe ear and stimulus ear diagram is displayed.
In each of the following cases inspect the audiogram and the reflex
thresholds. Note that the air conduction thresholds are designated by a
red line for the right ear and a blue line for the left ear.
 Refer to Example 1 
Right Ipsilateral: Reflexes occur at normal Hearing Levels as
expected.
Left Contralateral (stim-L, probe-R): The left ear is presented with the
reflex activating signal while it is monitored in the right. Again, the
cochlear loss in the left is too great to elicit a reflex. It is of no
consequence that the right ear (where the reflex is being monitored by
the probe tip) has normal hearing. Remember, the cochlear status of
the non-test ear does not influence a contralateral acoustic reflex.
Left Ipsilateral: The left ear has too great a cochlear loss to produce a
reflex from a 110 dB HL tone.
Right Contralateral (stim-R, probe-L): Right contralateral means the
right ear is presented with the reflex eliciting sound and it is the right
ear for which we record its reflex threshold. The presence of the reflex
in this condition is monitored in the left ear. Thus, with the reflex
stimulus in the right ear and the probe in the left ear we would expect
to observe stapedial reflexes at normal Hearing Levels. The cochlear
status of the non-test left ear has no bearing on the observation of a
reflex. The stimulation of the right cochlea activates a neural chain of
events that by-passes the left cochlea.
Consider these findings for a sensorineural hearing loss:
Refer to Example 2 
Right Ipsilateral: The reflex sensation level is 45 dB (dB reflex
threshold minus 50 dB hearing threshold) and consistent with SNHL.
Left Contralateral (stim-L, probe-R) and Left Ipsilateral: The same
findings seen in the right ear will apply to the left in this symmetrical
cochlear loss.
Right Contralateral (stim-R, probe-L): The observed reflex threshold is
consistent with hearing loss of cochlear origin in the right.
Consider these sensorineural hearing loss findings:
 Refer to Example 3 
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Right Ipsilateral: While we expect the reflex to occur within a range of
reduced sensation levels in the sensorineural ear, the upper limit at
which we are willing to test may not yet be sufficient to elicit a reflex.
As with this ear, the reflex may have been obtained beyond the 110 dB
HL ceiling we have set.
Left Contralateral (stim-L, probe-R): Stimulating the left ear produces
reflexes at reduced sensation levels. The right ear thresholds do not
influence our ability to observe the effects of a stapedial contraction in
the right.
Left Ipsilateral: The reduced reflex sensation levels are consistent with
SNHL.
Right Contralateral (stim-R, probe-L): The same reasoning holds for
right ipsilateral and contralateral testing since in each instance it is the
right ear that elicits the reflex.
Conductive Hearing Loss.
While elicitation of the acoustic reflex is expected in all but the
severest of sensorineural losses, for most conditions affecting the
conductive structures of the middle ear, the reflex is absent or elevated
in threshold. Two generalizations regarding conductive hearing loss
and the acoustic reflex are:
1.When the probe or "monitor" ear has a conductive pathology, the
acoustic reflex will be absent. Even with neural pathway integrity and
the resulting activation of the stapedius muscle, if the conductive
condition either stiffens the ossicular chain and TM or decouples the
ossicular chain from the TM, the acoustic reflex will not be observed.
In the first instance, the ear already stiffened by otitis media is beyond
further stiffening from the effects of a reflex contraction. If the
ossicular chain is decoupled from the TM as is the case with ossicular
discontinuity, the change in compliance produced by the reflex
contraction is not transferred to the TM where its influence would
affect the probe tone.
2.When the reflex eliciting signal is presented to the ear with the
conductive pathology, reflex thresholds will be elevated or absent.
That is, when the ear being stimulated has a 30 dB conductive loss, it
will take 30 dB more sound energy to reach the cochlea and initiate the
neural events precedent to a reflex contraction (to be monitored in the
other ear). If the conductive elevation in threshold is great enough,
there may be insufficient reflex eliciting intensity to produce a
contraction, resulting in "absent reflexes."
Consider the following case: Unilateral otitis media with effusion:
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Chapter Nine  97
 Refer to Example 4 
Right Ipsilateral: The reflex threshold of 80 dB HL is expected since
there is no conductive pathology in the right ear. The state of the left
ear is irrelevant for ipsilateral right stimulation.
Left Contralateral (stim-L, probe-R): In this situation the reflex may be
elevated or absent. Consider that when this transient middle ear
condition is resolved and the ear has normal air and bone conduction, a
reflex threshold of 85 dB is measured. To overcome the current 30 dB
conductive "pad" while stimulating the left, a 115 dB tone (85 + 30)
would have to be used. Note that it is important to record the highest
sound level used that failed to produce a reflex, since this sound level
may convey the probable reason for an absent reflex.
Left Ipsilateral: The absent reflex in the left to ipsilateral stimulation is
consistent with our first rule, that when the probe ear has a conductive
pathology, the acoustic reflex will be absent. The conductive type
audiogram and type B tympanogram are consistent with otitis media.
Right Contralateral (stim-R, probe-L): The right contralateral reflex is
absent. While the right ear's normal conductive and cochlear function
likely generate sufficient neural energy to elicit a reflex, it is the left
conductively-involved ear that is the probe ear, hence the "absent
reflex." Here the fluid-filled left middle ear is already stiffened to a
degree such that even a strong reflex contraction would not alter the
middle ear admittance.
Now consider the second case of unilateral otitis media:
 Refer to Example 5 
Right Ipsilateral: The reflex threshold of 80 dB HL (and SL) is
consistent with normal peripheral auditory function.
Left Contralateral (stim-L, probe-R): Stimulating the left ear at an
intensity sufficient to overcome the 10 dB pad while monitoring in the
normal right middle ear produces a reflex at 95 dB. If reflex threshold
is usually 80 dB in the left ear, then a 95 dB HL tone would overcome
the 10 dB conductive threshold elevation and elicit a reflex.
Left Ipsilateral: The reflex is absent since excessive stiffness in the left
obscures a reflex. The conductive loss itself does elevate the reflex
threshold, but the increased stiffness is the overriding factor for reflex
absence.
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Right Contralateral (stim-R, probe-L): While stimulating the right ear
likely produced a bilateral reflex contraction, the conductive condition
in the left probe ear obscured it.
Consider the following case of bilateral otitis media:
 Refer to Example 6 
For each of the reflex testing conditions in the 2x2 matrix the probe
ear has a conductive condition producing extreme stiffness with
resultant absent reflexes. For instance, the right-ipsilateral condition
would produce an absent reflex because the immobility of the TM in
the right would not yield to the effects of an acoustic reflex. In the
right-contra situation, the left ear is the probe ear, and the same
generalization holds true. Even if sufficient intensity could overcome
the conductive loss in the right and produce a contraction, the effects
of that bilateral contraction would not be observed because of the
stiffness of the left TM.
Consider the following case of unilateral negative middle ear pressure:
 Refer to Example 7 
Right Ipsilateral: The extreme negative pressure changes the middle
ear dynamics enough to prevent observing a reflex. Note that the
negative middle ear pressure has not produced a measurable air-bone
gap on the audiogram, yet the acoustic reflex is absent.
Left Contralateral (stim-L, probe-R): The probe ear's stiffness from
negative pressure obscures observation of a reflex. It is interesting that
negative pressure may or may not yield an air-bone gap. As in this
case, even if no measurable conductive loss exists, the tympanometric
abnormality is often sufficient to yield the "absent reflex" state.
Left Ipsilateral: Reflex observed as expected.
Right Contralateral (stim-R, probe-L): The reflex falls within the
normal 70-95 dB HL range, but we might suspect that it is slightly
elevated due to the added stiffness of the stimulated right ear. It is
worthwhile to note that the vibratory dynamics of the middle ear are
altered by even the slightest of abnormalities that in this case are
manifest in the right tympanogram but not on the audiogram.
Observe the following case of bilateral occluded tympanostomy tubes
(referred to as PE tubes hereafter):
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Chapter Nine  99
 Refer to Example 8 
Occasionally an otologist may request a determination of PE tube
patency. If the tube is patent, we will observe immittance findings
similar to those of a TM perforation; flat tympanograms and excessive
canal volume, with varying degrees of confirmation from audiometric
air and bone conduction.
When the PE tube is plugged, we often observe a type B tympanogram
and normal canal volume. The present case illustrates this diagnostic
pattern. Absent reflexes occur bilaterally as would be the case for any
middle ear condition producing the degree of stiffness of these
tympanograms. Remember, for each condition in this 2x2 matrix the
probe ear has a conductive loss.
Consider the following case of unilateral PE tube patency:
 Refer to Example 9 
Right Ipsilateral: Reflex response observed as expected.
Left Contralateral (stim-L, probe-R): Observance of an elevated reflex
is consistent with the conductive disruption in the stimulated ear
requiring a greater intensity to elicit the reflex response. If the
conductive condition were of greater severity, a stimulating tone of
100 dB would have been an insufficient reflex stimulator.
Left Ipsilateral: With the probe in the left ear, we would anticipate
absent reflexes here for the reasons detailed above.
Right Contralateral (stim-R, probe-L): The general rule with the probe
in an ear with a patent PE tube is to find an absent reflex. Consider that
the reason for the tube was to evacuate the middle ear cleft of exudate
and to permit air exchange. As such, the middle ear cavity is likely
quite stiff and therefore unresponsive to further stiffening via stapedial
contraction. Consider also that since the PE tube is patent, the probe
tone is circulating throughout the external and middle ear cavities. If
the ossicular chain was mobile enough to respond to stapedial
contraction, the resultant change in probe tone intensity would likely
be insignificant and not be considered reflective of a reflex response.
Consider the following case of bilateral cerumen accumulation:
 Refer to Example 10 
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Prior to immittance testing, otoscopic observation may reveal a
cerumen occlusion that obscures a view of the TM. Tympanometry
will show whether the cerumen is fully or partially occluding the
external canal. Reduced canal volume and a flat tympanogram are
consistent with occluding cerumen. If only a pin hole of air space
around the cerumen exists in the canal, the probe tone may circulate
and monitor TM dynamics during tympanometry and reflex testing.
In this case, the normal tympanograms and reflex thresholds dictate
that the cerumen has not fully occluded the external canal. This
determination is useful in that since we know the TMs are intact, the
canals may be irrigated. Had we discovered a TM perforation,
cerumen management would have taken a different approach. Note
that the reflex sensation levels are consistent with a sensorineural loss.
Observe the following case of bilateral otosclerosis:
 Refer to Example 11 
The diagnostic signature of otosclerosis is reduced static admittance,
absent or elevated reflexes, and an audiometric conductive loss.
Reflexes may be present in the early stages of stapedial fixation.
Reflexes are absent bilaterally in this case since in all test conditions,
the probe ear is in an ear having conductive loss. Thus, in each of the
2x2 reflex test conditions a stapedial contraction would have no further
influence on an ossicular chain already significantly reduced in
compliance by the ossification process.
Observe the following case of unilateral otosclerosis with
stapedectomy:
Refer to Example 12 
During the stapedectomy procedure the stapedial tendon is severed,
rendering the stapedial reflex nonfunctional. That is, a reflex
contraction would have no means to influence the movement of the
stapes with the attaching tendon's disconnection.
Right Ipsilateral: The reflex is absent as this ear has been
stapedectomized. The air-bone gap in the right ear may or may not be
of significant improvement over the presurgical audiogram. Yet, the
primary consideration in the reflex's absence here is the fact that the
ear has undergone a stapedectomy, rendering the stapedial reflex
inoperative. Even if the air-bone gap had been completely closed with
no measurable conductive loss, the reflex would be absent.
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Chapter Nine  101
Left Contralateral (stim-L, probe-R): The bilateral reflex elicited with
stimulation to the left will not be observed in the right ear due to the
severed right stapedial tendon.
Left Ipsilateral: The slightly reduced reflex SL is consistent with mild
SNHL.
Right Contralateral (stim-R, probe-L): Once the 10 decibel conductive
component of the hearing loss is overcome by the intensity of the
reflex activating tone, the right cochlea generates sufficient neural
activity to initiate the neuronal chain of events to produce a
contraction in the left ear, where the right contralateral reflex is
monitored.
Observe this case of unilateral ossicular disruption: Most cases of
ossicular discontinuity from skull trauma involve incudostapedial joint
separation. Since the stapedial tendon inserts on the neck of the stapes
(which is medial to the common site of ossicular disruption), stapedial
contraction will have no effect on the rest of the ossicular chain.
Therefore, a "reflex absent" state will be observed in the probe ear
with ossicular discontinuity.
The ossicular disruption at the incudostapedial joint effectively
decouples the stapedial tendon's influence from the tympanic
membrane. Thus, as with other conductive conditions, the "absent
reflex" conclusion refers solely to the absence of an observed reflex
via the immittance equipment. With ossicular disruption the stapedial
muscle may well contract, but the effects on the ear's immittance are
not transmitted to the TM and therefore not "observable" by the
immittance meter.
Refer to Example 13 
Right Ipsilateral: The reflex is observed at normal Hearing and
Sensation Levels.
Left Contralateral (stim-L, probe-R): The elevated reflex threshold
reflects the requirement of overcoming the effects of the conductive
loss in the stimulated ear. Once a sound level capable of eliciting a
reflex is reached, it is monitored in the right ear with no complications.
Left Ipsilateral: Again, the probe and conductive ear pairing precludes
the observance of a reflex.
Right Contralateral (stim-R, probe-L): With the probe ear as the
conductive ear, no reflex will be observed.
Retrocochlear Hearing Loss (VIIIth Nerve Pathology)
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The acoustic reflex pattern associated with VIIIth nerve pathology is
dependent upon the location and size of the lesion. Acoustic neuromas
typically involve the VIIIth nerve but may spread to brainstem
structures as well.
Reflex decay is the reduction in reflex amplitude greater than 50% of
the initial compliance change over 10 seconds to a 10 dB SL reflex
eliciting tone of 500 or 1000 Hz. For example, in an ear with a reflex
threshold of 85 dB HL at 1000 Hz, the reflex decay test would be
administered at 95 dB HL to this ear. In VIIIth nerve pathology we
expect the reflex to be absent to stimulation between 70-100 dB HL on
the affected side in about 65% of patients. Approximately 20% may
have normal reflexes, while in 15% the reflex is present yet displays
abnormal reflex decay. Thus, approximately 80% of VIIIth nerve
patients will display abnormal reflex test patterns manifest in either
absent or elevated reflexes or significant reflex decay (Jerger and
Jerger, 1974; Olsen et al, 1975).
Recall that reflex sensation level is reduced in the cochlear involved
ear. One identifying feature of an acoustic neuroma ear is that the
reflex occurs at normal sensation levels. Thus, with a 30 dB loss the
reflex might occur at 115 dB HL (30 + 85 dB SL). If one tests for
reflexes only to 110 dB then an erroneous "absent reflex" would be
recorded for this ear. This situation again highlights the need to specify
the upper intensity at which the reflex was not observed.
With this simulation reflex decay may be tested in the ipsilateral and
contralateral modes. In the early days of immittance testing when
ipsilateral reflex testing was not available and/or suspect because of
limitations of the instrumentation, the only means of assessing reflex
decay was in the contralateral mode. That is, if a right acoustic
neuroma was suspected, the right ear would be stimulated with a reflex
eliciting tone while the probe in the left ear monitored the reflex
response. Realize that the entire afferent pathways of the right ear and
the efferent reflex pathway in the left ear must be normal to measure
the status of the right 8th cranial nerve. If the left ear had excessive
negative middle ear pressure, or adhesions from childhood otitis
media, or any number of other conditions, one could not test reflex
decay of the right ear.
Using the ipsilateral mode, however, one could test right reflex decay
and monitor such decay from the same ear. Of course, the contralateral
reflex testing mode always yields more information about the integrity
of lower brainstem pathways (whether testing reflex threshold or
reflex decay) than does the ipsilateral test, yet there may be clinical
situations in which reflex decay data cannot be acquired without
relying on the ipsilateral testing mode.
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Chapter Nine  103
To perform a normal stapedial reflex decay test, follow this procedure.
(Note: If the simulation is not set up to test Immittance on Case 2 as on
p. 58, click Case, Select Case, and choose Case 2. Now click Test,
then Immittance. Replot the tympanogram by clicking Plot.)
Furthermore, the stimulus ear should be Ipsilateral and the Probe Ear
should be Right. Click Reflex decay. Reflex decay is measured with an
intensity that is 10 dB above the reflex threshold. When the ipsilateral
reflex threshold was obtained at 1000 Hz on p. 69, it was 75 dB.
Accordingly the proper intensity at which to measure the reflex decay
is 10 dB above that threshold, or 85 dB. So adjust the intensity to 85
dB. Click Plot. Observe that the amplitude of the tracing did not decay
by more than 50%, yielding a normal response.
To perform an abnormal reflex decay test, follow this procedure.
Again with Case 2 select the Left ear. The ipsilateral reflex threshold
for the Left ear obtained at 1000 Hz on p. 69 was 75 dB. Replot the
tympanogram for the Left ear, if necessary. Then click Reflex decay.
Set the intensity to 10 dB above the reflex threshold, or 85 dB. Click
Plot. Observe that the amplitude of the tracing decayed more than
50%. In fact it decayed almost 100% (i.e., nearly returned to the
baseline.), thus yielding an abnormal response.
In summary, reflex decay testing assesses the reflex adaptation of the
ear stimulated by a 10 second tone. It is preferable to stimulate one ear
and monitor any decay in the other ear (contralateral mode), but the
status of the probe ear may preclude this mode of testing on occasion.
In such situations, ipsilateral reflex decay testing is appropriate.
Observe the following case of unilateral right acoustic neuroma:
 Refer to Example 14 
Right Ipsilateral: The right ipsilateral reflex is absent because the
right-sided acoustic neuroma interrupts normal synchronous firing of
the eighth nerve. An insufficient neural code is sent to the reflex
centers above the eighth nerve.
Left Contralateral (stim-L, probe-R): This reflex is normal since the
left contralateral reflex arc essentially involves the left auditory nerve
and right facial nerve. The right acoustic neuroma is not a factor when
the left ear is stimulated and the reflex response monitored in the right.
Left Ipsilateral: Normal reflex thresholds observed since no part of the
dysfunctional right eighth nerve is stimulated.
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Right Contralateral (stim-R, probe-L): The involved ear has poor
neural conductivity, and therefore the higher reflex pathways are not
stimulated with a threshold reflex input. While the facial nerve on the
left is intact, the neural disruption on the right precludes its activation.
Summary
You have completed a comprehensive simulation course in the
essentials of aural acoustic immittance. Next we complete the basic
audiological test battery with an introduction to speech audiometry.
Chapter Ten
Speech Audiometry
Standard speech tests require a relatively minor amount of time to
administer in terms of the complete audiological test battery, yet the
results of speech tests reveal vital information about the listener’s
capacity to communicate. Two types of tests are routinely given as part
of the regular protocol, but there are many specialized tests as well.
The two most common tests are the speech recognition threshold
(SRT) and the word recognition test (WR).
Speech Recognition Threshold
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Chapter Ten  105
Just as thresholds can be found for the pure-tone frequencies, the
threshold for speech can also be measured. The nature of the speech
materials used and the manner in which they are presented are of
paramount importance to the outcome of the test. For the most part
disyllabic words spoken with spondaic stress are used. That is, the
word list consists of two syllable words pronounced with equal
emphasis on each syllable. The original set of words was eventually
reduced to a set of thirty-six, know as the W1 word list.
Measuring a threshold involves very low intensities, and two syllable
words spoken with equal stress on each syllable (called spondees)
constitute an easy-to-recognize stimulus. To further facilitate
perception, the listener is informed of the words in the set of test
stimuli before the test begins and asked to repeat them. One way this
can be accomplished is to deliver the words to the listener through the
earphones at a comfortably loud intensity. Alternately, the clinician
may choose to recite the words to the listener in a face-to-face
situation. In either case care should be taken that the listener is
understanding the words based solely on auditory and not visual cues.
Thus, covering the mouth may be necessary. During this
familiarization phase, if the listener misses a word, it is repeated until
the listener comprehends it. If the listener is ultimately unable to repeat
a particular word, it should be eliminated from the set and not used.
The specific steps involved in conducting the threshold test will be
covered below.
The SRT provides two valuable pieces of information. First, it serves
to confirm the validity of the pure-tone thresholds (more on this
below). Some individuals falsify the intensity at which they detect
pure-tones; that is they do not respond at threshold but rather at
suprathreshold levels; i.e., they fake a hearing loss. Speech thresholds,
in contrast, are more difficult to misrepresent because speech is a
much more complex acoustic signal, and it has an intrinsic meaning.
Second, the SRT forms the baseline intensity upon which other speech
tests can be conducted. For example, if a listener’s SRT is 25, and you
want to perform another test at an intensity 40 dB greater than the
threshold, or 40 dB sensation level (SL), then the intensity dial is set at
65 dB for the ensuing test.
Word recognition
The more significant of the speech tests is word recognition, or the
capacity of the listener to understand speech. The result of such a test
is recorded as a percentage that is intended to represent the listener’s
ability to perceive everyday speech.
The type of stimuli to use in order to measure word recognition is
much more controversial than for the SRT. Generally speaking, the
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materials can be subdivided into two broad groups: individual words
and multiple words (phrases or sentences). The objective is, of course,
is to assess the degree to which the listener can understand speech in
ordinary conversation. The means of converting that goal into a
reasonably simple and economical clinical tool is challenging.
In addition, everyday speech occurs in a variety of listening conditions
varying from the quiet of the dinner table to the moderate noise of the
urban street corner to the extreme noise of a heavy construction site.
Thus, speech is often measured both in quiet, meaning that just the
speech signal is presented to the listener, and again in noise, meaning
that a competing noise is mixed with the speech signal and delivered
simultaneously to the test ear. We emphasize that both the speech and
the noise are delivered to the same ear.
Unlike the SRT, the WR test is given at a suprathreshold level, ideally
a level that will optimize the listener’s ability to comprehend the
words. The most commonly used level is 40 dB above the speech
threshold, or 40 dB SL, although other levels are used as well. In fact,
WR may be measured at several sensation levels, both in quiet and in
noise.
The most commonly used materials used to determine word
recognition consist of a set of 50 monosyllables. There are several
word lists that have been used historically, such as the W-22 and the
NU-6 lists. There are other lists too, including lists that are comprised
of sentences instead of individual words. When a monosyllabic word
list is used, it is generally presented with an accompanying carrier
phrase, that is the test stimulus is preceded by a phrase such as “Say
the word a-word”, where a-word is one of the words on the list. The
point of the carrier phrase is create a more natural speech event; we
converse in sentences, not in isolated words. Furthermore, the stimulus
(the word at the end of the phrase) is not to be emphasized, but rather
is to be spoken as the final word of any conversational sentence. Each
word is presented only once, and a tally is kept of the number of words
repeated correctly by the listener and the number missed. The resulting
score is converted to a percentage of correct responses.
The lists of 50 words were carefully constructed to contain a balance
of the sounds that occur in everyday English. Moreover the
construction of a given test, like the NU-6, consisted of the
development of several lists of different words which are of equivalent
difficulty. That is, there are multiple lists of words, and using any of
the lists should result in the same score on the same individual. The
need for multiple lists arises when doing several tests on a listener. If
the exact same word list were used repeatedly, the person’s score
would be expected to improve as the person become increasingly
familiar with the words after repeated exposure to them.
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Chapter Ten  107
Recorded vs. Live Voice Presentation
To assure consistency among repeated tests on the same listener and
across different listeners, speech materials are available in a recorded
format. Originally called “taped speech”, the tapes have now been
supplemented by CD formats as well as digital formats stored in the
memory of some audiometers or audiometers interfaced with
computers. Recorded speech offers the advantage of generating
precisely the identical stimulus every time the test is administered.
In contrast, the clinician may choose to say the stimulus words. This
method is known as monitored live-voice speech audiometry. In
ordinary conversation the intensity of our voices varies by a few
decibels. Because it is imperative that the speech stimuli be at a
constant intensity for a given intensity dial setting, the clinician uses a
VU meter to monitor the level of her or his voice in order to maintain a
correctly calibrated signal. Thus, the level of the clinician’s voice is
monitored to assure a consistent intensity from word to word. One
major advantage of a live-voice presentation is that it allows much
greater flexibility than does the recorded format.
Even with monitoring, the acoustical event that is generated by the
clinician is likely to vary somewhat from presentation to presentation,
and certainly from clinician to clinician. This fact creates the incentive
for using recorded materials. A given clinic will usually adopt one
method or the other as established clinic policy.
The Audiology Clinic provides an SRT and a WR score for most
cases. The WR score represents the results of a single test done in
quiet. These scores can be seen by plotting the audiogram for the
particular case. A subset of ten spondees is used for the SRT in order
to conserve disk space.
Descending Threshold Protocol
The method of obtaining the SRT that is recommended by the
guidelines issued by the American Speech-Language-Hearing
Association (ASHA, 1988) makes use of a descending threshold
technique. The signal level can be varied in 5 dB or 2 dB steps. The
audiometer simulated by The Audiology Clinic only permits 5 dB
changes, so instructions for utilizing that increment will be specified.
The guidelines should be consulted to obtain the details of the 2 dB
method, which is similar.
There are two stages to the determination of the threshold. First is the
preliminary phase.
1.Set the intensity to a level that is 30-40 dB higher than the estimated
threshold and present one word.
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2.If the word is missed, increase the intensity by 20 dB and again
present one word. Repeat this step until the word is repeated correctly.
3.Decrease the intensity by 10 dB and present one word. If the word is
missed, present a second word.
4.Continue as in Step 3 until both words are missed.
5.Increase the intensity by 10 dB and record this intensity as the
starting level.
At this point the test phase begins.
1.Present five words at this level and note the correct or incorrect
response to each word.
2.Decreased the intensity by 5 dB and present five more words.
3.If the first six (6) words are not repeated correctly, then increase the
intensity by 10 dB and record this value as the new starting level.
4.Continue presenting five words at each level scoring the responses
until all five words are missed.
5.Calculate the threshold as follows: SRT = starting level - number of
words correct + 2
It should be pointed out that the guidelines recommend this procedure
to assure consistency among clinics. However, the modified HughsonWestlake procedure that was described in Chapter 2 has also been used
to measure the SRT. The procedure for establishing the SRT using that
method is exactly like that used to get a pure-tone threshold.
Experiment with both methods or use the formula required by your
clinic or as directed by your instructor or supervisor.
Calibration of the Speech Signal
As previously mentioned, before conducting speech tests it is
mandatory to calibrate the speech circuit regardless of whether livevoice or recorded speech is performed. We will use recorded speech
stimuli. If your computer has the proper sound card, both the test
stimulus and the listener’s response can be heard. If there is not a
sound card, then the word spoken by the clinician and repeated by the
listener will appear in a bubble on the screen. Further, if you have
audio capability but wish to defeat it, on the Options menu observe
whether there is a check mark next to Sound on. If so, click Sound on
to remove the check mark, and the sound will be muted and the words
will appear in bubbles as described above.
It is necessary to calibrate the speech circuit before doing a speech
test. This is because different clinicians speak with different
intensities. Even different recorded materials produce varying output
levels. In order to maintain the calibration of the intensity of the
Instruction Manual Audiology Clinic V2
Chapter Ten  109
speech signal, the small VU meter on the clinical audiometer must be
monitored to confirm that when the word is spoken, the indicator
peaks at 0 dB (as shown in Figure 10-1 below). This fact assures that
an intensity dial setting of, let’s say, 45 dB results in a word with an
intensity of 45 dB being delivered to the listener.
Figure 10-1. VU meter.
To calibrate the speech signal the audiometer must be in Speech Mode,
so click on the Speech button in the Mode window of the audiometer.
The frequency window will display the letters “Spch”. Now click
Options, then Calibrate speech. The calibration window will open. It
contains a large scale on the left side. The scale has a pointer, which
will be randomly positioned. Click Play words. Either you will hear
the words being spoken or see them in a bubble. Your task is to
monitor the pointer of the VU meter on the audiometer as the words
are produced. It needs to peak at the red “0” on the dial. If the pointer
peaks below the “0”, move the pointer on the scale slowly upward
until the needle on the VU meter peaks at “0”. On the other hand, if
the pointer on the VU meter peaks above the “0”, drag the pointer on
the scale downward. (Click Instructions if you forget what to do.)
When the pointer of the VU meter on the audiometer peaks at “0”, the
speech circuit is calibrated, and the actual test may begin. In the
calibration window click Stop words, then close the window (click on
the X in the upper right corner). You will observe that all menus
become inactive during a speech event and are reactivated after the
event concludes. This is because the production of words involves a
complex event that cannot be interrupted.
Masking Speech Thresholds
Just as pure-tone thresholds must occasionally be masked, so must
speech thresholds be masked. The SRT is measured through the
earphones, consequently it is an air conduction event. Accordingly, the
masking rules for air conduction (Rule No. 1 and Rule No. 3) apply.
With regard to Rule. No 1 the decision is clear: mask (retest) when one
SRT is 40 dB or more poorer than the other for circumaural earphones
(60 dB or more for insert earphones). Nevertheless the situation with
regard to Rule No. 3 is more complex because you are comparing the
SRT in one ear to the bone conduction threshold in the other ear. But
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Instruction Manual Audiology Clinic V2
speech is a multi-frequency stimulus; the SRT cannot be compared to
the bone conduction threshold at a single frequency. It has been
shown, however, that there is a close correlation between the SRT and
the average of the thresholds at 500, 1000, and 2000 Hz. This average
is called the pure-tone average, and as a rule the SRT should be very
close to this value. The exception to the case occurs when there is a
substantial difference between the levels at which these three puretone thresholds are realized. An example would be the following: 500
Hz = 20 dB, 1000 Hz = 25 dB, and 2000 Hz = 75 dB. Clearly, there is
a precipitous drop in sensitivity between 1000 and 2000 Hz. In such
instances, a two frequency PTA is often calculated, using the two
frequencies that have the more similar thresholds, namely 500 and
1000 Hz. Accordingly, the two frequency PTA would be 22.5 dB.
As already mentioned above, the SRT serves to confirm the validity of
the pure-tone thresholds. There should be a close association between
the SRT and the PTA, barring the occasional configuration that has
vast differences among the thresholds in the 500 - 2000 Hz range. An
SRT and PTA that differ by more than 10 dB should alert the clinician
to a problem. An attempt should be made to resolve the difference,
which could be due to clinical error on the part of the clinician, a
misunderstanding on the part of the listener, or an equipment
malfunction. If the difference cannot be resolved, then the clinician
may suspect that the listener is deliberately falsifying the results
(usually the pure-tone thresholds). In addition, the results of other
tests, such as immittance, and other clinical information, for instance,
the case history, can be immensely helpful in reaching the correct
clinical interpretation of the status of the listener’s hearing.
Practice
It is possible to obtain SRTs on all of the 100 cases in this simulation
(50 cases in the Lite edition). The value obtained should agree with the
pure-tone average (PTA), which is usually calculated by averaging the
thresholds at 500, 1000, and 2000 Hz. With steeply sloping losses, or
losses that drop precipitously after 1000 Hz, a two frequency average
is sometimes used (500 and 1000 Hz). Because the SRT is constructed
to be an easy test, the low frequencies are of more importance than the
high frequencies, thus the rationale for the two frequency average. A
disagreement of 10 dB or more between the SRT and PTA,
particularly when the speech threshold is better than the pure-tone
thresholds, should alert you to the possibility of an error in testing, an
equipment malfunction, or worst of all a non-cooperative listener. It is
essential to explain the discrepancy. The situation of the malingering
listener is discussed in many audiology texts (Martin, 1994).
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Chapter Ten  111
Summary
Speech testing is an important aspect of the complete audiological
evaluation. Hopefully you have become proficient at measuring the
speech threshold. The word recognition test is even simpler to
administer and will be reserved for actual clinical situations.
112  Chapter Ten
Instruction Manual Audiology Clinic V2
P
Practice 12, 14, 20, 24, 41, 42, 47, 53, 60, 63,
66, 68, 87, 111
Index
R
Reactance 70–73, 70–74
Resistance 29, 70–72, 70–72
T
Technique 14–16, 14–16, 21, 23–24, 23–24,
34–35, 34, 35, 48, 53, 77, 91, 108
Transducer 10, 26, 51, 70, 79
A
Audiometer 4, 7, 10–14, 10–14, 18–19, 18–
19, 21–24, 26–27, 26–27, 33, 36, 51,
108–10, 108, 110
C
Crossover 8, 28–33, 28–33, 31–33, 36, 39,
42–45, 42–45, 49, 53–54, 53–54, 56–59,
56–59, 62
E
Earphones 10, 13, 16–18, 16–18, 21, 24, 26,
29–30, 29, 30–37, 32–36, 39–42, 39, 42,
44–47, 44–47, 51, 56–57, 56, 59–63, 59–
63, 106, 110
L
Listener 4, 10, 13–15, 13–15, 18–22, 19–24,
26, 28–29, 28–29, 32–33, 35–39, 35–39,
38, 43, 47, 49–51, 49–51, 53, 57–58, 57–
58, 64–68, 64–69, 105–7, 105–11, 109–
11
M
Masking 8, 13, 21, 28–30, 28–31, 34–63, 34–
63, 65–68, 65–68, 110
O
Overmasking 35, 41–45, 41–45, 52, 58–60,
58–60
Instruction Manual Audiology Clinic V2
Index  113