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Audiometry – Assessment B
3064-10/HLSP
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Community Services, Health,
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3064-10/HLSP Audiometry - Assessment B V1
3064-10/HLSP Audiometry - Assessment B V1
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Acknowledgments
TAFE NSW - Community Services, Health, Tourism and Hospitality Division would like
to acknowledge the support and assistance of the following people in the production of this
resource package:
Writer:
Bettina Turnbull
Audiologist
TAFE NSW
Project Manager:
Gary Wood
Program Manager
(Insert Program Area) Programs
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Fax: 02-9942 3257
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Reprinted 2008 with minor alterations and with the permission of Community Services, Health,
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ISBN 0 7348 1590 5
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3064-10/HLSP Audiometry - Assessment B V1
3064-10/HLSP Audiometry - Assessment B V1
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3064-10/HLSP Audiometry - Assessment B V1
3064-10/HLSP Audiometry - Assessment B V1
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TABLE OF CONTENTS
SUGGESTED LEARNING RESOURCES FOR THIS MODULE ....................................................... 1
Textbooks ....................................................................................................................... 1
TOPIC 1 – IDENTIFYING THE NEED FOR FURTHER TESTING .......................... 3
INTRODUCTION ................................................................................................................... 3
IDENTIFYING CLIENTS’ NEEDS ............................................................................................ 3
IDENTIFYING NEED FOR FURTHER TESTING ......................................................................... 4
Bone conduction masking .............................................................................................. 4
Air conduction masking ................................................................................................. 4
Speech masking ............................................................................................................. 4
Tympanometry ............................................................................................................... 4
TOPIC 2 – POST-BASIC ASSESSMENT – MASKING ................................................ 5
INTRODUCTION ................................................................................................................... 5
INTERAURAL ATTENUATION ............................................................................................... 6
BONE CONDUCTION MASKING ............................................................................................. 7
AIR CONDUCTION MASKING ................................................................................................ 8
When do you need to mask? .......................................................................................... 9
How much masking is needed? ...................................................................................... 9
How do you mask? ......................................................................................................... 9
Overmasking ................................................................................................................ 11
Scenario of overmasking ............................................................................................. 11
SUGGESTED ANSWERS TO ACTIVITIES ............................................................................... 14
TOPIC 3 – SPEECH TESTING ....................................................................................... 15
INTRODUCTION ................................................................................................................. 15
MOST COMFORTABLE LEVEL (MCL) ............................................................................... 16
RELIABILITY AND VALIDITY ............................................................................................. 16
SPEECH MASKING ............................................................................................................. 18
Overmasking ................................................................................................................ 19
Calculating MDSL and ESML ..................................................................................... 19
Practical ...................................................................................................................... 21
SUGGESTED ANSWERS FOR ACTIVITIES ............................................................................. 22
TOPIC 4 – TYMPANOMETRY ...................................................................................... 23
INTRODUCTION ................................................................................................................. 23
THE TYMPANOMETER ....................................................................................................... 23
The probe ..................................................................................................................... 24
THE DIFFERENT TYPES OF TYMPANOGRAMS ...................................................................... 25
SCREENING REFLEXES ...................................................................................................... 28
TOPIC 5 – THE SIGNIFICANCE OF VARIOUS TEST RESULTS .......................... 31
INTRODUCTION ................................................................................................................. 31
RELIABILITY ..................................................................................................................... 31
VALIDITY ......................................................................................................................... 32
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3064-10/HLSP Audiometry - Assessment B V1
3064-10/HLSP Audiometry - Assessment B V1
Suggested learning resources for this module
Textbooks
TITLE
AUTHOR
PUB DATE
PUBLISHER
ISBN
:
:
:
:
:
TITLE
AUTHOR
PUB DATE
PUBLISHER
ISBN
Clinical Audiology An Introduction
Stach, B.A.
1998
Singular Publishing Group, UK
156593346X
Introduction to Audiology
Martin, F.N. & Clark, J.G.
8th Edition, 2003
Allyn & Bacon, Boston
0205366414
TITLE
AUTHOR
PUB DATE
PUBLISHER
ISBN
:
:
:
:
:
Audiology
Newby, Hayes
5th Edition, 1985
Prentice Hall, New York
0130519219
TITLE
AUTHOR
PUB DATE
PUBLISHER
ISBN
:
:
:
:
:
Hearing and Evaluation
Skinner, M.
1987
Prentice Hall, New York
0205135528
TITLE
AUTHOR
PUB DATE
PUBLISHER
ISBN
:
:
:
:
:
Basic Principles of Auditory Assessment
Hannley, M.
1991
Prentice Hall, New York
0205135528
Your college may have a copy that you can borrow or you can purchase it yourself.
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3064-10/HLSP Audiometry - Assessment B V1
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3064-10/HLSP Audiometry - Assessment B V1
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TOPIC 1 – IDENTIFYING THE NEED FOR FURTHER
TESTING
Introduction
This topic introduces you to basic information about assessments that are done after initial
screening audiometry. This includes both further testing and interpretation of those results.
In order to know whether further assessment is required, the audiogram as well as the
client’s history need to be taken into account. This leads to making decisions about what
further testing is required, the significance of the results, how to correctly report these
results, and identifying the need for referral.
Identifying Clients’ needs
After an initial history and audiogram it is time to look at the information you have
gathered and decide whether the client can be discharged, or to look at options for
improving hearing and whether a referral is required.
Each client has different needs, depending on their age, general health, severity of loss and
situation in life. Some needs are physical, but we also need to look at emotional,
psychological, sociological and financial needs. Remember, a person’s hearing loss causes
problems not just for themselves but also for their families.
Activity 1.1
The next time you see a client, write down all the needs of your client in each of the five
categories cited above and next to each need a way to overcome it. Eg.: Psychological –
client worried about cosmetic appearance of aid – show client actual size and colour of
hearing aid, discuss small hearing aid (if audiogram allows).
_________________________________________________________________
_________________________________________________________________
_________________________________________________________________
_________________________________________________________________
_________________________________________________________________
_________________________________________________________________
_________________________________________________________________
_________________________________________________________________
_________________________________________________________________
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Identifying need for further testing
Once you have done the initial testing (pure tone audiometry and bone conduction
thresholds) you will need to look at your results and consider whether further testing is
required.
The simplest type of audiogram is acquired by measuring with air and bone conduction,
using only pure tones. If all the thresholds fall within the ‘normal’ range, ie.<20dBHL,
there is no need for further assessment.
If, however, any of the thresholds exceed 20dBHL, or there is a difference between the
ears of more than 10dB at any two frequencies, more testing is required.
If there is an asymmetry between the ears or the air and unmasked bone thresholds differ
by more than 10 dB, masking will have to be applied. There are two types of masking:
bone conduction masking and air conduction masking.
Bone conduction masking
You will need to apply bone conduction masking if the bone conduction is 15dB or more
better than the air conduction threshold of either ear.
Air conduction masking
If there is a 40dB (or greater) air-bone gap at any frequency, then air conduction masking
is required at those frequencies.
Speech masking
If the average air conduction thresholds in one ear are significantly worse than the bone
conduction thresholds in the other ear, you may have to do speech masking. The details of
these calculations are set out below
If any of the thresholds fall outside of the normal range, speech testing needs to be
performed.
Tympanometry
It is good practise to perform tympanometry on everyone, however, not all clinics do this.
If you do not routinely perform tympanometry, you should definitely perform it on those
people who show some conductive element in their hearing loss, that is a conductive
hearing loss or a mixed hearing loss.
Tympanometry should always be performed when testing children.
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TOPIC 2 – POST-BASIC ASSESSMENT – MASKING
Introduction
Why do we need to Mask?
When we hear sounds, most are heard by going through the normal pathway of the ear, that
is, by air conduction. However, some of the sound is also passed through the bones of the
skull, which is called bone conduction. This is the reason that our own voice sounds
different when we hear it recorded. We hear our own voice mainly by bone conduction,
which usually gives it a richer, deeper sound. When we hear ourselves on a recording we
are hearing our voice by air conduction which makes it sound thin and higher in pitch.
When we hear loud sounds, there may be a mixture of air and bone conducted sound.
When we are testing someone’s hearing, we need to be sure of what is recorded as air
conducted sound and what is bone conducted sound, and in which ear it was heard.
Masking is a technique which allows us to do just that.
Airconducted sound
To other cochlea
Bone conducted sound
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Interaural attenuation
When we apply an air conducted sound to one ear, we hear it only in that ear, until the
sound is so loud that some of it is transmitted through the bone and into the other ear
(remember, the cochlea is encased in a bony structure). In some people this occurs at
relatively soft sounds, in others the sound needs to be very loud before it ‘crosses over’.
This depends on the size and shape of the head. To be safe, we assume that it only takes
40dB for sound for it to cross over to the other side and is heard in the other ear (also
known as the non-test ear). This is called the interaural attenuation. If the interaural
attenuation is exceeded, sound will spill to the other cochlea. The amount of this sound is
called the cross over.
Air conducted sound at
60dB, interaural
attenuation =40dB
Bone conducted sound
to non-test ear, 20dB
(cross-over)
Sound presented with a bone
conductor will be heard by both
ears, or the better cochlea
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With air conducted sounds the interaural attenuation varies from 40-60dB. When using
headphones, we always assume 40 dB interaural attenuation, as this is the worst case
scenario. When using insert earphones, we can assume 60dB interaural attenuation, as
inserts reduce the amount of bone conducted sound.
When using the bone conductor the interaural attenuation is 0dB, that is because the sound
is applied directly to the bone. So a sound presented through a bone conductor will be
heard equally in both ears (if the hearing is the same in both ears), regardless of where the
conductor is placed on the head.
Bone conduction masking
When using the bone conductor, we obtain the cochlear threshold of the better ear,
regardless of on which side the bone conductor is situated. Therefore, if there is a
significant difference of hearing level (15dB or more) between the unmasked bone
conduction threshold and the air conduction thresholds in either ear, then we want to
establish whether the discrepancy is caused due to a conductive problem (middle ear) or a
sensorineural problem (cochlear or retro cochlear). To do this we need to isolate the worse
ear. That means we need to keep the better ear busy, so it’s not interfering in our
investigation of the worse ear. While we are keeping the better ear busy, we then test the
worse ear (or test ear).
We do this by introducing a masking noise into the better ear.

Masking noise is always presented via a headphone or insert phone, that is, by air
conduction.

The masking noise must be suited to the signal, ie. Narrow band noise for pure tones
and speech noise for speech signal.

The masking noise needs to be audible by the better ear, that is, it must be introduced
at least at air conduction threshold level.

It must be loud enough to take care of the cross-heard signal.

It must not exceed the interaural attenuation, otherwise the masking will spill into the
worse ear and affect threshold. This is called overmasking.

The earphone on the test ear side must be off the ear (usually up near the temple) as
occlusion may otherwise occur and skew test results.
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Air conducted narrow
band masking noise in
non-test ear
Bone conducted
signal not heard by
non-test ear due to
masking
Bone conducted sound
to test ear is heard
Air conduction masking
In the case of there being a large gap between the AC thresholds and the unmasked BC
thresholds, you will need to mask the AC thresholds. The worst case scenario for crossover occurring using AC is 40dB. Therefore if there is a 40dB gap, you must mask. The
principle is the same as BC cross-over, except that with AC testing the levels have to be
higher before they are transmitted by the bone (remember in the case of BC testing, the
sound is applied directly to the bone, whereas in AC testing they are not). In the case of a
dead ear, for example, you may be getting thresholds although the ear has no hearing. It
will mimic the hearing ear, but at levels of 40dB or more below. This is called a shadow
effect. When masking is introduced, the thresholds quickly drop down to the real levels.
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Air conducted narrow
band masking noise in
non-test ear
Bone conducted
signal (cross-over)
not heard by nontest ear due to
masking
9
Presented air
conducted signal to
test ear is heard in test
ear
When do you need to mask?
Bone conduction- always, except when AC and BC thresholds are within 10dB of each
other.
Air conduction- if the AC thresholds are 40dB worse than the unmasked bone conduction
thresholds.
How much masking is needed?
For both AC and BC testing:
masking level = Air Conduction threshold (Non-Test-Ear) + 10dB
How do you mask?
In order to obtain masked bone conduction (BC) thresholds, the bone conductor must be
placed behind the ear we want to establish the thresholds from ie. The test ear (TE). We
apply masking to the other (better) ear by placing the headphone on it. This is called the
non-test ear (NTE). We DO NOT place the headphone over the test ear but next to or
above it, as this may cause an inaccurate result due to occlusion. The arrangement looks a
little lopsided and may take some practice to achieve.
In the case of AC masking, the headphones are placed as usual, with pure tones applied to
the TE and masking to the NTE.
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Continuous masking noise is then introduced through the headphone to the NTE, 10dB
above the air conduction (AC) threshold and pure tones are introduced through the bone
conductor on the test ear. When threshold has been established in the TE, the masking
noise in the NTE is increased by 10 dB and the tone in the TE is repeated at threshold
level. If the client responds, turn up the masking level by a further 10 dB and present the
tone again, at threshold level. If the client responds again, the threshold is established.
This procedure is called establishing a masking plateau. A plateau of 30 dB must be
reached before the threshold is considered valid, ie. that none of the signal has ‘crossed
over’ from the NTE. (Remember that we started with 10dB to begin with, we then increase
twice more by 10dB to get a 30dB masking plateau).
If the client does not respond once the masking noise has been turned up, the signal in the
TE is increased at 5 dB increments until the client responds again. Then a new plateau
should be attempted. You must increase the signal on the TE, until a 30 dB plateau is
achieved, ie. The client must respond three times to the same level in the TE while the
masking noise is increased in the NTE.
Two scenarios of bone conduction masking
1.
10dB
10dB
2.
10dB
10dB
10dB
AC left = 20dB
AC right = 40dB
Masking
30dB
40dB
50dB
BC
20dB
20dB
20dB
AC left = 20dB
AC right = 40dB
response



Masking
30dB
30dB
30dB
BC
20dB
5dB 25dB
30dB
response
x
x

40dB
40dB
40dB
50dB
60dB
30dB
5dB 35dB
5dB 40dB
40dB
40dB
x
x



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Air conduction masking
1.
10dB
10dB
2.
BC = 20dB
AC right = 60dB
Masking left
30dB
40dB
50dB
AC
60dB
60dB
60dB
BC = 20dB
AC right = 60dB
Masking left
30dB
30dB
30dB
40dB
40dB
40dB
50dB
60dB
10dB
10dB
10dB
AC
60dB
5dB 65dB
70dB
70dB
5dB 75dB
5dB 80dB
80dB
80dB
response



response
x
x

x
x



Overmasking
Sometimes when we need to mask, there is the risk of overmasking. This is when the
masking level gets to a point where it is so loud that it spills back into the test ear. This
doesn’t happen very often, but is possible especially when there is a bilateral conductive
hearing loss. That is, a lot of masking is needed to be audible via the airconductive
pathway, but because the other ear has good bone conduction thresholds, the masking is
heard by the test ear. This means that the test ear now has trouble hearing the signal. In
this case, it is not possible to establish a masking plateau. Every time the masking is
turned up, the signal tone cannot be heard. It needs increasing, until it is heard again, but
then the masking noise will blot it out again. In this case, try to get a 20dB masking
plateau, but also write in the margin that a full masking plateau was not established due to
overmasking, or ‘Naunton’s masking dilemma’. You are at risk of overmasking when:
Masking level – 40dB > BC threshold of test ear
Scenario of overmasking
AC R+L= 60dB
BC R+L = 10dB
BC signal L
10dB 
Masking R
70dB
10dB
80dB
10dB x
5dB 15dB x
5dB 20dB 
10dB
90dB
20dB x
5dB 25dB x
5dB 30dB x
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Air conducted narrow
band masking noise in
non-test ear high due to
high air conduction
threshold
Bone conducted
masking noise
(cross-over) heard
by test ear due to
good BC threshold
in test ear
Presented air or bone
conducted signal to
test ear is not heard in
test ear due to
masking noise spilling
from NTE
TIP: To try and overcome a masking dilemma, you can use insert earphones, if available,
as this raises the interaural attenuation to 60dB.
Activity 2.1
Below you will find a number of terms associated with masking.
Next to each write a definition or description.
You can then check your responses with those given at the end of this topic
Terms
Definition/Description
Air-bone gap
Cross-over
Non-test ear
Test ear
Masking plateau
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Activity 2.2
Practice masking at your clinical visits whenever possible. Masking takes a while to get
the hang of, so the more practice you get, the sooner you will feel it coming to you
naturally.
Follow these steps for bone conduction masking:
1.
Place the bone conductor on the mastoid bone behind the test ear
2.
Place the headphone over the other ear (NTE) and to the front of or above the test
ear.
3.
Mask the NTE by starting at AC threshold + 10dB
4.
Test the bone conduction threshold going up in 5dB steps
5.
Add in 10dB of masking every time you get a response
6.
When you have done this three times at the same threshold, you can mark it in the
audiogram
For air conduction:
1.
Put masking into the better ear (NTE) at threshold + 10dB
2.
Retest worse ear (TE) in 5 dB steps
3.
When you have a response, add in masking in 10dB steps until you have done this
three times, with a response.
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Suggested answers to activities
Activity 2.1
Terms
Definition/Description
Air-bone gap
The difference between air conduction
thresholds and bone conduction
thresholds
Cross-over
The amount of signal that has crossed
from one side to the other (the amount by
which interaural attenuation has been
exceeded)
Non-test ear
The ear that is not being tested (the ear
that has masking applied to it)
Test ear
The ear in which threshold or speech
discrimination is being established
Masking plateau
Extra masking applied to NTE while
presenting signal at same level in test ear
Activity 2.2
Get your supervisor to check your technique.
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TOPIC 3 – SPEECH TESTING
Introduction
Speech testing is one of the basic test battery components. It helps us to assess the speech
intelligibility our client has. Speech intelligibility is mostly, but not always linked to the
audiogram. For example, a client who has a typical presbycusis loss with good low
frequencies and poor high frequency hearing is expected to do well on words such as CAN
but may have trouble with words such as THIN. There, are however, some cases where
the speech intelligibility is markedly better or worse than expected. In Australia we use
phonetically balanced word lists (PB word lists). These have been designed to carry a
proportional balance of phonemes as used in the English language.
When performing speech testing we like to test at different levels, so that we can graph a
speech curve. To graph a speech curve we should present at least 6 different speech lists
(of 10 words each) at different levels. We start 30dB above the three frequency average
(3FA) which consists of 500Hz, 1000Hz and 2000Hz, and then increase the level by 10dB
until the client scores their best score, and then below the starting level until the client
scores very poorly.
Typical speech curve
100%
50%
10
20
30
40
50
60 dBSPL
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Most Comfortable Level (MCL)
Some clinicians like to do speech testing at the client’s most comfortable level (MCL).
There are points for and against this.
The aim of rehabilitation in hearing is to get the client to hear to the best of their ability.
The variation in level within speech is about 30dB. This means that when you present
speech at 30dB above threshold, there is every chance that your client should hear
everything in the speech spectrum between 500-2000 Hz. In this technique we like to do a
speech curve, that is, we get the client to perform at different levels, so we can see where
their best performance is, and this can be taken into account when setting the hearing aid.
When using MCL in speech testing it is usually not done as part of a curve, ie only one or
two lists are presented at one level. This requires less time, but also yields less
information. Furthermore, the MCL of a client may be less than 30dB above 3FAHL and
the client is therefore then not given the chance to achieve their best score. However, the
argument for using MCL is that it is probably the level at which the client will end up
using the hearing aid, even if they hear better at a higher level, and is therefore a better
indication of the final result you can expect for your client.
There is no right or wrong in the different techniques, just different philosophies. What
method do you use in your clinic? Talk to the other clinicians and ask why they use a
particular technique. Perhaps you would like to use a combination of both.
Reliability and validity
The important thing to remember about speech testing is that it only gives us a rough
indication as to how a person is performing. A 5% or 10% improvement between levels is
not significant when you are only presenting 10 words at a time. Speech discrimination
results have problems with validity and can be impacted by issues such as native language
(ie those people whose first language is not English, and even those whose first language is
an English from elsewhere such as American, Canadian, Scottish or Caribbean), as well as
the pronunciation of the speech presenter (if done by clinician). There are also issues of
variability, which are dependent on changing levels if the speech is presented live voice,
compared to pre-recorded material which has been equalised. Also female vs male voices,
having a cold and the quality of the test set up can all affect speech discrimination scores
and their reliability. As well as all that, because of time restraints, we tend to only present
a limited amount of speech material, which renders small changes in scores insignificant
when calculated statistically.
Activity 3.1
Some lists are scored by words correct, others by phonemes. Can you find an example of
each type of material? Which do you think is the more sensitive material?
_________________________________________________________________
_________________________________________________________________
_________________________________________________________________
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Speech tests can be presented through tape/CD or live. List the advantages and
disadvantages of each method.
_________________________________________________________________
_________________________________________________________________
_________________________________________________________________
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Speech masking
Masking is also required when testing speech, if there is danger of cross-over occurring. In
the case of speech testing, we are testing at levels above threshold. This is called suprathreshold testing. We are therefore more prone to risk cross-over signal. In speech testing
we use the average hearing loss, that is the average of thresholds at 500Hz, 1000Hz and
2000Hz and add 30dB as our starting presentation level.
Speech presentation level = average AC (500Hz, 1000Hz, 2000Hz) + 30dB
Also known as 3FAHL
We then need to look at whether this presentation level is likely to spill into the other ear.
The spill or cross-over will be heard in the non-test cochlea, so we need to look at the
average bone conduction threshold of the non-test ear. We deduct 40dB (for air
conducted interaural attenuation) from the presentation level and compare it to the average
bone conduction in the other ear. If it is larger, the presentation signal will be heard by the
non-test cochlea, and we will need to mask.
If Presentation Level (PL) – 40dB (interaural attenuation) > average BC of NTE
To establish whether masking is needed, we also need to establish the minimum level at
which a speech score can be obtained. This is called the minimum discrimination score
level (MDSL). The reason we need to take into account the MDSL is because
Audiometers are calibrated with a pure tone. The speech signal is a broadband signal, and
as such has more energy at any given level than a pure tone of the same level. Therefore it
is possible and indeed often the case, that at the average threshold level, where a person
ought to be just hearing a speech signal, they might hear it better than expected, because
there is more energy in the signal. This would cause your speech discrimination score to
be better than expected.
The final formula looks like this:
When to mask.
If presentation level (average AC (TE) + 30dB) – 40dB - MDSL > average BC of NTE
How much masking do we need?
Well, we need to start with the 3FAHL of the non-test ear (NTE), the ear we want to
present the masking to. Then we need to add enough masking to overcome the cross-over.
The cross-over we calculated above, it is the difference between the presentation level etc
and the average BC of the NTE
That is:
From above
3FAHL (NTE) + cross-over ((PL-40-MDSL)-av BC(NTE))
When masking for speech we use speech masking noise, which is a broad band noise with
similar frequency weighting to speech. On top of this we need to add a ‘buffer’ called the
effective speech masking level (ESML). The ESML, again has to do with the fact that the
audiometer has been calibrated with a pure tone. Because the masking noise is calibrated
to pure tone levels, but has to mask a speech (broadband) signal, it may not be effective.
That is, ideally a 20dB masking noise should completely mask a 20dB speech signal, but
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this may not be the case. You may need extra masking, to ensure that the speech is really
masked. An explanation of how to establish ESML and MDSL for your audiometer will
follow below.
Therefore the formula for how much speech masking is:
3FAHL (NTE) + cross-over (which is presentation level – 40dB – MDSL – average
BC threshold in NTE) + ESML
Overmasking
With all masking techniques there is a chance of the masking noise spilling back into the
test ear. This is called overmasking. This sometimes happens when there is a large
bilateral conductive loss. It can happen more easily in speech testing because we are
testing at supra threshold levels, ie. Levels that are higher than hearing threshold. The
principle is the same, though, as for pure tone overmasking:
Overmasking if: masking level – 40dB > average BC (TE)
This is called Naunton’s masking dilemma. If this occurs, you must write this on the
audiogram or in your report.
Calculating MDSL and ESML
These levels only need to be established once, providing there are no changes at the annual
calibration of your audiometer.
MDSL – the level at which a group of normal hearers begin to score (4%) with the speech
material used, ie. The minimum discrimination score level.
ESML – the level at which that speech material is effectively masked
A minimum of ten people with hearing within normal limits is needed to establish these
levels effectively. It will need to be done separately for each audiometer and different
speech materials.
To calculate MDSL present speech material at decreasing levels until the subject is
scoring only one or two phonemes. Take the lowest level at which any of the people tested
scored (we want to mask for the worst case scenario) as your MDSL.
N.B. It is important to do this for different speech materials used as PB word list MDSL
will be higher than a Boothroyd list MDSL because a whole word is needed for a minimum
score rather than just a phoneme.
To calculate ESML take the level at which a subject is scoring 100%. Then introduce
speech masking noise into the same ear until the score is reduced to less than 5 % (ie the
speech is ‘masked out’). The difference between the level of speech presented and the
masking level is the ESML. That is, if the speech material was 40 dB and the masking
level 50dB, your ESML is 10dB.
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NB: Some older audiometers do not allow masking to be presented ipsilaterally, ie. Into the
same ear as the signal. If this is the case with your audiometer, assume that the ESML =
0dB. However, it is important for you to understand the principle of ESML and why it is
taken into account.
Once you have established the MDSL and ESML for your audiometer(s), it is a good idea
to write them down on a strip of paper and tape them on or near the audiometer. This way
anyone testing can immediately see what your levels are.
Activity 3.2
Test 10 people with normal hearing and establish the MDSL and ESML for the audiometer
you are using. Use the tables below to help you record your results.
Calculating MDSL
Speech material:
Subject
Level dB at which lowest score is
achieved (lowest in column = MDSL)
1
2
3
4
5
6
7
8
9
10
MDSL
Calculating ESML
Speech material:
Subject
Level dB at which scoring 100%
Level dB at which effectively
masking column 2
Difference between
columns 2 and 3
Sum of column 4
Divide by 10
ESML=
1
2
3
4
5
6
7
8
9
10
You can check your answers with your teacher.
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Practical
Test speech masking using the following steps:
1.
Find the average threshold of 500,1kand 2k (3FAHL) for the ear you want to apply
masking to
2.
Add 30dB to the 3FAHL
3.
Take away 40dB (for interaural attenuation) Take away (or add! if the MDSL is
negative, (-(-)=+)) MDSL value
4.
Compare to the average BC thresholds in the other (test ear)
5.
If there is a gap you need to mask
6.
To calculate level for masking, add gap to 3FAHL to masking (non-test) ear
7.
Add ESML value
8.
Present masking noise at this level in masking ear and present speech to test ear at
30dB above average threshold.
9.
If you test at more than one level, the masking level will change accordingly, ie if
you present speech at presentation level + 10dB, then the masking level also has to
be increased by 10dB.
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Suggested answers for activities
Activity 3.1
AB (Arthur Boothroyd) word lists are marked by the phoneme, PB (Phonetically
Balanced) word lists are marked by the word. The AB lists are more sensitive, because if
you get part of the word right you get credit for it. If you get part of the word right in the
PB word list, you get no credit for it.
Presenting via CD or tape means that the voice is always the same, ie no colds or emotion
play a part, it is level equalised so that the levels are always even, it is always clear. The
disadvantages are that it is slow compared to live speech, the examiner’s voice may be
more familiar, it is faster.
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TOPIC 4 – TYMPANOMETRY
Introduction
Tympanometry is a test which looks at the middle ear and function of the eardrum. It is
part of a larger test battery known as impedance testing which also includes acoustic reflex
testing. For the purpose of this course we will only look at screening tympanometry and
screening reflexes.
The Tympanometer
The tympanometer is a machine that lets us measure middle ear function. There are three
measurements involved with tympanometry:
1.
tympanic membrane compliance
2.
middle ear pressure
3.
middle ear volume
Tympanic membrane compliance tells us how mobile the eardrum is. If there is fluid in
the middle ear, then the tympanic membrane will not move as readily, for example.
Middle ear pressure should be the same as the outside pressure, ie. 0 mmH2O. The
Eustachian tube which connects the middle ear to the pharynx opens a little each time we
swallow or yawn and thus equalises any differences in pressure that may occur, eg when
we drive down from a steep mountain, our ears ‘pop’. If there is an infection in the throat
which causes swelling around the opening of the Eustachian tube, air cannot get into the
middle ear. Since the middle ear is lined with skin which breathes, soon a vacuum is
created which gives us a negative middle ear pressure. Sometimes when we blow our
nose, extra air is forced into the middle ear and thus a positive middle ear pressure is
created. As far as pathology is concerned, we will concentrate on negative middle ear
pressure.
Please do not confuse negative middle ear pressure or even fluid in the ear with a middle
ear infection (otitis media). Fluid in the ear occurs when prolonged negative middle ear
pressure draws fluid from the skin into the middle ear cavity. This in itself is harmless.
An infection only occurs if bacteria travel up the Eustachian tube and then cause an
infection of the skin lining the middle ear. When fluid is present this may occur more
readily because of the warm, moist environment. If there is an infection, it is always
accompanied by pain and fever, and if pus forms, eventually the tympanic membrane may
rupture. Fluid in the ear alone is often reabsorbed with no pain or infection. Sometimes if
it remains in the cavity for extended periods of time, fluid will turn to ‘glue’, a sticky
substance which needs to be removed by a surgeon.
Middle ear volume is in itself relatively arbitrary, as it depends on how far the probe is
pushed into the ear canal. If however, the middle ear volume is significantly larger or
smaller than the normal range, it may indicate a perforated eardrum in the case of large
volume, or a middle ear growth (cholesteatoma) if it is very small.
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The probe
In order to make tympanometry measurements we need to insert a probe into the ear canal.
Therefore you must look into the ear canal before you start, first of all to look at the ear
drum, but also to check that there is no wax or other debris present. Next you need to
select an eartip that will fit the client’s canal snugly. If there is leakage, the test will not
work.
The probe of the tympanometer is made up of three parts.
1.
2.
3.
a tube that delivers pressure
a tube that delivers sound
a microphone outlet
eartip
Sound
Pressure
Microphone
probe
The tympanometer measures the compliance of the eardrum by putting pressure into the
earcanal and also producing a sound. As the pressure increases, the eardrum becomes less
compliant (it moves less) and the sound mostly bounces off. The microphone measures
how much sound comes back from the ear drum. Then we reduce the pressure, and in a
normal ear, the eardrum will be at its most compliant and the sound is conducted well and
does not bounce back. Then we reduce the pressure and again the eardrum becomes taut.
We graph this movement in a chart called a tympanogram.
Volume
cc
-200
0
Middle ear
with normal
pressure
+200 mmH2O
TM is most compliant at
0mmH2O
Sound + pressure
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Screening tympanometers usually do all this automatically, but you will still need to read it
and put the results in your report. If your tympanometer is not automatic you will need to
note down the cc values at –200mmH2O, at the peak of the curve (not necessarily
0mmH2O!!!) and at +200mmH2O. The height of the curve is measured from
+200mmH2O to the peak.
The different types of tympanograms
As an audiometrist you will only need to report the values of each of these three
measurements without having to report on abnormality, but it is useful for you to have an
understanding of the results.
Tympanograms come in three versions, Type A, Type B and Type C.
Type A denotes a tympanogram with normal middle ear pressure ie 0 mmH2O (+/100mmH2O).
Middle ear
with normal
pressure
TM is most compliant at
0mmH2O
Sound + pressure
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Type B denotes a tympanogram with no peak. This occurs when there is fluid or glue in
the middle ear cavity which prevents the eardrum from moving. In conjunction with a high
volume, a type B can also indicate a perforated eardrum. A type B tympanogram should
always be referred to a doctor.
There is no peak in the
curve because the TM can’t
move due to fluid in the ear
Vol.cc
-200
0
Middle ear
with fluid
200
mmH2O
TM has no compliance (is
unable to move) due to fluid
in middle ear cavity
Sound +
pressure
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A type C tympanogram has a negative peak and is commonly seen when the client has a
cold. For it to qualify as a type C the peak must be <-100mmH2O. NB.:In children this is
extended to -150mmH2O.
Vol. cc
-200
Middle ear
with negative
pressure
0
200 mmH2O
Negative pressure has to be
applied, before TM reaches
maximum compliance
Sound + negative pressure
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Screening reflexes
As part of the impedance test battery we perform acoustic reflex testing. Audiologists tend
to do ‘the works’, that is, diagnostic reflex testing which can give us clues as to the
integrity of the acoustic nerve. Reflex testing assesses a reflex arc of nerves that run
between the ears. When a loud sound is presented, a tiny muscle that is attached to the
stapes contracts, moving the stapes away from the oval window. It is designed to reduce
the amount of signal going into the ear. This reflex works in both ears simultaneously,
regardless of which ear the sound is presented to. This means that there are two ways of
measuring the reflex, contralaterally and ipsilaterally.
In the contralateral test the sound is presented to one ear and the reflex measured in the
other. In the ipsilateral test the sound is presented to the same ear where the reflex is
measured. Reflex testing is done with pure tones at different frequencies. In screening
tympanometry, screening reflexes are often part of the test setup and essentially confirm
the tympanogram result. Eg, if we get a type B tympanogram, we do not expect to get any
reflexes. We tend to also only use ipsilateral reflexes in a screening setup. The most
common reflexes tested are 500Hz and 1000Hz, although some also test 2kHz and 4kHz.
In a screen test, tones are presented only at one level and recorded as present or absent. (In
the diagnostic test, a threshold is obtained).
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Activity 4.1
Get your supervisor to show you how to do a tympanogram. It is important to choose the
correct probe tip size for each ear, as a seal must be established for the test to run properly.
Do a tympanogram on yourself and record the volumes at –200mmH2O, the peak, and at –
200mmH2O. Then calculate the height of the peak.
Do tympanograms on different people at your clinical visits and record your findings.
Check how the tympanometer at your clinic is set up for reflexes. At what level does it
present and at what frequencies? If your tympanometer is not set up for automatic
screening reflexes, what is the protocol regarding screening reflex testing at your clinic?
What are the three main components of a tympanometer (impedance bridge)?
Illustrate your answer with the help of diagrams.
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TOPIC 5 – THE SIGNIFICANCE OF VARIOUS TEST
RESULTS
Introduction
When you look at your test results you must keep in mind the reliability and validity of
those results.
Reliability
Reliabilty is related to how repeatable a test result is. Providing that your test equipment
and operating technique are intact, objective tests (such as tympanometry) are very
reliable. However, most of the tests you will do in the clinic are subjective tests, such as
audiometry and speech tests. These involve an element you cannot control fully, ie the
client. Things to take into consideration your client’s:
1.
hearing loss
2.
lucidity (alertness and ability to comprehend)
3.
language skills (if they are not proficient in English)
4.
motivation for test (does not want to do test or financial reward for HL)
5.
mental ability (mental handicap)
6.
physical ability (dexterity)
7.
age (very young or old)
8.
understanding of test protocol
Any of the above items may contribute to variations in test reliability. In order to increase
reliability, we need to be aware that variations may occur and how to minimise them.
Activity 5.1
Choose a client you are seeing and test their speech discrimination each time they come to
see you. When you have done 3-4 tests, compare them. Are they exactly the same? What
might have caused differences?
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Validity
Validity is a measure of how significant your test results are. This generally means how
statistically significant, but also the validity of the test stimulus. That is, whether your
audiometer has been calibrated properly and within the last 12 months, so you can be sure
that the pure tones are at the correct frequency and level. Another example is the speech
test. In order for the test to have statistic significance you must use a minimum number of
items. If you use only one list of 10 items, your test is not statistically relevant. Speech
tests, particularly, are subject to difficulties in validity if they are cut short. This happens
often in clinics, unfortunately, because of time restraints or client fatigue. Speech lists
should also be phonetically balanced to reflect the phonetic spread of the language you are
testing. Different speech tests are designed for specific purposes, so you must be sure you
are using the correct speech material for the test you wish to perform. If your client is of a
non-English speaking background, you must take into account that the speech test will not
reflect their true ability. Similarly, if the words you are using are unfamiliar to your client
you may not get valid results.
Activity 5.2
What speech material do you usually use in your clinic? Find out and write down what
factors have been built into the list to increase validity of the test results. How many
words/lists do you need to present for the result to be statistically significant?
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