Download COMD 3700 Basic audiology Lesson 4 The Measurement of Sound

COMD 3700 Basic audiology
Lesson 4
The Measurement of Sound
Highlighted information refers to a change between the audio
recording (using 10th edition) and the 11th edition of the textbook
1. COMD 3700 for distance education. This is lesson four, a
continuation of the measurement of sound. This will cover pages 5068 in your textbook
2. We will begin with a review of psychoacoustics. This was covered in
your ComD 3400 course, so I will just quickly review some of the
main points. Psychological acoustics or psychoacoustics includes
everything that can be perceived by the human ear from a sensory
standpoint. This refers to how the sound “feels” to us. Unlike physical
acoustics, which are the same with or without the human ear,
psychoacoustics requires a human ear. What we measure as
frequency, the ear perceives as pitch. Intensity measurements we
interpret as loudness, timbre as tonal quality. Frequency, the physical
measurement in Hz, or cycles per second, relates to the listeners
interpretation of pitch, the psychological measurement of whether a
sound is low or high. Raising the frequency of a sound makes the
pitch higher. Doubling the frequency raises the pitch one octave, but
does not double the pitch. Mels measure pitch. 1000 Mels is the pitch
of a 1000 Hz tone at a 40 dB sensation level for normal ears. A higher
pitched sound will have more Mels, a lower pitched sound, less.
Intensity, measured in dB IL, SPL or HL is a physical measurement.
Other words that describe intensity are amplitude and Maximum
Sound Pressure. But loudness is psychological. We perceive a small
increase in the intensity of a soft sound as a significant increase in
loudness. We require a greater change in intensity to make a loud
sound louder than to make a soft sound louder. If we make a sound
twice as intense, we would expect it to be twice as loud. This is not the
case. Most auditory systems follow a power law, sometimes
considered a logarithmic concept, because a 10 dB increase doubles
the loudness over most of the range of intensities. Remember that
when we double the sound pressure of intensity, a 6dB increase
occurs. A threefold increase in sound pressure, 10 dB, doubles the
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loudness. For example, a 1000 Hz pure tone at 80dB SPL is twice as
loud as 70 dB SLP, and half as loud as 90 dB SPL. Loudness levels
across frequencies are measured in phons. Loudness, up frequency
(not across), uses sones for measurement. So to review physical
acoustics vs. psychoacoustics, remember that the physically measured
loudness of a sound includes frequency, intensity, and duration.
These measurements remain constant regardless of whether hearing
is normal, or a conductive, mixed or sensorineural hearing loss is
present. Although the measurements remain fixed, they are perceived
differently by each patient because of individual hearing differences.
This perception is psychoacoustics.
3. Audiologists usually make 2 kinds of measurements. One is to test
the ability of patients with possible disorders of the auditory system.
The other type is to measure the sound pressure levels in the
environment. We are going to discuss the equipment used for both
types of testing. Before we start into measuring hearing and pure tone
audiometry, we need to talk about the equipment we use to measure
sound and how to calibrate, or looking at the integrity of, our testing
instrumentation. In an advanced class, talking about sound
measurement and calibration will be a lengthy portion of the class but
we want to talk about it in a brief form here in basic audiology. We
will talk about the equipment used in measuring the ability of a
patient’s auditory system and measuring sound pressure levels in the
environment as well as calibrating the audiometer.
4. We will start with measuring the amount of a patient’s hearing loss.
To do this we use an audiometer. I have heard people pronounce it
audio-meter. But we refer to this instrument as an audiometer.
Audiometers are made by a number of manufacturers, and vary in
complexity and control layout. The controls or indicators may be
dials, buttons or switches. For the most part, every audiometer has
the following:
Supra-aural and/or insert earphones and a bone receiver- we will
discuss these in greater detail later
Hearing Level indicator or attenuator- This controls the volume or
intensity of each tone you present. This dial measures the volume,
expressed in decibels Hearing Level or dB HL.
Frequency Indicator-Tests the following frequencies: 125, 250, 500,
750, 1000, 1500, 2000, 3000, 4000, 6000 an 8000 Hz. The sound
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produced at each frequency is a pure tone.
Ear Indicator-to select the ear to be tested, either left or right.
Interrupter switch-this controls both when the test tone is on and the
duration of the tone.
5. So let’s look at some audiometers. This is a very simplified face on a
basic pure tone audiometer. As you can see, we have a frequency dial.
That's the Hz dial frequency. On a simple audiometer like this, we can
select frequencies from 125 to 500 on up to 8000 Hz. Those are the
octaves or frequencies at which we test hearing. On the right side, you
see the other large dial. That is the hearing level dial measuring dB
HL. It runs from minus ten to 110. So we have our frequency range
and intensity range we can select. On top, we have the masking level
dial. We're not going to discuss this right now, but masking is the
noise that's put in the non-test ear to keep it from hearing the test
tone. Down at the bottom, is the tone interrupter. It's been called an
interrupter switch from the beginning of time. When these
instruments were first produced, the tone was constantly on. It had a
spring in it that would pop up. When you pushed down on it, it would
interrupt the tone. On most modern audiometers, the tone is off until
you depress an interrupter switch and then that causes the tone to be
delivered the client. Then you have the power switch with off and on.
A simple audiometer like this is designed for pure tone testing only
and is used as a screening audiometer.
6. Here, we are looking at a block diagram of this simplified pure tone
audiometer. This figure is found on page 56 in your textbook. These
are the basic components necessary to accomplish the task of
delivering a pure tone stimulus to the client's ear. On the left, you see
a pure tone generator. That is the same as the frequency dial. So we
need to generate a tone of given frequency. For instance, let's
generate a tone at 1000 Hz. That is delivered to an amplifier. Then
the amplifier amplifies the tone to the maximum level. So if the
audiometer is putting out 110 dB at the maximum level, that's the
level at which the amplifier is producing the tones, 110 dB.The
attenuator is the same as the hearing level dial. Here's where we
attenuate. Attenuate means to subdue or tone down or bring down.
Just like if you have a bright color and it's attenuated by adding white
to make it a softer color. This is what the dB dial is actually doing. We
take a high level amplified stimulus and attenuate it down to 70 dB or
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30 or 10 dB. When you have the dial, you're not pumping energy in as
you turn the dial up. You're allowing less attenuation so you're getting
a stronger signal. You send that to the silent switch. You saw the
interrupter switch on the previous slide. You see if you push the
switch, it will make the connection and deliver the tone. There will be
a switch somewhere on the audiometer, other than the basic one we
just used, where you can select either a bone conduction or air
conduction as the mode of delivery of the stimulus. So if you choose
air conduction, you can also choose to have the tone delivered to the
right earphone or the left earphone. Or if you choose to have the
stimulus delivered to the bone oscillator or vibrator it is just delivered
to one place. On the diagram it is the black square. You can deliver
tones to the individual by way of air or bone conduction. We talked
about these two pathways when we talked about tuning fork tests.
7. In a clinic environment when you are conducting advanced testing,
other than basic hearing screenings, then you will use a diagnostic or
clinical audiometer for testing pure tones as well as speech testing. A
speech audiometer is part of a clinical audiometer. We will discuss
testing with speech stimuli in a later lesson. In addition to the
equipment we listed earlier, a diagnostic or clinical audiometer will
also include:
Presentation Indicator-Changes the presentation of the tone from
interrupted (normal choice) to pulsed, continuous, or warble tones
Function Indicator-changes the audiometer function from
microphone to air conduction, to bone conduction or to speech
testing
Microphone-Allows you to communicate with the patient while the
headset occludes the ears
Monitor-Important when you use a two-room test suite, a sound
booth, or recorded speech tests
Masking Control-produces a white or other noise to isolate an ear
during certain tests
CD Player (or tape player)-used for speech testing
VU Meter-Monitors the input level by an averaging voltmeter called a
Volume units or VU meter. The meter reads in dB VU, implying an
electrical reference in watts.
8. So, let me summarize.
This is a diagnostic audiometer, sorry you
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can’t see the words indicating what the buttons on this audiometer
are for. But you can see the 2 dials for frequency and intensity. With
any pure tone audiometer, the objective is to deliver a tone at given
frequencies or pitches. We use the term frequency more often. The
frequencies we use to sample a person's hearing in audiometry, the
lowest is 125 Hz. Then we go up in octaves. 250 Hz, 500 Hz, 1000 Hz,
2000 Hz, 4000 Hz until the high end at 8000 Hz. With an
audiometer, we can select any of these octaves or tones. You can see
on the screen of the audiometer that 1000 Hz is chosen. With the
attenuator you can take any one of these octaves and adjust the
intensity level at that octave. Now we can deliver a 1000 Hz tone at 20
dB, 50 dB, 70 dB, 90 dB, for instance. Then we can switch to a
different octave. We can deliver 250 Hz at 0 dB, 15 dB, 25 dB. On this
audiometer 0dB is chosen. The pure tone or speech signal is routed
through supra-aural headphones, insert earphones, a bone
conduction vibrator or in the sound field. We will review those now.
9. Remember when we discussed the pathways of sound and said that
most of the time we hear via air conduction? When we are testing to
determine type and severity we start the testing of the air conduction
pathway using some type of earphone or rather a headset that
consists of two earphones on an adjustable band that fits over the
head. Earphones are color coded, red for right ear and blue for left
ear. Everything in audiology uses these color codes, so you’ll need to
remember that. I just remember R is for red & right. You can use
headphones that fit over the ear, Supra Aural, or insert receivers that
fit inside of the ear. There are definite advantages for using insert
earphones when conducting testing and in addition, they are usually
more comfortable for the patient. So whenever possible, I recommend
using them.
10. A bone receiver on a spring steel headband is usually used to test
the bone conduction pathway. It is usually placed on the mastoid
process. This is the bone directly behind the pinna of the ear. Bone
conduction testing can also be placed on the forehead using a plastic
strap that circles the head.
11. There are times when we will test using the sound field of the
audiometric suite or sound booth. This is usually done in speech
testing. The signal is fed into the booth using loudspeakers, rather
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than earphones. If pure tones are used for testing in the sound field,
then the stimulus of the sound must be changed to a warble, or wavy
tone. In this picture you can see the loudspeakers behind the patient
being tested.
12. So, we have discussed the equipment usually used to test the
auditory system of a person. Now we are going to discuss testing
airborne sounds to determine the sound pressure level in
decibels. To do this a sound level meter is used. Here are some
pictures of various sound-level meters. This testing may be done to
determine the level of sound in factories, schools, offices, stores,
airports, etc. Recently our students have been using them to measure
the intensity in the environment of sporting events. For those of you
familiar with USU, you won’t be surprised to hear that the intensity is
much louder at the basketball games than the football games! Sound
level meters are instruments that will collect the sound energy from
the surrounding environment, convert that energy to electricity, then
give us a decibel readout as to the intensity of the sound in the
surrounding environment. In other words, if we wondered how
intense a sound was in a given environment, we'd take our sound
level meter into that environment to measure the sound and the
intensity, and some of the characteristics of the sound in that
environment. Most sound level meters are battery operated and vary
in size from half a loaf of French bread down to as small as a mouse
you use on your computer. There are basic analog sound-level meters
and sophisticated digital sound level meters.
13. A sound-level meter can also be used to calibrate an audiometer.
Calibration is necessary to determine if an audiometer is performing
properly in terms of its acoustic output, attenuator linearity,
frequency accuracy and harmonic distortion. We need to know that if
the audiometer shows that it is emitting a 1000 Hz signal at 45 dB,
that it is actually doing it. At the USU Hearing Clinic we have all of
our equipment professionally calibrated once a year. However, we
want to make sure that in between the annual checks, the audiometer
is functioning properly. So we perform periodic checks on the
equipment. If an office is equipped with a sound-level meter and
couplers, then the calibration can be done at any time. We are going
to discuss some of the basic information regarding calibration.
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14. Here is a basic drawing of a very simple sound-level meter. As you
saw in the previous slide, most sound-level meters today are digital
and much more advanced than this picture. I am using this basic
drawing so that you can see the parts that we will be reviewing.
Observe the meter at the middle of the diagram. It says dB SPL in the
meter. The stylus on the meter is pointing at zero. That would
indicate there was some sound energy coming in. If we were in
silence, the stylus would drop to the left and we wouldn't measure
anything. The fact it's standing at zero indicates we're measuring
sound now. On the right, the meter is marked off in decibels steps. 1
dB 2, 4, 5. On the right hand, there's also equal demarcations. Move
down to the dial just below the meter, note that it says sensitivity dial.
We can adjust the sensitivity of the meter. If you move down, one
more dial there, you see the frequency dial. What we can do with most
sound level meters is actually take through a filtering process and
look at the intensity in the environment of a sound centered close to
the meter, there is a lot of sound energy around 1000 Hz, you can
select that band with the dial at the bottom of the meter.
If we would like to know what the sound intensity is at 500 Hz, we
could change the Hz down and get an indication of how much sound
energy was in the environment around 500 Hz or 250 Hz or 2000 Hz
or whatever. In order to transform the acoustical energy into
electricity, we need to have a microphone, which you can see is placed
at the top of the sound level meter. More particularly you can see the
diaphragm of the microphone sits atop the microphone. On top of
that is a 6 cubic centimeter coupler. Why 6 cubic centimeters? Sound
reacts to coupler or environmental size in predictable ways. If we
introduce an amount of sound energy into a small cavity, we'll
measure a relatively large sound in the cavity. If we increase the size
of the cavity, the sound pressure level in the cavity drops or lowers.
On the averages, when you put an earphone over the normal human
ear, you've occluded or covered the ear. The diaphragm in the
earphone over the opening of the ear canal causes a cavity to be
formed. The walls of the external auditory meatus and the tympanic
membrane on one end and the earphone on the other, that closed
cavity on average is 6 cc. Thus we use the 6 cc coupler when we do
calibration measurements on a sound level meter.
15. In this slide, we have the same sound level meter as we looked at
before and all of its components. Here we've taken an earphone from
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the audiometer head band and placed it atop the sound level meter
and positioned in over the cc coupler in preparation for calibrating
the earphone with its accompanying audiometer. We have the
earphone atop the sound level meter placed atop the 6 cc sound level
coupler, in preparation for calibration. We have also put a 500-gram
weight atop the earphone. That pressure of the weight is a standard
in the field. It's specified in the calibration standards. If you
increased or decreased the amount of weight put on top the earphone,
sound level pressure would vary slightly. Therefore, we have to go
with the standard and put a 500-gram weight on top.
16. In this diagram, you see the very basic audiometer on the left. We
have selected 1000 Hz as our audio output. We've selected 70 dB as
the intensity of our output. 70 dB is the suggested output if you're
going to stay with standard procedure. 70 dB output is standard for
each frequency we test. So we'll leave the dB dial at 70 and change the
frequency or Hz dial from 1000 to other frequencies. We'll deliver
that frequency at that intensity to the earphone. On this basic SLM
we've set the sensitivity dial to 75 dB in anticipation of this 70 dB
output from the audiometer coming into the sound level meter. The
sound level meter when stimulated by the audiometer reads on the
positive side 2.5.So you take the 75 dB that is on the sensitivity dial
plus 2.5. You get equals 77.5. We had an output of 70dB from the
audiometer and are measuring 77.5 on the SLM. The question is: is
this audiometer in proper calibration? The answer is yes. It's in
perfect calibration. The sound level meter is measuring in the world
of sound pressure level. On the left, the audiometer operates in the
world of hearing level. If you think back to the conversion we did from
SPL to HL. We learned that at 1000 Hz you add 7.5 to convert dB HL
to dB SPL. So, at 1000 Hz a 70 dB HL tone is equal to 77.5 dB SPL.
This audiometer with this earphone is in perfect calibration.
If we change frequencies and go to five hundred or 2000 Hz, we will
have to look on our conversion chart from SPL to HL to see the
difference there. We would then have to adjust the sensitivity of the
sound level meter in anticipation of that difference, then deliver a 70
dB HL tone to the sound level meter and we should read some level
above 70 dB. Remember HL is different from SPL and we have to add
some dB to get from one to the other. You would go through the same
process we just went through at 1000 Hz for all other frequencies to
determine whether or not an audiometer is in calibration.
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17. So, if we conduct all of the testing and everything is in calibration,
then you don’t need to do anything. You can feel confident that what
you are recording is accurate. However, what if you find that your
audiometer is not meeting the specified guidelines or is not
measuring as it should be? If calibration reveals marked differences
from specification, then it should be seen for recalibration. But if
there are just minor differences, then you can correct for those when
testing. We are going to discuss how to do that. When calibrating,
there is a certain level of tolerance. In terms of intensity, the standard
allows us plus or minus 2.5 dB margin of error. If we come up with
2.5 on either side of our desired level, we move to the next whole 5
dB. For example, if we are testing at 70dB at 1000Hz, we're
anticipating 77.5 dB HL, like we did on our previous slide, But what if
in fact we measured not 77.5 but 81.0? We would be strong by 3.5. So
in our calibration report we'd say to the next person who looked at the
audiometer, instead of 70 dB at 1000 we'd have to put a plus 5 there
to tell the next person the audiometer was strong by 5 dB. In the same
vein, what if we ended up with something less than 77.5 and exactly
2.5, which would be 75 dB? If we measure 74 dB, we would be on the
minus side so we'd report at 1000 Hz with the earphone that the
audiometer was minus 5 dB and we'd have to make adjustments
accordingly. So in calibrating intensity if the difference is plus 2.5 or
greater, we have to go up to the next five dB and minus 2.5 dB, we'd
have to drop down by five dB. Then we'd have to put that on a
correction chart so the next person who did testing with the
audiometer understood that the audiometer was not in perfect
calibration. It would specify that in fact, we were strong by 5 dB or
weak by 5 dB or some other dB level. It could be plus or minus 10 or
15 or whatever. With frequency, our tolerance is somewhere between
1-2% percent, depending on the type of audiometer according to the
1996 ANSI standards. So this would mean if we put the earphone on a
frequency counter at 1000 Hz, you could measure between 990 Hz
and 1010 Hz and still be in the tolerance level if you were using an
audiometer with a 1% frequency tolerance (This is stated incorrectly
on the recording). As an example I have made a simple calibration
chart that would represent the results of a celebration procedure. This
would be located with the audiometer that we calibrated. We've
indicated the results at 500, 1000, 2000, or 4000 with the right and
left earphone. You'd note that at 500 Hz the right earphone is in
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perfect calibration. In the right earphone at 1000, the audiometer is
strong or producing 5 more dB than it ought to. At 2000 Hz, the
audiometer is back in perfect calibration. At 4000 Hz, the audiometer
is weak in the right earphone. In the left ear the audiometer is weak
by 10 dB at 500 Hz. This is getting a little bit concerning, particularly
if it was 15. At 1000 Hz, the audiometer is strong by five dB. At 2000
Hz the audiometer is in perfect calibration. It is soft or weak or
producing 5 dB less than it should at 4000 Hz in the left ear.
18. So if you are conducting audiometric testing on an audiometer
that has a correction chart or factor that necessitates a correction to
be done, how is this recorded? We’ll answer that by learning about
what’s called the direction of the correction. What we're talking about
here is if the audiometer is not calibrated perfectly at zero, it's strong
by 5 dB or weak by 5 dB or another value, how do we adjust the final
result on the hearing chart before we report the patient's hearing to
someone else? If the audiometer is out by 5 dB, you can't take the dial
reading as being true, because it is not in calibration. You can't
report the dial reading as it stands and report that level to someone
else. You have to make the correction before you place it on the
hearing chart. So I created some samples here to help understand
this. For this example, we are using 1000 Hz. First, let's look at the
upper tier here. Note in example #1, the audiometer is reading 70 dB
HL and the sound level meter is reading 77.5 dB SPL at 1000 Hz. So,
the correction factor is zero. We anticipate the difference to be 7.5
because 7.5 is the difference between HL and SPL at 1000 Hz. In the
next example, #2, we're putting out 70 dB HL from the audiometer
but measuring 72.5 dB SPL on the sound level meter. The correction
would have to be minus 5 dB. We're weak by 5 from the audiometer.
If we move to example #3, the audiometer dial says we're producing
70 dB HL but the sound level meter says that's not true, you're
measuring 82.5 dB SPL, so we're strong by 5 dB. On example #2 and
#3 these correction factors would be noted. So now we proceed to test
a patient using these three audiometers. The lower part of the chart
represents the results from this patient. In example #1, when we test
the patient we find that the client's threshold at 1000 Hz is 30 dB.
We've calibrated the audiometer and know at 1000 Hz, it's in
calibration. So when we record the results of the hearing test, we can
put it on the audiogram at 30 dB. Everything is calibrated and in
order. Let’s move on to example #2. The dial on the audiometer says
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35 dB but the earphone is actually producing 30 dB. We know based
on our calibration that this audiometer is weak by 5 dB. It's out of
calibration. We think we're producing 35. But when we calibrate, we
measured -5. So we're actually only producing 30 dB. The
audiometer says the client's threshold is 35 but the audiometer was
soft so it is actually giving the patient a threshold that is less than he
should have or it's making him look worse than he ought to. So we
take the thirty-five from the audiometer and look at our calibration
chart. It says minus five because the audiometer earphone was down
by five. We take that five dB from the audiometer reading and place
the minus five. That puts the threshold on the audiogram at thirty,
that's his correct threshold-30 dB. Okay, now let’s look at number
three. Here, the audiometer says the threshold is 25 dB. But the
audiometer when calibrated was found to be producing 5 dB more
than it should have been. This audiometer is strong by 5 dB at 1000
Hz. Instead of placing 25 on the audiogram, we have to add 5 dB to
the patient's threshold before we record it. So then we'll record it as
thirty on the audiogram. Can you see that the earphone is producing
30-30-30? We have made the adjustments to audiogram as 30-30-30.
Everything is good irrespective of the false readings of the
audiometer, which read 30, 35, and 25. Number one is a true reading.
But #2 & #3 are not true readings, so we make that adjustment. So in
summary, you obtain the client's threshold on the audiometer. Don't
think about calibration just look at the threshold. Then go to the
correction chart. It will say minus 5 at 1000 on number two. Subtract
the 5 and record it on the audiogram. For example number three, take
the threshold, add five to it, and record it. You'll record it at thirty,
which is what's coming out of the earphone. This is the direction of
the correction. If you understand what you're doing, you'll do this
correctly. Otherwise, you can get in trouble. Find the threshold and
add or subtract the number on the correction chart and put that
amount on the audiogram.
This is the end of lesson 4. Hopefully what we have covered will
prepare you to begin learning about conducting hearing evaluations
in the next lessons. To me, this will begin the more exciting part of
audiology.
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