Download ComD 3700 Basic Audiology Lesson 7 Masking I Highlighted

ComD 3700 Basic Audiology
Lesson 7
Masking I
Highlighted information refers to a change between the audio
recording (using 10th edition) and the 11th edition of the textbook
1. This is the first part of a section on masking. We will be covering
chapter 6, pages 130-133 and 137. We will discuss the definition of
masking, the principles used in clinical masking and when it is
necessary.
2. I have mentioned masking several times throughout the lessons.
Although the term “masking” may be new to you, its effects occur
daily. For example, you are watching TV and someone turns the
dishwasher on. Now the TV is not loud enough for you to hear and
understand over the noise of the dishwasher. The dishwasher (the
masker) masks out the TV (the signal). We will discuss the concepts
of masking as it relates to audiometric testing. The textbook defines
masking as, “the process by which the threshold of a sound is
elevated by the simultaneous introduction of another sound.” To me
it is easier to understand masking using these two concepts:
1. Masking occurs when an unwanted sound causes a wanted
sound to be inaudible.
2. Masking is required to keep a better ear busy while testing a
poorer ear. The poorer ear receives the tone while the better
ear hears a controlled masking noise
3. The first concept in masking we will discuss is the test ear [TE] and
the non-test ear [NTE]. While performing an audiometric evaluation
you might assume that the sounds you are presenting to the right ear
is only heard by the right ear and vice versa. However, this is not
necessarily true. In fact, as you will learn in this lesson, it is common
to find that the sound being presented to one ear is actually being
heard by the other ear. To avoid confusion it is customary to call the
ear currently being tested the test ear (TE), and to call the opposite
ear, the one not being tested, the non-test ear (NTE). When there is a
possibility that the sounds being presented to the TE are really being
heard by the NTE this causes the outcome of a test to be suspect. So
we will be discussing why this situation occurs, how it is recognized
and how the clinician can ensure that the NTE is removed from the
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test or is not the ear responding to the stimulus. This designation of
test ear and non-test ear changes as we change the stimulus ear. So if
we are testing the right ear, the left ear becomes the non-test ear. If
the left ear is to be tested, the right ear becomes the non-test ear.
Think of the non-test ear as the ear that is not being tested, or the one
where the stimulus is not being delivered. These are not always the
same ear. Some students believe that once the left ear is the non-test
ear, it's always the non-test ear. But it is not. If we switch and test the
left ear then the right ear is the non-test ear. Here's an illustration to
show the concept of the test ear or non-test ear. You see on the left,
figure A, the earphone in pure tone testing is placed on the right ear.
That's the test ear. The left ear is therefore the non-test ear. If you
take the test ear as in figure B and we're doing pure tone audiometry
testing we place the earphone on the left ear. That's now the test ear.
The right ear becomes the non-test ear. The proper use of masking
during clinical audiometry ensures the clinician that the test results
reflect the performance of the test ear only.
4. Another concept to understand is cross hearing. Remember when I
said that it is common to find that the sound being presented to one
ear is actually being heard by the other ear? This phenomenon is
called cross hearing or shadow hearing. A definition of cross hearing
is, “The reception of a sound signal during a hearing test or at the ear
opposite the ear being tested”. This can occur during air conduction
or bone conduction testing. So it is possible during audiometric
testing for the stimulus presentation, or sound, to travel from the
poor ear to the better ear. Stop and think about what we're saying. We
put the stimulus in the poorer ear, but there's a possibility it can
travel to the better ear and be perceived by the better ear. The patient
will respond, not realizing that the NTE has actually perceived the
stimulus not the TE. If the clinician is unaware of this concept, then
they may report thresholds for the right ear that are in fact the
thresholds of the left ear instead. This can lead to very erroneous
results.
5. A good analogy to this is a vision test. You may go into have your
eyes tested and when you first look at the chart using both eyes, this is
like cross hearing. My son has had vision problems since he was born,
so I have sat through countless eye exams. In order to really tell what
each eye is seeing independently, the optometrist will cover the nontest eye. (And if you think this is easy to do to a 2 year old, think
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again!) So, in other words, one eye is tested while the other eye is
masked. In effect, we do that same thing in audiometric testing,
except that the auditory noise “blindfold” is a noise that is directed
into the NTE. The noise in the NTE stops it from hearing the noise
being presented to the TE. Just like covering it so it can’t see masks
the non-test eye, the non-test ear is masked by the noise covering it
up from hearing the stimulus.
6. Let's take an extreme example to illustrate at this point, and I
emphasize extreme, because we don't always have the luxury of this
type of situation. Consider a patient who has normal hearing in his
right ear and is completely deaf in the left ear. They have no
measurable hearing. Let's take a moment and look at this figure I've
put here and work our way through this a little bit. First, look at the
upper left side where it says normal hearing right ear. Under that, I
put that we've tested this ear. The threshold is 5-10 dB. It says nontest ear, but while we were testing that ear, it was the test ear. We've
now moved from the right ear and the left ear is the test ear. Just
know that we have previously tested and found thresholds at a 5-10
dB threshold in the right ear. Now we move our attention to the left
ear where we have a profound hearing loss. So the left ear is the test
ear. The right is the non-test ear. Now we begin to find the threshold
in the left ear. I will tell you that there should be no response from the
left ear. However, as we test, the right ear will hear the stimulus
before you get to the limits of the audiometer in the right ear. As we're
finding thresholds and working our way up to the limits what's
probably going to happen is at around 50-60 dB in the left ear, the
sound will cross the barrier of the head and be perceived by the right
ear. That's a condition we're going to need to be aware of and deal
with. Because if we don't understand cross hearing we assume the
stimulus of 50 dB is correct rather than no response.
7. So lets look at this in sample audiograms to make sure you
understand. If we take this patient and think about what we would
expect the audiogram to look like, we see the normal responses in the
right ear and the no response symbols in the left ear for both AC and
BC at the maximum testable levels. So, you might think this looks
right. However, this does not occur.
8. If the procedure for pure tone testing, covered in the previous
lessons, were used and the patient is tested without using masking
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then instead the audiogram will be more like this one. The responses
for the right ear are as you would expect. But look at the left ear
results. The left air conduction thresholds are in the 55-60 dB range
and the left BC thresholds are the same as for the RE. How can this be
if the LE is dead? The tone becomes loud enough to cross over the
head and the good ear hears the tone before the proper threshold has
been found. The patient may not even be aware that the good ear
responds, only that the tone is heard. The result is often called a
shadow curve or mirror audiogram. The thresholds obtained are not
the true thresholds. The point where the crossover occurs is being
recorded instead.
9. With proper masking, this is the true audiogram of the patient. The
right ear was masked so that it could not respond to the signal. So
now the no response masked results for the bone conduction and air
conduction of the left ear are correct.
10. So, we now understand that cross over occurs when you present a
sound to the TE, but the NTE hears the sound first. The crossover
level, or interaural attenuation, differs at each frequency, and varies
slightly from patient to patient. Interaural attenuation refers to the
loss of the acoustic energy of a sound as it travels from the TE, across
the head, to the NTE. Here we have the next concept we need to
understand, interaural attenuation. Inter means between. Aural
means the ear. So, between the ear attenuation. That describes a
condition where you diminish or dampen sound or interfere with the
transfer of sound. Sometimes the students like to think of this as
going from loud to soft attenuation. As the stimulus travels from one
side of the head to the other, some energy is lost in transmission. So
the head is a type of a barrier. This loss is called interaural
attenuation, sometimes abbreviated IA. IA varies with frequency and
individual patients. So think on that for a moment. The interaural
attenuation factor will change with different audiometric frequencies,
so 250 Hz, 500 Hz, etc. It will also vary with individuals. People have
different structural components and characteristic to the head.
Sometimes the placement of the bone oscillator on the head will make
a difference in interaural attenuation from one patient to another.
11. What I'm trying to do here is illustrate the concept of attenuation.
In audiometric testing we are concerned with interaural attenuation.
But here we have inter-human attenuation. You can see the speaker
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on the left. It's producing a good amount of sound. If you look down
at the bottom the Figure A, you can see its producing 85 dB. Then we
have a barrier between the two individuals. Figure B is unhappy
because he's receiving the sound in attenuated form. In fact, when we
measure the sound on side B, we only get about 20 dB. So we really
have 65 dB of attenuation being offered by the barrier between figure
A and figure B.
12. Let's look at this same concept of interaural attenuation not
because of a barrier between people but with a barrier between the
ears. As you can see the head is the barrier. Here, we have normal
hearing in the left ear and profound hearing loss in the right ear. So if
we put stimulus in the right ear, which has a profound hearing loss,
and increase the intensity of the sound, at what level will it become
intense enough to cross the barrier of the head and reach the normal
left ear? What we're saying is if we're testing by air conduction, what
will the interaural attenuation be? And if we're testing via bone
conduction, what will the interaural attenuation be? In other words,
how high would we have to raise the stimulus in the right ear to be
able to cross the barrier of the head and be perceived by the left ear?
13. To answer that, lets start by going back to our earlier patient. The
patient’s Right AC threshold at 1000 Hz is 10dB HL. Even though
their left ear is completely deaf, they also responded to a 1000 Hz
tone presented from the left earphone at 60 dB HL. This means that
the 60 dB tone presented to the left ear must have reached a level of
10 dB HL in the right ear. This means that the IA at 1000 Hz for this
patient must be 50dB (60dB-10dB=50dB). Similarly, the amount of
IA at 4000 Hz in this case is 55 dB. So is this the case for all patients?
14. Here are the results three different studies conducted to
determine the IA attenuation for pure tones. There are two different
studies with supra aural earphones and one study with insert
receivers. I'd like you to look at study A. It says that the IA, varying by
frequency is somewhere in the low sixties or approximately 60dB
interaural attenuation. In study B, they found the interaural
attenuation to be somewhat less, in the 45-65 dB range. These
represent studies where they've tried to calculate the amount of
energy that is lost when it crosses from one side to the other. This is
interaural attenuation. When tested using insert earphones, it was
found that they provide more IA than supra aural earphones.
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15. Rather than committing this chart to memory, a large group of
professionals who worked with interaural attenuation gathered at a
conference. By convention, they agreed to a rule that would cover all
possibilities. It was established that interaural attenuation for pure
tone conduction testing would be 40 dB HL. In other words, what
they're saying here is we know that studies show sound crosses from
one side of the head to the other at around 50 dB or so. But if there is
a difference between the ears, in air conduction, we will assume or be
wary of the possibility of cross hearing when that difference is 40 dB.
This is a little like when I read on the fertilizer package for my lawn. If
it tells me to put a certain amount of fertilizer per square yard, I will
put one half more. I want an extra green lawn. I do this with weed
killers in my grass to make sure I kill every weed. So, what has
happened here is that we know interaural attenuation is around 5060 dB. But to be on the safe side, it has been established for air
conduction around 40 dB. Remember when I said that there were
several reasons why many audiologists prefer to use insert
earphones? Well, this is one of them. Since insert earphones provide
more IA than supra aural headphones, 70 dB can be substituted for
40 dB. This means that masking will be required much less frequently
when using insert phones. Because there isn’t any way of knowing
which cochlea has been stimulated by a bone-conducted tone, cross
hearing during BC tests is always a possibility. Therefore, IA for bone
conduction has been established at 0 dB. So if you put a bone
oscillator or vibrator on the mastoid process or the head. It doesn’t
matter, even if it is placed on the forehead, the head offers no
attenuation. You can put this bone oscillator on mastoid process
behind the ear with the profound hearing loss. But, if you have a
normal cochlea on the opposite side, the sound will reach the good
ear just as if you put the oscillator right next to good ear. It doesn’t
matter with bone conduction. It crosses very well from one side to the
other.
16. Let's look more specifically at IA and air conduction testing. There
is a formula that determines whether or not masking is needed. It is
that the danger of cross hearing for air conduction tone presents itself
whenever the level of the tone in the test ear (TE) by air conduction,
minus the IA, is equal to or higher than the BC threshold of the nontest ear (NTE). Don’t be scared of this formula, it is not as
complicated as it appears. So, pretend that you have completed an air
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conduction and bone conduction test without any masking. Then we
go back and look at our threshold we've obtained. If using supra aural
earphones, we subtract 40 dB of attenuation from that threshold
value. If the result is equal to or greater than the bone conduction in
the non-test ear, then we have to suspect and take appropriate
measures for dealing with it in order to determine the actual
thresholds. A lot of people in their testing in actual practice do not
think of this formula. They think more the way we're going to look at
it in this audiogram. We're looking at an audiogram that has been
completed for the left and right ear with no consideration for cross
hearing. We just jumped right in and went through the procedure for
finding air and bone conduction thresholds for the right and left ear.
Now when you look at this think about the formula we talked about.
We suspect cross hearing whenever there is a difference between the
thresholds of the two ears. So, take the air conduction threshold of
the test ear or the worst ear and then minus 40 dB. Let's take 500 Hz
as an example. The threshold in the right ear is at 45 dB. Then we
subtract 40, based on the IA for supra aural headphones. That leaves
us 5 DB. Is 5 equal to or greater than 0 dB in the left ear? It is. So
that means we have to be suspicious that the 500 Hz tone delivered to
the right ear actually might have been heard in the left ear. In other
words, we look at the air conduction threshold in the test ear- the
right ear. We say is it 40 dB or greater than the bone conduction in
the opposite ear? So let's go to 2000 Hz. Our air conduction is 50 dB
HL. Go to the left ear bone conduction, which is 0 dB. Is there a more
than 40 dB difference between the air conduction in the right ear and
bone conduction in the left ear at 2000 Hz in the left ear? The answer
is yes. So we have to be suspicious of cross hearing.
17. Now let’s discuss cross hearing in bone conduction testing. In
bone conduction testing, the formula states that cross hearing can be
suspected whenever there is an air bone gap in the test ear greater
than 10 dB. So let's try and explain this with the audiogram we're
looking at right now. Is there a 10 dB or greater gap between bone
conduction and air conduction thresholds in the left ear? No, the air
conduction and bone conduction in the left ear are on top of each
other. In the right ear, is there an air bone gap of greater than 10 dB
in the right ear? Yes. So, let's go on to our formula, which says if
there's an air bone gap greater than 10 dB in the test ear, we have to
be suspicious of cross hearing. When we're all done testing we look at
the audiogram and notice that we have an air bone gap in the right
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ear. We have bone conduction scores at 0-5 dB and the air conduction
thresholds are at 35-40 dB. We have an air bone gap. Why? What are
the possibilities that would leave us with that? Consider this. Do you
remember us saying in bone conduction testing, there's no interaural
attenuation? That means if the left ear has a normal cochlea, it has a
normal inner ear. So when we test the right ear, it will show us
normal thresholds because there is a possibility that it could have
crossed over with no attenuation. It's been heard by the normal left
inner ear. Now we're not sure of that, but it's a possibility or one
consideration. So here we're left with the cross hearing dilemma. We
have to address it. We can see that from the audiogram and looking at
the air conduction threshold and from comparing air conduction
threshold with previous hearing tests that we have a normal left ear.
The right ear is the ear in question. So we have to think through this
and the possibilities. Why would there be cross hearing? Let's go back
one more step and look at air conduction in the right ear. Do you
think when we tested the right ear that the right ear stimulus was
crossing to the left ear? The answer is no. We went through this. If
you look at the air conduction and bone conduction in the opposite
ear, if it's 40 dB or greater, there might be cross hearing. But, if you
look at he threshold by air conduction in the right ear compared to
bone conduction in the left ear, there's not a 40 dB or greater gap. Air
conduction threshold in the right ear is fine. The problem here is with
bone conduction. What could be the problem? What if the right ear
when we finish testing is really a sensorineural hearing loss? That
would mean the bone conduction should be within 5 dB of the air
conduction thresholds. Why are they not? Because bone conduction
crossed the head and was perceived at the same level as in the left ear.
If we didn't understand cross hearing, we'd report these results as
normal hearing in the left ear and a mild conductive hearing loss in
the right ear. That's not true. So if this is a sensorineural loss then
when we followed protocol and mask out the left ear with appropriate
noise, which we'll do, those bone conduction symbols will fall right on
top of the air conduction symbols. That's when we'll know we're
looking at a sensorineural hearing loss. If we use masking and the
bone conduction thresholds in the right ear remain normal, then we
could have a conductive loss on the right side. But unless masking is
used to block out the left cochlea, we don't know. We're left with the
problem that with the air bone gap and without interaural
attenuation, we don't know if it is a conductive, sensorineural or
mixed hearing loss. So we have to mask or take out the left ear, retest
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the right ear, and see if it stays where it is or if it falls into a mixed
hearing loss or falls to where it's superimposed on the air conduction
thresholds, then we will know with certainty. I hope you can see the
logic behind the statement that we suspect cross hearing when there's
a 10 dB or greater air bone gap in the test ear.
18. I want to briefly mention central masking. It's something you need
to know about but won't encounter or deal with much unless you're
under the field of audiology. But so we don't leave you uneducated,
let's just read this. It has been shown that a small shift is seen in the
threshold of a pure tone when a masking noise is introduced into the
opposite ear. This increased shift averages about 5 dB. What that
means is if you test an ear and get a threshold and then introduce
masking into the opposite ear, you'll note the threshold in the
opposite ear has increased or worsened by five dB. It is believed that
the elevation of threshold is produced by inhibition that is sent down
from the auditory centers in the brain and has, therefore, been called
central masking.
19. I also just want to let you know that I realize this can be hard
information to study. Don’t get too overwhelmed with the specifics. I
just want you to understand the general concepts about masking. In
the on campus course the students have labs that allow them to test
other people and I think that helps to understand the concepts easier
and allows them to see the practical applications. So, although you
may not believe it based on some of the lessons that seem
difficult…Audiology can be fun! It is the practical use of all of this
information that makes the field exciting to me. But we have to start
with the basics. Please feel free to use the discussion boards or e-mail
me directly for clarification if there is a concept you don’t understand.
If there is something I can do to help make your studying more
productive, let me know. (Music) I have a 95-year-old grandfather
that is always positive. When the song, Don’t Worry, Be Happy was
released, probably before most of you were born, he used to play it
over & over to everyone that he knew. He loved it & felt like life would
be so much easier for all of us if we stay positive, despite difficulties.
Not that the task would become easy, but that a good attitude would
help it seem easier. So, that is my advice to you at this point in the
course. Hope this helps!
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