2. Masking and Bone Conduction
Let’s start off by considering what is going on
with Bone Conduction testing. Once we grasp
that, we’ll go to Air Conduction and apply the
same principles.
3. When we are testing for bone conduction
thresholds, we have the oscillator on one ear or
the other.
Just because we have placed the oscillator there
does not mean that this is the ear we are
testing.
4. Why not?
Remember that the BC oscillator is vibrating
both ears simultaneously.
So if you are presenting a tone of 30dB to the
right ear via the BC oscillator, it could possibly
be heard at 30dB on the other side.
5. • That is, the interaural Attenuation for BC
is O dB.
6. We have to find a way to “knock out” the other
side, or keep that other side busy, so that it can’t
hear that 30dB tone.
7. We are going to keep that other side (which we
will call the Non Test Ear, or NTE) busy by
bombing it with some masking noise.
8. We are going to do that with some noise. In
fact, we are going to do it specifically with
something called “Masking Noise.”
And for purposes of this lesson, we are going to
use Narrow Band Masking Noise, not “white
noise”
9. Just a brief explanation here:
White noise is noise that is made up of all the
frequencies across the spectrum, in roughly
equal parts.
Narrow band masking is also made up of several
frequencies, but just the ones that are centered
around our test frequency….perhaps just a few
Hertz on either side.
10. How can we do this?
Our audiometer* knows the truth and will be calibrated
to produce the appropriate narrow band masking every
time you change the frequency dial.
*(Most newer audiometers have narrow band masking
and you can hear it when you are doing your biological
check. Turn on the masking noise, and you should be
able to hear that the masking noise is becoming higher
and higher in pitch, even though it still sounds like “Sh-h-
h-h”, or as I like to call it, “The Windy Noise”.
11. If your stimulus is a tone, and you are listening
to your masking noise during your biological
check and the masking noise is NOT changing its
pitch as you change frequencies, then you
probably have “white noise”, or what we will call
“broad band noise”. (And yes, I know we could
have a detailed discussion about those
terms, but let’s just leave it at that for now.)
12. For purposes of this presentation, we are going
to assume that we have narrow band masking
on our audiometer because it is relatively new
and modern.
13. So, we are putting in a 30dB tone via bone
conduction to the left ear (the test ear) and both
ears can hear it because the Interaural
attenuation is 0dB.
Again, to recap, the stimulus is the same
loudness in the test ear as it is in the non-test
ear
14. • So,………we are going to deliver our narrow
band masking stimulus through a headphone
placed on the non-test ear.
Remember, we have a BC Oscillator on the test
ear, and now we have a headphone on the non-
test ear. The tone is being presented through
the oscillator and the noise is being presented
through the headphone.
15. How much masking noise are we going to need?
Or, in other words,
What is going to keep that non test ear “busy”
enough so that it can’t hear the 30dB signal we
are presenting on the other side?
18. • Next, we have to go back and look at the pure
tone AIR conduction audiogram we just did.
And let’s assume that our thresholds in both
right and left ear is 30dB
• And we have to ask ourselves…will my patient
be able to hear 30dB of masking? (In other
words, is his threshold 30dB or better?)
19. • If he is going to be able to hear 30dB of
masking, then add a little “cushion” of 10dB
just to be sure that your “noise bombing” is
covering up that 30dB.
• If his threshold is worse than 30dB, you are
going to have to present your masking noise at
his threshold +10dB.
20. So, back to our BC threshold testing
You present the tone of 30dB to the test ear via
the oscillator and put in 30dB+10 (or 40 dB) of
masking in the Non test ear via a headphone.
You need to try to find the threshold of the test
ear. Since your patient’s AC threshold is 30dB,
his BC threshold should be no worse than 30dB.
22. • To summarize:
1. Look at your AC audiogram. Make note of
the AC thresholds for both side.
2. Start your BC testing at the threshold for that
frequency (in our example it was 30dB AND
present 30dB+10db (i.e., 40dB) of masking to
the non-test ear, assuming your inspection of
AC thresholds told you he’s going to be aware
of 40dB on that side.
23. You present the tone via the oscillator, and,
keeping the masking noise where it is for the
other side, if he can hear the tone, reduce it by
10dB until no response, then raise by 5dB, just
like you do with AudSim.
You haven’t even had to change the masking
noise……just be sure it’s on.
24. • Once you have bracketed using the method
you learned with AudSim, and you have found
your BC threshold, then raise your masking
noise by 5 dB for three presentations. If
nothing has changed, you are done with that
frequency.
25. Now, what about AC testing?
• Well, it is pretty close to the same procedure
except that now, the level at which we are
presenting to the TEST EAR is NOT the same
level that could be picked up on the other
side.
26. The example
Remember, this is AIR conduction testing, not Bone.
We have found a threshold of 60dB in the left ear,
but the right ear threshold was only 20dB
“Houston, we have a problem” (I think that’s from
the Apollo 13 movie, but no matter, just be aware
that it is not my original material or a quote for
which I am taking personal credit.)
27. • When we are doing AC testing…we are not
looking at an interaural attenuation of 0dB, like
we had in BC.
• Now we know that sound presented to the test
ear loses an average of 40dB through/around the
head. (and once again, we are using this as a
conservative value. Yes, research does tell us that
there is some variabiity according to frequency
and some other factors, but his is going to be our
rule.
28. • The threshold on the test ear was 60dB and the
threshold on the non-test ear was only 20dB……
• And because the tone/signal is losing 40dB in
interaural attenuation (meaning through or
around the head) when we put in 60dB to the
test ear, our patient might be able to hear it on
the other side, because it is going to be 20dB
loud on that non-test ear, and our patient can
hear tones at 20dB.
29. • “What to do? What to do?”
(Cinderella probably said that at midnight,
when her coach starts to turn back into a
pumpkin)
30. • We need to put a “noise bomb” on the non
test ear in order to keep it busy, so it can’t
listen.
• Our masking level, therefore, needs to be AT
LEAST 20dB, because that is what the signal is
going to be after it has crossed over.
• Let’s just add 10dB to that as a cushion, or
MEM, if you understand Carol Silverman,
making it a masking level of 30dB.
32. • If the threshold is still 60dB in the test ear,
even though you have 30dB on the opposite
side, great.
• Now raise your masking by 5 three more times
and retest the threshold each time. If the
threshold in the test ear does not change, you
are finished. (Well, not entirely finished, but
finished with that frequency.)
34. • If everytime you raise your masking dial by
5dB, your threshold goes up by 5dB, you are
going to have to keep repeating this procedure
until your THRESHOLD does not shift over
three successive increases in masking.
• Most of the time, if you have followed my
instructions, you will not see that shift, but it
is possible.
35. But, my preceptor says that’s not
right…
• This whole masking thing can get a lot more
complicated if you really want it to.
• I am only trying to teach it in a way that is
easy to understand.
• Someone could look at my method and call it
into question because they do not do it that
way or because they are aware of some
unusual condition.
36. • Because you are new to this and easily
confused, try to keep my method in your
heads.
• It is simple
• It will work almost all the time
• Doing it this way will foster understanding of
the concepts .
37. • In other words,
I am trying to teach you the principles
behind why we need to mask and how to do it.
What these other people are here for is beyond
my scope!
38. • I will invent a discussion thread in which I
want to entertain your thoughts and your
questions.
• Thank you for your time and attention.
