Masking for the HIS Student

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.

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.

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.

• That is, the interaural Attenuation for BC is O dB.

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.

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.

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”

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.

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”.

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.)

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.

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

• 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.

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?

And the answer is:

• At least 30db of narrow band masking.

• 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?)

• 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.

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.

• Continue this for every frequency.

• 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.

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.

• 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.

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.

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.)

• 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.

• 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.

• “What to do? What to do?” (Cinderella probably said that at midnight, when her coach starts to turn back into a pumpkin)

• 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.

• Next, find the threshold again

• 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.)

• BUT

• 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.

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.

• 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 .

• 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!

• I will invent a discussion thread in which I want to entertain your thoughts and your questions.

• Thank you for your time and attention.