Calculating Sufficient Masking Levels


In this section, I’m starting to talk about formula masking, but there is one big exception. In this section I am going to talk about the formula for masking a given level of signal and “pretend” that is the one and only stimulus that I need to mask.   So, we’ll use case examples such as presenting a 70 dB tone to one ear and ask how would we formula mask to keep that 70 dB tone from being heard.

That’s a good and necessary question to answer at this point. In many of these examples that 70 could be the test ear’s threshold level.  I just want to “go on record” now as saying that in formula masking we will think about calculating the non-test ear noise level before we find the test ear threshold… we’ll have to guess the threshold.  But that is too much information for now.

There are many articles on the average occlusion effect, and differing opinions on whether one, when plateau masking, should add a "pad" or extra safety factor of 10 or 15 dB.  In this tutorial, I have elected to use the occlusion effect values of:
    250 Hz = 15 dB
    500 Hz = 15 dB
     1k Hz = 10 dB
With a "pad" of 10 dB.  If you use other values, that's fine.  I chose these because the majority of the students I am teaching at this moment happen to have been taught these values.

I will use an air-conduction interaural attenuation value of 50 dB, and am assuming insert earphones are used.

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1)   Let's look at several masking cases and determine if the masking level used was sufficient.  In this first case, the tympanograms are normal and acoustic reflexes are present, so sensorineural pathology is likely.  The tone is presented by air conduction.  Rather than using a plateau approach, the audiologist desides to put in 40 dB of masking in the NTE

Is the 40 dB EM adequate?



2)  Note that 40 was more than enough masking. The audiologist would have been safe using 30 dB EM.  

In the above scenario, presented in question #1, the masking level was 40 dB less than the air-conduction presentation level.  Magic number?  Well, sort of.  Assume that the air-conduction signal crossing over will be attenuated 50 dB.  You could subtract 50, and present that much contralateral masking, but then if there is any minor calibration error or air-bone gap, you are in trouble. So, by adding 10 more to the masking level you have a bit of a pad.  And of course:
     Presentation level to test ear - 50 interaural attenuation + 10 dB extra
is the same as
    Presentation level to the test ear - 40 dB

So, let's see when this works and when it does not.  How about in this next case of asymmetrical sensorineural loss.


Presentation level (100) - 40 = 60 dB EM.  Is the adequate?




3)  Will that work if the loss in the non-test ear's loss isconductive or mixed? Let's assume that the non-test ear has a flat tympanogram and absent reflexes with the probe in that ear.  You don't know the bone-conduction threshold, so it would be safest to assume that it is 0 dB HL.

Is the 60 dB EM still appropriate?



4) So, in question 3 we demonstrated that the air-bone gap reduces the masking at the non-test ear.  Can we add in the estimated size of the air-bone gap to the formula and mask properly?  That is, can we set the masking level using this formula for air-conduction masking for insert earphones?

Test ear presentation level - 40 dB + non-test ear air-bone gap 

In this case, that would mean using 100 dB EM.  I know that sounds loud, but remember, the patient won't hear 40 dB of it, if there really is a 40 dB conductive component.

Is 100 dB EM adequate?  Is there any fear of overmasking?



5) So, we have established a masking formula forAIR CONDUCTION MASKING for INSERT EARPHONES:
Test presentation level - 40 + nontest ear air-bone gap.

When testing bone conduction, would this formula work?  Let's explore this using an example of the non-test ear having normal hearing.  Let's assume the test frequency is 1000 Hz and that the occlusion effect is 10 dB.


Will the air-conduction formula work?



6) So a modification to the rule is needed forbone conduction masking. The correct formula is:

Stimulus presentation level - 0 (since there is no interaural attenuation) + occlusion effect value OR estimated air-bone gap in the non-test ear (NTE) + another 10 dB for safety. 
    An ear with a significant conductive loss does not have an added occlusion effect (OE) when you mask, (though the bone conduction threshold may already be lowered because of it) so you can omit the OE if the non-test ear has a significant conductive component ears. So, the rule is add the larger of the occlusion effect size or the air-bone gap size.

    Why use the air-bone gap rather than the test ear threshold?  Let's take the case of a 70 dB bone-conduction signal in the test ear.  The bone conduction signal crosses over at 70, regardless of whether the non-test ear cochlea's sensitivity is 0, 30 or 70.  The masking at the cochlea needs to exceed the crossed-over level of 70, not the hearing threshold level of the non-test ear.  We need to overcome the non-test ear conductive component if any, and add a little more for safety. (Of course, if the crossed over level wasn't heard, then you wouldn't need to mask.) 

     If you don't know the non-test ear air-bone gap, you have to make assumptions from the immittance test results. That’s pretty hard to do! You could try, e.g. the tympanograms is flat so I’ll guess there is a 50 dB conductive loss.  More commonly, you just assume that the entire loss is conductive –assume that the bone conduction threshold in the NTE is 0 dB HL.  Another approach is to obtain the unmasked bone conduction thresholds first when tympanometry is abnormal (or the patient has symptoms of conductive loss.) That is, even before doing air-conduction testing, test unmasked bone.  You could then make the educated guess that the unmasked bone conduction threshold could well be the non-test ear threshold. That would be the “worst case scenario” when it comes to deciding how much masking is needed.  


    Next let’s try some cases, and let’s start out with the easy case – where we don’t think there is an air-bone gap in the NTE because the NTE thresholds are normal and immittance is normal.

     Stimulus presentation level + occlusion effect value OR estimated NTE air-bone gap + another 10 dB for safety. 



Let's assume that the occlusion effect is 10 dB at this frequency (1k Hz). What masking level should be used?



7)   Let's examine a case with an air-bone gap in the non-test ear, with presumed cochlear loss in the test ear.

What masking level would you use? Does the level that the formula yields cause any problems?



8)  Is it futile to even attempt to plateau?  No.  What if the patient's actual IA is 60 dB? 


What would the range of appropriate masking levels be -- that is, where would the plateau be found?



9)   Ready to practice?  For this audiogram, what is your formula masking level at 250 Hz, testing air conduction of the right ear, masking the left ear?  The patient had normal tympanograms bilaterally and left ipsilateral reflexes, so you are assuming the loss will besensorineural in the left ear.  Reflexes are absent sound to the right ear, in the ipsilateral condition.  Let's not reveal the contralateral reflexes, and assume that you still have some concern about whether the loss might be mixed for the right ear, inspite of the normal tympanogram.  Can you formula mask?  If so, what is your masking level in the left ear?


What is your formula masking level at 250 Hz, testing air conduction right ear, masking left?

Now, that was good practice but it’s time to talk about what we really would do in this case.  Do I think the right ear threshold will shift? Yes. It’s going to shift by at least 5 dB… the amount of the central masking effect. It might even shift more than that.  When it shifts, I have to readjust my masking level.  I could do that, but it would be inefficient. So what I really would do is, after seeing the AC threshold without masking come in at 65, I would make my calculation for a higher signal level. I would set my masking so that as long as the threshold doesn’t come in higher than…. Oh, say, 90 dB HL… then I’m OK. So what I’d really do is set my formula for 90, not 65. 



10) What is the formula masking level for 1000 Hz, testing air conduction in the right ear?  Since the left ear threshold may well shift, I will calculate my masking level based on my guess that it may shift a little, but probably won’t shift above 110 dB HL.  What masking level in the NTE (right) ear would be appropriate to use? 

 Is over masking a concern?



11) The patient, a 70 year old female, has a history of bilateral progressive hearing loss that she attributes to "aging."  She had a cold and sinusitis, and says that it has caused greater loss of hearing in the left ear.  While the right tympanogram is type A, the left is type B and otoscopy shows a grossly abnormal eardrum.  It appears reddened and bulging.  Reflexes are present only in the right ipsilateral condition.  

When testing air conduction, can you formula mask the right ear, while testing the left ear, at 250 Hz?  Let's guess that her left ear threshold may shift to 90 dB HL and use that to calculate the masking levels.



12) What is the formula masking level for 4000 Hz (air conduction)?  The maximum that the audiometer puts out is 120 dB HL, so use that when making your estimates of how much masking is needed, so that you won't have to adjust the masking level even in the "worst case" scenario of the patient not hearing even the highest intensity signal that you can produce.



13) How would the masking change if you had to use supra-aural earphones?  Assume a 40 dB IA.



14) In this next case, let's assume that both tympanograms are low compliance and reflexes are absent bilaterally.  Can you formula mask?  What level will you use for air-conduction at 250 Hz, using insert earphones, when assessing the right ear?




15) How about if this is probably left conductive, right sensorineural?  (Let's assume instead that the patient has unilateral right Meniere's disease for years, and the left ear may or may not be starting to have involvement with Meniere's or might have conductive loss. The right tymp is normal, and thresholds are only slightly worse than seen at the last eval, which showed a sensorineural loss. At a time when the left ear was within normal limits.   The left tymp is low compliance, so you will proceed with caution, considering the worst case scenario (for your masking, not for the patient!) that the left ear loss is purely conductive.  Are you willing to formula mask the right ear, air conduction, and if so, what level?  250 Hz please.



16)  Now, let's look at some examples of formula masking forbone conduction.  Recall that the formula is now:

Presentation level + (NTE air-bone gap or OE) + 10.


What masking level should be used when beginning bone conduction testing with the oscillator on the right ear, with masking in the left ear?  Start with 250 Hz.  Assume that the loss is going to end up as sensorineural, so calculate the amount needed for formula masking if the threshold is 30 dB HL by bone in the right ear at 250 Hz.



17)  Masking is simpler in the high frequencies, where no significant occlusion effect will occur.  If the right bone is tested at4000 Hz, and you assume the loss will be sensorineural, (let's assume that it has been at all the other frequencies you tested so far in this hypothetical case).  What is the limit of your audiometer?  Do you need to assume a 90 dB threshold if your audiometer max output is 70 dB HL?  No, of course not, so assume you will input up to 70 dB of signal.  How much masking do you need in the left ear to preclude the NTE from hearing signals to 70 dB?



18) Let's mask bone conduction for a unilateral conductive loss, first with the conductive loss in test ear - that's do-able, it's when the conductive loss is in the non-test ear (or both ears) that dictate plateauing.

The left ear tymp is flat, thus your expectation is that the loss is conductive.  I think you're ready for another challenge. Let's try see if we can use that "efficiency trick" -- rather than assuming that the bone will be exactly equal to that in the non-test ear, let's see if we can put in a masking level that would still be "good" even if the left ear loss ends up sensorineural.  (So if it does, you don't need to adjust the masking level.)  That will give you higher levels of masking and greater risk of cross-back. But let's try it. Again, you don't need to assume a signal level of 95, if the bone vibrator can't go beyond 70.  Let's evaluate1000 Hz left bone conduction  (mask the right ear) because the likelihood of vibrotactile low-frequency thresholds makes that frequency easier to discuss than 250 or 500 Hz.



19) Let's take the 2k threshold.  If 1k bone threshold had been 50 dB HL,  a 45 dB air-bone gap, perhaps it will be similar at 2k Hz.  I will assume that I need to be able to mask for signals up to 65 dB HL (since air-bone gaps often narrow in the high frequencies, I'm trying to be efficient by saying the signal could be louder than my real guess, which is that the air-bone gap is 35 or 40, that the threshold is 60-65 dB HL by bone.)


What masking level at 2k is needed if the bone signal that is presented might be as high as 65 dB HL left?



20) Let's see what happens when the loss is unilateral conductive, with the non-test ear the one that is involved.  If your are following along, you should be predicting that this is not a "pretty cases" and you should plateau. But let's continue. The left tymp is flat, reflexes absent probe left.  The right ear has a normal tymp and present reflexes in the low to mid frequencies ipsilaterally.

Can you formula mask for 250 Hz testing by bone (test right bone, mask the left ear)?



21) Well, you know that if there is a significant bilateral conductive loss -- where the conductive loss magnitude approaches that of the interaural attenuation -- you have a masking dilemma.  But what if the loss is bilateral conductive and not so severe.  Can you formula mask -- say for 15-20 dB air-bone gaps?


Let's test bone left mastoid and mask the right ear, 250 Hz.  I might need to put in bone conduction signals as high as 35 dB HL, so let's see what masking level -- if any -- can safely mask that.



22) Want more practice?  Here is the blank masking head for air conduction testing, and one for bone conduction testing.  You can right click to copy the image, then save them in another file / print them.

Also below are some various audiograms to practice from, and a blank audiogram for you to make up your own cases.  Of course, you can go back and use the audiograms above at all the other test frequencies as well.  You can create your own scenarios about whether the immittance results.










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Content (c) 2005, Teri Hamill, Ph.D., Nova Southeastern University