The presence of conductive loss in the non-test ear means that air-conducted masking is reduced in intensity as it travels through the outer and middle ear systems. To make sure that you have enough noise at the cochlea, you have to overcome any conductive component.

Conducting immittance testing before audiometry is a good way to determine if you may need to add an estimate of the size of the non-test ear conductive loss to your masking calculations. If the non-test ear had normal tympanograms, and even more importantly, present ipsilateral reflexes, then it’s a safe bet that the ear has sensorineural loss. You don’t need to add in an air-bone gap estimate in calculating the minimum masking level.

Patient symptoms, otoscopic evaluation, and audiogram shape also can lead one towards or away from suspicion of conductive loss. This first section will focus on the minimum level of masking that is needed; we will then think about the maximum level. If you are unsure of whether there may be conductive loss, you can opt to use a masking level that is closer to the maximum than it is to the minimum. If you use the “maximum approach” you don’t have to have a perfect estimate of the conductive loss size.

The safest assumptions will be to consider the worst case scenario when there is evidence of conductive loss (e.g. abnormal tympanometry and absent reflexes): make the calculations as if the loss in the non-test ear IS conductive. If the calculations indicate that masking may cause overmasking, then don’t formula mask: plateau mask.

After you obtain the entire audiogram, you will want to do one last review of your masking levels. If you had used a minimum masking approach, and if a conductive hearing loss component was present when you had not anticipated it, you’ll need to test again. (But with more masking noise, the total test time may still be less than if you had plateau masked.)

Add in the air-bone gaps only if they are “real.” Remember that there is a 5 dB test-retest variability to hearing threshold testing, so, once you are finished with all your testing, if you notice the occasional 5 or 10 dB air-bone gap, don’t be concerned. You will be adding in a 10 dB safety pad – using 10 dB more masking than the absolute minimum, so this will help if there are small air-bone gaps. I would recommend adding in air-bone gaps of 15 or higher. It is safer to add in the air-bone gap than omit it, so if you are “auditing” your final test results before taking the patient out of the booth, if you have any question that there was insufficient masking, add in those air-bone gaps to your formula and re-test the air-conduction thresholds that are in question. Air-conduction testing uses a 50 dB IA value in the formulae, and most patients have a much greater IA, so for air-conduction testing, it’s not so critical that you consider minor air-bone gaps. When we move to bone-conduction testing, then it is important. You don’t have the overly-cautious interaural attenuation value to mitigate against the probability of undermasking.

This thought generally doesn’t even need to enter your mind until the air-conduction test signal level is 50 dB or higher. (More on that in the next section.) You next asked yourself:

This requires thinking about the bone-conduction threshold of the non-test ear, since crossover happens when the air-conducted sound vibrates the skull and creates bone-conducted sound.

Example: at 1000 Hz we have:
Left ear AC threshold = 70 dB HL
Right ear             = 10 dB HL  
The 20 dB cross over (assuming use of insert earphones with a 50 dB interaural attenuation) could be heard.  

Is there conductive loss in the non-test ear (NTE)? If so, what is the largest size it could realistically be? Add the estimated NTE air-bone gap to the crossover level. The noise presented by air conduction will lose intensity (in the amount of the air-bone gap size) as it passes through the outer and middle ear. To ensure it is at the desired level when it reaches the cochlea, it has to be increased by the size of the NTE air-bone gap.

Example: at 1000 Hz we have:
Left ear AC threshold = 70 dB HL
Right ear             = 10 dB HL  
If immittance is normal, and there are no symptoms of conductive loss, then I would not add any estimated air-bone gap size to my formula. 

If instead this adult patient had abnormal immittance, I would consider that the bone-conduction threshold may be 0 dB. 
I will add 10 dB to my formula.  
  

I call this a “pad” or “safety pad.” If my estimate of the air-bone gap is slightly low, or if the masking noise is slightly off calibration, or if there was a small unrecognized non-test ear air-bone gap, this extra little bit of noise reduces the likelihood that I will undermask.

Example: at 1000 Hz we have:
Left  ear AC threshold = 70
Right ear AC threshold = 10 dB HL.
Immittance is normal.

The total formula: 
    MMin = TE signal level – 40 + NTE ABG

Above, the steps were to:
take the signal level, subtract 50, then add 10 as a safety pad, 
Which simplifies to:  signal level – 40.

Formula = Signal level – 40 + NTE ABG 
        = 70 -40 + 0
        = 30 dB EM
If immittance had been abnormal, 
to mask the 70 dB signal I need :
        (70 - 40 + 10) = 40 dB EM

When determining the need for masking and estimating the air-bone gap and setting the mimimum level, you may ask yourself whether it is appropriate to assume that bone conduction is better than 0 dB HL. For example, could a 45 dB HL pure tone cross over and be heard if the bone-conduction threshold is -5 dB? If the patient is a child, this may be appropriate. Or if estimating non-test ear air-bone gap size, should you consider that perhaps the bone-conduction score is really -5 or -10 dB HL? Again, you can if it is a child.

How cautious you want to be depends on you and your preceptor’s general approach. You can be cautious from the start and make these super-conservative assumptions (particularly if it’s a child). You could also “play the odds” – assume that the patient will be typical (bone-conduction threshold not better than 0 dB HL). After you have completed testing both ears, your last step should be to see if you failed to mask when masking was needed, or didn’t sufficiently account for a small air-bone gap. If either is the case, you would just retest with sufficient masking.

However, remember that the assumption of a 50 dB interaural attenuation is already a very conservative assumption, (unrealistically so for low-frequency pure tones.) In reality, the minimum interaural attenuations are higher – refer back to Chapter 2, Figure 3. That means that the amount of masking you are using, which was based on the 50 dB IA number, is higher than you probably need. And if you find a low-frequency asymmetry of 50 dB between the test ear air-conduction threshold and the non-test ear bone-conduction threshold, in reality, it is not truly crossover. (In the 1000 to 3000 Hz range, the IA could be as low as 50 dB.)

Setting the minimum masking level ensures that you are not undermasking – that the non-test ear is prevented from hearing the crossed-over signal. It is equally important to make sure that the presence of masking noise in the non-test ear is not crossing back resulting in overmasking.

The interaural attenuation for masking noise is the same as it is for pure tones (i.e. a minimum of 50 dB for insert earphones). For example, a noise level that is 70 dB EM could cross back to the test ear cochlea at 20 dB EM. As with crossover, the crossed-back signal is sent via bone conduction. This means we need to compare the crossed-back noise level to the TEST ear BONE-conduction threshold.

To determine the maximum level of noise to safely put into the non-test ear without concern for overmasking, estimate the best (lowest dB number) TEST ear bone-conduction threshold that is reasonably expected, add the interaural attenuation, then subtract 5 dB. Why subtract 5 dB? If the dB EM at the cochlea is exactly equal to the pure tone level, you will have over masking. You need to reduce the masking level 5 dB to find the maximum level of noise that can be used without risk of overmasking.

Maximum Masking Level MMax = TE BC threshold + IA – 5 dB

For insert earphones, this simplifies to :
    TE BC threshold + 45 dB 

And for supra-aural earphones:
    TE BC threshold + 35 dB

The maximum level is often very high. Let’s examine the case of an 80 dB HL sensorineural loss. The maximum masking level that can be used before overmasking with insert earphones is 80 + 50 – 5: 125 dB EM.

For air-conduction minimal masking levels, you are advised to estimate that the test ear air-conduction threshold will come in at least 20 dB higher than the unmasked threshold. Don’t do that when thinking about the bone-conduction threshold. You need to consider the “worst case scenario” – that is, the best bone-conduction threshold you are likely to find. If immittance was normal and the patient symptoms are pointing away from any conductive involvement, then I would use the unmasked air-conduction threshold as my estimate of the bone-conduction threshold. (I may not get a full 2 out of 3 Hughson-Westlake threshold before masking, but I’ll use that as the approximation of threshold.)

If you want to be particularly cautious, you can estimate that the bone-conduction threshold will be even lower (e.g. 10 dB lower).

To make it easier to read the formula, I am writing “BC threshold” in formulae, but really what I mean is cochlear sensitivity. For example, if the air-conduction threshold is 90 dB HL, and the audiometer bone-conduction output limit is 70 dB HL, use the 90 dB HL number in the formula.

If you predict that, when you are conducting bone-conduction testing, you may find vibrotactile thresholds, again, think about cochlear sensitivity when calculating MMax – not the “threshold that is probably feeling not hearing.”

(Well, if you had a prior audiogram with masked bone-conduction results, that could be an exception to the rule that you have to consider that the entire loss could be conductive. Use the prior bone-conduction threshold in that case (it's unlikely that cochlear sensitivity has improved significantly since you last tested the patient.))

Formula masking is easiest and safest for sensorineural losses – if estimating the TE or NTE air-bone gap is too uncertain, and/or the air-bone gaps are large – then plateau masking is preferable. Formula masking still works when there are minimal air-bone gaps, even in each ear. Just remember the saying: “When in doubt, plateau it out.”

Using the maximum level instead of the minimum masking level is efficient – it will eliminate having to turn up the masking noise if the hearing threshold is higher than what you used in your calculation of the minimum masking level. However, the maximum level may be very high, and you don’t want to cause your patient discomfort. In the example of 80 dB unilateral sensorineural loss, MMax was 125 dB EM. Putting 125 dB of effective masking noise into the normal-hearing non-test ear would be painful.

Even with hearing loss in the non-test ear, the MMax level may cause loudness discomfort. If the patient has asymmetrical hearing loss, with profound cochlear loss in the test ear (e.g. poorer ear sensorineural loss of 110 dB HL and better ear threshold 50 dB HL), while you could use effective masking levels that are as high as the audiometer output limits without fear of overmasking, loudness grows quickly in ears with sensorineural loss. A UCL of 100-110 dB HL would be expected for the non-test ear with 50 dB of cochlear loss. Don’t use levels near MMax if it would be uncomfortable (unless you are required to do so to reach MMin.)

If you don’t have experience in estimating UCLs, there’s an app for that too, and that app is currently free (PC or Mac OSX platforms). It’s found on the www.audstudent.com website. You can estimate MCL (most comfortable loudness level) and UCL (uncomfortable loudness level) for various hearing losses, sensorineural and those with conductive components.

Take the noise level you are presenting. Subtract 45 dB (35 dB for supra-aural earphones), which is 5 dB less than the interaural attenuation value. Do you have concern that the bone-conduction threshold may be at or below that level? If so, you may be overmasking. Instead, use the maximum masking noise level approach or switch back to plateau masking.

As the example above illustrates, if you have significant bilateral conductive loss, it’s not safe to formula mask. You have to revert to plateau masking

Some clinicians routinely use the MMax approach (with the level reduced to ensure that loudness discomfort isn’t a problem.) If you use that strategy, do a final check that the masking was sufficient to mask the crossover. This is basically calculating the minimum masking level using the measured threshold:

Masked threshold – IA + 10 dB + NTE ABG. If you did not use at least this amount of noise, you are potentially undermasking.