Plateau masking is the “gold standard” of masking. If you found the unmasked threshold, used the appropriate initial masking level, increased the masking level 15 dB, and the threshold has not changed with those three 5-dB increases, you are assured that you have masked correctly.

However, in cases of unilateral severe / profound hearing loss, plateau masking takes a long time. Consider the case of bone-conduction testing of a unilateral “dead” ear when the better ear has a 0 dB threshold. The unmasked 2k Hz threshold is likely 0 dB HL. Assume that the NTE air-conduction threshold is normal. You would start with 10 dB of masking, which probably elevates the bone-conduction threshold to 20 dB HL (10 due to peripheral masking and 5 dB increase from central masking, and the signal needs to be 5 dB above the noise to be heard). Each time you increase the non-test ear noise in 5 dB steps, the threshold increases in 5 dB steps.

This process is repeated until the bone-conduction signal level is at the audiometer maximum, i.e. 75 dB HL. Most likely, the contralateral noise is 75 dB HL at the time you obtain the no-response. Masking levels used were 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75.

To ensure that the patient didn’t anticipate the tone and respond just due to the increase in the masking noise level, the audiologist also had to test at a slow rate, pausing after turning up the noise by a variable amount of time to ward off false-positive responses. And heaven forbid that the patient is one prone to false-positive responses! Finding one threshold can take several minutes. There is a faster way!

When conducting formula masking, the audiologist puts a calculated amount of noise into the non-test ear to prevent cross hearing.

There is more than one way to formula mask. The audiologist may formula masked based on the initial unmasked threshold, but to be time efficient, the audiologist considers that the threshold will likely shift once masking is used and base the calculations on that expected threshold. There are other variations to the formula masking approach. The audiologist can determine the highest level of masking noise that can be used, and/or the minimum level that is needed, use either of those levels, or put in a level somewhere between that minimum and the maximum level.

The audiologist may need to make an adjustment to the noise level – e.g. if it was predicted that the loss was moderate and threshold testing is showing greater hearing loss, the masking noise may need to be increased; or vise-versa, if the assumption was that the loss was profound, and thresholds are coming in better than expected, the audiologist may need to decrease the noise level – if the high noise level could have overmasked. But relatively few masking level intensity adjustments (if any) are made when using a formula masking approach. This offers a time savings.

Formula masking requires some thinking, but it becomes easier and easier with practice. The audiologist will always be thinking about what signal levels could be crossing to the non-test ear, and what masking noise level is needed to mask the crossover. An additional consideration is needed – whether using a loud noise in the non-test ear could be uncomfortably loud.

Another concern is that if the masking noise is intense enough, it too can vibrate the skull and create cross-hearing. In the case of overmasking, the crossed-back-to-the-test ear sound interferes with hearing the test ear signal.

Formula masking means thinking about how much crossed over, and then finding a masking noise level that is enough to mask the cross over, but not so loud that it is uncomfortable or could overmask. With that one level of noise in the non-test ear, threshold is found. If threshold was what you expected – what you based your calculations on – you are done! No up 5, up 5 . . .

Once the basics of hearing testing are mastered, it is enjoyable to have the mental challenge of figuring out how much masking is needed, and fun to see introductory-level audiology students’ looks of amazement when the audiologist seemingly uses intuition in nearly automatically setting the masking level to “the” level needed. But reaching that zenith takes a bit of time and effort. Why put forth the effort? Keep reading.

Increasing your test efficiency means seeing more patients, but it also maximizes your diagnostic results. Whether testing children or adults with limited mental capabilities, cooperation with testing occurs only for a limited amount of time. Being efficient can mean the difference between complete and incomplete test results.

Patients appreciate short test times. While taking a hearing test is not difficult, it does require attention. It’s a rare patient who would want to have a larger number of test tones to detect. Formula masking saves both you and the patient time.

When conducting word recognition testing, you cannot plateau mask. Increasing the masking noise and determining if the word recognition score stayed the same or improved or worsened wouldn’t work – word recognition scores vary from list to list even at the same intensity. It’s not feasible to determine if the scores are changing meaningfully as the contralateral noise level is adjusted. Also, it would be time consuming and tedious.

When estimating hearing thresholds using auditory brainstem response (ABR) testing, one has to be as efficient as humanly possible in order to obtain the needed thresholds. Repeating testing with varying levels of contralateral noise is out of the question. The audiologist has to know how much noise to use to mask any possible crossover of the test signal to the non-test ear. Routine use of a formula masking approach during pure-tone testing is the best way to ensure that you are able to mask during ABR testing, which is a situation that involves concentration on many things in addition to masking.

The profession of audiology has not adopted one single formula for air-conduction masking testing, nor one for bone-conduction masking. As a result, each professor/clinical preceptor may want their students to use a slightly different approach. That’s the bad news. The good news is that all the approaches are based on the same underlying ideas.

It sounds harder than it really is. We will take it a step at a time, and in the next chapter, we begin with air-conduction formula masking. You have already worked on the first steps if you have completed the mQuest learning game, levels 1 through 9, the plateau masking games

There are occasions where the audiologist wants to mask using a formula, but in making the mental calculations, determines that the minimum level is either at or close to the maximum that can be safely used without cross-back interfering with the test ear hearing the signal, or sometimes those calculations tell you the impossible: The minimum to use is already above the maximum to use without concern or reservation. In those cases, the audiologist reverts to plateau masking if that is an option. (So, please don’t replace your understanding of how to plateau with how to formula mask – you need to know both!)