Audiologists put masking noise into the non-test (opposite, contralateral) ear when it is possible that the stimulus is being heard by that non-test ear. When testing by air-conduction, this “cross hearing” occurs when the vibration of the air molecules is intense enough that the skull is set into vibration. The cochlea are embedded within the bones of the skull; once the skull is vibrating, the energy is sent to BOTH the test ear and non-test ear cochleas (Figure 2-1).

Figure 2-1. The air-conducted test signal, if intense enough, can create vibration of the skull, which permits bone-conduction hearing. Since the skull is fused, this vibration activates fluid motion within both the ipsilateral and the contralateral cochlea: The non-test ear’s cochlea is stimulated by bone conduction. Because a sound produced by an insert earphone vibrates only a portion of the ear canal, it takes a greater intensity test signal to create the cross-hearing than if a supra-aural earphone is used.

If the sound is loud enough to cross over to the non-test ear, it is also going directly to the test-ear cochlea. When the sound level is above a certain intensity, it will vibrate the skull, and this sound vibration will by-pass the outer and middle ear. This is the concept behind the “maximum conductive loss”. A purely conductive loss cannot cause total deafness; the air-conducted sound becomes bone conducted and by-passes the conductive system. (Most conductive loss allows some sound transmission through the middle ear, so the maximum loss usually seen is not a true maximum conductive loss.)

The audiologist’s concern is that the bone-conducted sound (created by the loud air-conduction vibration) will be heard by the non-test ear rather than the test ear. If the moderately loud air-conducted sound becomes an above-threshold level bone-conducted sound that is detected by the opposite ear, then the audiologist is not testing the ear to which the stimulus is being sent. In this case, noise needs to be put into the non-test ear to prevent hearing of the crossed-over test ear signal. The amount of noise needed depends on the amount that may be crossing over.

Patients differ in their skin, soft tissue and skull characteristics; not everyone will start to experience cross-hearing at exactly the same intensity level. Studies have been conducted on persons with complete unilateral hearing loss to determine how loud sound has to be in the “dead ear” to be perceived in the good ear. An example audiogram of one such person is shown in Figure 2-2.

Figure 2-2. Audiogram of a person with unilateral sensorineural hearing loss, shown with and without of the masking of the “dead” right ear. Insert earphones were used.

Recall that the cross hearing for air-conduction occurs when the non-test ear cochlea hears the pure tone sent via the test ear earphone/headphone. The term interaural attenuation needs to be defined. It is the loss of sound energy that occurs during the process of the sound crossing to the non-test ear cochlea. To determine air-conduction interaural attenuation,

• compare the unmasked air-conduction thresholds of the poorer hearing ear
• to the bone-conduction threshold of the better ear.
In the example above, compare right unmasked air-conduction thresholds to left bone conduction thresholds.
• At 1000 Hz, the right ear unmasked air-conduction threshold is 90,
• the left ear bone-conduction threshold is -5.
• This 95 difference is this patient’s interaural attenuation at 1000 Hz.

Interaural attenuation refers to how much sound energy is lost, in this case, as it is transformed from an air-conducted signal to a bone-conducted signal that has crossed the head. When we know the interaural attenuation value, we can determine how loud a sound will be at the non-test ear cochlea (and then, determine if that will be audible or not.) For this patient, if we present a 1000 Hz, 120 dB HL air-conducted pure tone, it will be heard as 25 dB HL (120 – the 95 interaural attenuation) at the non-test cochlea.

Several studies have tested a number of patients with unilateral profound hearing loss to determine the range of interaural attenuation that different patients can have. Figure 2-3A shows the averaged results from some of these studies.

We have no way of knowing before testing a patient if he or she will have low or high interaural attenuation values. You could determine the interaural attenuation if you first test unmasked air conduction and compare the threshold to the better ear’s unmasked bone-conduction threshold. (This assumes that after putting masking in the non-test ear the true hearing threshold is even poorer, which means cross hearing did occur.) But that step is not done clinically. It would take too much time and testing.

Since we don’t know if the person has a high or low interaural attenuation value before testing, we have to assume the worst-case scenario exists: Assume that the patient you are testing has a low interaural attenuation value – the patient has cross-hearing at the lowest possible level. These levels are shown in Figure 2-3B.

Figure 2-3. Average interaural attenuation results when testing using supra-aural earphones (a mix of models TDH 39 and 49) and for insert earphones (mix of deeply inserted, shallow insertion, and depth not specified).

Figure 2-3B. Not everyone has average interaural attenuation values.
The lowest values seen in any of the studies is shown.

Figure 2-3. Maximum values (the highest value in any one of the studies) are displayed in this last figure. The total number of subjects tested was 34 for TDH headphones and 40 for insert earphones.

See reference list for study values used.

Audiologists have traditionally made the generalization that the lowest interaural attenuation value is 40 dB for supra-aural earphones and 50 dB for insert earphones. These assumptions are very cautious (conservative). In the vast majority of cases, the value will be higher. Figure 2-3C shows that for some patients, interaural attenuation values for insert earphones can exceed 100 dB.

In reviewing Figure 2-3B, you see that the minimum interaural attenuation for insert earphones is 50 dB only for 2000 and 3000 Hz. One could use the chart below of the minimum interaural attenuation values per frequency, and only use contralateral masking if the signal level is high enough that there may be cross hearing at that frequency. However, in the professional communities in which I have worked, this is not typically done. Therefore, if I were to use this (scientifically sound) technique, my colleagues might question what I was doing, and a few might question my competence. Since masking is really not that hard (trust me, it soon won’t be), it’s not all that onerous to assume that the interaural attenuation could be as low as 50 dB for insert earphones and use this value when masking, regardless of frequency. However, there will be times when you may want to know what those minimum levels really are; the next section summarizes them to serve as an easy reference.

The general rule is to compare the air-conduction threshold of the test ear to air-conduction threshold of the non-test ear. If the test ear threshold is ≥ 50 dB above the non-test ear threshold, then masking is needed. However, if the non-test ear has conductive hearing loss, and thus better bone-conduction sensitivity than air-conduction sensitivity, then the “compare air to air” rule will not detect all the times when masking is needed. Observe Figure 2-4.

Figure 2-4. Using the “mask the non-test ear when testing the poorer ear if the air-conduction thresholds differ by 50 dB or more” rule would not alert the audiologist that the low-frequency thresholds of the right ear may also be due to potential for cross-hearing. The secondary rule is to “mask the non-test ear when testing the poorer ear if the air-conduction threshold is 50 dB or more higher than the bone-conduction threshold of the non-test ear.”

It’s prudent to review the completed audiogram before ending testing, to double check that masking was conducted when needed. Once all the bone-conduction thresholds have been obtained, it is easy to see if you have forgotten to mask based on the “test-ear air to non-test ear bone” rule.

Note that in Figure 2-4 unmasked bone-conduction scores are shown. How do you know if that’s really the left cochlea’s hearing sensitivity? You don’t. But since it could be the left ear, then we need to mask.

For clarity then, let’s state the second “when to mask for air-conduction” rule again. If there is a 50 dB or more difference between an air-conduction test ear threshold and the non-test ear bone-conduction threshold, then masking is needed. And if there may be this size difference, it’s prudent to mask rather than waiting until all the bone-conduction thresholds are obtained.

The Masking Calculator (mCalc) is an app that allows you to check whether masking is needed. This would be a good time to start using the app. Note that you adjust the air- and bone-conduction thresholds: The masked/unmasked symbols are not used. In this application, you are doing a “what if” and assuming that the thresholds are as they would be if masking were used – the levels you adjust in the app are assumed to be the “real” thresholds. You can input impossible scenarios, such as a unilateral profound conductive loss, if you desire.

The masking calculator is at this Web location: Start mCalc

At this stage of learning, you should adjust the thresholds and click on the “Mask Air” buttons. As shown in Figure 2-5, the interaural attenuation is always assumed to be 50 dB for air-conduction. The crossover will be calculated and displayed next to the non-test ear bone conduction threshold (shown as “Co” in the app). If the crossover is above the bone-conduction threshold, masking is recommended.

Figure 2-5. Screen shot of the mCalc application. Click on “Mask Air?” to determine if masking is needed. If the air-conduction threshold of the poorer ear is 50 dB or more above the bone-conduction threshold of the better ear, then masking is needed. Use this app to examine different scenarios to see if you correctly predict when masking is and is not needed.

There is a game that tests you on the mCalc concepts. Level 1 of the Game-Based Learning App, called mQuest, gives you practice at recognizing when to mask in cases of sensorineural loss: the first “when to mask rule” is sufficient (50 dB or more difference between air-conduction thresholds). Levels 2 and 3 may require use of the second rule (compare test ear air to non-test ear bone); the examples include mixed loss. Note that air-bone gaps in the test ear are irrelevant. You will compare the air-conduction threshold of the test ear (whether or not it has a conductive component) to the bone-conduction threshold of the non-test ear. 100% mastery is needed to move up in game levels.