Because the bones of the adult head are fused at the cranial sutures, it’s generally stated that if you vibrate one bone of the head, you will vibrate them all equally. This means that if you put the bone oscillator on the right mastoid, the vibration goes to both cochleas. Therefore, there is no interaural attenuation in bone-conduction testing. That statement is not completely true. There can be some interaural attenuation for bone conduction, but it’s typically not a lot – usually 10 dB or less. Examine Figure 3-1, the same audiogram shown in the prior chapter, and note that 4000 Hz shows 15 dB of interaural attenuation for bone conduction.

Figure 3-1. At 4000 Hz the right ear unmasked bone-conduction threshold is 5 dB, which is 15 dB higher than the left ear unmasked threshold. This shows that this patient has 15 dB of interaural attenuation. However, at the other frequencies, the interaural attenuation is 0 dB HL. The most conservative (careful) approach is to assume there is no interaural attenuation – that the entire intensity of the sound is carried to the non-test ear.

The 2004 American National Standards Institute (ANSI) audiometric calibration standards assume that masking was used when testing bone-conduction hearing. Introducing masking noise to the non-test ear makes the listening task a little harder for a patient – it elevates the test ear threshold slightly relative to the threshold that would be obtained without putting noise in the opposite ear. This is called the central masking effect. It’s a small effect when the contralateral noise is at near-threshold-levels, but it often creates a 5 dB elevation of threshold. The fact that ANSI assumes you are always using contralateral masking means that if you don’t test using contralateral masking, bone-conduction thresholds may well be 5 dB better. (The threshold will not be elevated by the central masking effect.) The person with perfectly normal hearing would have thresholds of -5 dB HL, not 0 dB HL. (Note again, Figure 3-1.) This can create the appearance of minor air-bone gaps. This is one reason that some audiologists advocate masking bone-conduction thresholds routinely.

Another argument for using masking, and for testing bone conduction in each ear, is the definition of a complete hearing test: air- and bone-conduction threshold testing in each ear. If taken literally, this would require testing each ear’s bone-conduction thresholds, using contralateral masking to ensure that one is indeed testing the ear that one intends to test. However, that means conducting more testing than needed, which is an inconvenience to both the patient and audiologist.

When testing bone-conduction hearing without masking, the sound is of similar intensity at each cochlea since interaural attenuation is ~0 dB. If there is a better hearing ear, then that is the cochlea that detects the signal. I’m assuming you know the definition of conductive involvement: an air-bone gap of 15 dB or more. (If not, you’re not ready to study masking.) If the unmasked bone-conduction test results rule out conductive involvement, then no further testing is needed. If there is a possibility of conductive loss, then we need to know whether the air-bone gaps are unilateral or bilateral.

Examine Figure 3-2. The loss is bilateral and symmetrical. No information would be gained by testing the left ear by bone-conduction. It cannot be significantly better than the unmasked right ear thresholds, since bone-conduction interaural attenuation is usually 0 dB and seldom more than 10 dB. Bone-conduction masking is not needed

Figure 3-2. There is no need to mask the right ear bone-conduction thresholds, or to test the left ear bone-conduction thresholds. Since the unmasked bone-conduction thresholds represent the better cochlea if there is one, the left ear’s bone conduction thresholds will not show a conductive component even if it were to be tested.

In contrast, Figure 3-3 shows a case where bone-conduction masking is needed. There may be air-bone gaps for both the right and left ears.

Figure 3-3. Masking is needed for bone-conduction testing for each ear. In the left ear, there are apparent air-bone gaps at 1000 Hz and below, but the bone-conduction thresholds may reflect the right ear’s cochlear sensitivity. Left ear bone-conduction testing requires that masking noise be put into the right ear to prevent it from detecting the crossed-over bone conduction signal, in case the right ear cochlea does have normal hearing. The right ear also requires masked bone-conduction testing at all frequencies. If the bone oscillator were placed on the right mastoid, similar or identical unmasked thresholds would be obtained, and again, it would not be known if the thresholds were truly for the right cochlea, or if it was a cross-hearing response from the left cochlea.

The rule for when to mask is “if you need to determine if there is truly a conductive hearing loss, if you aren’t sure which cochlea is responding and it matters, then mask.” We can make the rule even more specific – mask if there is any possibility there could be a 15 dB or more air-bone gap. Because it is traditional to start testing bone-conduction without masking, the findings as shown in Figure 3-3 are typical. We don’t yet know if there is or is not an air-bone gap without having yet masked, so we can also say if there is a 15 dB or more difference between an air-conduction threshold and the unmasked bone-conduction threshold, then use contralateral masking.