CI Atlas · Audiological Evaluation · Module 03

3Masking — the art of isolating the test ear

Masking is where audiometry becomes a craft. A tone loud enough to test a poor ear can cross the skull and be heard by the good ear instead, so the threshold you record may belong to the wrong cochlea entirely. The remedy is to busy the non-test ear with noise — but masking too little leaves the cross-hearing in place, and masking too much spills back and shifts the very threshold you are trying to measure. Between those errors lies a plateau of correct masker levels, and finding it is the skill. The whole problem hinges on one number, the interaural attenuation, which is small for the old supra-aural cushions but large for insert earphones — one more reason inserts have taken over. This module sets out when to mask, how much, and the dilemma that arises when both ears block sound.

TCross-hearing

Crossover occurs when sound presented to the test ear is heard by the non-test cochlea through bone conduction. The recorded threshold then reflects the better ear, not the one under test. Masking noise placed in the non-test ear removes it from the task so the test ear answers for itself.

CInteraural attenuation & the rules

Whether masking is needed is governed by interaural attenuation — the energy lost crossing the head: only about 40 dB for supra-aural earphones but often ≥60–70 dB for inserts. The air-conduction rule: mask when the test-ear AC threshold minus the non-test-ear bone threshold reaches the interaural attenuation. For bone conduction, interaural attenuation is near zero, so masking is needed whenever there is an air–bone gap in the test ear.[2020]

CThe plateau method

Hood's plateau method finds the true masked threshold by raising the masker in steps: while under-masked the threshold rises with the masker (still cross-hearing); across the plateau it stays put (the real value); and once over-masked the noise crosses back and pushes it up again. A wide plateau is a confident result. Over-masking also adds a small (~5 dB) central-masking shift to beware.

Hood's plateau method finds the true masked threshold by stepping the masker. While too little masking is in the non-test ear, the test ear is still cross-hearing, so its apparent threshold rises in lock-step with the masker (under-masking). Add enough and the threshold stops moving — the plateau is the real value. Push the masker too high and it crosses back to the test ear, pulling the threshold up again (over-masking). A wide plateau means a confident result; in bilateral large air–bone gaps the plateau can vanish — the masking dilemma. Schematic.

CThe masking dilemma

The masking dilemma arises with bilateral large air–bone gaps: the level needed to mask the non-test ear is itself high enough to cross back to the test ear, so no plateau exists. The practical escape is the high interaural attenuation of insert earphones, which often restores a usable plateau — a clinical reason the transducer choice of the previous module matters here.

Sound presented to one ear loud enough will cross the skull and be heard by the other — the reason masking exists. The amount lost in transit is the interaural attenuation: roughly 40–80 dB for air conduction (higher with insert earphones, which is why they reduce the need to mask) but essentially 0 dB for bone conduction, so bone-conduction testing almost always needs masking. When the level reaching the non-test ear (by its bone conduction) meets or exceeds that ear's bone-conduction threshold, the response may be cross-hearing — and the test ear must be re-measured with the other ear masked. Schematic.

Case 10.3 · Whose threshold is it?

A right ear with a 75 dB air-conduction threshold is tested with supra-aural phones; the left ear has normal bone conduction. No masking is applied.

Why might the recorded right-ear threshold be invalid?

Self-assessment — Module 32 questions

Question 1 · Trainee

When is masking required for an air-conduction threshold?

Question 2 · Clinician

What is the plateau (Hood) method?

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