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Clinical masking: interaural attenuation, when to mask, the plateau and the masking dilemma
When the sound to the test ear is strong enough to cross to the other ear — mainly by vibrating the skull — noise is given to the non-test ear to "keep it busy"; masking is right when the threshold stays stable as the noise is raised (the plateau), and fails when there is too little noise (undermasking) or too much (overmasking).
Why it matters in the exam
Exam questions test definitions and rules: in air-conduction testing sound crosses to the other ear mainly by skull vibration; interaural attenuation with supra-aural earphones is about 40 dB and more with insert earphones, and for bone conduction it is about zero; so in air conduction with supra-aural earphones you mask when the difference between the test ear and the other ear's bone threshold is 40 dB or more. Effective masking is the noise level that shifts the masked ear's threshold to exactly that level — 50 dB of effective masking will raise its threshold to 50.
Phenomena are asked too: the plateau is the stage at which the noise is raised and the threshold does not change; in overmasking raising the noise also raises the threshold of the test ear; in undermasking responses still come from the other ear; the masking dilemma arises with a moderate bilateral conductive loss, and its chance is lowest with insert earphones in air conduction; central masking, a small effect of the noise on the threshold through the brainstem, cannot be avoided; and a shadow curve is a response of the other ear to sound that crossed over to it. When there is no shadow curve at all in an apparently "dead" ear, suspect a functional loss.
Key points
- Crossover to the other ear is mainly through skull vibration.
- Attenuation: supra-aural about 40, inserts more, bone about 0.
- Effective masking shifts the threshold to exactly the noise level.
- Plateau = stable threshold; under and over are the two errors.
- Masking dilemma with a moderate bilateral conductive loss; central masking is unavoidable.
Summary table
| Concept | Definition | Number or rule |
|---|---|---|
| Crossover to the other ear | Sound reaches the other ear mainly through skull vibration | In bone conduction — almost always possible |
| Interaural attenuation | How much the sound weakens on the way to the other ear | Supra-aural about 40 dB; inserts more; bone — about 0 |
| When to mask (air) | When the difference between the tested air threshold and the other ear's bone threshold exceeds the attenuation | With supra-aural: 40 dB or more |
| Effective masking | The noise that shifts the masked ear's threshold to exactly its level | 50 dB effective masking — the threshold there will be 50 |
| Efficient noise | Narrow noise around the test frequency (the critical band); for speech — noise shaped to the speech spectrum | In word recognition testing the masking level is constant |
| Plateau, undermasking and overmasking | Plateau — stable threshold; under — response from the other ear; over — the noise raises the test ear's threshold | Hood's plateau method |
| Masking dilemma, central masking and shadow curve | Dilemma with a moderate bilateral conductive loss; central masking unavoidable; shadow — the other ear's response | No shadow in a "dead" ear — suspect a functional loss |
A free exam question
In what respect do insert earphones help with masking?
- A. The masking noise itself
- B. Calibration in a 2-cc coupler
- C. The comfort of their foam tips
- D. Interaural attenuation (IA)
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Sources
- Katz (ed.), Handbook of Clinical Audiology, 7th edition (2015) — Clinical masking
- Hood, Journal of Laryngology and Otology, 1960 — The principles and practice of bone conduction audiometry (the plateau method)
- Yacullo, Clinical Masking Procedures, 1996 — Interaural attenuation, effective masking and the masking dilemma
Sources are given for further reading; the text is written afresh and not copied from them.
Reviewed by the Examqo AI Professor · 2026-11-02. The text may contain errors; when in doubt, check the source.