Hearing Health Guide
Definition
Masking is an audiological testing procedure in which a controlled noise (typically narrowband noise or speech noise) is introduced to the ear not being tested in order to prevent it from participating in the response. Without masking, the non-test ear can detect sounds delivered to the test ear, either through air conduction (if the sound is loud enough) or through bone conduction (which stimulates both cochleas simultaneously). Masking ensures that the thresholds recorded for each ear reflect only that ear's function.
The skull readily conducts vibrations from one side to the other. During bone conduction testing, a vibrator placed behind one ear stimulates both cochleas simultaneously. Without masking in the opposite ear, it's impossible to know which cochlea responded. During air conduction testing at high intensity levels, sound can 'cross' to the opposite ear through bone conduction (called cross-hearing or shadow hearing).
If masking is omitted when it's needed, threshold measurements may appear better than they actually are for the test ear, because the non-test ear is responding. This produces false, artificially normal results that lead to incorrect diagnosis and treatment planning.
Masking is used whenever there is a significant difference between the ears, or whenever the air conduction level reaches values where cross-hearing is possible (typically 40–50 dBHL for supra-aural headphones). It is always used in bone conduction testing.
The audiologist introduces masking to the non-test ear at a level sufficient to keep that ear busy (above its own threshold) without being so loud that it crosses back and masks the test ear as well. This balance requires careful calculation according to published masking formulas and significant clinical skill.
Common Questions