The human ear serves as both a detector and a generator of sound. Tiny hair cells in the inner ear convert incoming acoustic vibrations into nerve signals, but as the cells move in response to sound waves, they themselves produce faint sounds, which are known as otoacoustic emissions.

By listening to these feeble signals, researchers can study in remarkable detail how the inner ear works. Now, detection of these emissions shows promise as a means of evaluating a wide range of common hearing problems involving damage to hair cells.

The sound source generates either a click or a tone, and the microphone picks up the resulting ear-generated sound. In an ear with normal hearing, the faint output sound is nearly identical to the input sound.

This type of test may prove particularly valuable because many hearing difficulties involve damage to hair cells. Such damage can be caused by exposure to prolonged or excessively loud noises, various drugs, or bacterial and viral infections.