There’s a fascinating article on the evolution of hearing in The Scientist that also contains an interesting gem – your ears produce sounds as well as perceiving them.
In addition to amplifying hair-cell activity, these active mechanisms manifest as spontaneous movements of the hearing organ, oscillating even in the absence of sound stimuli. Such spontaneous movements actually produce sound that is emitted through the middle ear to the outside world and can be measured in the ear canal.
Spontaneous otoacoustic emissions were evaluated in 36 female and 40 male subjects. In agreement with the results of previous surveys, emissions were found to be more prevalent in female subjects and there was a tendency for the male subjects to have fewer emissions in their left ears.
It also turns out that spontaneous otoacoustic emissions are used to test hearing in newborn babies.
It’s an interesting problem, because normally hearing is tested by giving people tests where they are played various sounds or silence at certain time points and they have to signal whether they think they heard the sound, and correct decisions are counted.
This clearly doesn’t work with babies but one way of testing hearing is by measuring the response of nerve signals that connect to the auditory brain stem.
This training video shows how it’s done but in the last section there’s a test which relies directly on detecting the ‘echoes’ created by otoacoustic emission when a tone is played directly into the ear.
Link to The Scientist article on the evolution of hearing.