Otoacoustic emissions are small sounds caused by motion of the eardrum in response to vibrations from deep within the cochlea. The healthy cochlea creates internal vibrations whenever it processes sound. Impaired cochleae usually do not. Some healthy ears even produce sound spontaneously as internal sounds are processed and amplified. As described later, the cochlea’s capacity to generate sound is intimately associated with its achievement of normal auditory threshold, and the underlying mechanism is very easily damaged.

To record the sounds made by the cochlea an earphone and microphone combination probe is fitted into the ear canal. The middle ear has to be working efficiently in order to conduct the minute cochlear vibrations back to the ear drum - acting like a stethoscope. A good fitting of the probe is important. Closure of the ear canal by the probe greatly increases the sound pressure created by any ear drum vibration. It also excludes unwanted external sounds.

Normally the ear to be tested is given mild acoustic stimulation to evoke an otoacoustic emission. Clicks, tones, noise and even speech all elicit an OAE response. There is a unique OAE response to every stimulus. Depending on the nature of the sound presented, different signal processing techniques are effective in extracting the OAE from the stimulus and other noises. The common technologies are ‘TEOAE’ when clicks or tone bursts are used, ‘DPOAE’ when dual tone stimuli are used, and ‘SFOAE’ when single tone stimulation is used. It is important to remember ‘TEOAE’, ‘DPOAE’ and ‘SFOAE’ instruments deliver different views of the same auditory process and a combination of measurements is needed to get a complete picture.

The essential fact about OAEs is that their presence is always good news about cochlear and middle ear function. It usually means hearing is within normal limits around the stimulus frequency evoking the response - but this is not guaranteed. There can be problems further along the auditory pathway and there is much still to learn about OAEs and cochlear physiology. In the following pages we look at the use of OAEs today for newborn screening and for clinical investigation, and at the auditory physiology and biophysics behind OAEs and OAE technology.

This article has been extracted from the publication ‘Understanding & Using Otoacoustic Emissions’, written by Professor David Kemp and published by Otodynamics, and is reproduced with the author’s permission. Copyright remains with the author.