Otoacoustic emissions are sounds made by our inner ear as it works to extract the information from sound to pass on to the brain. These biological sounds are a natural by-product of this energetic biological process and their existence provides us with a valuable ‘window’ on the mechanism of hearing, allowing us to detect the first signs of deafness - even in newborn babies.
Sounds made by healthy ears are quite small - quieter than a whisper and usually less than 30dBSPL. They arrive in the ear canal because the middle ear receives vibrations from deep inside the cochlea. This causes the eardrum to vibrate the air in the ear canal creating the sounds that we can record.
To record otoacoustic emissions, or ‘OAEs’, a ‘probe’ is inserted in the ear canal. The probe closes the ear canal, keeping the OAEs in and any noise out. The probe both stimulates the ear with precisely defined sounds and records the sounds made by the ear via a tiny microphone.
Separating the applied sound from the ear’s own sound is a delicate business and needs computer processing power.
Today this is achieved by a variety of otoacoustic instruments. Hand-held and pocket-sized screeners are available which provide a quick indication of the status of the ear and are widely used for infant screening. Because OAEs are blocked by middle ear immobility, these instruments alert to both conductive and sensory dysfunction. Some OAE screeners provide a single indicator of function across speech frequencies, as does screening ABR. Others provide a basic frequency breakdown. Although OAE screeners are sensitive to threshold elevations as small as 20dB, they do not provide a measure of the actual threshold.
Simple OAE screening instruments conceal the fact that otoacoustic emissions are quite complex phenomena - whether they are evoked by tones or clicks. Click evoked OAEs (TEOAEs) consist of a complex response waveform which can be broken down into different frequency bands (typically half octave), telling us about cochlear status in each band. Distortion product OAEs are evoked by a pair of tones (typically one-third-octave apart) which are stepped across the frequency range to be examined. Each pair of tones may produce several DPOAEs. One of these (typically the one at 2f1-f2) is plotted on the ‘DP gram’.
Both TEOAEs and DPOAEs provide frequency specific data on cochlear function.