The normal cochlea does not just receive sound; it also produces low-intensity sounds called OAEs. These sounds are produced specifically by the cochlea and, most probably, by the cochlear outer hair cells as they expand and contract.
The 4 types of otoacoustic emissions are as follows:
--> Spontaneous otoacoustic emissions (SOAEs) - Sounds emitted without an acoustic stimulus (ie, spontaneously)
--> Transient otoacoustic emissions (TOAEs) or transient evoked otoacoustic emissions (TEOAEs) - Sounds emitted in response to an acoustic stimuli of very short duration; usually clicks but can be tone-bursts
--> Distortion product otoacoustic emissions (DPOAEs) - Sounds emitted in response to 2 simultaneous tones of different frequencies
--> Sustained-frequency otoacoustic emissions (SFOAEs) - Sounds emitted in response to a continuous tone
Pure-tone (PT) audiometry measures throughout the outer ear, middle ear, cochlea, cranial nerve (CN) VIII, and central auditory system. However, OAEs measure only the peripheral auditory system, which includes the outer ear, middle ear, and cochlea. The response only emanates from the cochlea, but the outer and middle ear must be able to transmit the emitted sound back to the recording microphone.
OAE testing often is used as a screening tool to determine the presence or absence of cochlear function, although analysis can be performed for individual cochlear frequency regions. OAEs cannot be used to fully describe an individual's auditory thresholds, but they can help question or validate other threshold measures (eg, in suspected functional [feigned] hearing loss), or they can provide information about the site of the lesion.