Otoacoustic emissions are acoustic signals produced in the cochlea, that can be recorded in the ear canal using a sensitive microphone and some noise cancellation techniques. They appear in nearly all normal hearing listeners. Otoacoustic emissions (OAE) are a consequence of the nonlinear and active pre-processing of sound in the cochlea. People with a cochlear hearing loss show reduced or no emissions. This gives OAE the potential to act as an objective test of hearing function.

Different OAE types are classified according to the stimulation paradigm. Transient evoked emissions are weak sounds in response to a short, transient stimulus, like a click, a tone burst or a chirp. In this case, stimulus and emission are separated in time. With broadband stimulation, nearly all normal hearing listeners emit a broadband response. Click-evoked emissions are now widely used for screening of infant hearing, since they can sufficiently differentiate between populations of normal hearing and hearing impaired subjects. However, a quantitative prediction of hearing thresholds for individual listeners based on click evoked OAE is difficult.
         
Simultaneous evoked OAE are emissions in response to stationary external stimulation, typically with sinusoids. One very important type is the distortion product OAE (DPOAE), which allows a separation of stimulus and response in the frequency domain. DPOAE can be recorded as one or even several combination tones when stimulating with two sinusoids at different frequencies. The most prominent component is the distortion product at the frequency 2f₁-f₂, when the stimuli are sinusoids at f₁ and f₂ with an appropriate frequency ratio (around 1:1.2).

About 40% of normal hearing people also have spontaneous OAE, i.e. narrowband signals emitted at a very low level without any external stimulus. This epiphenomen of the active processes in the cochlea is usually not linked to the phenomenon of tinnitus.