The three different kinds of otoacoustic emissions (OAE) measured in the human closed ear canal and their characteristics are described. Spontaneous OAE are picked up by sensitive microphones as very faint tonal components at about 70% of normal ears. Simultaneous evoked OAE are measured as rippled frequency response at low levels which disappears at higher levels. Delayed evoked OAE can be observed at about 97% of normal subjects as a delayed response to short sound bursts, but at low levels only. The correlation between these different emissions as well as their relation to the fine structure of threshold in quiet are discussed. A model simulating the nonlinear cochlear processes with active feedback describes these effects as well as many characteristical dependences like tuning curves, delay times, spectral compositions, suppression-period patterns or (2ƒ
1-ƒ
2)-difference tones in good agreement with human data. This way, otoacoustic emissions are used as an effective tool in research of the hearing system's peripheral processing.
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