Knight, Richard David; (2001) Mechanisms of otoacoustic emissions. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Otoacoustic emissions (OAE) are sounds which are emitted by the ear in response to acoustic stimulation as a byproduct of normal mammalian auditory function. Published research into OAEs leaves many of their detailed characteristics unreported or of uncertain explanation. For example, the mechanisms by which the emissions occur and how many emission mechanisms or categories there are have not yet been clearly established. The purpose of this study was to obtain comprehensive OAE data in a format which would permit specific questions regarding the mechanism of their generation and emission to be answered. Both distortion and transient evoked OAEs responses (DPOAE and TEOAE) were obtained from healthy human ears. It was found these two responses can have similar characteristics but only if a restricted set of DPOAE stimulus parameters are employed, implying that under these conditions the underlying emission mechanisms are closely related. Another significant finding is that lower sideband DPOAE (e.g. 2f1-f2) obtained with stimuli f2/f1>1.1 has fundamentally different phase characteristics (and hence origins) to all other DPOAEs. A new frequency/area representation of detailed DPOAE intensity and phase data has been developed. This revealed that lower and upper DPOAE sidebands are a continuation of each other for f1~=f2, implying a continuity of emission mechanism. This and the transition of behaviour pattern in lower sideband DPOAE for f2/f1>1.1 supports a one source/two emission routes model for DPOAE. DPOAEs arriving in the ear canal by these two routes have been successfully separated and analysed and inferences are drawn regarding the location of DP generation. Amplitude fine structure was demonstrated to be partly due to interference between the two components but also an additional mechanism is involved, perhaps interference within emission generation regions or internal reflections. A simple transmission line model demonstrated that the hypothesis can explain the results seen to a good approximation.

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