Modelling the cochlear origins of distortion product otoacoustic emissions

Abstract

Distortion product otoacoustic emissions (DPOAEs) arise within the cochlea in response to two
stimulus tones (f1 and f2) at frequencies such as 2f1 ? f2 and 2f2 ? f1. Each DPOAE derives from two
contributing mechanisms within the cochlea: a distributed distortion source and a reflection source.
They are used for hearing screening, but a better understanding of their cochlear origin and
transmission could potentially extend their clinical application to facilitate objective hearing loss
assessment, differential diagnosis of sensorineural hearing losses and improved auditory
rehabilitation using hearing aids.

In this thesis a numerical model of the human cochlea is developed to study the generation
of DPOAEs. It is based on a pre-existing active nonlinear model, the micromechanics of which are
carefully re-tuned to simulate the response of the human cochlea to single- and two- tone
stimulation. Particular attention is paid to the form and position of the nonlinearity within the
model to best match experimental results. The model is also reformulated to verify its stability and
ensure computational convergence of the iterative frequency domain solution method. Its
predictions are validated against estimated time domain simulations and documented experimental
DPOAE measurements. Additionally a novel method is developed for decomposing each
frequency component of the cochlear response into forward and backward travelling waves, which
is applied to investigate the multiple sources of both the 2f1 ? f2 and 2f2 ? f1 DPOAEs.

The model is used to explain and predict a variety of phenomena observed in experimental
DPOAE studies. It also confirms for the 2f1 ? f2 emission, that the two source mechanisms are
spatially separated and that the only significant reflection contribution is associated with the 2f1 ? f2
travelling wave. In contrast, it predicts that the two source mechanisms will overlap in the case of
the 2f2 ? f1 DPOAE, which can be influenced by reflection of both the primary and 2f2 ? f1
travelling waves.

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