Research project: Cochlear modelling of distortion product otoacoustic emissions

Distortion product otoacoustic emissions (DPOAEs) are emitted by the cochlea when it is stimulated by two pure tones (f1 and f2 where f2>f1) simultaneously. These DPOAEs can be detected in the ear canal and have frequencies which are integer combinations of the stimulus frequencies (eg. fdp=2f1-f2). We have developed a classical, lumped-parameter cochlear model of the type illustrated in Figure 1, to study the cochlea and simulate the production of DPOAEs. The model incorporates the nonlinear behaviour of the outer hair cells and so the simulations exhibit compressive amplitude growth with increasing stimulus level as well as harmonic and intermodulation distortion.

The model has been tested by simulation of known physiological characteristics of human DPOAEs, such as the amplitude spectra and the optimal f2/f1 ratio required for maximal amplitude distortion products. We intend to apply the model to investigate the results of various DPOAE experiments to improve our understanding of the generation of DPOAEs within the cochlea, and the mechanism by which they are emitted. For example we have confirmed that the fine structure, which is seen when DPOAE amplitude is plotted as a function of f2, can be attributed to the mixing of distortion products from two different sources within the cochlea; a wave-fixed (distortion) component located near the f2 characteristic place and a place-fixed (reflection) component originating from the fdp characteristic place. We hope that an improved understanding of DPOAEs may facilitate their use in clinical hearing assessment and rehabilitation in the future.

Figure 1. A discrete model of a coupled cochlea. Each single micromechanical element is composed of a spring-mounted oscillator.

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