The Origin of Fractional Distortion Components in the Cochlea

Abstract

This paper describes the properties of pairs of fractional distortion components that are generated in chinchilla and guinea pig cochleas and suggests a possible source of these pairs. The pairs consist of two components with frequencies that are equally spaced around one-half the excitation frequency and that sum to the excitation frequency. The pairs exhibit a threshold of appearance, their frequencies can drift with time, and at excitation intensities well above threshold the pairs develop into noisy bands instead of remaining as discrete frequency components. An analysis of the stability of an acoustic flow over a flexible boundary suggests a possible source of these pairs. The analysis shows: (1) for a sufficiently large acoustic excitation, the flow is unstable owing to an exponentially growing one-half subharmonic wave, and (2) pairs of fractional components may appear in the flow just before the flow becomes unstable. On the basis of this analysis, it is suggested that the fractional distortion pairs observed in the cochlea are related to a developing flow instability. [This work was performed at the Biomedical Engineering Center and supported by the NIH.]