Wave Reflection in Passive and Active Cochlea Models

Abstract

In the formulation of a cochlea model, ‘waves-to-the-right’ (i.e. from stapes to helicotrema) are generally treated as equivalent to ‘waves-to-the-left’. In the resonance peak region of the model the conditions for wave propagation vary so rapidly that there is every reason for waves to be reflected. Yet very little evidence of reflected waves is observed, even in an active model. This property is studied by considering various models equipped with a perfectly absorbing wall at the stapes location so that reflected waves can’t make the model unstable. A short-wave model does not give rise to reflections. However, a model in which short and long waves are possible shows a preference for waves in the ‘normal’ direction of propagation (waves-to-the-right), these undergo little or no reflection. Waves in the opposite direction may be reflected when they enter the long-wave region from the short-wave region. The same model is also used to study which degree of impedance irregularity may cause a reflection (an evoked acoustic emission). This degree turns out to be extremely small in an active model: an irregularity of 0.5 per cent extending over the width of two hair cells is sufficient to cause a reflection with the same intensity as the incident wave.