The plateaus in basilar membrane vibration data—explained

Abstract

In recent measurements of basilar membrane tuning curves both the amplitude and the phase approach a constant value for frequencies above the transition frequency. To analyse the origin of these plateaus it is common to use the property that the basilar membrane‐stapes displacement ratio has the same form when plotted against x (=distance to stapes) or log f (=log frequency). An analysis of the situation beyond the characteristic place then shows that the phase plateau is caused by the mass‐controlled character of the membrane impedance there. The magnitude of the ratio decreases sharply in that region; the added mass effect of multi‐dimensional modelling leads to a change in slope, but not to a plateau. The x vs log f transformation however looses its validity for frequencies above the highest resonance frequency in the cochlea. For those frequencies the membrane response tends to become frequency independent, which yields the plateau. Hence amplitude plateaus measured in one animal at different places must begin at the same frequency, and must have a decreasing level with increasing x. This is in agreement with the experimental data.