Basilar membrane and middle-ear vibration in guinea pig measured by capacitive probe.

Abstract

Measurements of amplitude and phase were made on the basilar membrane and middle ear of guinea pigs. A subminiature capacitive probe giving wide dynamic range and linearity was used in conjunction with a slave filter to provide continuous frequency response records and a lock−in amplifier to provide high−sensitivity measurements on the basilar membrane down to 40 dB SPL. The results confirm the general features of previous measurements using various optical and Mössbauer techniques but also demonstrate that the mechanical tuning properties of the basilar membrane are equally poor at low SPLs and under excellent physiological condition of the cochlea, as indicated by the threshold of the gross cochlear action potential to clicks. The basilar membrane response conforms to the differential cochlear microphonic rather than the much sharper single cochlear nerve−fiber threshold curve. Linearity of response at the peak of the mechanical curve was observed over an 80 dB range: significant nonlinearities were observed only for frequencies on the high−frequency slope, and then only for sound levels above 110 dB SPL. The existence of an amplitude and phase plateau was confirmed. The peak basilar membrane/incus ratio averaged 29 dB independent of cutoff frequency over a range from 16−43 kHz, with an rms vibration amplitude at the peak of 500 Å at 100 dB SPL over cutoff frequencies in the 16−26−kHz range. Subject Classification: 65.20, 65.26, 65.24.