The Variation of Phase Along the Basilar Membrane with Sinusoidal Vibrations

Abstract

An optical means of measuring phase relations in the cochlea is described. With the stapes set into sinusoidal vibration, the phase relations, measured along the length of the cochlear partition, show that a traveling wave has been set up. The form of resonance which appears does not correspond at all with that to be found in a simple vibrating system. Measurements are presented for a frequency of 200 c.p.s. of the amplitude and phase of vibration at various places along the cochlear partition. From these is derived a curve showing the longitudinal bending of the partition with the traveling wave at two different points in time. Very small changes in the form of vibration can easily be demonstrated by the measurements of phase. By this means it is shown that the basilar membrane cannot be released from tension since it is not normally under tension. The elastic properties of the various distinguishable tissues within the cochlear partition were measured. These measurements show that only the elasticity of the basilar membrane changes through the length of the cochlear canal. The other portions of the partition are quite constant throughout the entire distance. Further, the non-linearity in the vibration of the partition was investigated with results which indicate that the observed lowering of pitch of a tone when the loudness is increased cannot be produced by mechanical means.