Tympanic Membrane Vibrations in Cats Studied by Time-Averaged Holography

Abstract

The vibratory characteristics of the cat's tympanic membrane (TM) were studied in fresh cadavers and living animals with the aid of time-averaged holography. Initially, technical difficulties were presented by the TM's small size, low stiffness, and its transparency. Time-averaged holograms of the TM were obtained for various sound-pressure levels at a number of frequencies between 600 and 6000 Hz. Reconstructions of such holograms show an image of the TM superimposed by isoamplitude contour lines giving the absolute displacement amplitude at any point on the TM. Vibratory patterns remain essentially unchanged in their first mode up to a frequency of 2000 Hz, with higher modes occurring above that value. At all frequencies, the TM vibrates maximally in the posterior-superior quadrant and less in the anterior and inferior ones. These results show that, even in its first mode of operation, i.e., below 2000 Hz, the TM does not vibrate like a stiff plate as observed by von Békésy in human cadavers. Rather, the present findings support the curved-membrane concept of Helmholtz. Contributions to the total transformer action of the middle ear by the area ratio: TM/stapes footplate, the ossicular lever ratio, and that due to the curved membrane action are reevaluated.