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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.

Tympanic-membrane (TM) vibrations were studied in fresh human-cadaver specimens by means of timeaveraged holography. As with an earlier study in cats IS. M. Khanna and J. Tonndorf, J. Acoust. Soc. Amer. 51, 1904 (1972) ], the findings supported the curved-membrane hypothesis of Helmholtz. Despite a careful search for experimental artifacts, no satisfactory explanation was found to account for the fact that the present results are at variance with the stiff-plate concept of yon B(•k6sy. With respect to their quantitative evaluation, the human-cadaver data were not as clear-cut as the earlier cat data on account of anatomical differences. A number of points, which on first glance do not seem to agree with Helmholtz's concept are being discussed in the light of present or other recent findings, and explanations are being offered. These topics include: (1) the compliance of the TM; (2) the effects of differential air pressure across the TM upon middle-ear transmission; (3) the effects of TM perforations; and (4) the fact that Helmholtz's cadaver experiment, which was thought to support his hypothesis, could not be duplicated. SUBJECT CLASSIFICATION: 4.2, 4.2.1.The authors wish to dedicate this paper to the memory of Georg yon B•k•sy. labeled the "lower fold."In line with these findings, the transformer ratio of the middle ear was thought to be given by the product of the ossicular lever ratio, malleus to incus, and the area ratio, tympanic membrane to stapedial footplate. Since the lower fold contributes little if anything to the area ratio, the term "effective area" was coined. It referred to that portion of the TM which is tightly coupled to the manubrium. Its size was found to be about two-thirds of the total TM area (von B•k•sy, 1941; Wever et al. 1948). Recent findings in this laboratory (Khanna, 1970; Khanna and Tonndorf, 1972; Tonndorf and Khanna, 1970) cast doubt upon the general validity of the above concept of TM vibrations. Time-averaged holography, which allows to record the vibratory pattern of a given membrane as a whole at one instant (Powell and Stetson, 1965), showed that, at least for the case of cats, the TM is not displaced like a stiff plate. At low frequencies (f< 2000 Hz), the holographic findings were compatible with the "curved-membrane principle" of Helmholtz (1868), which the stiff-plate concept of von B•k•sy was though to have superseded. Helmholtz had argued, on the basis of the wellknown catenary principle, that the TM itself might act as a transformer. That is to say, larger displacements of lesser force in the middle of the anterior and posterior curved sections of the TM should lead to smaller displacements of larger force of the manubrium--and this was exactly what our own experiments had indicated, as least with respect to the changes in displacement.Helmholtz had attempted to verify his hypothesis by conducting a dc displacement experiment in a human cadaver specimen. His own positive results were not confirmed by Wever and Lawrence (1954), although they employed superior instrume...

A new, powerful, magnetostrictive bone conduction (BC) vibrator is described. At low output levels, it has a wide frequency range (5 Hz to above 20 kHz), and for midfrequencies undistorted outputs reach up to 50 g in terms of acceleration. BC thresholds and SL’s (the latter determined by AC/BC cancellations) were measured with this vibrator. Both varied with contact force, but much more markedly with contract area, in unoccluded as well as in occluded ears. Sound attenuation boxes, after von Békésy, were used in the unoccluded state. Results of the AC/BC cancellations permitted determination of the external-ear and the inner-ear BC components. With the ear unoccluded, variations with contact area (and/or force) were mainly related to changes of the inner-ear responses, but with the ear occluded, they were related to those of the BC-generated sound pressures in the ear canal for f&lt;2 kHz. At AC and BC threshold, SPL’s in the ear canal had identical values and did not vary with contact area or force for f&lt;2 kHz. For a comparable small contact area, BC acceleration thresholds were similar to those specified in ANSI S3.13−1972. For larger contact areas, however, they were lower, by as much as 25 dB, at some frequencies. These results indicate that acceleration is not a valid indicator of the BC input to the skull. Subject Classification: [43]65.22, [43]65.80, [43]65.64.

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