Hearing in the frog: dynamics of the middle ear

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Abstract
The motion of the amphibian eardrum in response to acoustic stimulation was investigated, by means of laser speckle vibrometry. We first demonstrate that the vibration of the eardrum can adequately be described as a damped harmonic oscillator, with resonance frequency at about 1800 Hz and a mechanical resonance Q of 6.2. By diffusing helium in the mouth cavity or by forcing the mouth open, we then show that the resonance characteristic of the eardrum is mainly due to the middle ear cavity, composed of the Eustachian tubes and mouth cavity. Together they act as a Helmholtz resonator. Finally, we present evidence for intertympanic coupling. The vibration of one eardrum causes a concomitant motion of the other. It is postulated that this coupling forms the basis for the observed variation in vibrational amplitude of the tympanic membranes according to the direction of incident sound.