Tympanic Muscle Effects on Middle-Ear Transfer Characteristic

Abstract

The change in the magnitude and phase of sound transmission through the guinea-pig open-bulla middle ear is measured when two levels of independent, isotonic, tympanic muscle contraction are effected by direct electric stimulation of the muscle bodies. Both middle-ear muscles attenuate the passage of low-frequency vibration giving the largest attenuation for frequencies below 300 Hz, the tensor tympani producing 28 dB maximum attenuation, the stapedius producing 10 dB. In the 1- to 3-kHz frequency range, both muscles are capable of generating an apparent gain in transmission: the tensor tympani giving a gain of 5 dB at 2.5 kHz for maximum contraction, the stapedius 0.5 kHz at 2.5 kHz for maximum contraction. The phase shifts for all contraction cases were leading phase functions. Changes in the magnitude and phase of the transmission are modeled by a second-order low-pass system where the break-frequency increases and the damping ratio and low-frequency magnitude decrease with muscle contraction. The transfer function with maximum tensor contraction had a break frequency of 2.5 kHz with a damping ratio of 0.31, while the stapedius maximum contraction gave a break-frequency of 1.57 kHz and a damping ratio of 0.71. These transmission changes are accounted for the most part, in the model, by a decrease in compliance of the middle ear, but there are also increases in resistance and inertance over the flaccid muscle state.