Tympanic muscle effects on middle-ear transfer characteristic
Alfred L. Nuttall · The Journal of the Acoustical Society of America · 1974
Using the cochlear microphonic potential from the basal turn as an indicator, changes in the magnitude and phase of sound transmission across the open-bulla middle ear of the guinea pig were determined for two levels of isotonic tympanic muscle contraction. Muscle action was produced by independent electric stimulation of the muscle bodies. Both the stapedius and the tensor tympani muscles attenuated the passage of low-frequency vibration giving the largest attenuation for frequencies below 300 Hz; the tensor tympani produced an attenuation of 28 dB or more; the stapedius produced 10 dB or more. For some frequencies, in the range 1 to 3 kHz, both muscles were capable of generating an apparent gain in transmission; the tensor tympani causing a gain of 5 dB at 2.5 kHz; the stapedius 0.5 dB at 2.5 kHz. The phase shifts of the cochlear microphonic potential relative to the sound stimulus were leading phase functions for all contraction cases. A second tympanic muscle contraction level, of lower magnitude, gave qualitatively similar but smaller results. Changes in the sound-transmission magnitude and phase were modeled with second-order low-pass system characteristics in which the break frequencies increase, the low-frequency magnitude decreases and the system becomes underdamped with muscle contraction. These transmission changes are accounted for in the model primarily by a decrease in the compliance of the middle ear, but small increases in resistance and inertance over the flaccid muscle state were also indicated.