Study of the Acoustic Reflex in Human Beings. I. Dynamic Characteristics

The contralateral acoustic reflex, elicited by wide-band noise and pure tones whose frequencies ranged from 200-2000 Hz, was measured and studied by using dynamic recordings of the change of the input acoustic impedance of the eardrum. Step and sinusoidal (envelope) stimuli with the above carriers at different sound pressure levels (SPLs) were used, and four human subjects with normal hearing were tested. It was shown that the dynamic properties of the acoustic reflex are both frequency- and magnitude-dependent. In general, within the dynamic range of the reflex, the speed of on-response (with step input) seemed to be proportional to the SPL of the stimulus. This proportionality was more clearly observed in the on-response of the reflex elicited by stimuli having pure tone carriers than wide-band noise carriers. The sinusoidal responses of the reflex tended to indicate that the apparent bandwidth of the reflex mechanism increases with SPL. The most distinct frequency-dependent characteristic of the reflex was observed in the on-re-sponses of the reflex with different carriers, namely an underdamped oscillatory response was seen when the carrier frequency fell between 400-800-Hz range and an overdamped on-response outside this range. It was suggested that the magnitude dependence of the reflex could be attributed to a magnitude-dependent nonlinearity of the muscle mechanism, while the frequency-dependent characteristic was due to frequency-dependent attenuation of the sound transmission due to the activity of the acoustic reflex.