Factors influencing the acoustic-immittance characteristics of the acoustic reflex.

Measurements of the aural acoustic-immittance (admittance and impedance) characteristics of the middle-ear transmission system in humans during the quiescent (static) and reflexive states were made (N = 36) utilizing a signal-averaging technique. Three pure tones (750, 1000, and 2000 Hz) and broadband noise stimuli elicited the acoustic reflex in 2-dB steps at sound-pressure levels from 84-116 dB (tones) and 66-116 dB (noise) during ascending- and descending-intensity level runs. The contralateral middle-ear activity was monitored with a 220-Hz probe by digitizing the conductance and susceptance outputs of an admittance meter. A computer corrected for the ear-canal volume utilizing measurements made at ear-canal pressures of 0 and --350 daPa and then converted the conductance and susceptance values into admittance and impedance units. The results were reported in absolute and relative immittance units, including components, as a function of both stimulus sound-pressure level and intensity level above the acoustic-reflex threshold. The static immittance of the middle ear changed nonlinearly over time to lower admittance or higher impedance values. The influence of this static-immittance shift on the reflex magnitude was discussed. The largest mean reflex magnitude and the slowest rate of growth were observed with broadband noise, although eight of the 36 subjects demonstrated the largest reflex magnitude in response to one or more of the tonal stimuli. Although static-immittance values and acoustic-reflex thresholds were poorly correlated, the reflex magnitudes were proportional to static immittance. The variability of the reflex measures was similar to the variability of the static-immittance values. Finally, bi-directional changes in resistance during the reflexive state were observed and discussed.