The effects of aging on the magnitude of the acoustic reflex.

Aural acoustic-immittance (admittance and impedance) measurements during the quiescent and reflexive states were made using a computer sampling technique on 18 subjects with normal hearing in each of two age groups (less than 30 years and greater than 50 years). Seven pure-tones (250-6000 Hz) and broadband-noise stimuli served to elicit the acoustic reflex at sound-pressure levels from 84-116 dB (tones) and 66-116 dB (noise) in 2-dB steps during ascending and descending runs. The contralateral middle-ear activity was monitored with a 220-Hz probe by digitizing the conductance and susceptance outputs of an acoustic-admittance meter. The computer corrected for the immittance characteristics of the ear-canal volume by utilizing measurements made at an ear-canal pressure of -350 daPa and then by converting the conductance and susceptance values into admittance and impedance units. The results are reported as the immittance change between the quiescent and reflexive states as a function of both the activator sound-pressure level and the activator-pressure level above the reflex threshold. There were no significant differences between the static-immittance values for the two groups. Although acoustic-reflex thresholds for the two groups were the same in the low- to mid-frequency region (250-2000 Hz), the reflex thresholds for the greater than 50-years group were elevated significantly (approximately 8 dB) for 4000 Hz, 6000 Hz, and noise activators. In all conditions, the magnitude of the acoustic reflex was substantially smaller for the greater than 50-years group as compared with the less than 30-years group. The variability of the reflex magnitude was large for both groups of subjects. Saturation of the individual growth functions, which was frequency dependent, occurred twice as often with the greater than 50-years group as with the less than 30-years group. The relationship between the magnitude changes in conductance and susceptance from the quiescent to the reflexive state was the same for the two groups. Finally, the magnitude differences among the reflex-growth data were not related to differences in static immittance.