Determination of acoustic reflex response morphology requires an aural acoustic immittance instrument capable of accurate measurement of complex immitance. Unfortunately, conventional analog instruments are either (1) incapable of measuring complex immittance or (2) do so inefficiently and with long time constants. Accurate data can be obtained, however, with a digital immittance instrument. Acoustic reflex response measurements were obtained using a digital acoustic immittance instrument (226-Hz probe tone) and a laboratory computer. Activating signals (1-s duration) were four tones and broadband noise presented in 5 dB increments (maximum 120 dB SPL) from − 5 to + 30 dB re: acoustic reflex threshold (ART). Ten subjects were tested per decade of age from 20 years through 79 years. Results for the 2-kHz tone and BBN at 20 dB re: ART indicated that (a) slope of onset increased with signal level, (b) response slope during signal presentation was greater for noise than for the 2-kHz signal, (c) baseline, attack time, onset latency, and amplitude of maximum response decreased for the oldest age group, and (d) release time and recovery time increased for the oldest age group. [Work supported by Rehabilitative Research and Development Service of Veterans Administration.]
Intelligibility tests using a Modified Rhyme Test were performed with commercially available hearing aids with various values of time constants of compression and of harmonic distortion during overshoots. For normal-hearing subjects, signal-to-noise ratios of −2 and +2 dB were used. Subjects with sensorineural hearing impairment listened in quiet at their most comfortable listening level and at two additional sound-pressure levels (10 and 20 dB lower than the first one). Different hearing aids affected the performance of both groups of listeners similarly. At low signal-to-noise ratios or low sound-pressure levels, better performances were observed for shorter time constants of compression and larger compression ratios. At higher signal-to-noise ratios or higher sound-pressure levels, the occurrence of harmonic distortion during overshoots deteriorates the performance, with greater deterioration occurring for impaired hearers than for normal hearers. [This research was supported by the NIH Biomedical Sciences Support Grant (FH 07088) and by the Prosthetic and Sensory Aids Service of the Veterans Administration.]
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