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Influence of phonation on high-intensity sound transmission in the auditory system McBay, Heather Dorrelle
This investigation studies the effect of phonation and of some activities eliciting middle-ear muscle contraction on high-intensity sound transmission in the normal human auditory system. For the most part it is concerned with the influence of phonation on TTS from a continuous pure-tone stimulus. The main experimental technique consisted of measuring subjects' hearing thresholds before and after a 5 min, 500-Hz, 118-dB SPL exposure, this exposure being sometimes accompanied by the performance of a specific activity such as phonation. Threshold tracings were obtained by using a Békésy-type procedure, and threshold was measured at 7 times after cessation of the exposure tone. Analysis of the results indicates that TTS from phonation (humming) during exposure was significantly less than TTS from the exposure tone without any supplementary activity, for a variety of humming activities: humming at 125 Hz (males) or 250 Hz (females); humming loudly at these same frequencies; or humming at 250 Hz (males) or 500 Hz (females). TTS from humming loudly and humming at the higher frequencies was consistently, although not significantly, less than TTS from humming at the lower frequencies. For females, phonation (humming during exposure was more effective in decreasing TTS than for males. Repeatedly turning the head during exposure, which is believed to elicit MEM contraction, resulted in less TTS than no activity during exposure. Similar slight decreases in TTS were observed when the following activities which elicit middle-ear muscle contraction were performed during exposure: chewing, smiling, swallowing. Listening to recorded humming during exposure did not significantly alter TTS from the exposure. The activity of exhaling after preparing to hum did not significantly alter TTS from the exposure. In addition to the TTS studies, measurements of acoustic impedance during the exposure tone and of acoustic reflex thresholds were obtained. Various hypotheses concerning causes for the reduced TTS from phonation during exposure are discussed. Attenuation provided by middle-ear muscle contraction during phonation does not appear sufficient to decrease TTS to the extent that humming does. Sound may be attenuated by inefficient stapes vibration during phonation and TTS may therefore be reduced. Two other possibilities are suggested to account for the TTS decrease: interference between humming and the exposure tone; and interference (by humming) with the central control of middle-ear muscle activity. More evidence will be necessary to satisfactorily determine which, if any, of these mechanisms is actually in effect.
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