Influence of Loud Contralateral Stimulation on the Threshold and Perceived Loudness of Low-Frequency Tones

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Abstract
Two experiments were performed to measure the attenuation due to the acoustic reflex at intensities near threshold. In the first experiment subjects tracked the threshold for tones at 100, 500, and 1000 cps continuously. A 100 db 2000 cps tone was then suddenly introduced into the opposite ear for 30 or 90 sec. Threshold shifts observed were inconsistent with past experiments on the acoustic reflex, which had shown appreciable attenuation of intense sounds. In order to rule out the factor of subject response lag, a second experiment was performed in which thresholds were determined by the method of limits for brief tones at 125, 500, and 1000 cps with and without concomitant contralateral stimulation with a 2200 cps, 105 db tone. Again, threshold shifts were small. It was hypothesized that the acoustic reflex may differentially attenuate loud and soft tones. To investigate the influence of the reflex on loud tones, 2 experiments were performed. In the first, 11 observers were presented with a series of 500 cps pulses, 50 msec. in duration, separated by 1-sec. intervals. Immediately before and during alternate pulse bursts (test tones), a 2200 cps tone loud enough to activate the acoustic reflex (activating tone) was introduced in the contralateral ear. Observers were asked to judge whether these pulses were equal, fainter, or greater in loudness than the alternate pulses (comparison tones) presented without the contralateral activating tone. The same procedure was performed with the activating tone absent. Results indicated that the activating tone had no measurable effect at 70 and 75 db. The tones were almost always reported as equal in intensity. At higher levels, however, there were significantly fewer reports that the tones were equal and significantly more reports that the comparison tone was louder, and these tendencies significantly increased as the level of the test and comparison tones were increased. In the second experiment, a point of subjective equality for test and comparison tones was determined for 14 listeners at levels of test tones ranging from 70 to 105 db. Apparently attenuation was increased (i.e., a relatively louder comparison tone was required to match the test tone) as the level of the test tone was increased. Although the results of both experiments supported the experimental hypothesis, a possible alternative explanation is discussed.

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