Experiments on the Bone-Conduction Threshold in a Free Sound Field

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Abstract
Earlier investigators of the free-field bone-conduction threshold occluded the auditory meatus with plugs and muffs and reasoned by various arguments that the real-ear attenuation obtained was less than the attenuation provided for air-conducted sound. The conclusion was that the sound perceived reached the auditory system via bone conduction. The present experiment''s checked these conclusions for the frequencies from 1000 to 8000 cps. With the auditory canal occluded by plugs and muffs the whole skull, or sections of it, was covered with a 2-inch layer of absorbent cotton. The muffs were not covered. A higher free-field threshold with the cotton indicated that without cotton the sound was reaching the ear via bone. Cotton over the entire head raised the bone-conduction theshold by 10 to 18 db from 2000 to 8000 cps. Bone-conduction thresholds determined previously were confirmed from 1000 to 8000 cps. The front and back of the head were approximately of equal sensitivity with respect to bone conduction. Covering only the side of the head facing the sound source raised the threshold more than covering the "shadow" side alone.

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