Abstract
The change of the effective volume of the auditory canal caused by a strong sound was studied with the aid of a small acoustic pipe of T form. The pipe was held against one ear while an earphone was set on the other. First, the normal effective volume of the auditory canal was measured when no sound was introduced. The measurement was repeated with the strong sound issuing from the earphone. A change in the effective volume was observed when the sound intensity reached about 80 db. With stronger sounds the change was in proportion to the increase of the intensity of this auxiliary sound. In the lower frequencies below ca. 600 cps however the effective volume actually decreased, for instance by about 0. 1 cc at 400cps when the sound intensity was 120 db, as contrasted to an increase of about 0. 1 cc at 1000 cps with the same intensity. This is in spite of the fact that the resonance frequency of the middle ear mechanism remains unchanged at about 800 to 1200 cps. These phenomena may be explained by the increase of resistance in the middle ear mechanism. The influence of the change of the effective volume on the sense of hearing was measured. A bone-conduction and an air-conduction receiver were attached to one ear, and to the other ear an air-conduction earphone for emitting strong sound. First, the bone and air-conducted tones were adjusted to the same level as perceived by the one ear. Next, the strong sound was introduced to the other ear and again the bone and air-conducted tones were balanced to the same hearing level by means of attenuators. Perception of both the air and boneconducted tones was dulled with the accompaniment of the auxiliary sound when it exceeded about 80 db, the extent of this attenuation being more pronounced in the case of the bone as compared to the air-conducted tone. This relative difference in perception between bone and air couducted sound increased with the intensity of the auxiliary sound, being about 4 db at 100 db and 7 to 10 db at 120 db. This effect changed little with the frequency of the strong sound. The decrease of the sensitivity of both air and bone-conducted sound is considered to be due to the action of the inner ear.
