音響外傷のC₅ dipと外耳の集音機能

抄録

Sound pressure amplifying function of the external ear was measured by using a probe tube microphone as the ratio of sound pressure at the tympanic membrane to field sound pressure at the position of the helix of the pinna. The maximum ratio was found as much as 28 to 21dB at frequencies of 2, 500 to 3, 000Hz in four persons with normal hearing. The frequency of this maximum ratio corresponded with the resonance frequency of the air contained in the external ear as a horn calculated from its dimension because the resonance frequency varied with individual variation of the dimension of the external ear in the tested persons. This maximum ratio was found to increase vibration amplitude of the tympanic membrane as much as this ratio in dB when the ratio of sound pressure at the tympanic membrane to field sound pressure was measured at the threshold of the same person. Therefore, this maximum peak of sound pressure amplifying function of the external ear was expected to cause a distinct dip in an audiogram of aural fatigue after exposing the ear of the tested person to an intensive white noise of flat spectrum of 87.5±2.5dB produced by a loud speaker.
The maximum dip in a detailed audiogram of aural fatigue was found to be about a half octave higher than the resonance frequency of the external ear which had been measured by a probe tube microphone. Moreover, C₅ dip appeared when the detailed audiogram was replotted exclusively according to the test tones of a clinical audiometer. It was concluded that the sound pressure amplifying function of the external ear at the resonance was the cause of C₅ dip in acoustic trauma.