Sound Absorption at the Surface of the Body of Man and Animals

Abstract

The results of measurements of sound absorption by the body surface of man and fur-bearing animals are reported for the frequency range 100 to 12,000 c.p.s. The acoustical impedances and the absorption coefficients of the surfaces were determined from the resonance characteristics of an air-filled tube. The end of the tube was closed first by a rigid wall and then by the unknown surface. From the change in the resonance characteristic induced by the unknown surface the desired quantities were calculated. In order to speed up the measurements, which is important with living test material, a second method was used. In this method the air column was excited by sound pulses, and the decay time of the sound pressure was measured from the photographs of the cathode-ray trace. At the higher frequencies an aluminum rod, pressed against the body surface, replaced the air column. The desired quantities were calculated from the change in longitudinal resonance characteristics in a manner similar to that employed for the air column. The absorption coefficient of the human body surface varies frown 10 percent to 0.1 percent and decreases at higher frequency. The resistances per cm2 varies between 103 and 105 ohms and are dependent upon the area of application and the pressure upon the skin and muscles. The measurements on rats show that up to 1000 c.p.s. the absorption coefficient decreases and then increases up to 6000 c.p.s. The behavior below 1000 c.p.s. is determined largely by the tissues beneath the skin while the increased absorption above 1000 c.p.s. is the result of the presence of the fur. This is demonstrated by measurements on shaved animals where the absorption decreases steadily as the frequency increases, whereas in the case of furred skin supported on a rigid wall the absorption increases as the frequency increases. These absorption coefficients permit the estimation of the sound energy absorbed by a whole animal. The sound intensity required to cause heat death can then be calculated approximately.