Theoretical investigation on the sensitivity of a microphone using the change in the total reflection of light by sound

Abstract

A new microphone, which uses the optical total reflection on the boundary surface between glass and air, has been proposed. The intensity of reflected light changes by the sound pressure, as the fluctuation of air density due to sound changes the refractive index of air and the change in the refractive index changes the angle of total reflection. The sound pressure can be measured as the change in the intensity of reflected light. In this paper we investigate theoretically the sound pressure sensitivity of the proposed microphone in certain types of the shape of curved boundary surface. As the sensitivity can be improved considerably by employing a plane wave light source, such as laser light, and the curved surface with a large radius of curvature, the microphone is expected to be practically used. The microphone detects the sound pressure without mechanical vibration and can be made small, so its upper limit in the frequency range is very high in principle.