This paper proposes a microphone of a new principle which uses the optical reflection at the edge of a thin glass rod or a glass fiber. The optical reflection area changes by the change in the density of air due to the sound presence. The change ratio in the reflection area depends on the shape of the edge. This paper investigates theoretically the effect of the edge shape on the sensitivity for the spherical edge and the parabola type edge. The possibility of the practical use of the microphone is expected though the sensitivity is low. The sensitivity may be improved by the invention of edge shape and the detection method. This microphone is expected to have good high frequency response as it can be made small and has no mechanical vibration. The experimental study is now under preparation.
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