円形導波管内を伝わる音波と電磁波との相互作用

概要

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A plane electromagnetic microwave propagating through dielectric media can be little affected by a small perturbation of the relative permittivity of the media. However, in a waveguide the electromagnetic wave can be observed to reflect around the cut-off frequency of the waveguide by very small perturbations. The microwave is reflected by medium which is perturbed by the sound wave, and the application of this phenomenon can be used a new type microphone. First, the propagation characteristics of the sound wave in a circular waveguide was investigated theoretically and experimentally. The theoretical value and the experimental one was in very agreement, and that the sound wave in the circular waveguide maintaines a piston motion at less than frequency of 4,458Hz. At the higher frequencies, however, higher modes of non-piston motion may appear in the guide. In order to enlarge the microwave signals reflected by the sound wave, it is effective to use the critical frequency at which the differrential coefficient of the reflection coefficient of the microwave with respect to the incident frequency becomes large value. The results of the experiments show that an amplitude of the microwave reflected by the sound wave is not flat in the range of audio frequencies, but similar to the Fletcher's loudness contours. Therefore, these characteristics are so effective as a microphone that the aural sensitivities of human can be compensated.
山梨大学の論文