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Proposal of Optical Wave Microphone and Physical Mechanism of Sound Detection

An optical wave microphone with no diaphragm, which uses wave optics and a laser beam to detect sounds, can measure sounds without disturbing the sound field. The theoretical equation for this measurement can be derived from the optical diffraction integration equation coupled to the optical phase modulation theory, but the physical interpretation or meaning of this phenomenon is not clear from the mathematical calculation process alone. In this paper the physical meaning in relation to wave-optical processes is considered. Furthermore, the spatial sampling theorem is applied to the interaction between a laser beam with a small radius and a sound wave with a long wavelength, showing that the wavenumber resolution is lost in this case, and the spatial position of the maximum intensity peak of the optical diffraction pattern generated by a sound wave is independent of the sound frequency. This property can be used to detect complex tones composed of different frequencies with a single photo-detector. Finally, the method is compared with the conventional Raman-Nath diffraction phenomena relating to ultrasonic waves. AES 135th Convention Best Peer-Reviewed Paper Award Cowinner

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