Measurement of Audible Acoustic Wave by Laser Light

The experiment to detect the acoustic wave of an audible frequency region in air by a visible laser beam was carried out, showing that the audible waves with relatively long wavelength from 34 mm to 1700 mm could be directly detected by using a diode laser light of 670 nm and 6 mW. The frequency property was measured for each sound pressure from 75 dB to 100 dB and it is shown that the amplitude of output signal is linearly proportional to the sound frequency, which is consistent with the theoretical prediction. Furthermore, property of directivities in the planes vertical and parallel to the optical axis of laser beam propagation and the spatial profiles of the laser diffraction pattern for each incident angle of a sound wave were measured. The directivity property in the vertical plane for each frequency (200Hz to 10kHz) is very similar to each other and the full angle of half-maximum of directivity is about 120°. On the other hand, the directivity in the parallel plane for each frequency is different from each other. The directivity for low frequency is relatively broad. The full angle of half-maximum of directivity for 10 kHz is smallest and about 90°.