Measurement of Sound Velocity by using Phase Detection of an Optical Beat

Abstract

New methods for measuring sound velocity are described. Photo-electric detection of the first order spectrum in the double diffraction of a laser beam by two successive ultrasonic waves makes it possible to generate a light beat at a difference frequency of the two ultrasonic waves. From a theoretical discussion of the phase of this light beat three measurements of sound velocity are predicted. That is a comparison of two sound velocities one of which is known, the absolute measurement of the velocity in a small limited volume in the medium, and the instantaneous value of the velocity which changes with time. These methods are confirmed experimentally using an NaCl solution. A change in sound velocity down to 0.05ms^-1 can be detected when it is varying with time.