Measurement of the Velocity of Sound in Water by Optical Methods

Upon passing through a sound beam, a collimated light beam becomes modulated in phase. The resulting interference produces an amplitude modulation of light that may be detected by a photomultiplier. This amplitude modulation has a spatial periodicity very nearly equal to the wavelength of the sound. The sound velocity may be determined by measuring a translation of the transducer and number of wavelengths indicated by the photomultiplier output. The nature of the systematic error produced by the free‐field diffraction is a quasiperiodic function of the distance from the transducer; being able to account for this error permits accurate optical determinations to be extended to frequencies below 1 MHz. [This work was supported by the Office of Naval Research and the National Science Foundation.]