A new ultrasonic interferometer for velocity measurement using optical diffraction and an optical heterodyne system is described. The velocity of pure water at 20 degrees C has been measured in the frequency range 0.5-5 MHz, and the velocity obtained increases with decreasing ultrasonic frequency. In order to clarify the behaviour, a new theory for the ultrasonic interferometer is given and the usefulness of the instrument is shown by using it to measure the velocity dispersion in benzene.

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