An apparatus is described for measuring acoustic wave fronts in water at frequencies in the vicinity of 1 mc. The sound waves are radiated by a piezoelectric crystal and are picked up by a movable microphone. The voltage output from the microphone is compared in phase with the voltage feeding the piezoelectric source, by means of a phase discriminator. The signal at the microphone is due both to the transmitted acoustic wave and a stray electric signal. These two are combined in the microphone in such a way as to produce a full wavelength modulation in the resultant electric signal. A wave front in the sound wave is then traced by moving the microphone along such a line or surface as will cause the phase discriminator to indicate constant phase difference. A mathematical analysis of the problem is given and the method of measuring the shape of acoustic wave fronts in water is described. The apparatus and method which are described reduce the time required for measuring wave fronts to about one‐third that previously needed. Typical experimental results are given.