Phase comparison at high ultrasonic frequencies is found to be particularly suited to velocity measurements for small solid specimens such as single crystals. For moderate accuracy and a minimum of equipment a pulse overlap method is used for determining the frequency at which the path length contains an integral number of wave-lengths. For greater precision, and for measuring small changes in velocity (as a function of temperature, for example) balanced circuits with calibrated phase shifters may be employed. The use of 14 λ-plastic seals between driver and specimen minimizes phase errors at the reflecting interfaces.