A method for determining the change in elastic constants as a function of some parameter such as temperature or pressure is described. The method is a modification of the pulsed radio frequency sound technique, in which the echoes are mixed with a signal from a free‐ringing quartz crystal in order to extract small phase changes in the echoes. Conventional ultrasonic attenuation equipment, which ordinarily is too imprecise for such work, is utilized. The precision is shown to be about 2 parts in 105.