Some techniques for measuring the velocity of sound in single‐crystal materials utilize multiple internal reflections of an ultrasonic pulse between two plane parallel surfaces of the specimen. If a thin layer of cold worked material is present at these surfaces, it is possible for the initial pulse to be distorted upon successive reflections in a way which would make the apparent transit time too short. The characteristic distortion of the initial pulse is shown in an unrectified oscillogram and consists of a forerunner pulse with a dominant frequency controlled by the thickness of the cold worked layer. Velocity determinations based on the arrival time of this forerunner will be too high. In the example considered the error amounts to about ½ percent for cold worked surface layers approximately 0.005 in. thick.