Hot-wire measurements in the free stream of a supersonic wind tunnel were made in the Mach number range of 1.6 to 5.0. I t is shown that the mass-flow fluctuations increase very rapidly with increasing Mach number. If the fluctuation field is assumed to consist of sound waves—an assumption tha t is consistent with the measurements—the sound intensity is approximately proportional to M, within the range of the experiments. Furthermore, the orientation of the field is found to be different from the Mach line direction; it corresponds to a sound-source velocity of approximately one-half the free-stream velocity for the higher Mach numbers. I t is shown that the turbulent boundary layer along the nozzle and the tunnel walls is responsible for this sound field.