When a small microphone is used in an ultrasonic beam, the direct acoustic signal is mixed with electric pick‐up and reflected signals. The use of pulses may not entirely remove these interferences. The mixing process is analyzed here for a particular situation and experimental evidence is adduced to support these conclusions: (1) that the main response pattern of the microphone shows space variations of both half the acoustic wavelength and the full wavelength; (2) that the former variation dies out at sufficiently great distances from the source at a rate showing that a single echo path (source‐microphone‐source‐microphone) is responsible along with a constant electric signal; and (3) that the three signals can be resolved analytically.