A temporally and spatially periodic light intensity distribution generates thermally in an absorbing sample a corresponding density variation which can be considered a standing sound wave. This thermal excitation of ultrasonic waves by modulated laser light has been investigated theoretically and experimentally. The sound waves were excited by two light beams of equal intensity derived from a mode‐locked ruby laser. The amplitude, velocity, and damping of the sound waves were examined by Debye Sears diffraction of argon laser pulses.