Heterodyne detection of a monochromatic laser beam scattered by molecules in a transparent gas gives a signal that is proportional to the spatial Fourier transform of density fluctuations for a wave vector determined by the optics. In a turbulent air jet, density fluctuations are due to either acoustic waves or to entropy fluctuations. This technique captures both. The method can then be used as a nonintrusive microphone to study the propagation of acoustic waves inside the jet. At each point in the flow the direction of propagation of the acoustic wave is determined, and the acoustic field can be described. This will eventually help to localize the sources of the waves in the flow and hopefully determine the hydrodynamic event that generates these waves.