We develop a theoretical treatment of the effect of pressure fluctuations on the phase of an electromagnetic wave propagating through them. The pressure variations may be either acoustic or turbulent in nature. We then describe an experiment in which acoustic noise excitation of a weakly resonant system is measured by laser interferometry. Wide‐band acoustic noise energy is generated by a “free jet” emptying into a long pyrex pipe which is continuously pumped on by a large‐capacity mechanical vacuum pump. The acoustic noise propagates down the pipe into a region traversed axially by a laser beam in which acoustic resonances occur. The phase of the coherent light is measured relative to a stable path and is related to the density variations produced by the random acoustic field. The spectrum of the phase fluctuations in the path gives explicit information about the acoustic noise. Theory and experiment are compared.