An acoustic field which varies sinusoidally in time is completely determined by the complex values of its pressure on two concentric spheres. We have developed an experimental procedure which carries out such a measurement. If the source of sound is located inside the inner sphere, we experimentally obtain an expansion in spherical waves whose outgoing and incoming (that is, reflected) components are independently determined. This paper describes both the apparatus and the underlying theory, and presents illustrative results on the wave reflected from the wall of an anechoic chamber.