Acoustic spiral wave field

While most low-amplitude acoustic waves in uniform media can be considered to travel straight from source to receiver, the energy flow and propagation directions in a spiral wave field change as the wave moves away from its source [P. H. Ceperley, Am. J. Phys. 60, 938–942 (1992)]. Thus, one would expect that directional microphones and acoustic imaging systems would give false information as to the location of the source in such a wave field. Two synchronized oscillators, four small speakers, and a horn were used to create a spiral wave in the laboratory. A table top robot arm and microphone were used to map out this sound field, which was then displayed in color using MathCad. The apparent propagation direction was measured using a directional microphone. [Work supported by ONR.]