Abstract

The directional characteristics of a piston mounted on a surface of zero impedance are studied. Zero and infinite acoustic impedance surfaces represent two extreme cases; although the most typical surfaces have infinite impedance. Zero acoustic impedance can be produced at discrete frequencies by using quarter-wave resonators. A two-dimensional model was simulated by multipole synthesis, which was then confirmed using experimental results. Because energy cannot flow tangentially on a surface with zero impedance, the directivity pattern was greatly sharpened. The results support the idea of using surface impedance as a means of controlling loudspeaker directivity patterns.