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.
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