Cancellation of Air-Borne Acoustic Plane Waves Obliquely Incident upon a Planar Phased-Array of Active Surface Elements

The active reduction of sound inside and outside an enclosure is of much interest to researchers in acoustics. The approach most often used is to control the sound in a given region (sensed by a series of point microphones) with destructive interference from a set of point sources whose strengths, phases, and locations can be adjusted. However, the Active Surfaces Research Group at the National Center for Physical Acoustics has designed surfaces which react to an incident wave in a prescribed manner. If this reaction is such that the surface moves with a velocity that coincides with the normal particle velocity of the incoming wave, reflections are eliminated. Active surfaces have been built by this group for the control of normally incident plane waves. This dissertation reports on the construction and testing of a planar active surface capable of cancelling the reflections of obliquely incident acoustic plane waves. Theoretical analysis of the potential performance of the surface is given. The viscosity and thermal conductivity of the medium in which the surface operates are found to have negligible effect on the surface's performance. Since the phase of an obliquely incident wave varies over the active surface, an array of transducers is required to accomplish the desired control. The surface was shown to perform well for 30 -50 kHz sound. The cancellation covers a broad angular range.