Cancellation of air-borne acoustic plane waves obliquely incident upon a planar phased array of active surface elements

Abstract

The active reduction of sound reflected off an object is of much interest to researchers in acoustics. The approach 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 localized (point) sources whose strengths, phases, and locations can be adjusted. The work described herein involves extended surfaces which react to an incident wave in a prescribed manner. If this reaction is such that the surface radiates a wave 180 ø out of phase with and of the same amplitude as the reflected wave, the reflected wave is canceled. This paper reports on the construction and testing of a planar active surface capable of canceling the reflections of obliquely incident acoustic plane waves. Theoretical analysis is given, and 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 sound wave varies over an extended surface, an array of transducers is required to accomplish the desired control. The active surface described herein has the unique feature that the wave number and frequency of surface vibrations may be independently controlled. The surface was shown to perform well for 30-to 50-kHz sound. This technique also should be applicable to lower frequency sound, which is not easily controlled passively.

A technique for modelling multiple piezoelectric layers

“…Howarth et al [8,9] are the pioneers in introducing a piezocomposite active coating for underwater sound reduction that includes both piezoelectric polymer sensors and a piezocomposite actuator encapsulated with an impedance matching elastomer. The reflection of a plane harmonic wave propagating in an acoustic fluid and impinging obliquely upon a submerged laminated piezoelectric plate was investigated [10][11][12]. An array of transducers is required to accomplish the task because the phase of an obliquely incident sound wave varies over the surface.…”