An Analysis of the Parametric Acoustic Array for Spherical Wave Fields.
TEXAS UNIV AUSTIN APPLIED RESEARCH LABS
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The nonlinear interaction of two collinear acoustic waves to produce a modulation product in the lower sideband is examined both theoretically and experimentally. The modulation takes place in a portion of the medium specified by the beamwidth and the absorption of the primary radiations. The difference-frequency sound so created has significance in engineering applications because of the high directivity it derives from the great length of the zone of interaction. In this sense, the phenomenon may be thought of as a parametric, end-fired acoustic array. The theoretical approach of Westervelt is adapted to obtain solutions for the interacting fields composed to beams of monochromatic spherical waves emitted by circular piston projectors. The analysis is accomplished by an asymptotic method that is valid at long ranges from the interaction volume and by a more exact method involving a numerical integration that is valid in the farfield of the electroacoustic source. Beam pattern and sound propagation data resulting from several experiments conducted in a fresh water lake are presented to demonstrate the validity of the numerical solution. Author