An experimental study of parametric acoustic arrays with intermediate directivity in water

Abstract

Intermediate difference frequency directivity for the parametric acoustic array is identified with the transition from narrowbeam (Rutherford scattering) directivity to broadbeam (primary product pattern) directivity. Experimental data are presented in which parametric arrays with intermediate difference frequency directivity were formed for several primary frequency bands, step-down ratios, primary source configurations, and directivity ratios. The experimental results compare well with theoretical predictions. The difference frequency half-power beamwidth with intermediate directivity was observed to always be greater than the computed narrowbeam and broadbeam results; and low sidelobes, which are characteristic of intermediate and broadbeam directivity, were observed in the difference frequency beam patterns. Parametric acoustic source performance with intermediate directivity was demonstrated in the nearfield of the parametric array with a modest decrease in directivity and apparent source level as compared to farfield conditions.