Acoustic saturation of spherical waves in water

Abstract

The acoustic transmitting capacity of a medium is limited by nonlinear propagation effects. As the source amplitude is increased to very high levels, acoustic saturation sets in, a state in which the amplitude at a field point approaches a limiting value, independent of the source amplitude. A theoretical and experimental study of saturation of spherical waves is presented in this paper. Predicted saturation amplitudes for the fundamental component in an originally sinusoidal wave are obtained by matching the well known weak-shock solution to a linear-theory solution at the old-age distance rmax. The saturation formulas are verified by experiments done in fresh water with a 3-in.-diam piston projector operating at 454 kHz at source levels up to 135 dB re 1 μbar at 1 yd. Amplitude response curves were measured on the acoustic axis at six distances ranging from 0.76 yd to 111 yd. At the longer ranges the curves become isotonic, thereby demonstrating saturation. High-amplitude beam patterns show the effect of finite-amplitude attenuation in that the major lobe is broadened and minor lobe suppression is reduced. Finally, the saturation formulas are used to develop curves of maximum SPL as a function of range, with frequency a parameter, for both fresh and sea water.