Nonlinear Sound Scattering from Subsurface Bubble Layers

Abstract

This paper is a review of our recent theoretical and experimental investigations of the nonlinear sound scattering in a single bubble, as well as in bubble clouds and layers. The following effects are considered: nonlinear scattering at second harmonic and difference frequency in a single bubble, nonlinear determination of bubbles, nonlinear reverberation, parametric emission from a bubble layer and phase conjugation, effects of nonlinearity ‘concentration’ on acoustic wave propagation.

This paper is a brief review of our recent theoretical and experimental investigations concerning the nonlinear sound scattering by gas bubbles including the cases of single bubbles, bubble clouds and layers. As is well known, a bubble in liquid is a monopole scatterer with prominent nonlinearity. When a harmonic acoustic wave affects a bubble the spectrum of the scattered wave contains a second harmonic component, while a biharmonic wave also gives rise to signals of sum and difference frequencies. In the case of bubble clouds it is necessary to sum up the scattered nonlinear signals from separate bubbles, the signal having a coherent and a noncoherent part. The noteworthy effects which can be observed include: parametric radiation, the cross-interaction of two beams, the effect of phase conjugation. Another class of effects is related to the redistribution and coalescence of bubbles under the action of ‘averaged’ hydrodynamic forces. The space-time change of the bubble distribution function leads to nonlinear ‘self-action’ of acoustic field such as ‘self-transparency’, ‘self-diaphragming’ and ‘self-focusing’ of an acoustic beam. The paper is devoted to the consideration of some of these effects and their role in the acoustics of subsurface bubble layers.