Low-frequency acoustic wave generation in a resonant bubble layer

The nonlinear response of a bubble layer subject to harmonic and biharmonic excitation at frequencies smaller than the individual bubble resonance frequency is considered. The nonlinear resonance properties of the layer and generation of difference?frequency signal are studied analytically and numerically. It is found that, for bubble volume fractions ??10?3 and pump amplitudes of the order of 10?1 atm, the power of the low?frequency signal (including waves radiated in both directions) may reach 10% of the total power of the incident biharmonic wave. The efficiency is restricted by the rapid formation of shocks already at relatively low driving amplitudes, which may not occur in a more complete model accounting for the inertia of the bubble pulsations.