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 β≊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.