How does a liquid foam dance when insonified? Even if the acoustic wavelength is larger than the typical bubble size, the acoustic propagation is indeed far from trivial, and with much stronger dependencies on the foam physical parameters than on the chemical ones [1]. 

Experimentally the sound velocity and attenuation strongly depend on the bubble radius, R, and the acoustic frequency [1-3]. We propose a new model, based on the particular microstructure of liquid foams, to explain these experimental findings [3]. Two non-dispersive acoustic regimes and a resonance in between are observed. To distinguish the different regimes, the relevant length scale in not the acoustic wavelength in the foam, but the wavelength of the vibration wave on the liquid interfaces, at the forcing frequency [3,5-7].

At low frequency the sound velocity is very low, between 25 and 50 m/s, and it can be explained by a mixture law, so called Wood’s law [4]. At the highest frequencies, the acoustic wave cannot drag the Plateau borders but only the air and the small inertial mass of the soap film; the sound velocity is close to the sound velocity in the air. In between these two regimes the acoustic properties change drastically: a resonance of the foam structure occurs. At the resonance frequency, the Plateau borders and the acoustic wave move in phase, whereas the soap films move out of phase. This complex relative motion of the liquid channels and films gives rise to an interesting regime with a maximum of attenuation and a range of negative density [3].