Difference frequency generation due to collective resonant oscillations of a bubble cloud

Bubbly liquids exhibit a strong acoustic nonlinearity that can be used to generate low‐frequency sound in a parametric‐array arrangement. To study this possibility, two different bubble clouds have been generated in our laboratory. One is produced by a series of hypodermic needles giving a high void fraction bubbly medium with uniform bubbles sizes of about 1.1 mm radius. The other cloud is produced by means of a jet device that generates much smaller bubbles and a smaller void fraction. With the first cloud, the generation of waves with frequency around 200 Hz (corresponding to the natural frequency of the cloud’s breathing mode) was observed. The pump frequencies were around 2.2 kHz, close to the resonance frequency of the individual bubbles produced by the needles. For the jet‐generated cloud the observed difference frequency was around 2 kHz for pump frequencies ranging from 5 to 28 kHz. The generation of the difference frequency over this broadband is related to the broad bubble size distribution generated by the jet. A theoretical model based upon nonlinear sound generation by bubbles resonating with the pump frequency will be presented to account for the observations.