The acoustic excitation of newly-formed bubbles

Gas bubbles in water behave acoustically like lightly-damped oscillators with a natural frequency determined by their radius. When pinched off from another body of gas, their formation is usually accompanied by a pulse of sound at the bubble ’s natural frequency. Thus air bubbles formed beneath waterfalls, raindrops and breaking waves all radiate sound. One of the mechanisms driving the bubbles into acoustic oscillation is the rapid retraction of the neck of air formed immediately prior to formation. A model for the neck retraction, and it’s role in driving sound production by bubbles released from a nozzle and fragmenting in fluid shear, will be presented with experiments and theory. The potential to use the model to remotely monitor bubbles formed by breaking waves and released by marine methane seeps will be discussed. [Work supported by the Ocean Acoustics Division of the U.S. Office of Naval Research and the U.S. National Science Foundation].