Free oscillations of near‐surface bubbles as a source of the underwater noise of rain

A bubble that is oscillating with a small amplitude may be thought of as a damped, undriven harmonic oscillator. If the bubble is near a free surface, it may be considered as a simple acoustic source close to a pressure release boundary and will therefore radiate as a dipole. It has been shown in a previous paper [Pumphrey et al., J. Acoust. Soc. Am. 85, 1518–1526 (1989)] that this mechanism is the major cause of the sound produced by rain and probably makes a contribution to the Knudsen spectrum of ambient noise in the ocean. Experimental results are presented for such freely oscillating bubbles, mostly entrained by drop impacts, which show that they have the resonance frequencies and damping constants predicted by theory. The presence of the free surface affects these values to a certain extent. The radiation pattern was measured and was shown to be a dipole, as predicted. Calculations of the amplitude of the bubble oscillation indicate that it is small, confirming that the linear approximation is valid. The spectrum level caused by a rain shower was calculated and compared with field measurements.