Time and frequency analysis of air bubbles injected underwater

The acoustic signal generated by vibrating air bubbles injected in water allows for the passive detection of underwater sources such as divers and gas leaking from pipes. In this paper, an experimental analysis of the time and frequency characteristics of these signals is presented. The decaying acoustic signal of a single bubble created by nozzles of different diameters is described and extended to a train of bubbles from an array of nozzles. The oscillatory motion of the bubble surface generates an acoustic signal that has a damped sinusoidal characteristic. The spectrum of this waveform has a fundamental frequency and several harmonics. Therefore, the effect of ambient noise can be suppressed using a suitable band pass filter with a center frequency equal to the bubble fundamental frequency. It is shown that the acoustic signal generated by these bubbles can be utilized for passive detection and remote sensing (including depth estimation). Experimental data from a Saanich inlet experiment is analyzed to obtain realistic results. These results are compared with the theoretical analysis.