Short pulse acoustic excitation of microbubbles

Abstract

When a bubble is excited at or near its natural frequency it is a very effective scatterer and absorber of sound. In fact at resonance, scattering and absorption cross sections are of the order of 1000 times greater than the geometric cross section for typical marine microbubbles. As with any resonant system, a bubble takes a finite, albeit short time to ring up to steady-state oscillation and continues to oscillate for a finite time after the driving force ceases. A number of studies have been carried out to attempt to describe how acoustic absorption and backscatter depend on the duration of the driving force. Most notable of these are the backscatter measurements for short pulses in near surface sea water [Akulichev et al., Sov. Phys. Acoust. 23(3), 177–180 (1986)] and the forward transmission laboratory measurements for short pulse lengths at 120 kHz [H. R. Suiter, J. Acoust. Soc. Am. 91, 1383–1387 (1992)]. Akulichev observed a decrease in acoustic backscatter for short pulses at three discrete frequencies, whereas Suiter found no corresponding enhancement in forward transmission. This paper details a comprehensive study which measured acoustic attenuation through a well defined bubble cloud over the frequency range 20–200 kHz and for pulse lengths from 20 cycles down to a single cycle using a parametric transmitter. The experiment simulated the conditions for which a decrease in attenuation with decreasing pulse length might have been expected. No effect was observed for two different but well-defined bubble distributions.