Visualization of bubble dynamics in a spark-based underwater sound source by means of high-speed cinematography

The first direct visualization of cavitation phenomena in high-power paraboloidal spark-based sound sources is presented. The growth and collapse of a vapour bubble generated by an electric spark in water was recorded by means of high-speed cinematography, simultaneously detecting the acoustic signature. Films show that after the very first cycles there is a transition from a collapsing/rebounding bubble regime to a pulsating bubble regime, suggesting that during this time the bubble contents may vary considerably. Measured bubble sizes and collapse times for the first cycle are in substantial agreement with the model of bubble collapse first formulated by Lord Rayleigh. The energy conversion into sound is evaluated by considering the decrease of maximum radius between successive bubble cycles. The acoustic efficiency so obtained is consistent with values derived from purely acoustic data.