The onset of sound wave distortion and cavitation in water and sea water

Abstract

In order to determine the practical maximum power operating levels for a particular sonar array, an investigation was made of the onset of harmonic distortion and cavitation in the acoustic field near a high power transducer element. Observations were made using a 7 kHz transducer, first radiating into an acoustic tank containing aerated tap water, and then at sea at various depths. Significant amounts of harmonic distortion were observed in the pressure waveform at power levels of 10 −2 w cm −2 at 1·1 bar ambient pressure in aerated water. The subsequent increase in these harmonics with power was apparently the result of the growth of stable gas bubbles on the diaphragm of the transducer. Near cavitation about 30% amplitude distortion was evident and bubble oscillations at both ultraharmonics and subharmonics of the forcing frequency were present. The first subharmonic and the broad-band background noise were both found to increase markedly at about 10 −1 w cm −2 at 1·1 bar, at this level streamers of microcavities were first observed due to the catastrophic collapse of resonant bubbles on the transducer diaphragm. It was found possible to correlate aurally and visually some of the simple manifestations of the wave distortion and cavitation phenomena. Distortion and cavitation thresholds were also made at sea as a function of depth, pulse length and duty cycle. These last results suggest two cavitation thresholds: one dependent on the growth in time of gas nuclei to resonant size, the other dependent on the presence of subresonant gas bubbles which can be made to collapse catastrophically in a few cycles of the forcing frequency if sufficient power is applied.