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~o 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 10s t 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 thresholdsi 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.
1. Introduction
The National Institute of Oceanography is currently involved in the development of a long range sonar for geological purposes. For such a project it is important to know the power limitation imposed by the propagation medium since this will control the maximum practical operating power level of the sonar array at various depths.
Early on in this work it was believed that this limit should be set by the onset of cavitation. However, when observations of the pressure waveform radiated by the prototype projector element were seen it was clear that a sensible limit should depend on waveform distortion. It was found that 10 Yo amplitude distortion was occurring with the projector radiating only 1120 w em -2 at 7 kHz in a tank of static tap water at a depth of 1 m at ambient pressure. Similar distortion levels were found in near-surface tests in an open reservoir. As a result it was decided to determine the onset of waveform distortion and cavitation with this transducer in sea water at various depths, so that the phenomena could be investigated more fully and to enable an operational power limit to be set for the array.
Before these investigations were started the same piezoelectric sound projector had been run for three weeks at acoustic power levels of 600 w, i.e. 5 w cm -2, at a depth of 120 m in sea water. The distortion level in this trial was comparable to the near-surface measureinents in water when radiating only 1/20 w cm -2. Because of this pressure dependence of t8 257