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An experimental investigation is carried out of the variation of the amplitude and phase of the second harmonic pressure wave, generated at the focal region of a curved transducer as a function of the input acoustic pressure below and above the threshold of sonically induced cavitation. It is found that a shift of approximately 180° in the phase of the second harmonic pressure relative to that of the fundamental occurs at the onset of cavitation. It is concluded that observation of the phase of the second harmonic can be used for detecting the onset of cavitation.

It is shown from some preliminary experimental investigations that it is possible to use the appearance of acoustic subharmonic waves as a means of detecting the onset of mechanically induced cavitation.

Under certain conditions, subharmonic oscillation at one-half the frequency of the excitation may be generated by gas bubbles driven by an alternating pressure. An extremely simple acoustical method is suggested for determining the viscosity of highly viscous liquids. The method involves the measurement of the threshold pressure of the onset of subharmonic oscillation at a given driving acoustic frequency.