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This paper presents measurements of the acoustic emission from gas bubbles of controlled sizes “seeded” into water and glycerol-water mixtures and subjected to sound fields over a wide range of frequencies and intensities up to, and beyond, the transient cavitation threshold. The emission from unprepared liquids was also recorded for comparison. A mechanical device has been developed for generating small bubbles of uniform sizes at controlled rates. The results suggest that there are at least two mechanisms for generating signals below the excitation frequency (f0): (1) Subharmonics of f0 may be produced by forced oscillations of bubbles whose radial resonance frequencies are submultiples of f0; (2) bubbles of any size may be shock excited to emit signals at their radial resonance frequencies. Large bubbles of certain preferred sizes are normally present, and their resonances probably account for many low-frequency signals that are not integral submultiples of f0. It also seems certain that some other mechanism unconnected with resonant bubbles contributes to the relatively strong signals observed at the first subharmonic (f0/2).

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