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When a bubble in a liquid is subjected to a periodic sound field, the resulting bubble oscillations can interact with the sound field, giving rise to the primary Bjerknes force. A simple undergraduate-level derivation, and a graphical illustration of the underlying processes, are given.

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An undergraduate-level experiment is described for studying the sound emitted from bubbles formed by blowing gas through nozzles located within the bulk of a liquid. Hydrophone records give the frequency, pressure amplitude, and decay characteristics of the sound produced by a single bubble. The measured frequency agreed with the natural oscillation frequency of the bubble (the Minnaert frequency); the pressure amplitude gives the amplitude of oscillation of the bubble wall; the decay time broadly agrees with the value expected for radiative acoustic damping and thermal damping. The underwater sounds produced by natural brooks were recorded and found predominantly to consist of a succession of Minnaert-like oscillations, thereby allowing the bubble-size distributions to be obtained from the sound spectra.