Air injector geometry affects passive bubble acoustic signatures

Understanding the passive acoustic signals of gas bubbles has become increasingly imporant due to their growing relevance in ambient marine acoustics and medical areas, as well as the recent search for dark matter by Bubble Chamber Detectors. Here the acoustic signatures of 2.24, 1.83, 1.75, 1.66 and 1.43 mm radii bubbles are reported experimentaly. They were generated by two different air injectors in a quiescent liquid, a standard plastic syringe tube and 97 degrees-rotated metallic needles. The evolution of the sound pulse over time is presented alongside simultaneous images of the bubbles detaching from the injector and moving freely in the liquid. The beat-wave phenomenon is the standard interference pattern between two sounds of slightly different frequencies, generating a beat-period envelope of “rosary chain” form. In this study, beats are observed when the bubble exhibits changes in its shape through Cassini-oval, trapezoid, ‘guitar pick’, ellipsoid and oblate shapes, which is in agreement with earlier results on bubble fragmentation in a locally sheared flow. Finally, theoretical curve fits of freely-oscillating, lightly-damped bubble sound emissions confirm that the beats are due to changes in the bubble shapes.