Dual-perspective high-speed imaging and acoustic detection is used to characterise cavitation activity at the tip of a commercial 20 kHz (f0) ultrasonic horn, over 2 s sonications across the range of input powers available (20 - 100%). Imaging at 1 × 105 frames per second (fps) captures cavitation-bubble cluster oscillation at the horn-tip for the duration of the sonication. Shadowgraphic imaging at 2 Mfps, from an orthogonal perspective, probes cluster collapse and shock wave generation at higher temporal resolution, facilitating direct correlation of features within the acoustic emission data generated by the bubble activity. f0/m subharmonic collapses of the primary cavitation cluster directly beneath the tip, with m increasing through integer values at increasing input powers, are studied. Shock waves generated by periodic primary cluster collapses dominate the non-linear emissions of the cavitation noise spectra. Transitional input powers for which the value of m is indistinct, are identified. Overall shock wave content within the emission signals collected during sonications at transitional input powers are reduced, relative to input powers with distinct m. The findings are relevant for the optimisation of applications such as sonochemistry, known to be mediated by bubble collapse phenomena.
Keywords: Cavitation; Shock wave; Subharmonic; Ultrasonic horn; Vibration amplitude.
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