Revisiting the subharmonic route to acoustic chaos: broadband noise clearing via cavitation bubble synchronization

The cavitation activity within a tube transducer subject to a linearly increasing drive amplitude is studied experimentally, via high-speed imaging and parallel acoustic detection. A spectrogram of the cavitation emission signal collected over the duration of the ramped sonication, confirms a subharmonic route to chaos; the progression from harmonic emissions to broadband noise via a period-doubling bifurcation. Stroboscopic mapping demonstrates that the emergence of broadband signal is due to increasingly asynchronous bubble collapse across the population. Particular attention is given to a region of sudden broadband clearing in the spectrogram, coincident with strong subharmonic lines, approximately halfway through the sonication. The clearing is due to phase synchronization of the bubble oscillations. Broadband noise gradually re-emerges, often with further bifurcation to include period-quadrupled emissions, as oscillations dephase again following synchronization. Numerical modelling investigates the influence of amplitude ramping, inter-bubble distance and bubble dispersity on the oscillations and emissions from a 12-bubble system, based on an experimental observation. For a polydisperse system, it is found that a single bubble responding with period-2 oscillations initiates period-2 synchronization for the population, offering explanation for the consistent observation that prominent subharmonic emissions at half the driving frequency coincide with broadband clearing.