Covert cavitation: Spectral peak suppression in the acoustic emissions from spatially configured nucleations

Dual laser-nucleation is used to precisely configure two cavitation bubbles within a focused ultrasound field of f = 692 kHz, in proximity to the tip of a needle hydrophone. With both bubbles responding in the f/2 sub-harmonic regime, confirmed via ultra-high speed shadowgraphic imaging, an emission spectrum with no sub-harmonic content is demonstrated, for an inter-bubble spacing ≈λ. A spectral model for periodic shock waves from multiple nucleations demonstrates peak suppressions at nf/2 when applied to the experiment, via a windowing effect in the frequency domain. Implications for single-element passive detection of cavitation are discussed.