An analysis of the acoustic cavitation noise spectrum: The role of periodic shock waves

Abstract

Research on applications of acoustic cavitation is often reported in terms of the features within the spectrum of the emissions gathered during cavitation occurrence. There is, however, limited understanding as to the contribution of specific bubble activity to spectral features, beyond a binary interpretation of stable versus inertial cavitation. In this work, laser-nucleation is used to initiate cavitation within a few millimeters of the tip of a needle hydrophone, calibrated for magnitude and phase from 125 kHz to 20 MHz. The bubble activity, acoustically driven at f = 692 kHz, is resolved with high-speed shadowgraphic imaging at 5 × 10 frames per second. A synthetic spectrum is constructed from component signals based on the hydrophone data, deconvolved within the calibration bandwidth, in the time domain. Cross correlation coefficients between the experimental and synthetic spectra of 0.97 for the f/2 and f/3 regimes indicate that periodic shock waves and scattered driving field predominantly account for all spectral features, including the sub-harmonics and their over-harmonics, and harmonics of f.

Validity of the Keller-Miksis equation for “non-stable” cavitation and the acoustic emissions generated

“…4. This is due to the fact that the timings between respective shocks are not well replicated between the two representations, and therefore high frequency content is not adding at the correct phases in the simulated acoustic emissions [4,13]. From the results presented in Fig.…”

“…As discussed in [4], all spectral features except parts of the fundamental come from PSWs. Attempting to simulate a series of shock waves, and replicating the spectral features, is to a lesser extent achieved with the presented approach, compared to fitting individual bubble-collapse shock waves manually [4]. Nevertheless, it is appreciated that the crude simulation of the stable-inertially collapsing cavitation cloud is reproducing all spectral features within 3 dB, up to the 11 f 0 /2 harmonic of the sub-harmonic.…”

“…We have recently published an analysis of the acoustic cavitation noise spectrum, where we identify the role of periodic shock waves (PSWs), that are emitted from a stableinertially collapsing cavitation cloud [4]. It shows that all spectral features, expect parts of the fundamental frequency, are mediated by PSWs.…”

Validity of the Keller-Miksis equation for “non-stable” cavitation and the acoustic emissions generated

“…The HIFU field is somewhat perturbed by the presence of a NH, decreasing the pressure by ∼ 15%, compared to no NH in the emission collection position. This is assessed in detail with a fiber-optic hydrophone in [4].…”

“…In [4] the individual PSWs are fitted manually, achieving a high similarity between the synthetic signal and its spectrum when compared to the experimental measurement. In this paper we investigate the validity of the Keller-Miksis equation, and computed acoustic emissions, high-speed imaging data and acoustic data from a calibrated NH are used for respective validation.…”