Two Tresholds Of Multibubble Cavitation

It has been shown that cavitation zone generated by the high intensity focused ultrasound (HIFU) passes through different stages of evolution with either increasing pulse duration, decreasing pulse period or increasing driving voltage of pulsed ultrasound. In the first stage sonoluminescence (SL) is absent. Here sound absorption is weak in the cavitation zone and the hydrophone output is nearly constant if the transducer voltage is constant. The second stage corresponds to the onset of sonoluminescence and the smooth increase of its intensity. In the third stage, the sonoluminescence intensity increases in a sudden manner, accompanied by a synchronous rise of the ultrasound absorption in the cavitation zone. The sonoluminescence intensity reaches a maximum value and then decreases while ultrasound absorption increases smoothly upon further cavitation zone evolution. Two cavitation thresholds separate these stages: first is related to the SL appearance and the second—to the sudden increase of the SL intensity.