HIGH-RESOLUTION BUBBLE SIZING THROUGH DETECTION OF THE SUBHARMONIC RESPONSE WITH A 2-FREQUENCY EXCITATION TECHNIQUE

Sizing bubbles in fluid using a two-frequency excitation technique is not prone to the same drawbacks of some other sizing methods-it has a global maximum at the bubble resonance frequency and allows good spati al resolution. The bubble is insonated with a high fixed imaging signa l and a variable pumping signal tuned to the resonant frequency of the bubble, which are coupled at resonance by the high-amplitude oscillat ion of the bubble wall, with the formation of sum-and-difference terms . This paper examines both the resonance and off-resonance behavior of such combination frequency signals. A coupling of the subharmonic bub ble response with the imaging frequency is shown to be a much more acc urate and unambiguous detector of the bubble resonance than couplings involving the fundamental resonance. The characteristics of this subha rmonic signal are examined using an automated sizing method, and the d ependence of the response on the pumping signal amplitude and the freq uency step size between two successive pumping frequencies is examined . The location of a definite subharmonic threshold is reported and qua ntified both for single bubbles held on a wire and for free rising bub bles moving through the focus of the transducers. This amplitude is fo und to be orders of magnitude lower than that predicted by traditional volumetric pulsation models, but agrees very closely with the theoret ical onset of surface waves. (C) 1996 Acoustical Society of America.