Reduction of cavitation using pseudorandom signals

It is known that the scattering of ultrasound by cavitation bubbles reduces the efficiency of treatment by high-intensity focused ultrasound. In our experiments striving to reduce grating lobe levels of annular arrays we observed less microbubble formation at the focus of the transducer when pseudorandom phase-modulated CW signals were used rather than single-frequency CW signals. To confirm this unexpected result, we performed an experiment in a solution of luminol which is known to be a cavitation detector. A 5-cm diameter spherical transducer (1.1 MHz center frequency and 0.6 MHz bandwidth), focused at 197 mm was used. The ratio of the sonoluminescence intensity produced by a single-frequency signal to that produced by a pseudorandom phase-modulated signal increased with the intensity of the applied field and attained a value of nearly 50 for an intensity of 4.6 W cm(-2).