Studies of the cavitational effects of clinical ultrasound by sonoluminescence: 1. Correlation of sonoluminescence with the standing wave pattern in an acoustic field produced by a therapeutic unit

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Abstract
A Therasonic 1030 (Electro-Medical Supplies) therapeutic ultrasound generator operating at 1 MHz continuous wave was used to insonate aerated water at two temperatures, 22 degrees C and 37 degrees C. Using various acoustically reflecting materials, sound fields were set up with different standing wave components. Measurements of the acoustic pressure variations on the axis of the sound fields were made using a needle hydrophone and the results were compared with photographs of the spatial distributions of the image intensified sonoluminescent light output. The near field region was used, thereby simulating the clinical situation. Sustained sonoluminescence was observed for nominal intensities of 3 W cm-2, and acoustic reflections of greater than 40%. Under these conditions, if sonoluminescence did not appear spontaneously it could always be induced by rotating the transducer. Whenever bands of maximum light output formed they correlated closely with the pressure antinodes in the standing wave pattern. Very little light was produced by travelling wave fields.