A detector for monitoring the onset of cavitation during therapy-levelmeasurements of ultrasonic power

Abstract

Acoustic cavitation occurring in the water path between a transducer and the targetof a radiation force balance can provide a significant source of error during measurements ofultrasonic power. These problems can be particularly acute at physiotherapy levels (>1 W),and low frequencies (⩽ 1 MHz). The cavitating bubbles can absorb and scatter incidentultrasound, leading to an underestimate in the measured power. For these reasons, InternationalSpecification standards demand the use of degassed water. This imposes requirements that mayactually be difficult to meet, for example, in the case of hospitals. Also, initially degassedwater will rapidly re-gas, increasing the likelihood of cavitation occurring. For these reasons,NPL has developed a device that monitors acoustic emissions generated by bubble activity, fordetecting the onset of cavitation during power measurements. A commercially available needlehydrophone is used to detect these emissions. The acoustic signals are then monitored using aCavitation Detector (CD) unit, comprising an analogue electrical filter that may be tuned todetect frequency components generated by cavitating bubbles, and which provides anindication of when the measured level exceeds a pre-defined threshold. This paper describesstudies to establish a suitable detection scheme, the principles of operation of the CD unit, andthe performance tests carried out with a range of propagation media.