Validated ultrasonic power measurements up to 20 W

A project has been completed to develop reference methods for the measurement of ultrasonic power with a validated measurement uncertainty of < 7% at power levels of 1 to 20 W over the frequency range 1 to 3 MHz of collimated beams. The project is the result of collaborative research between the Physikalisch-Technische Bundesanstalt, Germany (PTB, DE), the National Physical Laboratory, UK (NPL, UK) and the Netherlands Organisation for Applied Scientific Research, Prevention and Health (TNO-PG, NL). The work has been undertaken under the 4th Framework Programme of the European Community (EC). Primary standard designs of radiation force balances based on both absorbing and reflecting targets have been constructed. To avoid heating effects, the measurements should be done relatively quickly (10 to 20 s). The methods have been validated using ultrasound (US) transducers that demonstrated an adequate short and long-term stability; a method to detect cavitation based on monitoring the acoustic signals produced by bubble oscillation and collapse has been confirmed. It has been shown that only the detection of the subharmonic can be used in practice as cavitation detector. Different procedures for obtaining degassed water have been investigated. A method showing significant promise to be used in a clinical or manufacturer's environment involves the addition of sodium sulphite (Na2SO3). During the validation process, commercially available radiation force balances and ultrasonic physiotherapy devices have also been evaluated. Limitations of current measurement methods and practices, including power measurements made on transducers exhibiting a diverging beam, have been identified. It has been shown that a reflecting target is not appropriate to measure powers of transducers with a ka-value < 30. Based on beam shape and target distance, it has been shown also that proper power measurements using a 45 degrees convex-conical reflecting target can never be performed for transducers with a ka-value < 17.4.