Abstract
The important parameters of ultrasonic beams used in current medical applications are briefly surveyed. Two designs of radiation balance are described which are suitable for the absolute measurement of acoustic power in diagnostic and therapeutic type beams respectively. The sensitivities of the two instruments are of the order of 0.3 mW and 0.05 w. A description is also given of the construction and use of a small piezoelectric hydrophone probe which is suitable for measurements of beam shape and pulse shapes (spatial and temporal distribution of acoustic energy). The two types of measurement provide complementary information and it is considered that their use in combination provides a good practical approach to the problem of ultrasonic beam calibration.

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