A beam-plotting facility has been developed to provide a reference measurement system for determining the acoustic output of medical ultrasonic equipment. It consists of two coordinate-positioning systems controlled by stepping motors and a minicomputer. One system is used for holding and manipulating an ultrasonic transducer and the other for a hydrophone. A membrane hydrophone made from polyvinylidene fluoride of 9- mu m thickness with an active element of 0.5-mm diameter is used for most measurements. The hydrophone is connected to an amplifier and digitizer, also controlled by the minicomputer, and the whole system has a measurement bandwidth of 75 MHz (-3 dB). A detailed description of this system is given together with a full assessment of measurement uncertainties and the methods used to correct for the effects of nonlinear distortion and spatial averaging. Typical overall uncertainties (95% confidence) for the determination of the peak-positive acoustic pressure, peak-negative acoustic pressure, spatial-peak pulse-average intensity and spatial-peak temporal-average intensity are +or-13%, +or-8%, +or-17%, and +or-23%, respectively.
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