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A laser interferometer designed to measure acoustic displacements at megahertz frequencies, which has been the basis of a primary standard for the calibration of ultrasonic hydrophones for over ten years, is described. The interferometer is of the Michelson type and is designed to measure the acoustic particle displacement by sensing the movement of a thin plastic membrane placed in the field of an ultrasonic transducer. The acoustic pressure is derived from the measurement of displacement and the hydrophone is calibrated by substituting it for the pellicle. Various sources of uncertainty are described, including acoustooptic corrections, the frequency response of the interferometer, the acoustic properties of the thin membrane, and the lack of ideal plane-wave conditions. Highest calibration accuracy is achievable for membrane hydrophones, with a relative standard uncertainty, for a confidence level of 95 %, of 0.040 at 0.5 MHz, 0.035 from 1 MHz to 7 MHz, 0.046 at 20 MHz and increasing to 0.250 at 60 MHz.The dissemination of the primary standard calibration method, which uses membrane hydrophones as secondary standards, is also described. These hydrophones are shown to have predictable performance properties and long-term stability, making them ideal secondary standards and choice as gold-standard reference devices worldwide.

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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-mum 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 +/-13%, +/-8%, +/-17%, and +/-23%, respectively.