Primary calibration of membrane hydrophones in the frequency range 0.5 MHzto 60 MHz

A laser interferometer designed to measure acoustic displacements at megahe rtz frequencies, which has been the basis of a primary standard for the cal ibration of ultrasonic hydrophones for over ten years, is described. The in terferometer is of the Michelson type and is designed to measure the acoust ic particle displacement by sensing the movement of a thin plastic membrane placed in the held of an ultrasonic transducer. The acoustic pressure is d erived from the measurement of displacement and the hydrophone is calibrate d by substituting it for the pellicle. Various sources of uncertainty are d escribed, 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 conf idence 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 me mbrane hydrophones as secondary standards, is also described. These hydroph ones are shown to have predictable performance properties and long-term sta bility, making them ideal secondary standards and choice as gold-standard r eference devices worldwide.