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

Abstract

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.