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.

Technique for the calibration of hydrophones in the frequency range 10 to 600 kHz using a heterodyne interferometer and an acoustically compliant membrane

“…The transmission losses at frequencies of 10 MHz and more were small but measurable, and allowed a correction to be made when calibrating miniature ultrasonic hydrophones at megahertz frequencies. 17 At the lower frequencies of interest in the work reported here ͑below 500 kHz͒, the transmission loss is expected to be negligible. To predict the transmission losses at lower frequencies, a simple layered model was used to represent an infinite water layer, a Mylar © layer, an aluminium layer, and a second infinite water layer.…”

“…15 At the National Physical Laboratory, UK, such a method is used for the primary calibration of miniature ultrasonic hydrophones. 16,17 The methods use optical interferometry to measure the displacement of a thin plastic membrane ͑termed a pellicle͒ placed in the farfield of an ultrasonic transducer. The membrane is used to reflect the optical signal beam of a Michelson interferometer, the pellicle being thin enough to be acoustically transparent at the frequency of interest.…”

Acousto-optic effect compensation for optical determination of the normal velocity distribution associated with acoustic transducer radiation

“…34 It became the basis of a primary calibration method for high-frequency hydrophones. 8,35,36 A second method uses a laser Doppler vibrometer to detect and measure the velocity of a pellicle or transducer surface. 9,33,37 These two interferometric methods involve phase and frequency modulation.…”

“…It is appreciated that the acousto-optic effect in the measurement of transducer or pellicle motion can be very significant, 30,31,[33][34][35]42 more than a mere perturbation of the measured or observed velocity. A recognized obstacle to quantification of this effect has been the spatial complexity of the radiated acoustic field between pellicle and laser beam window, i.e., in the optical measurement volume.…”

“…(ii) Bacon 31 used a beam displacer in the form of a rotatable glass block together with a translatable lens. This system was also used by Preston et al 35 (iii) Other parallel beam-displacement techniques can be imagined.…”