National Physical Laboratory

Underwater acoustics primary standards

The primary standard for free-field hydrophone calibrations in the frequency range 1 kHz to 500 kHz is the method of three-transducer spherical-wave reciprocity. This requires the use of three hydrophones, at least one of which must be a reciprocal transducer; that is, its transmitting and receiving sensitivities are related by a constant factor. The hydrophones are paired off in three measurement stages, at each of which one device is used as a transmitter and the other a receiver. For each pair of hydrophones, a measurement is made of the ratio of the voltage across the terminals of the receiving device to the current driving the transmitting device. By use of the reciprocity principle as applied to the reciprocal transducer, the sensitivity of any one of the three hydrophones can be determined from these purely electrical measurements. This calibration method is ultimately traceable to electrical standards. The uncertainty in this method can be better than 0.5 dB (expressed for a confidence level of 95%).

In the frequency range 20 Hz to 2 kHz, the primary standard for pressure calibrations of hydrophones is realised by the method of coupler reciprocity. In this case, the reciprocity calibration is undertaken with the hydrophones inserted into a fluid-filled coupler of dimensions which are small compared to the acoustic wavelength providing a uniform acoustic pressure. The uncertainty in this method is typically 0.4 dB (expressed for a confidence level of 95%) (See Underwater Acoustic Coupler Reciprocity Facility).

The primary standard for free-field hydrophone calibrations in the frequency range 500 kHz to 20 MHz is provided by optical interferometry. This method allows the determination of the acoustic displacement in the field of an acoustic source using a specially-designed Michelson laser interferometer. The interferometer uses an electro-optic feedback circuit to eliminate the effects of low frequency vibration and measure the sub-nanometre displacements encountered. The acoustic pressure is then calculated from the displacement and the free-field sensitivity of the hydrophone can then be determined from the ratio of the output voltage produced by the hydrophone when placed at that point in the field to the acoustic pressure. The uncertainty in this method is typically between 3% and 6% depending on frequency in the range 500 kHz to 20 MHz (expressed for a confidence level of 95%). This method is ultimately traceable to standards of length through the wavelength of the laser light.

Currently, NPL is undertaking work to develop the next generation of primary standards for underwater acoustics by extending the optical methods to lower frequencies. This work is described in:

DQL-AC 001, April 2004 (ISSN: 1744-0599), Fundamental standards for acoustics based on optical methods - final report . by Theobald, P, Thompson, A D, Robinson, S P, Barham, R G, Preston, R C, Lepper, P*, Swift, C*, Tyrer, J*, Greated, C*, Campbell, M*, Schlike, T*, Yuebing, W

Mutual Recognition and harmonisation of standards

To ensure harmonisation of these standards with the UK's trading partners, NPL participates in Key Comparisons organised under the auspices of the Consultative Committee on Acoustics, Ultrasound and Vibration (CCAUV) of the CIPM. Such comparison exercises allow the determination of the degree of equivalence between the standards of each country, as required by the Mutual Recognition Arrangement (MRA). NPL has acted as pilot laboratory in two Key Comparisons of hydrophone calibrations: CCAUV.W-K1 and CCAUV.U-K1. The results are available on the web-site of the Bureau Internationale des Poids et Mesures (BIPM)

Last Updated: 21 Apr 2011
Created: 6 Jun 2007


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