National Physical Laboratory

Next generation of primary standards for underwater acoustics

An optical interferometer probes through an optical window in the NPL small water tank facility with a laser measurement beamResearch work is currently being undertaken towards the development of a new optically-based primary standard for the calibration of underwater hydrophones based on heterodyne interferometry and laser Doppler anemometry.

The existing primary standard for the free-field calibration of hydrophones is based on the three-transducer reciprocity method for frequencies up to 500 kHz. This method provides the absolute sensitivity of the devices under calibration with traceability through electrical standards. However, there is a requirement for a new method that will enable the calibration of existing hydrophones (and also new devices from emerging technologies) to frequencies up to 1 MHz.

In recent years, NPL has been performing research into the development of a new optically-based primary standard that covers frequencies from as low as 10 kHz to as high as 1 MHz. The method is based on laser Doppler effect measured by a custom-design optical heterodyne interferometer.

In this case, a water matched acoustically compliant membrane is suspended in the NPL small water tank facility placed at the far field of a suitable transducer providing the acoustic excitation such that the pellicle faithfully follows the acoustic oscillation. Outside the small tank, an optical interferometer probes the pellicle through an optical window with a laser measurement beam. The reflected beam is mixed in the interferometer with a laser reference beam that is frequency shifted and by analysing the resulting signal, the Doppler shift demodulation provides the acoustic particle velocity and hence free-field pressure.

The optical technique provides the potential for direct and absolute calibration of hydrophones directly related to the unit of acoustic sound pressure, does not rely on existing measurement artefacts and offers high dynamic frequency and pressure range. Current work concentrates on the instatement of the system in NPL's small water tank facility and the investigation of potential uncertainties associated with the new method.


  • Particle velocity measurements using heterodyne interferometry and Doppler shift demodulation for absolute calibration of hydrophones
    Koukoulas, T., Theobald, P., Robinson, S., Hayman, G., and Moss, B.
    POMA, Acoustical Society of America, 17, pp. 70022: 1-10, 2012
  • Technique for the calibration of hydrophones in the frequency range 10 to 600 kHz using a heterodyne interferometer and an acoustically compliant membrane
    Theobald, P.D., Robinson, S.P., Thompson, A.D. Preston, R.C., Lepper, P.A. and Yuebing, W.
    Journal of the Acoustical Society of America, 118 (5), pp. 3110-3116, 2005
Last Updated: 16 Aug 2016
Created: 4 Apr 2013


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