National Physical Laboratory

Underwater Acoustic Coupler Reciprocity Facility

A New Underwater Acoustics Primary Standard

The new low frequency primary standard for hydrophone calibration is based on the method of coupler reciprocity. The system consists of a fluid filled steel cylinder housing a reference device at either end with an intersecting port through which the hydrophone under calibration is fitted. The system is designed to allow calibrations to be performed at elevated hydrostatic pressures (as well as near-atmospheric pressure) and over a range of temperatures.

The operating frequency range of the system is 20 Hz to 2 kHz, with third octave steps chosen for convenience, although fine frequency calibration is equally achievable. The coupler reciprocity system provides calibrations with an accuracy of ±0.4 dB. The hydrostatic pressure range is between 0.1 MPa and 7 MPa, the equivalent water depth of 10 m to 700 m.

The variation of the response of a hydrophone with temperature can be just as significant as that with depth. A temperature control system enables calibrations in the coupler to be undertaken at temperatures ranging from 10 °C to 40 °C.

The coupler may therefore be used to characterise the response of a hydrophone in terms of temperature and hydrostatic pressure (depth). Limited work has highlighted some significant variations in response for commercially available hydrophones.

The graph on the left illustrates how the receive sensitivity characteristics of a hydrophone can vary with hydrostatic pressure. The graph is of a hydrophone calibration performed at a single frequency and over a range of hydrostatic pressures. The gradient of the orange portion of the curve gives a hydrostatic pressure dependency of approximately -0.3 dB/MPa, whereas the green portion of the curve gives -0.6 dB/MPa. This change in pressure dependency is due to the hydrophone being held at 2 MPa for a couple of hours prior to continuing the pressure increase (typically increasing at a rate of 0.5 MPa every 10 minutes). Although not shown here, the hydrostatic pressure was reduced from 3.5 MPa to 0.5 MPa in 0.5 MPa steps at 10 minute intervals, bringing the hydrophone receive sensitivity back to the original start point.

The hydrostatic pressure dependency of a hydrophone at 800 Hz in the pressure range 0.5-3.5 MPa gauge.

Hydrophone calibrations performed in the coupler have now been compared to those carried out using an established air-pistonphone with good agreement. The hydrophone calibration graph below is of a commercially available device with uncertainty error bars of ±0.4 dB.

Limitations of the coupler lie in the physical size of hydrophone that can be fitted to the system. Work has begun on investigating methods of disseminating the calibrations performed in the coupler through the use of our Acoustic calibration and testing at simulated ocean conditions (in the NPL Acoustic Pressure Vessel), which can accommodate large devices.

Hydrophone calibration performed in the NPL coupler at an elevated hydrostatic pressure of 0.5 MPa gauge.

Further Information

For further information, contact Justin Ablitt

Last Updated: 30 Mar 2012
Created: 6 Jun 2007


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