Providing and disseminating standards
The existing primary standard for the free-field calibration of hydrophones is based on the three-transducer reciprocity method for frequencies from 250 Hz to 500 kHz. This method provides the absolute sensitivity of the devices under calibration with traceability to electrical standards. Three electroacoustic transducers are paired off in three measurement configurations, in each case one device being used as a source and one as a receiver. The acoustic transmit or receive sensitivity of any one of the devices may be obtained from purely electrical measurements of the electrical drive currents of the source transducers and the open-circuit voltage of the receiving transducers at each measurement stage. The method relies on the assumption that at least one of the devices is transmitting and receiving response which are related by a constant factor. The primary standards are realised in the controlled conditions of the our laboratory tanks.
Next generation of standards using optical techniques
Research is currently being undertaken into the development of a new optically-based primary standard for the calibration of underwater hydrophones based on heterodyne interferometry. The new optically-based primary standard will cover frequencies from 100 kHz to 1 MHz. In the method, an acoustically compliant membrane (pellicle) is suspended in the water tank placed at the far field of a suitable transducer providing the acoustic excitation such that the membrane faithfully follows the acoustic oscillation. Outside the tank, an optical interferometer probes the pellicle through an optical window with a laser beam. The reflected beam is mixed in the interferometer with a reference beam and by analysing the resulting signal the the acoustic particle velocity and hence free-field pressure can be determined. The optical technique provides the potential for absolute calibration of hydrophones directly related to the unit of acoustic sound pressure, and does not rely on the performance of existing measurement artefacts.
Low frequency calibration standards
NPL provides low frequency pressure calibration of hydrophones using comparison in a closed chamber in the frequency range 5 Hz to 315 Hz. We also have a coupler reciprocity calibration facility which can be operated over the range 20 Hz to 2 kHz. Low frequency calibrations can also be provided in a free-field environment in the NPL laboratory tanks using signal modelling techniques to extend the frequency range down from 1 kHz to 250 Hz. The different methods have been compared over the overlapping frequency range.
NPL contributes to the work of standards committee IEC TC87 and has led the work to produce a number of key standards in Working Group 15, including IEC 60565:2006 (Calibration of hydrophones). NPL holds the Convenorship of Working Group 15 which is currently working to revise this standard.
Validation of standards
NPL standards are validated by rigorous study of uncertainties and by comparison between independent methods. In addition, NPL participates in international key comparisons between NMIs to validate the results under the auspices of the International Committee of Weights and Measures (CIPM). A repeat comparison, CCAUV.W-K2, is in progress for free-field standards in the range 250 Hz to 500 kHz.