A comparison of hydrophone calibration by free-field reciprocity and by optical interferometry

Abstract

As the national standards laboratory in the UK, the National Physical Laboratory has established two primary standard methods for the free-field calibration of hydrophones. In the frequency range 500 kHz to 20 MHz, the primary standard is realized using optical interferometry; in the range from 5 to 500 kHz, it is realized by the method of three-transducer spherical-wave reciprocity. Although the ranges of the primary standards are defined as above, both methods can be used in the frequency range 200 kHz to 1 MHz, enabling a comparison to be undertaken between these two independent methods. The comparison was made by calibrating reference hydrophones using both methods and comparing the results. In general, the results agree to within ±0.4 dB or better, well within the combined uncertainties of the methods. In this paper, a comparison of results is presented and a discussion given of the major sources of uncertainty in the methods.