International comparison of free-field hydrophone calibrations in the frequency range 10 kHz to 315 kHz

This paper describes a European comparison of free-field hydrophone calibrations in the frequency range 10 kHz to 315 kHz. A total of twelve participants from seven European countries took part by calibrating three reference hydrophones, with project coordination provided by the National Physical Laboratory (NPL), UK. The agreement between the results was generally encouraging, with a majority of the results lying within 1 dB of the grand mean. However, some large variations were observed, and the uncertainties in the calibrations were typically underestimated by the participants, with the maximum differences from the grand means almost invariably exceeding the quoted overall uncertainties. A discussion is given of possible reasons for the disagreement, and of useful further work identified as a result of this comparison.