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 calibra tions in the frequency range 10 kHz to 315 kHz. A total of twelve participa nts from seven European countries took part by calibrating three reference hydrophones, with project coordination provided by the National Physical La boratory (NPL), UK. The agreement between the results was generally encouraging, with a majorit y of the results lying within 1 dB of the grand mean. However, some large v ariations were observed, and the uncertainties in the calibrations were typ ically underestimated by the participants, with the maximum differences fro m the grand means almost invariably exceeding the quoted overall uncertaint ies. A discussion is given of possible reasons for the disagreement, and of useful further work identified as a result of this comparison.