An international key comparison of free-field hydrophone calibrations in the frequency range 1 to 500 kHz

A description is given of the results of a Key Comparison of primary free-field standards for underwater acoustics at frequencies from 1 to 500 kHz. This is the first such Key Comparison exercise in the field of underwater acoustic calibration and measurement. Laboratories from UK, Germany, USA, Russia, China, Canada, and South Africa participated by calibrating three reference hydrophones, with project coordination provided by the National Physical Laboratory, UK. The agreement between the results obtained from the comparison was generally encouraging, with the calibration values reported by the laboratories agreeing within quoted uncertainties over the majority of the frequency range, and the results generally lying within a 0.5-dB band for frequencies up to 300 kHz. A discussion is given of the general sources of uncertainties in the calibrations, in particular those which are thought to have contributed to the differences in the results between laboratories. The results of the participants have been used to estimate the equivalence of national measurement standards within this field.