European comparison of ultrasonic hydrophone calibrations

A European comparison of 1 mm hydrophone calibrations in the frequency rang e 0.5 MHz to 15 MHz is summarized. The project, organized by the Bureau Com munautaire de Reference (BCR) of the Commission of the European Communities , involved five participants from different countries measuring the free-fi eld sensitivity of three types of miniature ultrasonic hydrophone. Three di fferent absolute calibration methods were used in the project, which was co mpleted and published in 1991. The uncertainties quoted at the 95 % confide nce level ranged from 2.3 % to 18 %, depending on the frequency, method and laboratory. Agreement between laboratories was generally within a few part s in 10(2) in the range 0.5 MHz to 5 MHz but individual results differed by up to 15% from the grand mean at 10 MHz and 15 MHz.