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Russian and Chinese national standards have been compared by exchangflzg hydrophones for calibration in the frequency range from 1 Hz to 630 kHz. The results agree very closely, which confirms that the estimates made of the errors in the two standards are reliable.A comparison of hydroacoustic measurements was made in 1997-8 by comparing the calibrations of piezocemmic hydrophones on the standards held by the All-Russia Technical Physics and Electronics Research Institute (VNIIF'I'RI) in Russia and Hanchou Applied Acoustics Institute (HAAI) in China. HAAI sent hydrophones of types 8100 and 8103 made by Bruel and Koer in Denmark together with a hydrophone of RHS 2 type developed by that institute, which has a spherical sensing element of diameter 20 mm. VNIIFTRI sent HAAI hydrophones designed by the Institute types GI-20 and GI-22, which have spherical sensing elements correspondingly of diameters 20 and 7 turn, together with a GI-30 high-frequency hydrophone whose active element is a radially polarized thin-walled (0.2 mm) cylinder of diameter and length 2 mm.The calibrations were performed with the equipments in the primary and secondary national standards over the frequency range 1 Hz to 630 kHz. The measurements were made at standard frequencies in a three-octave series. Table I gives the calibration methods and any distinctive features of the equipment used to calibrate the hydrophones.We now consider briefly the features of the methods. Piezoelectric compensation methods were developed at the Russian Institute [1] and are recommended by the International Electrotechnical Commission IEC, and they have been used to calibrate hydrophones on the Chinese standards (at 1 Hz to 2 kHz) and the Russian ones (0.8 Hz to 4 kHz). The equipments that employ this method have much in common: closed chambers of approximately equal dimensions, balancing converters in the form of two coaxial piezoceramic cylinders with elastic coupling between them, and similar forms of supporting hardware, with the measurements managed under the control of a personal computer.The Chinese standard equipment employs a two-channel oscillator to excite the radiator and the null detector in the balancing converter, which has independent amplitude and phase regulation for each channel, and where particular attention is given to errors of measurement for the voltage (_+0.1 dB) and phase (-4-0.1 ~ for the null detector.The distinctive features of this method in the Russian standards are related to corrections for the nonuniforrnity of the sound pressure distribution in the chamber [3]. The corrections incorporated the height of the null detector, the dimensions of the hydrophone's active component, the shifts in the acoustic centers of the hydrophone and null detector from the geometrical center of the chamber, and also the measurement frequency. This enabled us to reduce the errors of measurement and to extend the frequency range to 4 kHz. * Deceased.

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