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.
A description is given of the COOMET project 473/RU-a/09: a pilot comparison of hydrophone calibrations at frequencies from 250 Hz to 200 kHz between Hangzhou Applied Acoustics Research Institute (HAARI, China)—pilot laboratory—and Russian National Research Institute for Physicotechnical and Radio Engineering Measurements (VNIIFTRI, Designated Institute of Russia of the CIPM MRA). Two standard hydrophones, B&K 8104 and TC 4033, were calibrated and compared to assess the current state of hydrophone calibration of HAARI (China) and Russia. Three different calibration methods were applied: a vibrating column method, a free-field reciprocity method and a comparison method. The standard facilities of each laboratory were used, and three different sound fields were applied: pressure field, free-field and reverberant field. The maximum deviation of the sensitivities of two hydrophones between the participants' results was 0.36 dB.Main text. To reach the main text of this paper, click on Final Report.The final report has been peer-reviewed and approved for publication by the CCAUV-KCWG.
This is the final report of a Russian-Chinese international comparison of hydrophone calibration methods conducted by exchanging hydrophones between the Hangzhou Applied Acoustics Research Institute (HAARI) and the All-Russian Scientific and Research Institute for Physical-technical and Radio-technical Measurements (VNIIFTRI). The comparison was organized in 1997/98 in order to assess the current state of the art for hydrophone calibration in the Russian Federation and China. Six hydrophones of different types were calibrated using the Russian and Chinese Underwater Pressure National Standards in the frequency range 1 Hz to 630 kHz. The following methods and facilities for hydrophone calibration were used during the comparison: piezoelectric compensation in a closed chamber, vibrating column in an open chamber, and free-field reciprocity in the laboratory tanks. Some features and nuances of these facilities are considered. The agreement of 140 calibration data was checked for six hydrophones. The mean and maximum differences between the participants' results were 0.24 dB and 1.6 dB, respectively.
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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