The reciprocity coupler system in the Low-Frequency Facility (LOFAC) at the Underwater Sound Reference Divsion (USRD) of the Naval Undersea Warfare Center is a closed-chamber environmentally controlled system for the absolute calibration of standard hydrophones at low frequencies. There is a need to extend this capability to field hydrophones that are not specifically designed for use in a reciprocity coupler. The transfer reciprocity method described here is an answer to this need. An error analysis is presented indicating the current state of the existing reciprocity coupler including the transfer reciprocity method. The analysis shows that there is a systematic error which can be eliminated by correcting the sensitivities obtained using the transfer reciprocity method. This correction has a frequency independent component of 0.31 dB and a frequency dependent component of 0.1 dB at 700 Hz and 1.0 dB at 2000 Hz. After correction for the systematic error, the uncertainty in sensitivity is ±0.25 dB for frequencies below 1000 Hz, growing to ±0.35 dB for frequencies near 2000 Hz.

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.

The normal velocity distribution of the surface of a free-flooded ring transducer completely determines its radiation characteristics in a given medium. The SHIP program is an algorithm for rapidly predicting the radiation characteristics of a free-flooded ring when the known surface velocity distribution is axisymmetric. The character of the radiation at low frequencies is extremely sensitive to the details of the velocity distribution which, in turn, depends not only on the geometry of the ring, but also on the manner in which the ring is driven and on the electromechanical parameters of the ring material. Gross differences in the directivity patterns predicted for different drive conditions occur at frequencies well below the first radial resonance for typical rings. Directivity patterns were obtained for a number of rings fabricated for different commonly encountered drive conditions (circumference, thickness, or height) and compared with patterns predicted by the SHIP program using simple models for the velocity distribution appropriate to each drive. Good agreement was obtained between the measured and predicted directivity patterns.

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Numerically predicted radiation characteristics of uniformly vibrating free-flooded rings are compared with experiment at frequencies above the monopole region but still below any resonances. Low-frequency velocity distributions for piezoceramic rings for several commonly used drive conditions are derived. Analytical formulas for the monopole radiation resistance, farfield pressure, and free-field voltage sensitivity are presented. A technique is developed for measuring the piezoelectric constants of free-flooded ceramic cylinders. It is predicted and experimentally verified that a free-flooded circumferentially polarized ring has maximum sensitivity in the axial direction for a band of very low frequencies. Thus, such a system may have application as a low-frequency directional hydrophone.Subject Classification: 20.55.