High-power, underwater transducers utilizing polarized piezoelectric ceramic material are usually limited in drive amplitude so that depolarization does not occur, but application of a dc bias field in the polarization direction allows the use of higher ac drive fields. To demonstrate the feasibility of biased operation as a means of achieving higher power, a thin-walled, spherical-shell transducer was constructed of Channel 5800 and tested in NUWC's Acoustic Pressure Tank Facility at a hydrostatic pressure of 1400 psig ͑9.65 MPa͒. The transducer was successfully driven to 33 V/mil ͑1.3 MV/m͒ rms with an accompanying bias field of 31 V/mil ͑1.2 MV/m͒. The source level was 206 dB re: 1 Pa•m at 61 kHz, corresponding to a 10-dB improvement over the unbiased drive limit of 10 V/mil ͑0.4 MV/m͒ rms and 196 dB re: 1 Pa•m.

Tonpilz transducer head piston diameters are usually about one-half wavelength in diameter, so that ‘‘rho-c’’ radiation loading will be approximately achieved. However, for this underwater tracking application, the transducer is required to fit inside the throat of a 360-deg horn. The piston diameter is limited to 1 in., or about 0.22 wavelength at the resonance frequency, 13 kHz. The design is based on a lumped-element model. The PZT-8 ring stack is 2 in. long with an outer diameter of 0.5 in. and an inner diameter of 0.25 in. The head mass is 0.25-in.-thick aluminum, and the tail mass is tungsten, with a mass of 86 g. The transducer is isolated from its cylindrical housing with a syntactic foam spring. An O-ring piston seal is used, and a similar O-ring provides support for the tail mass. The results of free-field measurements will be presented and compared with the model, which will then be used to predict the performance after installation in the horn. [Work supported by Naval Air Systems Command.]

The direct measurement of farfield acoustic radiation from underwater transducers in a shore-based facility is often impossible because of the limited size of measurement tanks. The Trott nearfield calibration array (NFCA) concept solves this problem by allowing the farfield radiation to be determined from measurements made close to the transducer. We describe here the development of a 5- to 50-kHz synthetic cylindrical NFCA that is suitable for measurement tanks as small as about 2.5 m in each dimension and that can be operated at hydrostatic pressures up to 6.8 MPa. The NFCA is synthesized using a single line of 48 piezoelectric ceramic hydrophone elements spaced 2.54-cm apart center-to-center and amplitude shaded in a special way. The transducer to be calibrated is driven electrically and rotated near the line about an axis parallel to the line. The response of the line is measured at equally spaced angular positions, thereby creating a virtual cylindrical array surrounding the transducer. We then obtain the azimuthal farfield pattern of the transducer by computer processing the angular responses together with a set of complex shading coefficients. We evaluated the line experimentally by calibrating a large piston transducer in a cylindrical measurement tank less than 2.5 m in diameter. The resulting computed farfield patterns are in excellent agreement with corresponding patterns obtained from direct farfield measurements made in a lake.

PERIOZ ~CO4ER-i Interim report on a continuing USRD TYPE F63 TRANSDUCER i NRL oroblem.

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