885096005

TUESDAY MORNING, 30 APRIL 1991

LIBERTY A, 8:45 TO 11:50 A.M.

Session 2EA

Engineering Acoustics: Transducers and Arrays

James M. Powers, Cochair

Naual Underwater Systems Center, New London, Connecticut 06320-5594

George S. K. Wong, Cochair

Institute for National Measurement Standards, National Research CounciL Ottawa, Ontario KIA 0R6, Canada

Chair*s Introduction—8:45

Contrihuted Papers

8:50

2EA1. High-powcr test ot a barrel stave flextensional transducer.

Mark B. Moffett and William L. Clay, Jr. (Naval Underwater Systems Ctr., New London, CT 06320)

Barrel stave flextensional transducers are potentially useful as compact, low-firequency, high-power projectors. An equivalent circuit model that includes a higher-mode, extensional compliance is used to estimate the maximum radiated power. Becausc the mechanical quality factor, Q, is Iow (on the order of 3 or 4), the source level of such a projector is limited by the maximum electric field that the piezoclectric ring stack driver can safely handle without dcpolarization or significant dielectric losses (about 400 kV/m for Navy type III lead zirconate titanate). A barrel stave flextensionai projector 18 cm long and 9 cm in diameter with a mass of 4.1 kg in air was tested to 200 psig (1.4 MPa) in the pressure vessel at NUSCTs Dodge Pond Field Station. A source level of 194.7 dB//l ^Pa-m was obtained at 1.56 kHz for an applied rms volt-age of 5 kV. The projector figure-of-merit was about 14 W/kg-kHz-£, and this number would be expected to apply to a larger, lower-frequency projector of commensurate dimensions.

9:05

2EA2. Applications of very bigh-energy density electrostrictive ceramics for underwater projectors. A. P. Ritter, S. M. Pilgrim, P. Kuhn, S. R. Winzcr (Martin Marictta Labs., 1450 S. Rolling Rd., Baltimore, MD 21227), and J. Sewell (Martin Marietta Aero & Naval Systems)

Performance of Navy sonar transducers is limited by the inherent energy density of the driver materiał—especially for those size-constrained applications that require very high source levels and/or very Iow frequencies. Ultimately, the maximum sound-pressure level will be limited by the ainount of power that can bc gcucratcd frnm the ceramic driver at its masimum engineering limit of 10-15 V/mil ( —0.5 MV/m). Incremental improvements in transducer performance may be possible througli design refinements, however, revolutionary large-scale improvements require new approaches to overcome the basie PZT materia! limitations. Electrostrictive ceramic materials. such as the PMN-based compositions being developed at Martin Marietta Laboratories, have energy density values an order of magnitude higher than Navy PZTs and therefore could significantly improve transducer performance (i.e., maximum attainablesource level) if substituted for PZT in conventional transducer designs. Transducer model calculations for comparable PZT- and PMN-driven transducers show — 10-dB gain m the transmitting voltage response for the PMN transducer. These model

1058 J. Acoust. Soc. Am., Vol. 09, No. 4, Pt 2, April 1991 predictions have been verified by experimental data obtamed under a joint Martin Marietta/Navy materials development program.

9:20

2EA3. Using piezoclectric film and ultrasound resonance to measure the elastic moduli of spherical ceramic particles. P. S. Spoor, M. J. McKenna, and J. D. Maynard (Dept. of Physics, Penn State Univ., University Park, PA 16802), and John R. Hellmann (Penn State Univ., University Park, PA 16802)

The search for altemate sources of energy has prompted interest in smali ceramic beads, callcd "proppants,” which were developed as a means of “propping” open cracks during the hydraulic fracturing of l>edrock in the vicinity of oil wells; recently, they have been considered as possible thermal transfer media for use in solar reccivers [J. R. Hellmann et at.. “Evaluation of Spherical Ceramic Particles for Solar Thermal Transfer Media,” SAND86-0981, Sandia National Laboratories, January 1987). To monitor the effect.s of repeated thermal stresses on the proppants, one would like to have a reliable measure of their elastic constants; however, their spherical shape and smali size (—500 /im) make conventional techniques, such as pulse-echo, inapplicable. Using a special piezoclectric film transducer and a smail-sample resonance tech-nique (J. D. Maynard, J. Acoust. Soc. Am. Suppl. 1 85, S20 (1989)], the authors have been able to determine the elastic constants and their variation as a function of heat treatment. (Work supported by the Office of Naval Research and NSF Grant DMR 9000549.)

9:35

2EA4. Optimally formulated high efificient planar projector arrays. P.

M. Joseph and P. R. Saseendran Pillai (Dept. of Electron., Cochin Univ. of Sci. and Technol., Cochin 682 022. India)

Closely packed multi-element transducer arrays are extensively used in underwater applications for achieving better directionality and longer transmission rangę. With the current tendency of extending their oper-ation toward lower frequency and particularly near resonance, the acoustic interaction among the elements grows stronger and in tum will degrade the predicted transmitting characteristics. This troublesome ef-fect is much alleviated in uniform planar arrays by restructuring it with the optimal interelement sparing at which the interaction force is minimum (P. M. Joseph and P. R. Saseendran Pillai, Acoust. Lett. 12 (11), 190-193 (1989)]. It has been seen from the results of computation that a further reduction in interaction can be achieved by incorporating the nonuniform array concept. A simple method for predicting the opti-

121st Meeting: Acoustical Society of America 1858