TECHNICAL NOTES AND RESEARCH BRIEFS

Paul B. Ostergaard 10 Glenwood Way, West Caldwell, NJ 07006

Editor's Note: Original contributions to the Technical Notes and Research Briefs section are always welcome. Manuscripts should be double-spaced, and ordinarily not longer than about 1500 words. There are no publication charges, and consequently, no free reprints; however, reprints may be purchased at the usual prices.

Pressure-coupled acoustic-transducer assembly [43.35.Yb, 43.38.Fx]

Prior high-temperature measurement systems have typically in- cluded acoustic transducers hard coupled to delay lines by use of epoxies, brazing alloys, indium alloys, ceramics, and silicones. The acoustic cou- plings in such a system involve stationary bonds to the delay line and limit the mobility and use of the transducer assembly. An improved acoustic- transducer assembly developed for uge at NASA Langley Research Cen- ter is easy to assemble, is relocatable, and can be used at high tempera- tures.

In the improved assembly, a piezoelectric acoustic transducer is pressure-coupled to a delay line or fixture through a soft metal like alu- minum, copper, or gold. The transducer subassembly includes a layered structure (Fig. 1) of the coupling material, the transducer, a thin disk of the coupling material that acts as a cushion for the transducer, an elec-

Pressure Bolt

Transducer .... • .- Subassembly I..•_"1 i _-_--i I--"--"1 I-•, '1 I-'•'l

Pressure • Delay Line [ or Fixture

Insulating Container

Outside Container Insulated Feedthrough

• Pressure Plate • Insulation/Damping Material • Electrode

I ' • • Soft Metal ; ....... •,,.•."'""• • Acoustic Transducer

• • Coupling Material =lllllllllllllllllll•

FIG. 1. Coupling pressure is applied by tightening the bolt onto the transducer subassembly.

trode disk with a coaxial cable lead attached, insulation/damping mate- rial, and a pressure plate.

The fabrication of the transducer subassembly begins with the at- tachment of the coupling material to one end of a hollow metal cylinder that serves as the outer container of this subassembly. The coupling ma- terial can be any soft material that will withstand the operating temper- ature of the transducer. A cylindrical insulator is placed inside the outer metal container and serves to electrically insulate the transducer and hold the layers of the subassembly in place. Coupling pressure is applied via a rounded bolt threaded through a metal fixture. A 6oaxial-cable lead is attached to the electrode disk, and the shielding is attached to the metal cylinder so that the cylinder acts as an electrical ground plane as well as an enclosure for the assembly. The acoustic signal can be enhanced by matching the thickness of the coupling material to a quarter wavelength.

This assembly is designed to be used in the ab0ustic characterization of materials at high temperatures. It can be used at temperatures up to the melting temperature of the coupling metal. (This melting temperature must be less than the Curie temperature of the transducer.) Because this transducer is not hard-coupled to delay line, it can be moved easily from one location to another. Also, pressure coupling precludes the problem of matching the coefficients of thermal expansion of the transducer, coupling material, and delay line.

This work was done by F. Raymond Parker of Langley Research Center. No further documentation is available.

Advanced-degree dissertations in acoustics Editor's note: Abstracts of Doctoral and Master's theses will be

welcomed at all times. Please note that they must be double spaced, limited to 200 words, must include the appropriate PACS classification numbers, and formatted as shown below (don't make the editor retype them, please!). The address for obtaining a copy of the thesis is helpful. Please submit two copies.

Study of a single-fiber fiber optic lever microphone [43.38.Kb, 48.58.Vb, 43.38.Zp]--Trung Duc Nguyen, Old Dominion University, Norfolk, VA 23508, August 1993 (M.S. Engineering). This thesis describes the single-fiber fiber optic lever microphone, which is used to measure acoustic pressure at temperatures up to 538 øC (1000øF). The micro- phone is an optical-intensity modulated sensor, utilizing a single fiber to transmit and receive light, as opposed to multi-fiber array, for stability purposes. The microphone is designed to have a membrane radius of 914.4 pm (0.036 in.) for a bandwidth of 45 kHz or 406.4 pm (0.016 in.) for 120 kHz. The dynamic range of the microphone is typically from 130 to 190 dB. The thesis includes computational programs, based on the mechanical and optical transfer functions, to predict the theoretical re- sponse of the microphone. Laboratory calibration data at room temper- ature and 538 øC (1000 øF), along with a field test result, are presented. The background noise consists of the thermal noise sources from the electronic circuit and the shot noise from the photodiode. A portable computer is adapted to serve as a data acquisition system for service in the field.

Thesis advisors: Oscar R. Gonzalez, Allan J. Zuckerwar.

The adaptive beamformer: Evaluation of a noise-reduction scheme for hearing aids [43.72.Ew, 43.66.Ts]--Martin Kompis, ENT Department, University Hospital of Zurich, 8091 Zurich, Switzerland, July 1993 (Ph.D.) (Language: German). The adaptive beam-

1167 J. Acoust. Soc. Am. 95 (2), February 1994 0001-4966/94/95(2)/1167/2/$6.00 ¸ 1994 Acoustical Society of America 1167

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