Warfare Systems Command, the Naval Postgraduate School, and most importantly by several branches of the Department of Energy: Advanced Industrial Concepts, Materials Science in Basic Energy Sciences, and our local Industrial Partnership Center. A discussion with Pat Arnott helped us define the scope of this work, and comparisons to the results of parallel-plate-stack codes written by A1 Migliori and Dick Martin were useful in the early stages. Suggestions by Kim Godshalk, Charles Jin, Tom Hofler, and Jeff Olson have led to significant improvements in DELTAE's capability and usability.

•A. D. Pierce, Acoustics (Acoustical Society of America, Woodbury, NY, 1989).

2 N. Rott, "Damped and thermally driven acoustic oscillations in wide and narrow tubes," Z. Angew. Math. Phys. 20, 230 (1969).

3G. W. Swift, "Thermoacoustic engines," J. Acoust. Soc. Am. 84, 1145 (1988). N. Rott, "Thermally driven acoustic oscillations, Part III: Second-order heat flux," Z. Angew. Math. Phys. 26, 43 (1975), as extended in Ref. 3. Thermophysical Properties of Matter: the TPR C Data Series, edited by Y. S. Touloukian (Plenum, New York, 1970).

6T. J. Hofier, "Thermoacoustic refrigerator design and performance," Ph.D. thesis, University of California at San Diego ( 1986); T. J. Hofler, "Concepts for thermoacoustic refrigeration and a practical device," Proceedings of the 5th International Cryocoolers Conference, Mon- terey, CA (1988). G. W. Swift, "Analysis and performance of a large thermoacoustic engine," J. Acoust. Soc. Am. 92, 1551 (1992).

8j. R. Olson and G. W. Swift, "Similitude in thermoacoustics," J. Acoust. Soc. Am. 95, 1405 (1994).

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.

Generation, diffraction, and radiation of subsonic flexural waves on membranes and plates: observations of structural and acoustical wave fields [43.20.Rz, 43.20.Px, 43.40.Rj]--Thomas J. Matula, Department of Physics, Washington State University, Pullman, WA 99164-2814, December 1993 (Ph.D.). Electromagnetic acoustic wave transducers (EMATs) are described for generating low-frequency tone bursts on metallized membranes in air and elastic plates in water. Bursts on the membrane have phase velocities much less than the speed of sound in the surrounding air and are accompanied by plane evanescent waves. The frequency and time-domain responses of the EMAT and the depen- dence on gap spacing between the coupling coil and the membrane were studied. Wave-number selective optical and capacitive probes were used to measure the wave properties. Versions of these transducers are insen- sitive to long wavelength motion of the membrane. Diffraction of the burst by a sharp edge in air was observed as a function of the gap between the membrane and a razor edge. The scattered pressure decreases expo- nentially with increasing gap as expected from an approximate analysis of edge diffraction of evanescent waves. In related work an EMAT is used to generate 28-kHz tone bursts of bending waves on an aluminum plate. The bursts propagate down into water where the surrounding wave field is probed. Observations described indicate that there occurs a branching of energy as the wave crosses the air-water interface. Radiation from sub- sonic flexural plate waves due to the discontinuity in fluid-loading is observed. It is partially' analogous to the transition radiation of fast charged particles crossing a dielectric interface. The angular radiation pattern resembles that of a line quadrupole. Near the interface there exists an interference between the two energy branches in water that produces a series of pressure nulls. The pressure nulls are associated with a •r phase change in the wave field and are indicators of wave-front dislocations. A computation of the wave field in an unbounded fluid due to a line-moment excitation of a plate is comparable with the null pattern observed but differs in certain details.

Thesis advisor: Philip L. Marston.

Physical theory of narrow-band sounds associated with aneurysms [43.28.Ra, 43.80.Qf]--T. Douglas Mast, Graduate Program in Acoustics, The Pennsylvania State University, University Park,

PA 16802, August 1993 (Ph.D.).A theory is presented for the mechanism of narrow-band sounds associated with intracranial aneurysms in hu- mans. These sounds have frequencies on the order of 500 Hz and the bandwidths corresponding to quality factors on the order of 30. An an- eurysm can be modeled as a lumped-element oscillator in which kinetic energy is associated with motion of blood in the opening and potential energy is stored in the flexible walls of the sac. However, quality factors of aneurysms as lumped-element oscillators are much smaller than ob- served quality factors of aneurysm sounds, so that aneurysm sounds can- not be explained as a simple resonance phenomenon. The best explanation for aneurysm sounds is a self-excited (limit cycle) oscillation of the sys- tem comprised of an aneurysm and an unstable arterial flow. Limit cycles of the aneurysm-fiow system are found using describing-function analysis. In these limit cycles, arterial flow disturbances lock onto the natural frequency of the aneurysm, resulting in a narrow observed bandwidth, even for a time-varying mean flow. This effect makes it possible to dis- tinguish aneurysm sounds from other arterial sounds. Results of the the- ory agree qualitatively with observed characteristics of aneurysm sounds. The theory has potential for application in noninvasive acoustic diagnosis of aneurysms and characterization of aneurysmal tissue.

Thesis advisor: Allan D. Pierce.

The combustive sound source [43.30.Lz]--Preston Scot Wilson, Mechanical Engineering Department, The University of Texas at Austin, Austin, TX 78712-1063 (M.S. Engineering).This thesis describes a unique low-frequency underwater sound source, called the combustive sound source (CSS). Electrolysis of water produces hydrogen and oxygen gas, which is captured in a combustion chamber and ignited with a spark. The ensuing combustion produces high-intensity, low-frequency acoustic pulses. The thesis begins by discussing the background of the project and electrolysis. Two experiments are presented that relate the acoustic output of CSS to fundamental combustion theory. The dependence of the radi- ated acoustic waveform on the volume and depth of the bubble was also investigated. The first bubble period of the CSS pressure signature agrees with Raleigh-Willis theory in trend, but not in absolute value. Empirical equations are presented which predict the first bubble period for three different situations. Through high-speed filming of the CSS bubble, the motion of the bubble is shown to be related to the acoustic output in the classic manner, with pressure peaks associated with minimum bubble volumes. Finally, several other factors that affect the acoustic output of CSS are discussed. These include the shape of the combustion chamber, the ignition source, the oxidizer, the presence of high-pressure bubble collapses, and the presence of high-frequency components.

Thesis advisors: Janet L. Ellzey, Thomas G. Muir, Mark F. Hamilton.

3672 d. Acoust. Soc. Am., Vol. 95, No. 6, June 1994 Technical Notes and Research Briefs 3672

Redistribution subject to ASA license or copyright; see http://acousticalsociety.org/content/terms. Download to IP: 128.114.34.22 On: Sat, 22 Nov 2014 11:29:01