reference, the information is coded using the DPSK method (differential phase shift keying). At the reception, the carrier is restored from the re- ceived signal to avoid the problem of Doppler effect during the transmission in water, and after an appropriate processing, is used to demodulate the signal and, to put it in a logical form, the logical restored signal is then decoded using the DPSK method. To test the sensitivity of the system to the parasite echoes, an experimental setup has been used. The emitted beam is separated into two parts by two plane reflectors. The difference in the phase and the relative amplitude are adjustable. The use of a derivative circuit for putting in logical form allows the sensitivity of system to be lowered for the parasite echoes.

Thesis advisor: Professor G. Maze.

Copies of this thesis may be obtained from Professor J. Ripoche, L.A.U.E, Universit• du Havre, Place Robert Schuman, 76610 Le Havre, France.

Statistical study of Hindi speech sounds[43.70.Kv]--Israr Khan, Department of Physics, Aligarh Muslim University, Aligarh-202 002 (India), July 1990 (Ph.D.). The thesis presents the statistics of a sizable corpus of 135 014 running words of present printed Hindi text. The random samples were picked up from the different columns of daily newspapers from differ- ent regions of the Hindi speaking areas, excluding the editorials and jour- nalistic articles and some from popular magazines. The data were analyzed by a computer to obtain frequencies of occurrence of characters, diagram sequences, word count, word syllabic structures, occurrence of consonant clusters, consonants in word-initial and word-final positions and three most probable left and right characters. The statistical information obtained of word counts was utilized to construct five monosyllabic phonetically bal- anced (PB) word lists of 50 words each. Perception tests were carried out to test the proposed lists. For study of Hindi vowels, ten meaningful C1VC2 context words, selected from corpus and from human vocabulary survey, and commonly used/hV.d/nonsense context for Hindi native were compared by carrying out spectrographic study and perception tests. The meaningful words were found to have similar spectrographic parameters but better per- ception performance than nonsense/hV.d/context. The statistics will be used to select material for developing an acoustic phonetic data base of Hindi for speech research community.

Thesis advisor: S. K. Gupta.

Copies of the thesis may be obtained from S. K. Gupta, Department of Physics, Aligarh Muslim University, Aligarh-202 002 (India) on payment of electrostat and postal charges (approx. $20).

Modeling of the acoustic radiation of subsonic and supersonic free turbulent flows [43.28.Ra]--Christophe Bailly, Ecole Centtale Paris, France, March 1994 (Language: French). This work deals with the modeling of the acoustic field radiated by free turbulent subsonic and super- sonic flows. The aim of this study is finally to develop methods applicable to industrial configurations, as in numerical fluid mechanics. First, one uses a statistical model coupled with an integral resolution of Lighthill's equation. Two different approaches have been developed for free turbulent flow either with subsonic or supersonic convection Mach numbers. In the first ap- proach, one calculates the turbulent mixing noise. This model produces the acoustic intensity and power, frequency spectra and space-frequency distri- bution of acoustic sources. In the second approach, one calculates the Mach wave radiation due to the supersonic convection of turbulent large eddies. This model gives the acoustic power and directivity. A synthesis of these two models allows a complete description of the free jet acoustic radiation. Computation of the aerodynamic fluid is performed with a compressible g-e turbulence model. This approach is applied to subsonic and perfectly ex- panded supersonic cold round free jets, such as 0.35•<M•<2.0. The case of an M =2.0 and Tjet/To=2.5 hot supersonic jet is also treated. Comparisons with available experimental data show a very good agreement. In the second part, linearized Euler equations around a stationary mean flow are used to compute the acoustic field. Acoustic source terms are identified by analogy with aeroacoustic wave equations. The mean flow is calculated by solving averaged Navier-Stokes equations while the turbulent field is synthetized. The system formed by Euler's equations and their source terms deduced from the stochastic turbulent field, is numerically solved. This method is finally applied to the case of a subsonic jet. Comparisons with experimental

data are very encouraging. These two methods are similar in a sense that one systematically uses local mean flow data for acoustic prediction. However, they are basically different in that they describe turbulence and how the acoustic field propagates. Thus the second approach takes into account the mean flow effects on the acoustic waves propagation. In practice, these developments are already used for acoustic prediction of industrial systems. Thesis advisor: S•bastien Candel.

Copies may be obtained from Philippe Lafon, AMV/P63, Electricit6 De France, Direction des Etudes et Recherches, 1 av. du Gtntral de Gaulle, 92140 Clamart, France.

Numerical simulation of the effects of the turbulence on the

propagation of sound in the atmosphere[43.28.Bj, 43.28.Fp]-- Patrick Chevret, Laboratoire de M•canique des Fluides et d'Acoustique de l'Ecole Centrale de Lyon-URA CNRS 263, 36, Avenue Guy de Collongue, B.P 163 69131 Ecully Cedex (Ph.D.). Sound level in the first hundred meters of the atmosphere is difficult to predict because propagation is significantly affected by ground reflections and refraction associated with the mean gra- dients and fluctuations of temperature and wind speed. In our numerical model, the medium is considered as the sum of the mean gradient and turbulent fluctuations of the refraction index. The turbulence is represented as a set of independent realizations (generated by the superposition of ran- dom Fourier modes) in which the sound is propagated by making use of a wide-angle parabolic equation. The discretization method (split-step Pad•) incorporates the advantages of the split-step Fourier method (the speed) and of the implicit finite difference method (the possibility to model the ground impedance). Our calculations have been applied to two particular classes of atmospheric propagation problems for which outdoor measurements have shown that strong effects of the turbulence are present. Our results are mainly comparisons with those measurements. First, the modifications caused by the turbulence on the interference spectra in the line of sight region near the ground were studied. Second, the case of an upward refract- ing atmosphere was simulated and it was shown showed that, in the pres- ence of turbulence, an important diffusion of sound into the shadow zone exists. These calculations were done with a Gaussian correlation function.

To take into account the inertial region of the turbulence spectrum, a modi- fied von Karman spectrum was then employed and, in the case of the propa- gation in a shadow zone, a better agreement was observed with measure- ments.

Thesis advisor: Ph. Blanc-Benon.

Copies may be obtained from the address above.

Design and analysis of space-time and Galerkin/least-squares finite element methods for fluid-structure interaction in exterior

domains[43.20.Px, 43.20.Rz, 43.20.Tb, 43.40.Rj]--Lonny L. Thompson, Department of Civil Engineering, Stanford University, Stanford, CA 94305-4020, April 1994 (Ph.D.). This work is concerned with the design and analysis of new finite-element methods for structural acoustics involv- ing the interaction of elastic structures submerged in an infinite acoustic fluid. Space-time finite-element methods based on a new time-discontinuous Galerkin/least-squares (GLS) variational formulation for solution in the time-domain are developed and analyzed for stability and convergence. The methods are especially useful for the application of adaptive solution strat- egies in which unstructured space-time meshes are used to track waves propagating along space-time characteristics. New time-dependent and exact nonreflecting boundary conditions are developed based on the exact imped- ance relation through the Dirichlet-to-Neumann (DtN) map in the frequency domain. Several numerical examples involving transient radiation and scat- tering are presented. A Galerkin/least-squares finite-element method is also developed for time-harmonic acoustics in the frequency domain. A multidi- mensional Fourier analysis is used to select optimal GLS mesh parameters that lead to dramatically improved phase accuracy for waves propagating in multidimensions. Optimal GLS parameters are given for hierarchical p-version and spectral discretizations. Practical guidelines for phase and amplitude accuracy in terms of the spectral order and the number of ele- ments per wavelength are reported.

Thesis advisor: Peter M. Pinsky.

3813 J. Acoust. Soc. Am., Vol. 96, No. 6, December 1994 Technical Notes and Research Briefs 3813

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