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mum configuration of element locations that reduce the interaction to a lower level, without altering the aperture dimension, is proposed in this paper. Here, the element locations are dispersed in accordance with Gaussian distributed random mimbers by keeping the optimum spacing as the mean. Even though a large standard deviation va!ue will greatly rcduce interaction, a reasonable value is chosen, as large deviałions produce deteriorated beams. This optimally formulated array does not exhibit any grating lobes nor does its radiation pattem diffcr much from the conventional A/2 spaced and restructured anays.

9:50

2EA5. The generation of sound by a dipole moving near the edge of a half-plane. Gerrit Schouten (Aerospace Dept., Delft Univ. of Technol., Kluyverweg I, 2629 HS, The Netherlands)

The shape and the spatial amplitudę distribution of the sound pulse generated by the passage of a dipole (or ring vortex) near the sharp edge of a half-plane are computed and presented in plots. Instead of using a low-frcquency approximation to the Grecn’s function a direct approach to the poteutial of a moving dipole in a branched spacc is used together with an image dipole. In a compressible medium (with a finite velocity of sound) the wave is physically present. In the limit of an incompressible medium the wavelength becomes infinite and the wave degenerates into a timc history of the pressure variation. The incompressible pressures agree with those obtained from direct differentiations of the classical results of Sommerfeld [Proc. London Math. Soc. 30, 121-163 (1897)]. The wavc results are in conflict with some results presented in the literaturę [T. Kambe et al., J. Fluid Mech. 155, 77-103 (1985)] obtained with a low-frequency approach.

10:05

2EA6. Charactcrization of anisotropy in wood composites. V. Bucur (Centro de Recherches Forestiercs de Nancy, Champenoux, 54280 Seichamps, France)

The aim of this report is to study the anisotropic behavior of wood composites using the ultrasonic vc!ocity method and acoustic emission method. Velocities of ultrasonic longitudinal and transversal waves were used for the estimation of five elastic constants. Complementary, stim-ulated acoustic emission, induced by breaking 0.5-mm pencil lead on the surface of the specimen, was employed to measure five parameters of the acoustic emission signal (duration, counts number, energy, amplitudę, and nse time). The anisotropy was estimated as the ratio of velocities, of acoustic invariants, and acoustic emission parameters.

10:20

2EA7. FI o w noise induced in large arrays via the flexure of the support. B. Dubus (lnstitut Superieur d’Electroniquc du Nord, 41 BIvd. Vauban, 59046 Lille Cedex, France) and R. E. Montgomery (Naval Res. Lab., Orlando, FL 32856)

The noise induced by the turbulcnt boundary layer in sonar arrays is usually split into a direct path where the fluctuating pressures directly excite the hydrophones after traveling through an elastomer layer backcd by a rigid surface, and the indirect path where the excitation of the support by the turbulent boundary layer induces the noise in the array. To dcscribc this last phenomena, the force-modal transform method [M. C. Junger and D. Feit, Sound, Structures, and Their Interaction (MIT Press, Cambridge, MA)] has been extended to dcscribc the

flcAiirc of a submerged multilayercd platc and the wave numbcr spectrum of the noise sensed by the array. Several configurations are ana-lyzed showing the effect of the stiffness and the damping of the support and of the distribution of the hydrophones. From these results, somc Solutions to reduce the noise level are proposed. (Work partially sup-ported by Direction des Recherches Etudes et Techniques, Paris.]

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2EA8. Wave propagation in a viscoelastic medium with a high concentration of pores. Y. Ma, V. K. Varadan, V. V. Varadan (Res. Ctr. for the Eng. of Electron, and Acoust. Mater. and Dept. of Eng. Sci. and Mech., Penn State Univ., University Park, PA 16802), and C. Audoly (G.E.R.D.S.M. Le Brusc, 83140 Six-Fours, France)

Wave propagation in a viscoeIastic and porous medium, whosc prop-crties are temperaturo and frequency dependent, was investigatcd. The propagation characteristics of compressional waves in such a medium cannot be predicted using single scattering inodels when the porosity of the medium is considerable. Different from a previous study [Y. Ma, V. K. Varadan, and V. V. Varadan, ASME J. Heat Transfer 112, 402-107 (1990)], which was for electromagnetic waves, this investigation pro-vided another proof of enhanced attenuation due to dependent scattering for the acoustic case. Most importantly, this is a problem of an elastic wave propagation in a lossy host medium with lossless scatterers which has seldom been considered. Numerical results of phase velocity as well as of attenuation of such a medium for different temperatures and different frequencies are presented. Differences in results between single and multiple scattering when the porosity is inereased are also shown. Numerical results considcring multiple scattering compare fa-vorably with those from the experimenta! work which was done inde-pendently by G.E.R.D.S.M. in France. [Work supported by CEEAM.]

10:50

2EA9. Computation of sound radiation from an arbitrary body with mixed boundary conditions using program chief. W. Thompson, Jr. (Penn State Univ., Appl. Res. Lab., P.O. Box 30, State College, PA 16804) and C. M. Siders (Naval Res. Lab., Orlando, FL 32856-8337)

The Computer program chief has been modified to allow the computation of sound radiation from an arbitrary-shaped body where the normal velocity is prescribed on only part of the surface of the body while a locally reacting complex impedance is assumed to cover the remainder of the surface. This impedance can rangę in value from zero (pressure-release boundary condition) to cssentially infinity (rigid boundary condition). Different values of impedance are allowable on different portions of the same surface. Some results of computations of far-field directivity pattems and the radiation impedance loading on simple-shaped radiators with the mixed boundary conditions are presented. (Work sponsored by ONT.]

11:05

2EA10. Finite element model for a planar fiber optic hydrophone. Paul J. Zoccola and Paul C. Shang (FIow Noise Branch, David Taylor Res. Ctr., Bethesda, MD 20084)

A finite element model has been dcveloped for estimating the acoustic sensitivity of a planar fiber optic hydrophone. The sensor consists of a single-mode fiber surrounded by a thin layer of Silicon and a hytrel jacket. This strand Ls eoiled and embedded in an elastomer layer. The model will allow use of nonisotropic materials also; these materials aro less suitable for analytical modeling. The finite element model uses lin-ear solid elcmcnts and uses symmetry about two or three planes, de-pending on the configuration. Because the sensor is designed for low-frequency measurements, hydrostatic pressure loading is used. Scnsitivity is calculated using the strain on the fiber. The results aro compared to closed form Solutions for cylindrical configurations and to experimental results. They show that for most practical purposes, sen-sitivity is mostly controlled by the volume of the elastomer surrounding the fiber.

11:20

2EA11. Theoretical study of a fiberacoustic waveguide with continuous variation of the radial acoustic velocity. A. G. Yin, V. K.

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J. Acoust. Soc. Am., Vol. 89. No. 4, Pt. 2, April 1991

121st Meeting: Acoustical Society of America

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