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84-1769

A Fiaite Differenc* Sohition for the Propagation of Sound in New Sonic Flows

S.I. Hariharan and H.C. Lester Inst. for Computer Applications in Science and Engrg., Mail Stop 132C. NASA Langley Res. Ctr.. Hampton, VA 23665, J. Acoust. Soc. Amer.. 75 (4). pp 1052-1061 (Apr 1984) 11 figs, 1 table. 12 refs

Kay Worda: Sound waves, Wave propagation

An explicK time/«jace finrte dłfference procedurę is uaed to model the propagation of aound in a quasi-one-dimen-aional duet containing high Mach numbar aubaonie fk>w. Noniinaar acoustic equatk>ns ara darivad by parturbing the tima-dapandant Euler aquations about a itaady. eompraaaibla maan flow. The govarning difference ralationaara ba sad ona fourth-order, two-step (predictor-corrector) MacCormack scheme. Oifference equatk>ns for tha aource and termination boundary conditions ara darived from the appropriata characteristic ralationihips. Tha solution algorrthm functions by switching on a time harmonie sourca and allowing tha differanca aquations to Karate to a steady State. A signifkant advantage with this approach is that tha noniinaar tarms csn ba ratainad and evaluated with only modett additionał Computer cott above that required for a linaar model calculation.

84*1771

Sound Field in a RectanguUr Enclosure with Dif-fuaely Reflecting Boundmries

R.N. Miles

The Boeing Co.. P.O. Box 3707, Seattle. WA 98124, J. Sound Vib.. 22. (2), PP 203-226 (Jan 22. 1984) 10 figs. 1 table. 6 refs

Kay Words-' Enciosures, Rooms, Sound waves, Wave ref lec-tion, Wall*

The sound field is determined in a three dimensional rectan-gular anclosura with nonuniformly absorbent and diffusely reflecting wali*. Tha method consist* of solving tha geomet-rical acoustic* intagral equatk>n for enciosures containing either steady State or time-varying sourcas. Thasa Solutions ara an improvement over classical geomet rical acoustics in which it is assumed that tha reverberant field isdiffusa in both the steady State and during decay. The conditions in which tha classical mathods can laad to errors ara shown and simple approximate formutas ara prasantad which com-prisa a correction to Sabina's dacay time formule for the a yaraga absorption ooafficiant of reverberation cham bert with absorbent test samples.

84-1770

Distortion and Harmonie Generation in the Nearfield of a Finite Amplitudę Sound Beam

S.I. Aanonsen, T. Barkve. J.N. Tjotta, and S. Tjotta The Univ. of Bergen, 5000 Bergen, Norway. J. Acoust. Soc. Amer..(3). pp 749-768 (Mar 1984) 11 figs, 2 tables. 39 refs

Kay Words: Sound wavas, Wava propagation

Dittortion and harmonie generation in tha naarfiald of a finite amplituda aound bmm ara conjkJered, aaauming tima-pariodic but otharwise arbKrary on-aource conditions. Tha basie equationt of motion for a loasy fluid ara simplif lad by utilizing tha parabolic approximation. and tha aolution ia darły ad by seeking a Fourier seria* expension for tha aound presaure. Tha harmonics ara governed by an infinKa set of couplad dłffarantial aquationa in tha amplituda*, which ara truneatad and so)ved numarlcally. Amplituda and phase of tha fundamantal and the first faw harmonie* ara calculated aiong tha baam axis, and acroaa tha baam at v*rious ranga* from tha sourca. Two casa* for tha sourca ara considered and compared.

84*1772

The Combined Effects of Fluid Loading and Córa-pliant Coating on the Acoustic and Structural Field s of an Infinite Ranę Elastic Surface

D.G. Crighton

Univ. of Leeds. Leeds LS2 9JT, UK, J. Sound Vib., 22(2). pp 237-252 (Jan 22. 1984) 14 refs

Kay Words: Membranes (structural membert), Coating*. Fluid-inducad axcKation, Elanie system*, Sound wavas, Wava propagation

Th i* pa par daals wKh tha combined effects of fluid toeding and compłiant coating on an alastic surface. takan to ba a membrana, under uniform lina forca excitation. Tha param-et ara dafłning a coating mey be choaen either to dacoupla tha fluid from tha surface yibration and thereby raduca tha far field rediation. or to cancel fluid loading affact* and restore tha far field to tha level and directivity corra*»nding to tha vtoratk>n of a surface without fluid loading. In both cases there may ba attandant changas In tha structural vk>ration leval», either ciosa to tha excKatk>n or far from K. Thasa changas ara ex«mined jn a oomprehensive fathion. tha drlya and transfer admłttances and tha acoustic far field bting calculated for exchatk>n at presertbed forca ampIKuda, at preseribad yalocity ampIKuda, and for preseribed input powai.