885095953

10:45

8PA10. Variational scatterinR calculations for a prolate spheroid. D.

E. Freund and R. A. Farrel (Johns Hopkins Univ. Appl. Phys. Lab., Johns Hopkins Rd., Laurcl, MD 20723)

Many theoretical melhods for computing the scattering amplitudę, T, employ an initial approximation (trial field) to the exact fields within, or on the surface of, the scatterers. Variational principlcs are unique in that łhey possess the important feature known as quadratic convergence. That is, first-order errors in the trial fields manifest them-selves as only second- and higher-order errors in the desired quantity T. Results that were remarkably accurate at all wavelengths were achieved previously [D. E. Freund and R. A. Farrelt, J. Acoust. Soc. Am. 87, 1847-1860 (1990)1 by using the Schwinger variational principlc in con-junction with simple, physically motivated, trial fields for the case of scattering from a soft sphere. Here, the efficacy of these simple trial fields is investigated further by applying them to the morę sevcre test of scattering from an acoustically soft prolate spheroid with a piane wave axially incident. Accurate forward- and backscattcring results arc found for sphcroids with aspect ratios ranging from 2:1 to 100:1. [Work sup-ported by the U. S. Navy under Contract N00039-89-C-0001.1

11:00

8PA11. The manifestation of certain classes of resonances as standing waves on elastic shells. M. F. Werby (NOARL, Codę 221, Stennis Space Center, MS 39529) and H. Ubcrall (Catholic Univ., Washington, DC 20064)

In an earlier work it was shown that certain classes of resonances excited on elastic solids correspond to standing-wavc patterns on the object surface. The observation was in fact implicit in the circumferen-tial naturę of such resonances madę elear by the early theoretical works of Uberall et al and the experimental work of R. Goodman et al. The demonstration was madę possible by subtracting the rigid background (in partia! wave space) using the background proposed by Flax, Ober* all, and Dragonette for elastic solids. The background for elastic shells introduced in the upcoming book by Ubcrall on resonance scattering now makes it possible to demonstrate that this is also observed for Lamb-type resonances and this fact will be demonstrated for several submerged elastic shells.

11:15

8PA12. Opening ratę of the transverse cusp diffraction catastrophe in scattering from obiatę penetrable sphcroids. Cleon E. Dean (NOARL, Stennis Space Center, MS 39529-5004) and Philip L. Marston (Washington State Univ., Pullman, WA 99164-2814)

Sound scattered by an obiatę penetrable spheroid should produce a transverse cusp caustic in the region associated with the rainbow in optics (for relativc speed of sound c^/c < 1). The principal curvatures of the generic local wave front that produces the far-field transvcrse cusp are examined. This wave front is shown to generatc a caustic curve (U— U_c) ³ = where U and V are horizontal and vertical scattering angles, and U_c is the cusp point direction. The far-field opening ratę d_m is calculated for the transverse cusp. It is shown that d_x has a simple dependence on the parameters of the generic wave front. Define the aspect ratio q — D/H, where H is the height and D is the equatorial width of the penetrable spheroid. Generalized ray tracing is used to relate q to principal curvatures and shape parameters of the outgoing wave front and hence to d_v. Measurements of d_x in the optically anal-ogous problem appear to support the calculation. As q goes to 4^1.31, the critical valuc for the generation of a hyperbolic umbilic focal section, the predicted d_x goes to infinity. The naturę of the diver-gence was numerically investigated as was the ratę at which d^ vanishes as q approaches other critical values. The analysis suggests bcnchmarks for testing numerical scattering codcs. (Work supported by ONR.]

FRIDAY MORNING, 3 MAY 1991

INTERNATIONAL C, 9:00 TO 10:45 A.M.

Session 8PP

Psychological and Physiological Acoustics: Localization and Binaural Processes

Richard Freyman, Chair

Department of Communication Disorders, Uniuersity of Massachusetts, Amherst, Massachusetts 01003

Contributed Papers

9:00

8PP1. Effects of age on auditory spatial resolution in the horizontal piane. David W. Chandler and D. Wesley Grantham (Bill Wilkerson Ctr. and Vanderbilt Univ. Div. of Hearing and Speech Sci., 1114 19th Ave. South, Nashville, TN 37212)

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The purpose of this research was to determine if auditory spatial resolution in the horizontal piane changcs with age. Spatial resolution was measured in static conditions, represented by the minimum audible angle (MAA), and in dynamie conditions, represented by the minimum audible movement angle (MAMA). MA As and MAM As were measured for adult subjccts 60 to 80 years of age with normal (or near normal) hearing thresholds. All measurements were obtained in a dark-

1994 J. Acoust. Soc. Am., Vol. 89. No. 4, Pt. 2. April 1991 ened anechoic chamber. Thresholds were measured as a function of stimulus bandwidth, center frequency, and listening azimuth, for both MAA and MAMA. Additionally, MAMAs were measured as a function of the vclocity of the moving stimulus source. MAAs and MAMAs of the elderly will be compared with previous measurements for young normal adults that were obtained under the same stimulus conditions [D. W. Chandler and D. W. Grantham, J. Acoust. Soc. Am. Suppl. 1 87, S64 (1990)]. It is hypothcsized that auditory spatial resolution in the horizontal piane diminishes with age because of generalized deficits of central nervous system processing. This diminished resolution should be reflected by poorer MAAs and poorer MAMAs among the elderly subjects than among the young, normal-hearing subjccts. [Work supported by NIH.]

121st Meeting: Acoustical Society of America 1994