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Nov 1981

Volume 70, Issue S1, pp. S1-S109

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back to top Session J. Physical Acoustics II: Diffraction, Scattering, and Wave Propagation
Contributed Papers
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Diffraction of sound by an infinite rigid cylinder to small forward angles: A calculation using methods of physical optics (A)

Jacob George

J. Acoust. Soc. Am. Volume 70, Issue S1, pp. S21-S21 (1981); (1 page)

Online Publication Date: 12 Aug 2005

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We have used the Kirchhoff method of physical optics to calculate the acoustic intensity diffracted by an infinite rigid cylinder to small forward angles and have compared the results with those of exact calculations. The intensities predicted by the two methods are in excellent agreement. Kirchhoff predictions of a phase angle seem to be unreliable.
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Uses of monostatic and bistatic echoes from (calibration) spheres for inverse scattering (A)

G. Gaunaurd and H. Überall

J. Acoust. Soc. Am. Volume 70, Issue S1, pp. S21-S21 (1981); (1 page)

Online Publication Date: 12 Aug 2005

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We study the summed backscattering amplitude ∣∫(π, ka)∣ returned by tungsten carbide spheres in water, insonified by phase acoustic waves. After verifying the agreement of the Faran/Hickling classical theory with recent measurements [Dragonette et al., J. Acoust. Soc. Am. 69, 1186–1188 (1981)] we analyze the partial waves contained within the summed amplitude in the light of the resonance scattering theory. We computationally isolate the resonances both in the frequency ∫ and in the mode‐order n domains, by appropriate background subtraction. We generate and display the three‐dimensional “response surface” [i.e., Brill et al., J. Appl. Phys. 52, 3205–3214 (1981)] of the fluid‐loaded sphere and show how its two distinct families of resonance frequencies are well approximated by Love's “spheroidal eigenvibration” [A. Love, Mathematical Theory of Elasticity (Dover, New York, 1926), p. 283]. We display bistatic plots of the scattered amplitude ∣∫(θ, ka)∣ versus ka at certain fixed values of θ that make ∫n vanish, and versus θ at certain fixed values of ka that also make ∫n vanish. We then show how the information in these plots is used to determine the sphere's composition from its bistatic returns.
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Pressure patterns in the nearfield of a diffracting object (A)

R. V. Waterhouse

J. Acoust. Soc. Am. Volume 70, Issue S1, pp. S21-S21 (1981); (1 page)

Online Publication Date: 12 Aug 2005

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A steady train of plane waves impinges normally on a tight circular cylinder of infinite extent. The nearfield diffraction patterns for the normalized rms pressure were computed for values of ka from 1.5 to 6, where k is the wavenumber and a is the radius of the cylinder. Charts are given, showing the contours of rms pressure for ka = 1.5, 2, 3, 4, and 6, for each of two boundary conditions, pressure reflecting, and pressure release. Each chart shows the contour pattern out to a distance of 5 radii. The computed expressions are infinite series containing Bessel functions and their gradients. In some cases more than 30 terms in the series must be evaluated to get convergence to within 1%. The charts show that there are substantial areas where the pressure differs from the undisturbed value by 6 dB or more.
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Comparison of physical acoustics and geometrical theory of diffraction predictions for backscattering by impedance‐covered wedgelike scatterers (A)

D. A. Sachs

J. Acoust. Soc. Am. Volume 70, Issue S1, pp. S21-S21 (1981); (1 page)

Online Publication Date: 12 Aug 2005

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Radar scattering studies have shown that scattering predictions based on physical acoustics are generally invalid away from specularly reflecting directions. In this paper, comparisons are made between the predictions of physical acoustics and the geometrical theory of diffraction for acoustic backscattering by impedance‐covered wedgelike scatterers. Substantial differences are seen in the results at angles of incidence well off the secularly backscattering direction where diffraction by the wedge edge dominates the return. [Work sponsored by Naval Underwater Systems Center, New London, CT.]
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Generation and propagation of the coherently scattered boundary wave over a ridge (A)

Stephen J. Hollis and Herman Medwin

J. Acoust. Soc. Am. Volume 70, Issue S1, pp. S21-S21 (1981); (1 page)

Online Publication Date: 12 Aug 2005

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Recent theoretical studies by I. Tolstoy and experiments by H. Medwin and co‐workers have shown that a low‐frequency sound at near grazing incidence to a surface of rigid hemispherical bosses generates a coherently scattered boundary wave which can be much larger than the spherically divergent direct radiation wave. In the present experiment the axial grazing diffraction of the boundary wave over a slightly rough wedge is measured. Two components of the boundary wave are identified in the shadow region of the wedge: one (RS) develops along the rough upslope then diffracts over the crest in the same manner as the volume wave; the ratio of this boundary wave amplitude to the volume wave amplitude (BWA/VWA) grows as range r01/2 upslope and is constant with increasing range r beyond the crest. The other component (SR) is generated by the diffracted volume wave along the rough downslope and BWA/VWA grows initially as range r0.6 from the crest. For both components the amplitude ratio has a frequency dependence ∫2. Low wave‐number grazing propagation over a rough wedge thereby produces a boundary wave in the shadow region whose amplitude can be several times greater than that of a diffracted volume wave over a smooth wedge. [Work supported by Office of Naval Research.]
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Acoustic spectroscopy (A)

D. Brill, G. C. Gaunaurd, and H. Überall

J. Acoust. Soc. Am. Volume 70, Issue S1, pp. S21-S22 (1981); (2 pages)

Online Publication Date: 12 Aug 2005

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The resonances of elastic objects imbedded in a fluid medium have been shown to play an important role in the acoustic response of the object. We have previously discussed aspects of the inverse scattering problem, i.e., how to gain information regarding the material composition of the target from a determination of the resonance peaks. [J. Appl. Phys. 52, 3205–3214 (1981)]. Nuclear physics has shown that information on the shape of nonspherical objects may be obtained from analysis of the splitting of the giant (nuclear) resonances. We have used this idea to study the acoustically excited mechanical resonances of liquid targets by plotting the resonance spectral levels of liquid spheres. We have further studied the splitting of these resonances as the sphere deforms into spheroids of increasing eccentricity. These spectra are compared with those of cylinders of similar length/diameter ratios b/a, towards which they converge as b/a ≫ 1. The use of such an “acoustic spectroscopy” scheme for shape identification of acoustic targets is emphasized. [H. Überall is also at the Physics Department, Catholic University of America, additionally supported by Code 421 of ONR. D. Brill was supported by Code 2842 of DTNSRDC, Annapolis, MD 21402.]
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Scattering effects of a sphere in a cylindrical resonant cavity (A)

M. Barmatz and M. Gaspar

J. Acoust. Soc. Am. Volume 70, Issue S1, pp. S22-S22 (1981); (1 page)

Online Publication Date: 12 Aug 2005

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The resonant frequency, acoustic pressure and quality factor were measured for various plane‐wave modes of a cylindrical cavity as a function of sphere position along the tube axis. The apparatus consisted of a 5.7‐cm‐i.d. Plexiglass cylinder fitted with an adjustable plunger at one end. The sound energy entered along the tube axis and the sound pressure was measured with a microphone attached flush at the center of the plunger. Positional variation in the acoustic parameters relative to empty chamber values will be presented for various sample sizes, chamber volumes, and pressure levels. The changes in the resonant frequency depend on the ratios of the sample to chamber volumes, Vs/Vc, cross‐sectional areas, As/Ac, and diameter to length, Ds/Lc. For small values of these ratios the frequency shift is proportional to Vs/Vc. These measurements will be compared to a one‐dimensional theoretical model. [Work supported by NASA.]
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Resonance frequency shift of an acoustic chamber containing a rigid sphere (A)

E. Leung, N. Jacobi, and T. Wang

J. Acoust. Soc. Am. Volume 70, Issue S1, pp. S22-S22 (1981); (1 page)

Online Publication Date: 12 Aug 2005

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The resonance frequency shift of an acoustic rectangular chamber due to the pressure of a rigid sphere has been measured for the n = 1, 2 modes as a function of sphere size and position. The position dependence has sinusoidal and dc components. A simple excluded volume model and boundary perturbation theory were used to explain the data. The calculations can account for the sinusoidal behavior, but not for the dc shift. Also measured were the frequency shift, due to a thin disk of the same cross section and a cylinder of the same cross section and volume as the sphere, and the p2/p1 ratio at the wall as a function of sphere position. [Work supported by NASA.]
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Computer models for media transmitting plane waves (A)

Wilfred J. Remillard

J. Acoust. Soc. Am. Volume 70, Issue S1, pp. S22-S22 (1981); (1 page)

Online Publication Date: 12 Aug 2005

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Transmission, reflection, attenuation, and dispersion of small signal plane waves depend on the characteristics of the medium and its boundaries. Given sufficient time one could presumably use Smith chart‐type analysis and determine the effects on plane waves of variations in these characteristics. Since all of the operations that can be performed on Smith charts can easily be programmed as FORTRAN subroutines, it follows that one can model a given situation on a digital computer. The main program consists of initialization of the media parameter, followed by successive calls to the various subroutines. Examples are given of some of these models and their effects on plane waves.
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