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Nov 1989

Volume 86, Issue S1, pp. S1-S125

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back to top Session Y. Physical Acoustics IV and Engineering Acoustics III: Localized Wave Modes
Invited Papers
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Localized wave transmission physics and engineering (A)

Richard W. Ziolkowksi

J. Acoust. Soc. Am. Volume 86, Issue S1, pp. S61-S61 (1989); (1 page)

Online Publication Date: 13 Aug 2005

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Exact solutions of the scalar wave equation that describe localized transmission of wave energy will be reviewed briefly. These acoustic solutions can be optimized so that they are localized near the direction of propagation and their original amplitude is recovered out to extremely large distances from their initial location. Pulses with these very desirable localized wave transmission (LWT) characteristics may have a number of potential applications in the areas of directed energy applications, secure communications, and remote sensing. The following will be shown. (1) The underlying physics of the LWT effect is closely connected to the additional degree of freedom obtained by coupling the usually disjoint portions of phase space. Position and frequency spectra at different locations in the aperture are different but are highly correlated so that the effective frequency of the aperture is higher than expected and the resulting pulses from these aperture sources reconsitute the frequency content of the LWT packet, hence its pulse shape, as it propagates. (2) The LWT solutions do not violate any known uncertainty relations and satisfy a generalized full phase‐space uncertainty relation. (3) The independently addressible element LWT arrays can be designed to dramatically outperform conventional cw‐driven apertures. This includes pulse shape, peak amplitude, and energy fluence. New acoustic experimental data that corroborate these theoretical results will also be presented. [This work was performed by the Lawrence Livermore National Laboratory under the auspices of the U.S. Department of Energy under Contract No. W‐7405‐ENG‐48.]
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Nondiffracting Bessel optics: Theory and practice (A)

J. H. Eberly

J. Acoust. Soc. Am. Volume 86, Issue S1, pp. S61-S61 (1989); (1 page)

Online Publication Date: 13 Aug 2005

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The theoretical foundation for the nondiffracting Bessel beams identified in 1985 by Durnin will be explained briefly, and several practical realizations of these beams in optical contexts will be described.
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Experimental evidence of localized wave phenomena (A)

D. Kent Lewis, Steven E. Benson, Floyd C. Kirk, and Bill D. Cook

J. Acoust. Soc. Am. Volume 86, Issue S1, pp. S61-S62 (1989); (2 pages)

Online Publication Date: 13 Aug 2005

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Recent interest in creating beams that exhibit more localized behavior on the propagation axis than traditional solutions has led to many formalisms for creating and describing energy transport. One set of these solutions, the localized linear wave solution developed by Ziolkowski, has been under investigation through acoustic experiments for some time. Experiments using linear superposition to simulate two‐dimensional acoustic arrays are discussed. These experiments use two methods, acousto‐optics, to simulate an array of line sources and a point detector, as well as the traditional two‐transducer arrangement, to simulate square arrays of point sources. Preliminary experiments with an actual two‐dimensional acoustic array are also described. The technique of signal preprocessing to overcome damping problems and interelement interactions is discussed. [Work performed by the Lawrence Livermore National Laboratory under the auspices of the U.S. Department of Energy under Contract No. W‐7405‐ENG‐48.]
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Propagation of localized pulse trains in a circular acoustic waveguide (A)

I. M. Besieris, A. M. Shaarawi, and R. W. Ziolkowski

J. Acoust. Soc. Am. Volume 86, Issue S1, pp. S62-S62 (1989); (1 page)

Online Publication Date: 13 Aug 2005

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A brief overview of a novel approach to the synthesis of wave signals [cf. I. M. Besieris, A. M. Shaarawi, and R. W. Ziolkowski, J. Math. Phys. 30, 1254 (1989)] will be presented. This approach, referred to as the bidirectional method, was originally introduced in order to understand the salient features of Brittingham‐like solutions. Its scope is broader, however, and encompasses classes of problems altogether different from wave propagation in an unbounded homogeneous domain. The efficacy of the bidirectional method in geometrics involving boundaries has already been demonstrated [cf. A. M. Shaarawi, I. M. Besieris, and R. W. Ziolkowski, J. Appl. Phys. 65, 805 (1989)]. In this presentation, the propagation of localized pulse trains in an infinitely long, circular, acoustic waveguide will be examined in detail. The farfields radiated out of a semi‐infinite, circular, acoustic waveguide, excited by a localized initial pulse, will also be studied. These approximate solutions, which are computed using Kirchhoff's integral formula with a retarded Green's function, are causal, have finite energy, and exhibit a slow energy decay.
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Pulsed focused fields (A)

Leopold B. Felsen

J. Acoust. Soc. Am. Volume 86, Issue S1, pp. S62-S62 (1989); (1 page)

Online Publication Date: 13 Aug 2005

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Highly concentrated clumps of field energy that move through an environment without diffusing are of interest for a variety of applications. The basic problem is the synthesis of source distributions over an initial aperture to achieve this objective. Various analytical approaches have been used to explore how initial transient source field distributions can be tuned up so as to generate compact wavepackets that stay together indefinitely. The meaning of “staying together” and “indefinitely” depends strongly on how the problem is defined and has led to some startling conclusions in the early phases of investigation. At present, there seems to be agreement that no radically new phenomena are operative here but that interesting pulse shapes can be synthezied by clever spectral tuning of input conditions. Various techniques—based on direct time‐domain synthesis, real and complex spectral synthesis, and use of transient beam‐type basis elements—are reviewed within the context of causality and finite aperture size. Also discussed is a modeling scheme whereby a pulsed focused beam is generated analytically by assigning complex values to the source location and initiation time. This parametrization is useful for converting spherical pulse interaction with a perturbating environment directly into focused pulse interaction with that environment. [Work was supported by the Innovative Science and Technology Office through the U.S. Army Harry Diamond Laboratory.]
Contributed Papers
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An experimental study of the field profile of a Bessel beam ultrasonic transducer (A)

D. K. Hsu, F. J. Margetan, and D. O. Thompson

J. Acoust. Soc. Am. Volume 86, Issue S1, pp. S62-S63 (1989); (2 pages)

Online Publication Date: 13 Aug 2005

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The first experimental investigation of ultrasonic beams produced by a Bessel transducer is reported. Bessel sources are of particular interest in attempts to achieve diffractionless beams; Durnin et al. [Phys. Rev. Lett. 58, 1499 (1987)] showed that a Bessel beam of light was diffraction‐free. Using a novel technique of nonuniform poling, the polarization strength of a piezoelectric ceramic disk was made to follow the pattern of an axially symmetric, truncated Bessel function J0 in both amplitude and phase. Field profiles and propagation behavior of the Bessel ultrasonic beam were measured experimentally in a water immersion tank. The measured profiles agreed well with calculated results using a Gauss‐Hermite beam model. Effects of the number of lobes, frequency, and beam width on diffraction behavior were investigated. [Work supported by Basic Energy Sciences, U.S. Department of Energy.]
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Development of an acoustic array for generating ADEPT pulses in air (A)

Alan P. Poindexter, Bruce D. Baker, and Bill D. Cook

J. Acoust. Soc. Am. Volume 86, Issue S1, pp. S63-S63 (1989); (1 page)

Online Publication Date: 13 Aug 2005

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A six‐ring circular array with each ring driven by separate electronics has been developed for the purposes of exploring acoustic directed energy pulse trains in air. Each array clement is a small loudspeaker with 1‐in.‐diam cone. A different signal is to be applied to each ring simultaneously. These signals come from an MACII computer with an off‐the‐shelf interface board. The signal must be compensated for the loudspeaker's system reponse and interaction between array elements. [This work is supported by Lawrence Livermore National Laboratory.]
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Modification of the NPE computer code to describe the propagation of axisymmetric sound beams in infinite media (A)

Gee‐Pinn James Too and Jerry H. Ginsberg

J. Acoust. Soc. Am. Volume 86, Issue S1, pp. S63-S63 (1989); (1 page)

Online Publication Date: 13 Aug 2005

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NPE is a nonlinear progressive wave equation computer code developed by McDonald and Kuperman [Comp. Math. Appl. 11, 843–851 (1985)] to evaluate transient propagation in acoustic waveguides. It is suitable for two‐dimensional phenomena, as well as for radial propagation of azimuthally symmetric waves. The present study describes the modifications required to employ NPE for the evaluation of axially propagation axisymmetric waves, particularly those associated with sound beams radiated by a baffled piston. In addition to implementing a formulation in which the propagation is essentially parallel to the axis of a set of cylindrical coordinates, it is necessary to account for the transverse spreading of the beam into an infinite medium. Another issue is the manner in which the initial waveform input to NPE is obtained. The predictions of NPE for a linear sound beam when the input is obtained from the King integral, which is an exact solution in quadrature form, are compared to the results obtained for a simple input based on assumption of planar wave behavior in the vicinity of the transducer. The results in both cases are also compared to the analytical solution for the farfield radiation pattern.
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