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Journal of the Acoustical Society of America

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Mar 2004

Volume 115, Issue 3, pp. 933-1365

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Sound wave channelling in near-critical sulfur hexafluoride (SF6)

Stefan Schlamp and Thomas Rösgen

J. Acoust. Soc. Am. Volume 115, Issue 3, pp. 980-985 (2004); (6 pages)

Online Publication Date: 27 Feb 2004

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Strong density and speed of sound gradients exist in fluids near their liquid-vapor critical point under gravity. The speed of sound has an increasingly sharp minimum and acoustic waves are channelled within a layer of fluid. Geometrical acoustic calculations are presented for different isothermal fluid columns of sulfur hexafluoride (SF6) under gravity using a semiempirical crossover equation of state. More than 40% of the emitted acoustic energy is channelled within a 20 mm high duct at 1 mK above the critical temperature. It is shown how, by changes in temperature, frequency, and gravitational strength, the governing length scales (wavelength, radius of ray curvature, and correlation length of the critical density fluctuations) can be varied. Near-critical fluids allow table-top sound channel experiments. © 2004 Acoustical Society of America.
Show PACS
43.20.Dk Ray acoustics
43.20.Mv Waveguides, wave propagation in tubes and ducts

Multiple scattering in single scatterers

Liang-Wu Cai

J. Acoust. Soc. Am. Volume 115, Issue 3, pp. 986-995 (2004); (10 pages) | Cited 6 times

Online Publication Date: 27 Feb 2004

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Scattering by a multilayered scatterer is analyzed via a novel multiple-scattering approach. Based on the recognition that multiple scattering occurs within single scatterers having internal interfaces, the solution procedure follows the physical process, and yields analytically exact solutions. A simple two-layered scatterer subjected to SH incident waves is used to illustrate the detailed solution procedure. The solution is then verified by a two-layered circular cylindrical scatterer, whose exact analytical solution has previously been obtained by the author [J. Acoust. Soc. Am. (2004)]. The proposed approach opens new ways for analyzing scatterers of more complicated geometrical and physical compositions. © 2004 Acoustical Society of America.
Show PACS
43.20.Gp Reflection, refraction, diffraction, interference, and scattering of elastic and poroelastic waves
43.20.Fn Scattering of acoustic waves
43.40.Fz Acoustic scattering by elastic structures

Mode-exciting method for Lamb wave-scattering analysis

Arief Gunawan and Sohichi Hirose

J. Acoust. Soc. Am. Volume 115, Issue 3, pp. 996-1005 (2004); (10 pages) | Cited 2 times

Online Publication Date: 27 Feb 2004

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This paper presents an innovative method, called the mode-exciting method, to solve Lamb wave-scattering problems in an infinite plate. In this method, a set of Lamb wave modes is excited by appropriate boundary conditions given on the virtual edges of a finite plate. After solving numerically the elastodynamic problem defined in the finite domain, the numerical solution is decomposed into Lamb wave modes. The Lamb wave modes constitute a system of equations, which can be used to determine the scattering coefficients of Lamb waves for the original problem in an infinite plate. The advantage of the mode-exciting method is that a well-developed numerical method such as finite-element (FEM) or boundary element (BEM) can be used in the elastodynamic analysis for the finite region without any modification like coupling with other numerical techniques. In numerical examples, first the error estimation of the mode-exciting method is discussed by considering three types of error indicators. It is shown that among them, the power ratio of nonpropagating modes to propagating modes is the most suitable for the error estimation. Numerical results are then shown for scattering coefficients as a function of nondimensional frequency for edge reflection and crack scattering problems. © 2004 Acoustical Society of America.
Show PACS
43.20.Gp Reflection, refraction, diffraction, interference, and scattering of elastic and poroelastic waves
43.20.Mv Waveguides, wave propagation in tubes and ducts

Analysis of strong scattering at the micro-scale

Kasper van Wijk, Dimitri Komatitsch, John A. Scales, and Jeroen Tromp

J. Acoust. Soc. Am. Volume 115, Issue 3, pp. 1006-1011 (2004); (6 pages) | Cited 6 times

Online Publication Date: 27 Feb 2004

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Exploiting the fine structure of strongly scattered waves could provide a wealth of new information in seismology, ultrasonics, acoustics, and other fields that study wave propagation in heterogeneous media. Therefore, noncontacting laser-based measurements of ultrasonic surface waves propagating in a strongly disordered medium are performed in which the ratio of the dominant surface wavelength to the size of a scatterer is large, and waves that propagate through many scatterers are recorded. This allows analysis of scattering-induced dispersion and attenuation, as well as the transition from ballistic to diffusive propagation. Despite the relatively small size of the scatterers, multiple scattering strikingly amplifies small perturbations, making changes even in a single scatterer visible in the later-arriving waveforms. To understand the complexity of the measured waveforms, elastic spectral-element numerical simulations are performed. The multiple-scattering sensitivity requires precise gridding of the actual model, but once this has been accomplished, we obtain good agreement between the measured and simulated waveforms. In fact, the simulations are invaluable in analyzing subtle effects in the data such as weak precursory body-wave diffractions. The flexibility of the spectral-element method in handling media with sharp boundaries makes it a powerful tool to study surface-wave propagation in the multiple-scattering regime. © 2004 Acoustical Society of America.
Show PACS
43.20.Gp Reflection, refraction, diffraction, interference, and scattering of elastic and poroelastic waves
43.20.Jr Velocity and attenuation of elastic and poroelastic waves
43.35.Cg Ultrasonic velocity, dispersion, scattering, diffraction, and attenuation in solids; elastic constants
43.35.Pt Surface waves in solids and liquids

Atmospheric absorption in the atmosphere up to 160 km

Louis C. Sutherland and Henry E. Bass

J. Acoust. Soc. Am. Volume 115, Issue 3, pp. 1012-1032 (2004); (21 pages) | Cited 9 times

Online Publication Date: 27 Feb 2004

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This paper describes new algorithms, not previously available, for predicting atmospheric absorption of sound at high altitudes. A basis for estimating atmospheric absorption up to 160 km is described. The estimated values at altitudes above 90 km must be considered as only approximate due to uncertainties about the composition of the atmosphere above 90 km and simplifying assumptions. At high altitudes, classical and rotational relaxation absorption are dominant, as opposed to absorption by molecular vibrational relaxation that is the principle atmospheric absorption loss mechanism for primary sonic booms propagating downward from a cruising supersonic aircraft. Classical and rotational relaxation absorption varies inversely with atmospheric pressure, thus increasing in magnitude at high altitudes as atmospheric pressure falls. However, classical and rotational losses also relax at the high values of frequency/pressure reached at high altitudes and thus, for audio and infrasonic frequencies, begin to decrease at altitudes in the range of 80–160 km. This paper includes: (1) modifications to the existing algorithms in the ISO/ANSI standards for atmospheric absorption at high altitudes, and (2) algorithms for definition of mean atmospheric conditions, including humidity content at high altitude conditions. Also included are suitable values for the temperature-dependent physical parameters of the atmosphere, viscosity, and the specific heat ratio, involved in defining atmospheric absorption at temperatures found at high altitudes. It has been found that carbon dioxide plays a major role in the relaxation of O2 and N2 at high altitudes due to the absence of H2O. Molecular relaxation by CO2, not covered by the current ANSI or ISO standards, is the dominant source of molecular relaxation absorption at altitudes above 60 km at frequencies of 1 Hz and above 10 km at a frequency of 10 kHz. However, at such high altitudes, classical plus rotational losses dominate reaching maximum values at 80–160 km, depending on frequency. In this regime, vibrational relaxation is less important. More accurate predictions of absorption at altitudes above 90 km would require more sophisticated models for the variation in atmospheric viscosity and specific heat ratio above such altitudes. © 2004 Acoustical Society of America.
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43.20.Hq Velocity and attenuation of acoustic waves
43.28.Bj Mechanisms affecting sound propagation in air, sound speed in the air
43.28.Fp Outdoor sound propagation through a stationary atmosphere, meteorological factors

Boundary element modeling of the external human auditory system

Timothy Walsh, Leszek Demkowicz, and Richard Charles

J. Acoust. Soc. Am. Volume 115, Issue 3, pp. 1033-1043 (2004); (11 pages) | Cited 6 times

Online Publication Date: 27 Feb 2004

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In this paper the response of the external auditory system to acoustical waves of varying frequencies and angles of incidence is computed using a boundary element method. The resonance patterns of both the ear canal and the concha are computed and compared with experimental data. Specialized numerical algorithms are developed that allow for the efficient computation of the eardrum pressures. In contrast to previous results in the literature that consider only the “blocked meatus” configuration, in this work the simulations are conducted on a boundary element mesh that includes both the external head/ear geometry, as well as the ear canal and eardrum. The simulation technology developed in this work is intended to demonstrate the utility of numerical analysis in studying physical phenomena related to the external auditory system. Later work could extend this towards simulating in situ hearing aids, and possibly using the simulations as a tool for optimizing hearing aid technologies for particular individuals. © 2004 Acoustical Society of America.
Show PACS
43.20.Fn Scattering of acoustic waves
43.64.Ha Acoustical properties of the outer ear; middle-ear mechanics and reflex
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