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Aug 2000

Volume 108, Issue 2, pp. 463-851

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Acoustical wave propagator

J. Pan and J. B. Wang

J. Acoust. Soc. Am. Volume 108, Issue 2, pp. 481-487 (2000); (7 pages) | Cited 5 times

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In this paper, an explicit acoustical wave propagator (AWP) is introduced to described the time-domain evolution of acoustical waves. To implement its operation on an initial state of wave motion, the acoustical wave propagator is approximated as a Chebyshev polynomial expansion, which converges to machine accuracy. The spatial gradient in each polynomial term is evaluated by a Fourier transformation scheme. Analysis and numerical examples demonstrated that this Chebyshev–Fourier scheme is highly accurate and computational effective in predicting time-domain acoustical wave propagation and scattering. © 2000 Acoustical Society of America.
Show PACS
43.20.Bi Mathematical theory of wave propagation
43.20.Fn Scattering of acoustic waves
43.20.Gp Reflection, refraction, diffraction, interference, and scattering of elastic and poroelastic waves

Approximations for modal coupling in scattered fields from orifices

J. L. Horner, R. Lyons, and B. A. T. Petersson

J. Acoust. Soc. Am. Volume 108, Issue 2, pp. 488-493 (2000); (6 pages) | Cited 1 time

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Previous investigations have used Hankel transforms to establish the velocity potentials of the wave fields resulting from arbitrary angle plane wave impingement on a circular orifice in a rigid, thick wall. The scattered field from the orifice is examined, in particular the modal contributions to the amplitude of its velocity potential. For each m,n mode the amplitude is dependent upon the amplitude of the in-orifice waves and a driving term unique to each m,n mode. In establishing the amplitudes of the in-orifice waves, the effects of modal coupling are also considered. In this work these two components of the scattered wave amplitude are investigated on a modal basis and approximations given for coupling effects. These approximations are then used to calculate the scattered field and the results compared with conventional solutions that use full modal coupling. © 2000 Acoustical Society of America.
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43.20.Fn Scattering of acoustic waves

Sound propagation over layered poro-elastic ground using a finite-difference model

Hefeng Dong, Amir M. Kaynia, Christian Madshus, and Jens M. Hovem

J. Acoust. Soc. Am. Volume 108, Issue 2, pp. 494-502 (2000); (9 pages) | Cited 3 times

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This article presents an axisymmetric pressure-velocity finite-difference formulation (PV-FD) based on Biot’s poro-elastic theory for modeling sound propagation in a homogeneous atmosphere over layered poro-elastic ground. The formulation is coded in a computer program and a simulation of actual measurements from airblast tests is carried out. The article presents typical results of simulation comprising synthetic time histories of overpressure in the atmosphere and ground vibration as well as snapshots of the response of the atmosphere–ground system at selected times. Comparisons with the measurements during airblast tests performed in Haslemoen, Norway, as well as the simulations by a frequency-wave number FFP formulation are presented to confirm the soundness of the proposed model. In particular, the generation of Mach surfaces in the ground motion, which is the result of the sound speed being greater than the Rayleigh wave velocity in the ground, is demonstrated with the help of snapshot plots. © 2000 Acoustical Society of America.  
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43.20.Gp Reflection, refraction, diffraction, interference, and scattering of elastic and poroelastic waves
43.28.En Interaction of sound with ground surfaces, ground cover and topography, acoustic impedance of outdoor surfaces
43.28.Js Numerical models for outdoor propagation

Transport parameters for an ultrasonic pulsed wave propagating in a multiple scattering medium

Arnaud Tourin, Arnaud Derode, Aymeric Peyre, and Mathias Fink

J. Acoust. Soc. Am. Volume 108, Issue 2, pp. 503-512 (2000); (10 pages) | Cited 15 times

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A set of ultrasonic experimental methods was developed to characterize a multiple scattering medium in terms of ls, l∗, la, respectively, the elastic, transport, and absorption mean free paths and D the diffusion constant. Actually, these quantities are the key parameters for a wave propagating in a disordered medium. Although they are widely used in optics, they are less common in acoustics. The underlying model is based on the expansion of the average solution for the heterogeneous Green’s function equation. To validate this theoretical approach, a sample made of randomly located steel rods was used as a prototype. Through time-resolved measurements of the transmitted amplitude, the difference between the ballistic and the coherent wave is highlighted. In varying the sample thickness, ls is determined, the coherent and diffusive regime are distinguished, and the transition from one to the other is followed. Furthermore, as a limit to a description of the average intensity based on the diffusion approximation, the existence of a coherent backscattering effect is shown. This latter gives a method to estimate D and l∗. These quantities being determined, it becomes possible to infer la using average time-resolved intensity measurements. Finally, some applications to coarse-grain stainless steels are discussed. © 2000 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
43.20.El Reflection, refraction, diffraction of acoustic waves

Approximate expressions for viscous attenuation in marine sediments: Relating Biot’s “critical” and “peak” frequencies

Altan Turgut

J. Acoust. Soc. Am. Volume 108, Issue 2, pp. 513-518 (2000); (6 pages) | Cited 2 times

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Simple approximate relations are proposed for the viscous attenuation per cycle of the fast compressional and shear waves in the low-to-intermediate frequency range. Corresponding closed-form formulas are derived for frequencies at which maximum viscous attenuation per cycle occurs according to the Biot–Stoll theory of elastic wave propagation in marine sediments. In the new formulas, Biot’s approximation [M. A. Biot, J. Acoust. Soc. Am. 34, 1254–1264 (1962)] for the frequency-dependent viscosity correction factor F(f ) and the assumption of relatively low specific loss (Q−1<0.2) [J. Geertsma and D. C. Smith, Geophysics 26(2), 169–181 (1962)] are used to provide an accurate representation of the fast compressional and shear wave attenuation from low frequencies through a transition region extending to two or three times Biot’s critical frequency fc. The approximate viscodynamic behavior of marine sediments for the fast compressional and shear waves shows similarities to that of a “homogeneous relaxation” process for an anelastic linear element [A. M. Freudenthal and H. Geiringer, Encyclopedia of Physics (Springer-Verlag, 1958), Vol. 6]. © 2000 Acoustical Society of America.
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43.20.Mv Waveguides, wave propagation in tubes and ducts
43.30.Ma Acoustics of sediments; ice covers, viscoelastic media; seismic underwater acoustics
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