Volume 108, Issue 2, August 2000
Index of content:
- GENERAL LINEAR ACOUSTICS 
108(2000); http://dx.doi.org/10.1121/1.429577View Description Hide Description
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.
108(2000); http://dx.doi.org/10.1121/1.429578View Description Hide Description
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 scatteredwave 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.
108(2000); http://dx.doi.org/10.1121/1.429579View Description Hide Description
This article presents an axisymmetric pressure-velocity finite-difference formulation (PV-FD) based on Biot’s poro-elastic theory for modelingsound 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.
108(2000); http://dx.doi.org/10.1121/1.429580View Description Hide Description
A set of ultrasonic experimental methods was developed to characterize a multiple scattering medium in terms of 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, 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 These quantities being determined, it becomes possible to infer using average time-resolved intensity measurements. Finally, some applications to coarse-grain stainless steels are discussed.
Approximate expressions for viscous attenuation in marine sediments: Relating Biot’s “critical” and “peak” frequencies108(2000); http://dx.doi.org/10.1121/1.429581View Description Hide Description
Simple approximate relations are proposed for the viscousattenuation 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 viscousattenuation 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 and the assumption of relatively low specific loss [J. Geertsma and D. C. Smith, Geophysics26(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 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].