Index of content:
Volume 118, Issue 3, September 2005
- GENERAL LINEAR ACOUSTICS 
A time-variant impulse response method for acoustic scattering from moving two-dimensional surfacesa)118(2005); http://dx.doi.org/10.1121/1.1992687View Description Hide Description
A time-variant impulse response method is proposed, developed, tested, and shown to provide new insights into different scattering problems involving moving surfaces. The method proposed is general, conceptually straightforward, and can accommodate moving sources and receivers. While the time-variant impulse response method has been developed specifically in this study to extend the capability of the wedge assemblage (WA) model [e.g., R. S. Keiffer and J. C. Novarini, J. Acoust. Soc. Am.107, 27–39 (2000)], the framework can be applied to other scattering models including those based in the frequency domain. Calculations involving moving periodic surfaces and moving receivers are presented and compared with good results to a generalized grating equation and small wave-height approximation perturbation theory. The time-variant impulse response model is also applied to time-evolving sea surfaces, and the previously published results of Pourkaviani and Willemsen [J. Acoust. Soc. Am.90, 426–432 (1991)] are confirmed. It is also shown that windward-oriented backscatter geometries can lead to a Doppler spectrum that peaks at higher than expected Doppler shifts.
118(2005); http://dx.doi.org/10.1121/1.2000827View Description Hide Description
Existing techniques in correlation spectroscopy, such as coda wave interferometry and diffusing acoustic wave spectroscopy, determine the average motion of scatterers or change in the propagation velocity from the temporal change of multiply scattered sound. However, neither of them gives an indication of the spatial extent of the change in the medium. This study is an extension of the technique coda wave interferometry, where multiply scattered waves are used to determine the change in the wave field due to a localized perturbation in the propagation velocity. Here, the propagation of multiply scattered sound is described using the diffusion approximation, which allows the cross-correlation function of the unperturbed and perturbed wave fields to be related to the localized change in the propagation velocity. The technique is tested numerically for two-dimensional (2D) acoustic waves using synthetic seismograms calculated using finite-differences before and after a small perturbation in the propagation velocity has been introduced. Despite the relatively small size and magnitude of the change, multiple scattering greatly amplifies small perturbations, making changes in the phase or travel time of the wave field visible in the later-arriving waveforms. Potential applications of this technique include nondestructive evaluation of inhomogeneous materials and time-lapse monitoring of volcanoes and highly heterogeneous reservoirs.
118(2005); http://dx.doi.org/10.1121/1.2000828View Description Hide Description
Equations describing the radiation characteristics of a rigid disk in a finite open baffle are derived using a method similar to that used by Streng for a circular membrane based upon the dipole part of the Kirchhoff–Helmholtz boundary integral formula. In this case, however, a power series solution to the radiation integral is derived in order to eliminate the need for numerical integration. Hence, a set of simultaneous equations is obtained by simply equating the coefficients of the power series, which leads to two mathematical functions, one real and one imaginary, that can be applied to any radial velocity distribution. This provides an alternative method to obtain the sound scattered by a disk or the complementary hole in an infinite resilient screen according to Babinet’s principle. Using the principle of superposition (or Gutin concept), it is shown how the sound radiation characteristics of a disk radiating from just one side can be obtained by combining the radiation field of a disk in a finite baffle with that of a disk in an infinite baffle. This one-sided radiator may be interpreted as a disk in a thin, circular enclosure.
118(2005); http://dx.doi.org/10.1121/1.2001467View Description Hide Description
This paper aims to apply the radiative transfer method to acoustical diffraction by obstacles. Some fictitious sources are introduced at diffracting wedges and a transfer equation based on energy balance determines the diffracted powers. It leads to a set of linear equations on diffracted powers which can be solved in a finite number of steps. It is then possible to calculate the diffracted field anywhere. Some applications to diffraction by obstacles of various shapes are presented. Results of this method are compared with Geometrical Theory of Diffraction and BEM reference calculations. It is shown that this method is particularly efficient in case of multiple diffraction where the ray-tracing technique involves an infinite number of rays between a source and a receiver point.