Fast field program for a layered medium bounded by complex impedance surfaces

Abstract

The accurate prediction of sound levels propagating through a layered atmosphere is important in many diverse applications. A powerful prediction method is the Fast Field Program (FFP), in which the Green's function integral in the spectral domain is numerically evaluated by the Fast Fourier Transform. However, existing FFP's require the lowest layer to be semi-infinite liquid or solid, which does not accurately model the effect of the earth's surface on sound propagation. Chessel's [J. Acoust. Soc. Am. 62, 825–834 (1977)] empirical model of the ground surface as a complex impedance plane has produced good agreement between experiment and prediction for homogeneous atmospheres. In this paper, we generalize the FFP by incorporating Chessell's boundary conditions. Thus, our program is capable of calculating the sound attenuation of a point source in a layered medium bounded by a complex impedance surface. In a simple test case (both the source and microphone are above a complex impedance surface in a homogeneous atmosphere), our FFP solution is in excellent agreement with Donato's asymptotic solution [J. Acoust. Soc. Am. 60, 34–39 (1976)].

Meteorology and elephant infrasound at Etosha National Park, Namibia

“…The present study uses a numerical solution of the acoustic wave equation to predict attenuation in a thermally and velocity stratified medium above an impedance surface. The solution is a fast field program or FFP, first developed for underwater sound propagation and modified for atmospheric propagation by Raspet et al 14 and Lee et al 15 A detailed description of this modified FFP is given by Franke and Swenson, 16 and its employment as a tool to predict the attenuation of elephant infrasonic calls is described by Garstang et al 6 In brief, the assumption of a radially symmetric velocity field enables wind shear to be incorporated into a cylindrically symmetric, Helmholtz equation,…”

Evaluation of Simple Spherical Spreading Model for Near Vertical Acoustical Propagation

“…The FFP was developed for use in atmospheric acoustics around 1985. [2,31 The FFP allows researchers the opportunity to incorporate the effects mentioned earlier. The derivation of the FFP is beyond the scope of this report; however, there are several good references on the details of the derivation.…”

A hybrid BIE/FFP scheme for predicting barrier efficiency outdoors

“…The analytical models deal mainly with propagation in a homogeneous atmosphere above a plane impedance boundary or one with a simple sound speed gradient. 1 The numerical methods, which handle more general situations, include the fast field program [2][3][4][5][6] ͑FFP͒ for a range-independent environment, the parabolic equation method 7 ͑PE͒ for a range-dependent atmosphere above a plane boundary, and ray tracing techniques. 8 The first two techniques have found a wide spread use in both underwater and atmospheric acoustics.…”