Michael J. White
US Army Construction Eng. Res. Lab., P.O. Box 9005, Champaign, IL 61826-9005
Y. L. Li
University of Illinois at Urbana--Champaign, Urbana, IL 61801
Two new fast field programs (FFP) have been developed for numerical computation of anisotropic sound propagation through a windy atmosphere above ground. The first new FFP can be used to compute the near- and far-field sound pressures and use a two-dimensional fast Fourier transform with iterative refinement. The second new FFP is based on a far-field approximation requiring only a one-dimensional transform. For typical values of wind speed, this new expression shows that the component of wind perpendicular to the direction of propagation has little or no effect on propagation. However, for sound propagating into or with the wind, the wind should be treated as a flow, rather than as a perturbation on the intrinsic sound speed. The differences between the effective sound-speed model and the wind flow model appear as phase errors for high angle modes. In cases of ducted propagation with very few modes, the differences in levels are most significant. When many modes contribute to the level, the phase errors in the high angle modes are not as important and the effective sound-speed model is fairly accurate.