Abstract
Atmospheric acoustic normal mode computer codes are faced with finding the complex modal eigenvalues. Searching in the complex plane is difficult and requires special numerical techniques and custom software. A Legendre-Galerkin technique that reduces the problem to a complex matrix eigenvalue problem can be solved by commercially available software. This proposed Legendre-Galerkin method is described as the projection of the acoustic normal mode problem onto a recombined basis of Legendre polynomials. The modal approach is best suited for providing benchmark quality results in cases when guided modes dominate the problem. Such results are useful in establishing the validity and interpreting the characteristics of atmospheric acoustic fields computed with the parabolic equation method, for the same problem. The Legendre-Galerkin method is applied to cases with a ground based duct and an elevated duct. Measured wind speeds, from a costal experiment, provide the effective downwind and upwind sound speed profiles with these ducted characteristics.
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