The computational tools available for prediction of sound propagation through the atmosphere have increased dramatically during the past decade. The numerical techniques include analytical solutions for selected index of refraction profiles, ray trace techniques which include interaction with a complex impedance boundary, a Gaussian beam ray trace algorithm, and more sophisticated approximate solutions to the full wave equation; the fast field program (FFP) and the parabolic equation (PE) solutions. This large array of computational approaches raises questions concerning under what conditions the various approaches are reliable and concerns about possible errors in specific implementations. This paper presents comparisons of predictions from the several models assuming a complex impedance ground and four atmospheric conditions. For the cases studied, it was found that the FFP and PE algorithms agree to within numerical accuracy over the full range of conditions and agree with the analytical solutions where available. Comparisons to ray solutions define regimes where ray approaches can be used. There is no attempt to compare calculated transmission losses to measurements.
This paper presents a method for calculating the insertion loss (IL) of a thin barrier coveredwith absorbent material, either on the source side, on the receiver side, or on both sides. The method used combines a classical theory for the propagation of sound over ground with an approximate solution for diffraction around an absorbent barrier, which can take into account the specific impedance of each side of the barrier. The validity of the method was confirmed by comparing theoretical results with experimental measurements for various geometrical configurations and screen boundary conditions. The results show that, when the angles of diffraction are significant, the insertion loss (IL) of a hard barrier can be substantially increased by covering one of its surfaces with an absorbent material. This absorbent layer must be placed on the surface of the barrier associated with the greatest angle of the diffracted rays paths (source top-edge or receiver top-edge). When these angles are about the same on each side of the barrier, the increase will be the same if the absorbent material is placed on the source or on the receiver side. In this case, by covering both sides of the barrier, the increase of the IL due to the absorbent will double compared to a single covering.
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