A modified saddle point method for predicting sound fields above a non-locally reacting porous medium

This paper examines an asymptotic analysis for predicting sound fields above a rigid-frame porous medium, the so-called non-locally reacting porous medium. Their solutions can be represented by a direct wave term, a reflected wave term and a diffraction wave term. Exact and analytical solutions are available for the direct wave and the reflected wave from a perfectly hard ground. In the contrary, the diffraction wave term can only be cast in an integral form that is amenable to approximate analysis. A modified saddle-point method is explored to evaluate the diffraction integral asymptotically. Three different types of non-locally reacting surfaces, which are an extended reaction, a hard-backed layer, and an impedance-backed layer, were considered. The sound fields above these porous surfaces have the same form but they are different by an augmented diffraction term in the solutions. The analytical formula for the total sound fields, which can be stated in a closed form, offer a physically interpretable solution comprising of a direct wave and ground reflected wave terms. This latter term can further be split into a specularly reflected plane wave and ground wave components. A series of numerical comparisons have been conducted to validate the asymptotic analyses presented in this study. It has been demonstrated that the overall sound fields can be predicted well by the formula for all incidence angles and for a wide range of non-locally reacting porous surfaces.