Sound absorption by inhomogeneous porous layers

A three‐dimensional governing equation is derived from first principles that describes the effect of an inhomogeneity of acoustical parameters in rigid, porous sound absorbers. The equation contains a scattering term expressed in terms of the gradient of the material's complex fluid density. An expansion of the equation for weak inhomogeneities yields a source component containing the perturbations, giving rise to a secondary scattered acoustic field in the material. The analysis is then specialized to the exact solution of the oblique incidence problem for plane waves striking a layer of such a material with arbitrary (but continuous) inhomogeneity in the direction normal to the layer's surface. This analysis is applicable to the description of wicking (or fouling) of porous absorptive treatments in industrial or aircraft applications. The transformed governing relation is solved numerically assuming the acoustical parameters vary linearly with space in a 2.54‐cm layer of fiberglass. These results imply that the inhomogeneity degrades the layer's absorptive characteristics, as expected.