Generalization of Zwikker and Kosten theory for sound absorption in flexible porous media to the case of variable parameters

The paper suggests a generalization of Zwikker and Kosten theory for sound absorption in porous flexible media. We have considered the case in which the intrinsic characteristics of the medium, such as its porosity or compressibility, are varied along the direction of propagation of the sound wave. The eigenvalue problem was reformulated to encompass variable intrinsic properties, and several theorems which justify the numerical evaluation of the approximated solutions, were thoroughly analyzed and proved. We believe that this work might serve as a guideline for the design of acoustic blankets or barriers made of multilayer combination of nonwoven fiber webs of different characteristics.