Soundproofing schemes constituted of multilayer systems are used for acoustic insulation in many area. In the context of the prediction of noise levels in vibroacoustic systems, numerical models are developed for the low and medium frequency ranges. This paper deals with a multilayer system constituted of a porous medium inserted between two thin plates. An experimental data basis was carried out from a vibroacoustic experiment performed by ONERA. In this paper, we present the boundary value problem consisting of 12 coupled partial differential equations with boundary conditions. We present the analytical method allowing the local acoustic impedance equivalent to this multilayer system to be constructed. The method which is proposed is not usual. Finally, a comparison of the experimental results with theoretical calculations is presented. These experimental results have also been used to construct an algebraic model of this acoustic impedance.