The use of membranes attached to sound absorbing materials, with the aim of modifying its absorption properties, is quite a usual practice in acoustic conditioning applications. The behavior of the final composition formed by the sound absorbing base material and the attached membrane can serve to characterize the effect of the membrane, if the properties of the base material are known. This can be of great interest for several reasons. Firstly, the difficulty to characterize the materials separately, due to the thinness of the membranes. Secondly, the effect of the binding method used between the absorbing material and the membrane (glue, seams, etc.) can modify the properties of the membrane. This work presents a model that enables us to determine the acoustic impedance of the membranes from an initial analysis of the base material and a second analysis of the composition formed by the base material with the membrane. These analyses are carried out in an impedance tube by following the ISO 10534-2 standard and the results obtained allow modeling the attached membrane effect.