In this paper, we aim at reducing the cost of resolution of a system composed of an acoustic cavity coupled to a damping treatment. We use the characteristics of the waves propagating in the medium to condense the dynamics of the acoustic treatment on the surrounding cavity. These waves are predicted by using the Wave Finite Element method, and take into account lateral boundary conditions, spatial inhomogeneities and non-local reaction of poroelastic materials. The condensation procedure can be coupled with Component Mode Synthesis for the external medium. The efficiency of the method is discussed. It is shown that interesting reduction of computation times can be obtained, especially by selecting the waves which contribute to the coupling with the external media.