It is well known that when measuring sound transmission loss (STL) in a laboratory, among all test conditions, the location of a specimen in an aperture affects the results, due to the tunneling effect. Previous studies have considered this effect for flat single panels and double walls but the case of a panel with attached sound package seems to have received very little attention. This paper deals with the application of a modal approach to study the STL of a rectangular plate coupled with a porous material located inside a tunnel. The sound absorbing material is supposed to be described by a transfer matrix calculated using a Transfer Matrix Method, which relates interstitial pressure and total normal stress on both sides of the material. The model is validated by comparison with finite element/boundary element computations. Numerical results are shown to illustrate the validity of the proposed hybrid modal-TMM methodology and its use to investigate the niche effect in the presence of a sound absorbing material.