Abstract: A numerical mode-matching(NMM) method is extended to predict the acoustic attenuation performance of expansion chamber silencer with extended inlet and outlet.The corresponding theoretic formulation is derived and the computational code is written.The two-dimensional finite element method is used to extract the transversal wavenumbers and eigenvectors,and the mode-matching technique is employed to determine the transmission loss of expansion chamber silencer.Effects of different extensions and locations of inlet/outlet on the acoustic attenuation performance of the elliptical non-concentric expansion chambers with extended inlet and outlet ducts are investigated by using the present NMM approach and the FEM,and good agreements between them verified the accuracy of the present numerical mode-matching approach.The results show that the pass frequencies of the elliptical expansion chambers can be eliminated by choosing the extensions and locations of inlet and outlet,therefore the acoustic attenuation performance in the low-middle frequency range is improved.