In this paper, the effect of geometry and impedance on the acoustic behavior of wall and lined cylindrical ducts is investigated using a numerical model which enables one to compute the reflection and the transmission coefficients of such ducts using the multimodal scattering matrix. From this matrix, the acoustic power attenuation is deduced. By using these tools, the effect of duct diameter increase and duct diameter decrease of the wall or lined duct section is studied. The numerical results are obtained for two configurations of wall and lined ducts. Numerical coefficients of transmission and reflection as well as the acoustic power attenuation show the relative influence of each type of variation.