Acoustical scattering in waveguides is studied in this paper. The Wave Finite Element (WFE) approach is mainly used, since it allows the reduction of problems dealing with periodic waveguides. The paper deals with guided acoustical propagation, that is, propagation in a main direction is privileged. The scattering by a locally reacting lining is first studied. The liner can be characterised by its local impedance in this case. The equivalent surface impedance is therefore calculated. Then, scattering by a porous layer is considered. A full three-dimensional modelling of the lining is preferred since porous materials are bulk reacting. The scattering matrix of the lined part is computed, and acoustical scattering of high-order modes and conversion between modes are highlighted. The acoustic power attenuation is further evaluated. The response of ducts subjected to constraining boundary conditions is also calculated. Numerical results are presented and compared to those obtained with conventional approaches.