We develop a numerical method to characterize multiple-scattering effects in a duct. To reduce time computations, a FEM method coupled to an integral representation of the scattered pressure field, requiring the evaluation of the duct Green's function, is used, combined with the consideration of random scatterer configurations in the form of perturbed periodic arrays. This strategy reduces the mesh size and allows parallel computations. Application to the computation by an averaging process of effective transmissions is presented. When sending a plane wave, although waveguides involve mode conversion, the mean field is found to be a plane wave. Good agreement is found when comparing to analytical models, not initially designed for a duct, except for some frequencies, recognized to be the band gap frequencies of the underlying periodic arrays. © 2010 Taylor & Francis.