The influence of a perforated extension region on pressures radiated from the end of a duct is studied numerically using CFD. Planar 2-D geometry is considered and particular attention is paid to the case of pressure disturbances radiated from railway tunnels in cut-and-cover regions. The mechanism that causes this behaviour is described and it is shown to have an important influence of the effectiveness of a perforated extension region. It is found that such regions can strongly reduce the pressures radiated from a duct outlet, but that this benefit is offset by pressures radiated directly from the holes along the perforated region itself. In the particular case of tunnel design, practical studies of wave transmission are usually based on 1-D, plane-wave, analyses. Accordingly, attention is paid to assessing the limitations of such approaches in the case of wave propagation along a perforated region.