Abstract Reflections of plane waves from the open ends (portals) of axisymmetric pipes and plane two-dimensional (2-D) channels are investigated analytically, numerically and experimentally. An analytical approach developed by Rudinger [1] for pressure decay at an axisymmetric, flanged portal is extended to longer times, and equivalent analyses are developed for reflections from unflanged portals—both axisymmetric and plane 2-D. Predictions for the latter case are compared with numerical results from a computer program based on a 2-D method of bicharacteristics. The theoretical results are compared with measurements from a low pressure shock tube, which was used to investigate alternative end configurations including scarfed portals with and without flange plates. These confirm that the rate of pressure decay is much slower in the plane 2-D case and that flange plates further reduce the rate of decay, albeit slightly. Scarfed portals are shown to cause more uniform decay rates than 90° portals.