When a high-speed train enters a tunnel, it generates a compression wave which propagates along the tunnel ahead of the train. A small proportion of the wave subsequently emerges from the exit portal of the tunnel as an impulsive wave. This so-called micro-pressure wave causes annoyance through low frequency noise and vibrations in the surrounding area. The objective of this paper is to determine the optimum dimensions of an expansion chamber at the tunnel exit portal for the purpose of reducing the micro-pressure wave. The effects of expansion chambers are investigated numerically and experimentally. Optimum proportions of a right-cylindrical chamber are found and a clear relationship is obtained between the total volume and the degree of attenuation of the micro-pressure wave. (C) 1999 Academic Press.