Generation and alleviation of sonic booms from rail tunnels

The origins of sonic booms emitted from railway tunnel portals are described, and simple design expressions are provided to enable their amplitudes to be estimated, especially for the common case of relatively short tunnels. The paper is targeted primarily at designers wishing to make initial assessments without any need for specialist software or specialist expertise. However, many references are provided to sources of more advanced information. It is shown why the amplitudes of the disturbances tend to increase with tunnel length in the case of slab-track tunnels, but to decrease with length in the case of ballast-track tunnels. The potential effectiveness of various remedial measures is described, especially at tunnel entrances and exits. Some remedial methods would also improve comfort levels for passengers on trains in tunnels, but this is largely fortuitous because the sonic boom effect is influenced primarily by rates of change of pressure whereas passenger comfort is influenced primarily by absolute changes in pressure.