Tunnel boom

Tunnel boom is a phenomenon similar to a sonic boom that occurs at the exit of a high-speed train tunnel. The effect occurs because a train moving at high speed compresses and displaces a great deal of air; normally this air is pushed away harmlessly in all directions. However, when the train enters the tunnel, a high pressure zone is created when the displaced air has nowhere to go. This pressure travels along the tunnel at the speed of sound. In the case of a long tunnel this may mean that the effect is produced at the tunnel exit some time before the train. When this area of high pressure leaves the tunnel, it expands outward rapidly, creating a boom effect.

As tunnel boom can be a major disturbance to residents near the mouth of the tunnel and when a train passes through mountain valleys where the sound can echo throughout the area. Reducing it is a major challenge for trains such as Japan's Shinkansen and the French TGV. Methods of reducing tunnel boom include making the train's profile highly aerodynamic and widening the tunnel entrance.

Because the sound increases geometrically with the speed of the train, this has become a principal limitation to increased train speeds in Japan where the mountainous terrain requires frequent tunnels. Japan has created a law limiting noise to 70 dB in residential areas which applies to many tunnel exit zones.