Tunnel boom is a phenomenon of radiation of impulsive sound from the exit of a tunnel by high-speed trains. The perception of this sound by humans is similar to that of a sonic boom from supersonic aircraft. However, the physics of a tunnel boom is different. In particular, unlike sonic boom, it has no relation to train speeds exceeding the velocity of sound in air (this is not possible even for the fastest high-speed trains currently in operation). The effect occurs because a train moving at high speed compresses and displaces a great deal of air; normally this air diffuses in all directions. However, when the train enters a tunnel, a high pressure shock wave with a very large pressure gradient is created, which travels down the tunnel and arrives at the opposite exit some time before the train. When this shock wave reaches the tunnel exit, it generates a loud sound propagating in all directions from the tunnel, thus creating a boom.
Tunnel boom can disturb residents near the mouth of the tunnel, and it is exacerbated in mountain valleys where the sound can echo. Reducing these disturbances is a significant challenge for high-speed lines such as Japan's Shinkansen and the French TGV. Methods of reducing the phenomenon include making the train's profile highly aerodynamic and widening the tunnel entrance.
Tunnel boom 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.