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 diffuses in all directions. However, when the train enters a tunnel, a high pressure zone or shockwave is created, which travels down the tunnel and arrives at the opposite exit some time before the train. When this pulse leaves the tunnel, it expands outward rapidly, creating a boom. Like a sonic boom, it can create a loud explosion and can cause damage to nearby property (e.g. shatter glass).
Tunnel boom can disturb residents near the mouth of the tunnel, and 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.
Because the sound increases geometrically with the speed of the train, 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.
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