page_title: Piston effect
section_title: Tunnel boom
hierarchical_section_title: Piston effect / Tunnel boom
caption_attribution_description: English: A high-speed rail (TGV) tunnel with an entrance hood.Français&#160;: Le raccordement du viaduc de la LGV Méditerranée sur la ligne de la rive droite du Rhône avec le tunnel de Saint-Geniès, à Montfaucon (PK 608,600).
context_page_description: Piston effect refers to the forced-air flow inside a tunnel or shaft caused by moving vehicles. It is one of numerous phenomena that engineers and designers must consider when developing a range of structures.
context_section_description: Tunnel boom is a loud boom sometimes generated by high-speed trains when they exit tunnels. These shock waves can disturb nearby residents and damage trains and nearby structures. People perceive this sound similarly to that of a sonic boom from supersonic aircraft. However, unlike a sonic boom, tunnel boom is not caused by trains exceeding the speed of sound. Instead, tunnel boom results from the structure of the tunnel preventing the air around the train from escaping in all directions. As a train passes through a tunnel, it creates compression waves in front of it. These waves coalesce into a shock wave that generates a loud boom when it reaches the tunnel exit. The strength of this wave is proportional to the cube of the train's speed, so the effect is much more pronounced with faster trains.
  Tunnel boom can disturb residents near the mouths of tunnels, and it is exacerbated in mountain valleys where the sound echoes. Reducing these disturbances is a significant challenge for high-speed lines such as Japan's Shinkansen and the French TGV. Tunnel boom has become a principal limitation to increased train speeds in Japan where the mountainous terrain requires frequent tunnels. Japan has enacted a law limiting noise to 70 dB in residential areas, which include many tunnel exit zones.
  Methods of reducing tunnel boom include making the train's profile highly aerodynamic, adding hoods to tunnel entrances, installing perforated walls at tunnel exits, and drilling vent holes in the tunnel (similar to fitting a silencer on a firearm, but on a far bigger scale).
caption: A tunnel in the French high-speed TGV network with an entrance hood to mitigate tunnel boom.
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