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, France's TGV and Spain's AVE. 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.