Piston effect platforms with a single tunnel, however, air quality worsens when relying on the piston effect alone for ventilation. This still allows for potential energy savings by taking advantage of the piston effect rather than mechanical ventilation where possible. 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. The perception of this sound by humans is similar to that of a sonic boom from supersonic aircraft, but unlike a sonic boom, tunnel boom is not caused by trains exceeding the speed of
Speed of sound in ordinary air, deviating slightly from ideal behavior. In common everyday speech, "speed of sound" refers to the speed of sound waves in air. However, the speed of sound varies from substance to substance: sound travels most slowly in gases; it travels faster in liquids; and faster still in solids. For example, (as noted above), sound travels at in air; it travels at in water (4.3 times as fast as in air); and at in iron (about 15 times as fast as in air). In an exceptionally stiff material such as diamond, sound travels at 12,000 metres per second (27,000