High Speed Trains Can Make Underground Sonic Booms

High speed rail may be an efficient way to get large numbers of people between two high-traffic destinations, but it does pose some problems. One is the recently-confirmed existence of "ground vibration booms." These are sonic booms that happen underground.

Sonic booms are usually produced by airplanes, because sound travels at a certain speed in air. If sound were visible and you were to clap your hands, you'd see the sound waves as a spreading sphere around your hands. If you were to clap your hands on a plane moving through the sky, the sound would still spread in a sphere, but you'd be moving through that sphere — so the "wall" of the sphere that's in front of you would look closer, and the opposite wall would look farther away. Keep clapping and you'd keep producing spheres — as long as you're not traveling as fast as the spheres are.

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When you break the sound barrier in air, the high air pressure "wall" of sound in front of you is no longer in front of you. As you keep making noise the sound waves essentially pile up in front of you. You're no long making spheres, you're making a big cone of sound, a cone that is still spreading outwards. When people go from outside cone to inside the cone they hear a massive sonic boom.

The sonic boom doesn't have to happen in air. Sound propagates a different speeds through different media. It travels one speed in air, another in water, and another underground. As trains have gotten faster, they have reached the point where they are making their own cones of sound, and their own sonic booms. The only difference is, these sonic booms, called "ground vibration booms" travel through the Earth.

Anyone who has lived near jets knows that sonic booms aren't just annoying. They can shatter windows and do damage to property. What's more, the ground is no less populated than the air. These ground vibration booms can cause structural damage to the ground and ecological damage to the animals living there. It's not a reason to abandon high-speed rail, but it is something to be aware of as trains get faster.

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Top Image: Robbie Sproule

[Via Generation of Ground Vibration Boom by High-Speed Trains.]

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