Shock wave
A pressure front that moves faster than sound, with a sudden jump in pressure, temperature and density; explosions make them.
An ordinary sound wave is a gentle ripple of pressure travelling at the speed of sound, about 343 metres per second in air at 20 °C. A shock wave is a disturbance moving faster than that, with an abrupt, almost instantaneous jump in pressure, temperature and density at its front. Ahead of it the air is undisturbed; behind it, everything is suddenly compressed.
Explosions create them. When a charge detonates, the hot gases expand so violently that they slam the surrounding air into a blast wave, a shell of high pressure racing outwards, followed by a rush of wind. It is the overpressure of the blast wave that shatters windows and knocks down walls. Its strength and speed fall off quickly with distance, and far enough away it weakens into an ordinary, if very loud, sound wave that can carry a long way.
The size of explosions is often compared using the TNT equivalent: the mass of TNT that would release the same energy.
Supersonic aircraft drag a shock wave with them, heard on the ground as a sonic boom. Lightning heats the air so suddenly that it makes one too, which we hear as the crack of thunder.
A common misconception is that a sonic boom happens only at the moment a plane breaks the sound barrier. In fact a plane flying faster than sound carries its shock wave with it the whole time, sweeping a boom across the ground beneath its path.
See it in action
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