What is a sonic boom?

Sound travels at about 343 metres per second (around 1,235 km/h) in air at 20°C. When an aircraft goes faster, it outruns its own sound waves, which pile up into a cone-shaped shock wave. When that cone sweeps past people on the ground, they hear a sudden boom or double boom. The boom is not a one-time event at the moment the plane breaks the sound barrier; it follows the aircraft continuously along its path. Chuck Yeager first flew faster than sound in level flight in 1947, and booms are why supersonic flight over land is restricted in many countries.