What happens when a plane breaks the sound barrier?

Your question of the month in April (2014)
Martin, 8 years old

Have you ever noticed that there is sometimes suddenly a loud bang in the sky and the window panes even rattle? Then a jet plane might have just broken the sound barrier.

Jet planes create shock waves with their jet engines. These shock waves spread out in every direction and fly, so to speak, in front of the plane at the speed of sound. At the moment in which a plane reaches the speed of sound it is then travelling as fast as the shock waves it creates and a sound barrier develops in front of the plane. If the plane then accelerates, it catches up with its own shock waves and breaks this sound barrier.

The following happens: the shock waves cannot get out of the way quickly enough and are then compressed; the plane flies through them and creates a loud bang. It is then flying faster than its own sound. Clapping your hands or cracking a whip also produces sound, though it is a bit quieter.

In both cases you move the air so fast that it makes a noise. By the way, the speed of sound is called Mach1 after the Austrian physicist Ernst Mach. At 20°C it is around 1,235 km/h. That is almost ten times faster than a car that is going 130 km/h on the motorway.

And here is a legendary record: the extreme sportsman Felix Baumgartner became the first person to break the sound barrier in free fall in October 2012! Wearing a protective suit he jumped out of a special capsule that was hanging from a helium balloon at an altitude of around 39 kilometres. With a top speed of 1,342.8 km/h he was faster than the speed of sound and so broke the sound barrier.