How does the sound barrier work


Why does it bang when an airplane breaks the sound barrier?


The sonic boom of an airplane is the sound we hear acoustically when the shock wave caused by the movement of an airplane through the air reaches our ears.

How is this to be imagined? Air is a gas and consists of many molecules with a certain density. If a body, e.g. an airplane, moves through this gas, it pushes the molecules in front of it, compresses them and creates a so-called "pressure wave", i.e. a spreading local compression of the air. This pressure wave moves in front of him at the speed of sound, the speed of propagation of the sound being around 343 meters per second. If the speed of sound is accelerated and exceeded, the air can no longer compensate for the density fluctuations triggered by the moving body quickly enough. Now the density of the air continues to increase in a conical shape around the object (Machscher cone). This strong increase in density (sound barrier) can be heard by us as a loud bang as soon as the resulting shock wave reaches us. For the same reason, we also hear a loud bang when we whip. Here the shock wave emanates from the tip of the whip, which moves faster than sound. Thunderstorms, in turn, are based on shock waves triggered by rapidly expanding air heated by lightning.

It is also noteworthy that there is a sharp drop in air pressure in the space behind a moving object and, as a result, cooling. Depending on the air humidity, condensation water is created, which can be seen as a cloud, e.g. at the tip of the whip on wet days or as a disk of cloud surrounding the rear of the jet fighter.