The sound barrier is an aviation term. An aircraft flying at a speed equal to or less than the speed of sound builds up a wavefront ahead of itself. That moves - like any pressure disturbance in the air - at the speed of sound. If the plane flies faster than sound (ie supersonic), this wavefront takes on a cone shape. This cone-shaped wavefront is perceived as a loud bang as it passes. It is therefore not the case that the bang is audible when an aircraft (or other object) accelerates above the speed of sound.
In the years before people managed to fly faster than sound, some scientists believed that the pressure in the wavefront would become so great that an airplane would crash into it if it tried to overtake the wavefront ( so to fly faster than sound). This gave rise to the name sound wall or sound barrier.
On October 14, 1947, the American Chuck Yeager broke through this sound barrier in an X-1 rocket plane (the rocket was not launched from the ground, but raised by an airplane). With more effective streamlining, the sound barrier turned out not to be as impregnable as previously thought.
From World War II, there are known cases of the German Messerschmitt Me 262, a fighter aircraft with two jet engines, which shows that this aircraft could also exceed the speed of sound in dives. There is a report of this from a pilot at the time, Dr. Hans Guido Mutke. Analyzes of this were published in response to a German-American congress on 60 years of jet aircraft at the Deutsches Zentrum für Luft- und Raumfahrt. Mutke claims that he went through the sound barrier near Innsbruck on April 9, 1945 with his Me-262, but does not rule out that it also happened to other German pilots of this machine.
On August 16, 1960, US Air Force pilot Joseph Kittinger reached a speed of 988 kilometers per hour during a free fall after jumping from a height of 31,332 meters from a platform under a helium balloon. He just barely broke the sound barrier. Since then, several people, including Michel Fournier and Felix Baumgartner, have prepared or made attempts to jump with a special suit from a height greater than that from which Kittinger jumped (heights of 36 to 40 km are mentioned), the aim being to get the first become a paratrooper breaking the sound barrier. On October 14, 2012, exactly 65 years after the first human in an airplane broke the sound barrier, Felix Baumgartner succeeded in breaking the sound barrier as the first paratrooper. He jumped from a height of about 39 km and reached a speed of 1357 km/h (Mach 1.25).
For an ideal gas, the speed of sound only depends on the temperature, but in the air it also depends on the pressure and the nature of the medium.
This can be explained on a molecular level: when a sound source emits a sound, this is a pressure disturbance in the air. Movement energy is released from the sound source and this is partly transferred to nearby molecules. As a result of mutual collisions between molecules, the kinetic energy is further spread. The more energy the wave contains, the stronger the sound. At a higher temperature, the molecules already have a higher kinetic energy, in other words they move faster and therefore have a greater chance of collisions that are also harder in which energy is exchanged with each other. Sound also travels faster in solids, because the molecules are closer together and therefore exchange energy more quickly. The energy of a sound wave is thus propagated by the collisions of molecules in the medium.