The expression “sound barrier” refers to the speed of sound in a given medium. In the field of aerospace, which studies motion through air and space, the medium in question is air. Space is a vacuum, which is no medium at all. Sound can only travel if there is a medium to carry its information from one neighbouring molecule to the next. As such, sound does not travel outside our atmosphere, contrary to what silly movies like “Armageddon” would lead one to believe.
What is the speed of sound in air? It is dependent on two factors associated with the air itself: its density and its bulk elasticity. For standard atmospheric conditions, the speed of sound in air is about 340 m/s.
When an object travels through the air below this speed, its motion is said to be subsonic. The term Mach number (M) refers to the ratio of a vehicle’s speed to the speed of sound of the medium it travels through. A typical commercial airplane may travel around 170 m/s, or M = 0.5. Jets travelling beyond 340 m/s are supersonic. When an aeronautical engineer hears about Mach 4, he or she thinks of supersonic air travel, and not a Gillette razor.
