§ DICTIONARY · CONCEPT
Mach number
The ratio of a source's speed to the speed of sound in the surrounding medium.
§ 01
Definition
The Mach number M = v / c_sound is the dimensionless speed of an object relative to the speed of sound in its medium. M < 1 is subsonic. M = 1 is exactly the speed of sound — the 'sound barrier.' M > 1 is supersonic. M > 5 is usually called hypersonic.
The Mach cone trailing a supersonic object has a half-angle θ given by sin θ = 1/M. At Mach 2 the half-angle is 30°. At Mach 10 it is about 5.7°. The faster the object, the narrower the cone.
§ 02
History
Named for Ernst Mach, who first photographed supersonic shock waves in 1887 and worked out the cone geometry.