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supersonic velocity Velocity greater than that of the local speed of sound. In dry air at standard temperature and pressure (STP), this speed is c.331m/s (1080ft/s) or 1190km/h (750mph). Its magnitude is usually expressed as a Mach number. This is the ratio of the speed of a body to the speed of sound in a medium such as air. A velocity in excess of Mach 5 is said to be hypersonic. Any object travelling at supersonic velocity leaves behind a shock wave that a ground observer hears as a sonic boom.