Mach
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Mach [number] is the number of times faster than the speed of sound that an object travels.
For example, Mach 3 is the speed three times faster than the speed of sound, which was achieved by the SR-71 Blackbird.
Rockets commonly attain speeds faster than the speed of sound. Ordinary passenger airplanes do not. A "sonic boom" similar to thunder can be heard when an object "breaks the sound barrier" and travels faster than the speed of sound.