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An aircraft flying through the air generates pressure waves, which move at the speed of sound. At speeds below the local speed of sound these pressure waves warn the oncoming air that the aircraft is approaching. As aircraft speeds approach the speed of sound, these pressure waves no longer travel significantly faster than the aircraft, and a shock wave will begin to form on parts of the aircraft.
from: http://ezinearticles.com/?Aircraft-Design-of-Structures-and-Wings&id=3274833