Generalised animation of a moving wave source travelling at the same speed as the waves. An example would be an aircraft travelling at the speed of sound (Mach 1). The waves expand outwards at the same speed as the object moves and concertina together at the front of the object. This assembling wall of compression that cannot escape is called the sound barrier in the case of aircraft. If the plane exceeds the speed of sound it breaks through the sound barrier and becomes supersonic creating a conical shock wave (Mach cone), where the wavefronts trail the object.

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