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BUY THIS ANIMATION of DOPPLER SUPERSONIC SHOCK WAVE!
Supersonic Shock Wave: this animation shows how a shock wave propagates from a supersonic aircraft. The aeroplane compresses the air immediately in front and creates a shock wave (shown as a series of cloudy spheres with their equators emphasised for clarity). The wave fronts are shown propagating from their points of origin along the flight path. As the jet moves, so the resulting waves create a conically expanding wave front (the Mach Cone). This conical wave front sweeps the ground producing the characteristic sonic boom of a supersonic jet passing overhead. This aircraft noise can be a problem.
Compare this situation with that of a plane travelling at exactly the speed of sound and creating a sound barrier.
Shock waves are subtly different from sound waves but the Doppler effect is the same. The airplane is "outrunning" its shock wave and so creating the cone. In an aircraft the shock waves are more complex than shown with shock waves at the front and rear of the craft and at some points between.