Supersonic Shock Wave
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). 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 a real 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.