Four short animations illustrating Doppler effect:
1) Listener in motion
First, the listener is at rest, then he moves toward the source; finally, he moves away from the source.
2) Source in motion
The source (in red) move away from green listener and toward cyan listener.
3) Source in motion at the speed of sound, formation of a shock wave (sound barrier)
4) Source in motion at twice the speed of sound (Mach cone)
Other animations by Yves Pelletier