Figure: Doppler effect on a firefighters truck

A siren on a fire engine produces a constant frequency. Provided the fire engine is moving at a steady speed, its passengers will hear a constant tone. However, when standing beside a road and listening to the siren, the frequency produced by the approaching fire engine will be different from the frequency produced by the same fire engine as it gets further away. The noise coming from the fire engine's siren appears different to an observer at the roadside than it does to the fire engine's occupants. Someone at the road side will hear a rising and falling tone even though the pitch of the created sound is constant.

Similarly, if an approaching car sounds its horn at a constant frequency, the driver of the car will hear this constant pitch. However someone stationary outside the car will hear a higher pitch as the car approaches and a lower pitch as it passes and disappears into the distance. This phenomenon was called the Doppler effect after the Austrian mathematician, Christian Doppler, who first explained it in 1842.

Credit: Crocodile-clips http://www.crocodile-clips.com/

Click on "Basic properties of waves" and then "gallery" in Doppler Effect and Sonic Booms at http://www.crocodile-clips.com/absorb/AP5/index.htm?tab=3&segment=0