Doppler Effect principle - The Physics Classroom
The Doppler effect - University of California, Riverside
When wave energy like sound or radio waves travels from two objects, the wavelength can seem to be changed if one or both of them are moving. This is called the Doppler effect.
What is the Doppler effect? - Northwestern University
When a vehicle with a siren passes you, a noticeable drop in the pitch of the sound of the siren will be observed as the vehicle passes. This is an example of the Doppler effect. An approaching source moves closer during period of the sound wave so the effective wavelength is shortened, givinga higher pitch since the velocity of the wave is unchanged. Similarly thepitch of a receding sound source will be lowered.
The Doppler Effect - Extrasolar Planets - NAAP
Note: This app is not very realistic in one respect: As the Doppler effect should be seen as clearly as possible, the sound waves have a smaller velocity than in reality.