Imagine you are driving to school one day. You hear a shrill noise, and see cars pulling over the right. An ambulance is behind you in traffic. As the red lights get closer, you notice the siren is even more penetrating. The ambulance, siren passes you and the other cars. As it drives away, the siren seems much less sharp and almost quieter as it leaves. Why does the siren sound different in its approach versus its departure?
What you have observed is called the Doppler Effect. The Doppler Effect is the phenomenon of frequency of sound changing relative to the movement of the source and the observer. In the case of a siren, the sound of the siren gets higher in pitch or frequency as it approaches you. After the ambulance passes you, the siren's frequency decreases. That gives the sound a lower pitch.
Charly Whisky 18:20, 27 January 2007, CC BY-SA 3.0
There are many animals and technology that use the Doppler Effect. Bats and porpoises use the Doppler Effect in echolocation to orient themselves and to help them find food. Echolocation is the use of sound waves to locate objects. Ultrasound and Doppler radar use sound wave characteristics for imaging.