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The doppler effect was discovered by Christian Johann Doppler. He found that with motion, waves of light (and in our case sound) will compress when an object moves in opposition to the motion of the waves as shown above. When moving in the same direction of the waves, the waves expand.
This is the reason why when you hear a loud siren or train coming toward you, the sound is first lower pitched and becomes higher pitched as it nears your position.
The sound generated by the police car or train doesn't change, what changes is the frequency. Because the sound originator is travelling torward you, the waves become compressed, resulting in a higher pitch. When the sound originator travels away from you, the waves expand resulting in a lower pitch.
Why does this happen? The amount of time the sound has to travel to your ears changes. When the sound originator is travelling toward you, the sound has less time to reach you. So the period of the sound wave shortens and the waves become steeper and have a higher amplitude, because the length of the wave remains the same.
For sound originators which travel away from you the exact opposite occurs. The period of the sound wave lengthens because the time increases and the waves become shallower resulting in a lower amplitude.
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