What Causes the ‘Doppler Effect’ and How Does It Work in Everyday Life?

The Doppler Effect is when sounds or waves change pitch depending on whether something is moving toward you or away from you.

Imagine you're standing still, and a friend is riding a bike with a loud bell attached to it. When your friend rides toward you, the bell seems to ring louder and higher-pitched, like when someone shouts as they run up to you. But when your friend rides away from you, the bell sounds softer and lower-pitched, just like when a car drives past you on the road, it gets louder as it comes closer, then quieter as it goes away.

Think of a fire truck siren. When it’s coming toward you, the sound is sharper and faster, but once it passes you and moves away, the sound becomes slower and deeper. This happens because the waves from the siren bunch up when it's moving toward you (making the pitch higher), and stretch out when it's moving away (making the pitch lower).

You can even try it yourself! If you run while ringing a bell or blowing a whistle, people nearby will hear your sound change, just like the fire truck!