Doppler effect

Why does a siren's pitch change as it moves past you?

Imagine you're standing on the sidewalk as an ambulance with flashing lights races by. The siren's pitch suddenly shifts as it moves away from you.

The pitch change happens because the sound waves from the siren stretch out as it moves away, making them longer and thus lower in frequency.

Example

The siren's high pitch (around 700 Hz) drops to a lower pitch (around 400 Hz) as the ambulance speeds away.

Remember this

The Doppler effect explains why the frequency of waves changes for an observer when the source of the waves is moving relative to them.

Text adapted from Wikipedia, licensed under CC BY-SA 4.0.

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