How Sound Travels and the Doppler Effect
How sound travels, and why its pitch changes
0:51 · 720p · With AI narration and captions. This sample was made with the same system as this service.
Narration (full text)
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The Doppler Effect
Why does an ambulance siren sound lower after it passes by?
Sound Is Vibrating Air
Sound is a wave of vibrations traveling through air. When a speaker pushes the air, dense and thin regions travel outward one after another.
The Air Doesn't Travel
Air particles just sway back and forth in place. They don't travel along with the sound.
A Still Source
When the source is still, the waves spread out evenly in every direction.
A Moving Source
When it moves, waves bunch up ahead and sound higher. Behind it, the waves spread out and sound lower.
The Doppler Effect
This is the Doppler effect. It's why a siren suddenly sounds lower as an ambulance passes.
Higher Coming, Lower Going
Approaching sounds are higher and receding sounds are lower, because the spacing of the waves changes.
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