Every Crest Forgets the Source

You'll learnwhy an approaching siren sounds higher without the sound travelling any faster, and what the pitch actually does while a source approaches, arrives and passes.

Each crest a siren emits expands at the speed of sound from wherever the siren happened to be, and forgets the siren completely. Everything about the Doppler effect falls out of drawing those circles. Twelve short steps; press Next for each one.

12 STEPS · 6 QUICK CHECKS · CRESTS EXPAND AT v · SHORTER λ, NOT FASTER SOUND · v2

each crest expands at v from where it was born · shorter λ, not faster sound

Before you start

What you're looking at

A siren whose every crest is drawn as an expanding circle centred where the siren was when that crest left it.

The question

Why does an approaching siren sound higher — and does its pitch really climb as it gets closer?

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