Doppler: why the siren changes pitch
The ambulance plays one note and never stops playing it. You hear two: a high one while it comes towards you and a low one the moment it has passed. Nothing changed — you did.
What you are looking at
- The source emits at a constant rhythm: a new circle every so many frames, without exception. The only thing that changes is where each one starts from.
- Watch the grey dots left behind on the road. Each marks where a wave was born, and there it stays: once emitted, a wave forgets who emitted it.
- Ahead the circles bunch up, behind they spread out. That is not drawn in: it follows from the car catching up with its own waves.
01What is emitted and what arrives are different things
The siren vibrates 700 times a second and never stops. That is the emitted frequency, and it is a fact about the siren: it does not depend on where you are.
What reaches you depends on something else: how many wavefronts hit you per second. And that does change depending on whether the source is coming or going.
02The car chases its own waves
Once emitted, each wavefront expands as a circle from the point where it was born. That point never moves again. That is what the grey dots mark.
But the car does move. By the time it emits the next front it has advanced a little, so that new circle is born further along. Ahead the fronts bunch up; behind, they spread.
Bunched up means they reach you more often: higher frequency, higher pitch. Spread out, the opposite. There is nothing else to it.
03Why the change is heard all at once
While the car comes towards you, you hear the high pitch, and it is steady. While it goes away, the low one, also steady. The jump happens the instant it passes you.
That is why it sounds like a "neeeow" and not like a slide: the pitch does not fall gradually, there are two pitches and you cross from one to the other.
The animation uses a race car and not an ambulance for an honest reason: at 30 m/s the shift is 9% and nothing would be visible. The effect is the same, and it grows with the speed of the source.
Where this is used
The ultrasound at a pregnancy check-up
The machine sends ultrasound into the blood and measures how far the echo comes back shifted. That gives the speed of blood in an artery — and the fetal heartbeat you hear at a check-up is exactly this.
The speed camera
It sends a radio wave at your car and measures how shifted it returns. That difference IS your speed; the rest of the ticket is arithmetic.
Knowing the universe expands
Light from distant galaxies arrives shifted towards red: lower frequency, they are moving away. Measuring that shift is what turned cosmic expansion from an idea into a measurement.
The weather forecast
Doppler radar does not just see where the rain is: it sees whether it is approaching or receding, and how fast. That is how a tornado's rotation is spotted before it touches the ground.
Explaining it to a child
The idea, in plain wordsWhen something noisy comes towards you, the sound bunches up and you hear it higher. When it goes away it stretches out and sounds lower. The person driving the ambulance always hears the same thing.
- Stand on a pavement where cars go past fast and listen, still, to one coming. Watch for the exact instant it passes in front of you: that is the jump.
- Have them sing one long steady note while running towards you, then while running away. You will hear the change; they will hear none.
- Swap: now you run singing and they listen. It is the ambulance experiment, done by the two of you.
The question to leave them withIf you are sitting INSIDE the ambulance, do you hear the pitch change?
For the classroom
Ten minutes with the animation, ready to take into a room. Free under CC BY 4.0, like everything else here.
- Before anything else, ask: "when an ambulance goes past and the pitch drops, what does the driver hear?" Let it be argued for a minute. It usually splits the class.
- Play the animation and ask them to follow one single wavefront, not the ambulance. Have them see that each circle stays centred where it was born.
- Go back to the question. Now the answer —the driver always hears the same thing— is not a fact you handed over, it is something they deduced.
- If there is time: mention this is how we measure how fast a galaxy is receding. It is exactly the same sum.
At what exact moment do you hear the siren's true note?
Answer: When the ambulance is right beside you and, at that instant, is neither approaching nor receding. Before that you hear it higher; after, lower.
If instead of the ambulance moving, you moved towards a stationary siren, would you hear anything different?
Answer: Yes, the pitch would change too. What matters is the relative speed between the two, not which one moves — although the sums are not identical, because the air does distinguish who is moving relative to it.
Light from almost every galaxy reaches us shifted towards red. What does that say?
Answer: That they are moving away from us. It is the same effect as the siren, with light instead of sound, and it is the observation that revealed the universe is expanding.
Someone will say the driver hears the change too. That is the whole conception: if the change were in the source, everyone would hear it. Do not correct it straight away — ask the class whether they agree, and let them take it apart between them.
FísicaEterna classroom guide · CC BY 4.0 · fisicaeterna.cl
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The code behind it
It is written in Remotion, which renders video by writing React. The bar and the number are not drawn next to the figure: they come out of the calculation on the same geometry you are watching. If the drawing lies, the number lies with it.
Licence
This animation is under CC BY 4.0: you may copy, redistribute, adapt and use it commercially, as long as you credit the source.
Suggested attributionDoppler: why the siren changes pitch — FísicaEterna (fisicaeterna.cl), CC BY 4.0