The big idea: A racing car howls past the grandstand and the note of its engine suddenly drops the instant it flashes by. That change in the pitch a moving source seems to have is the Doppler effect.
Pitch and motion
- The engine never changed its sound, yet you heard the pitch fall.
- Pitch just means how high or low a sound seems — it rises when the frequency goes up.
- Coming towards you → higher pitch.
- Moving away → lower pitch.
The everyday clue: An ambulance siren sounds higher as it races towards you, then drops to a lower pitch the instant it passes and speeds away — even though the siren itself never changes.
Why it happens — wavefronts bunch up: Picture the sound spreading out as a set of evenly spaced rings — the wavefronts (each ring is one wave crest). As the source moves forward, each new ring is sent from a point a little further on.
Ahead of the source
- The rings are squashed together
- Shorter wavelength → higher frequency
Behind the source
- The rings are stretched apart
- Longer wavelength → lower frequency
Wavefront diagrams: source moving, observer moving
The source does not change: The source's own pitch is unchanged — it's the spacing of the rings reaching the listener that differs in front and behind.
Animated graph
Watch the graph build step by step in study mode.
It's the listener, not the source: The source always emits the same frequency f. Only the observed frequency f' changes — higher in front, lower behind.
Free preview
This is the free notes preview
You're reading the free notes. Aimnova Pro unlocks the full study experience — and you can try it with your first topic free to keep:
- FlashcardsLock in vocabulary and key terms with spaced repetition.
- Practice questionsAnswer exam-style questions and get instant AI marking.
- Mock exams & past-paper vaultSit full mocks and see exactly how examiners award marks.
- Personalised study planA daily plan built around your exam date and weak areas.
The Doppler effect also happens when the source is still and the observer moves.
Observer moving towards the source: The wavefronts in the air are evenly spaced, as usual. Moving towards them, you meet them more often, so you hear a higher frequency.
Moving away, you meet them less often: a lower frequency.
| Source moves | Observer moves | |
|---|---|---|
| Wavelength in the air | changes: shorter ahead, longer behind | unchanged |
| Why the frequency heard changes | the wavefronts are squashed or stretched | the observer meets them more or less often |
| Speed of sound in the air | unchanged | unchanged |
Light does it too: Light from a moving source shows the same effect: higher frequency (bluer) when it approaches, lower (redder) when it moves away. See 3.5.2.
A runner moves at steady speed along the line between two loudspeakers, P behind her and Q ahead of her. They emit different frequencies, yet she hears both at the same frequency. Explain which loudspeaker emits the higher frequency.
Model answer plan
See the answer broken into clear points, with guidance on how each point earns marks.
Never wonder what to study next
Get a personalized daily plan based on your exam date, progress, and weak areas. We'll tell you exactly what to review each day.
How this is tested — at SL the sound Doppler effect is mostly qualitative and graphical:
Paper 1A
- Identify the graph of the frequency a listener hears as a source passes — high-flat, then a sharp step down, then low-flat (never a smooth slope).
Paper 2
- Explain which way the pitch shifts — e.g. a driver between two sources hearing both at the same pitch — reasoning about approaching vs receding.
The classic trap: Thinking the pitch slides down gradually when the motion is along the line joining source and listener. It stays above f while approaching and below f while receding, with a sudden drop as the source goes by.
Read the graph as three parts: Approaching = flat and above the true pitch. Passing = a sharp step down through the true pitch. Receding = flat and below. A source that decelerates while approaching just makes the high level sag towards f.
Animated graph
Watch the graph build step by step in study mode.
Two identical buzzers move along a straight line, one ahead of a listener and one behind, both with the same source frequency. The listener stands still between them. One buzzer is moving towards the listener and the other is moving away. Explain which buzzer is heard at the higher pitch.
Model answer plan
See the answer broken into clear points, with guidance on how each point earns marks.