The big idea: A racing car howls past the grandstand and the note of its engine suddenly drops the instant it flashes by — the engine never changed its sound, yet you heard the pitch fall. That change in the pitch a moving source seems to have is the Doppler effect.
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, so ahead of it the rings are squashed together (shorter wavelength → higher frequency) and behind it they are stretched apart (longer wavelength → lower frequency).
The source's own pitch is unchanged — it's the spacing of the rings reaching the listener that differs in front and behind.
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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.
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The size of the shift is set by how fast the source moves compared with the speed of sound. The data booklet gives the equation for a moving source — you don't memorise it, you choose the sign.
- observed frequency — the pitch you hear (Hz)
- source frequency — the pitch actually emitted (Hz)
- speed of sound in the air (m s⁻¹)
- speed of the moving source (m s⁻¹)
Which sign do I use?: Look at the denominator v ± vs:
- Approaching → use the minus sign. A smaller denominator makes f' bigger (higher pitch). ✓ - Receding → use the plus sign. A bigger denominator makes f' smaller (lower pitch). ✓
Sanity check: approaching must give a higher number than f, receding a lower one.
A train horn emits a steady 400 Hz. The train moves towards a waiting passenger at 30 m s⁻¹. Take the speed of sound as 340 m s⁻¹. Find the frequency the passenger hears.
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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. 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.
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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.
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