Explain why two stationary observers, one standing directly in front of an approaching ambulance and one standing directly behind the same ambulance, simultaneously hear different frequencies from the ambulance siren, even though the siren emits sound at a single constant frequency.

IB DP Physics Standard Level (2023 syllabus) — C.5 Doppler effect · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

An ambulance travels at constant velocity along a straight road with its siren operating continuously at a single fixed frequency.

Model answer (4 marks)

The ambulance is a moving source. As it emits successive wavefronts it moves forward, so the front wavefronts are produced closer together in space – the wavelength in front is shortened. The observer in front therefore receives a higher frequency than the source emits.

Behind the ambulance the source moves away before each new wavefront is emitted, so the successive wavefronts are produced further apart – the wavelength behind is lengthened. The observer behind therefore receives a lower frequency than the source emits.

Thus two stationary observers, one in front and one behind, hear different frequencies at the same instant even though the siren emits a single constant frequency.

Examiner tips

  • Explain the motion of the source relative to each observer; mention wavelength change; link wavelength change to frequency change.
  • Use the words ‘compressed’ and ‘stretched’ to describe the wavelengths; state the consequence for each observer.
  • Show the causal chain: motion → wavelength change → frequency change.
  • Use the exact wording from the mark scheme where possible.

Common mistakes

  • Confusing the direction of motion with the direction of wave propagation; writing that the source moves towards the rear observer. Failing to mention that the wavelength is shortened in front and stretched behind. Using vague terms like ‘different’ without explaining why the frequencies differ.

Mark scheme (4 marks)

  1. The moving source emits successive wavefronts; the ambulance moves towards the observer in front before each new wavefront is emitted.
  2. This causes the wavefronts ahead of the ambulance to be compressed (wavelength decreases), so the observer in front receives a higher frequency than the emitted frequency.
  3. Simultaneously, the ambulance moves away from the observer behind before each new wavefront is emitted, causing the wavefronts behind to be stretched (wavelength increases).
  4. Therefore the observer behind receives a lower frequency than the emitted frequency, so the two observers simultaneously hear different frequencies despite the siren being constant.

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