Explain why an observer stationary beside a road hears a change in pitch of a car horn as the car travels towards and then past the observer.

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).

A car moving at constant speed sounds its horn continuously as it approaches and then passes a stationary observer standing beside the road.

Model answer (4 marks)

When the car moves towards the observer the source is moving in the direction of the wavefronts, so each successive wave crest is emitted closer to the previous one. This compresses the wavefronts in front of the car, giving a shorter wavelength at the observer. A shorter wavelength corresponds to a higher frequency, so the observer hears a higher‑pitched sound while the car approaches.

After the car passes, it moves away from the observer. The source now emits wavefronts behind it, so each crest is emitted further from the previous one. The wavefronts are stretched, giving a longer wavelength at the observer. A longer wavelength means a lower frequency, so the observer hears a lower‑pitched sound after the car has passed, producing the sudden drop in pitch at the moment the car goes by.

Examiner tips

  • Use the word ‘compress’ for approaching and ‘stretch’ for receding to show understanding of wavefront behaviour. Explain the link between wavelength and frequency (f = v/λ) to justify the pitch change. Mention the observer’s perspective (stationary) to emphasise the Doppler effect.
  • common_mistakes
  • :
  • Confusing the direction of wavefront compression with the direction of motion. Failing to state that the observer is stationary. Using the wrong relationship between wavelength and frequency (e.g., saying longer wavelength gives higher frequency).

Mark scheme (4 marks)

  1. The car (source) moves towards the observer, compressing the wavefronts in front of it, so the wavelength of sound reaching the observer is shorter than that emitted.
  2. A shorter received wavelength means a higher frequency is detected, so the observer hears a higher pitch than the emitted pitch whilst the car approaches.
  3. After the car passes, the source moves away, stretching the wavefronts behind it so the received wavelength is longer than that emitted.
  4. The longer received wavelength corresponds to a lower frequency, so the observer hears a lower pitch than the emitted pitch after the car has passed, producing the characteristic sudden drop in pitch at the moment the car goes by.

Key terms in this question

pitch

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