Explain why the pitch of a sound heard by a stationary observer changes as a train sounding a constant-frequency horn passes through a station without stopping.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
As the train approaches, the wavefronts in front of the horn are compressed, so the observed wavelength decreases.
A shorter wavelength means a higher observed frequency, so the pitch rises while the train is coming.
When the train passes and then recedes, the wavefronts behind the horn are stretched, so the observed wavelength increases.
A longer wavelength gives a lower observed frequency, so the pitch drops abruptly even though the horn emits at a constant frequency.
A shorter wavelength means a higher observed frequency, so the pitch rises while the train is coming.
When the train passes and then recedes, the wavefronts behind the horn are stretched, so the observed wavelength increases.
A longer wavelength gives a lower observed frequency, so the pitch drops abruptly even though the horn emits at a constant frequency.
Examiner tips
- Use the Doppler effect explanation, linking wavelength change to frequency change.
- Show the sequence: approach → compression → higher pitch; recede → stretching → lower pitch.
- Mention the abrupt change at the instant of passing.
Common mistakes
- Confusing wavelength with frequency; writing that the frequency emitted changes.
- Forgetting that the change is abrupt at the moment the train passes.
- Using vague terms like ‘speed up’ instead of ‘compress wavefronts’.”
Mark scheme (4 marks)
- As the train approaches, the wavefronts in front of the train are compressed / the observed wavelength decreases
- A shorter observed wavelength corresponds to a higher observed frequency, so the pitch heard is higher than the emitted pitch whilst the train approaches
- As the train recedes, the wavefronts behind the train are stretched / the observed wavelength increases
- A longer observed wavelength corresponds to a lower observed frequency, so there is an abrupt drop in pitch as the train passes, even though the emitted frequency remains constant throughout
Key terms in this question
Related
- All IB DP Physics Higher Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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