# 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.

> IB DP Physics Higher Level (2023 syllabus) — C.2 Wave model · Explain · 4 marks

## Mark scheme (4 marks)

1. As the train approaches, the wavefronts in front of the train are compressed / the observed wavelength decreases
2. A shorter observed wavelength corresponds to a higher observed frequency, so the pitch heard is higher than the emitted pitch whilst the train approaches
3. As the train recedes, the wavefronts behind the train are stretched / the observed wavelength increases
4. 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

- [pitch](https://www.gradenine.co.uk/glossary/pitch)

## Related

- [Revision notes for IB DP Physics Higher Level (2023 syllabus)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

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Source: [GradeNine](https://www.gradenine.co.uk/q/explain-why-the-pitch-of-a-84d61b13) · Published by Druglandscape Ltd.