# 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

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

## 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.

## Related

- [Revision notes for IB DP Physics Standard Level (2023 syllabus)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
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