# Explain why a stationary observer hears a change in pitch as an ambulance sounding a constant-frequency siren passes them at high speed, and describe how the observed frequency compares to the source frequency both as the ambulance approaches and as it recedes.

> IB DP Physics Higher Level (2023 syllabus) — C.5 Doppler effect · Explain · 4 marks

> An ambulance travels along a straight road at a speed that is a significant fraction of the speed of sound in air. The siren emits sound at a single constant frequency. A pedestrian stands stationary on the pavement as the ambulance passes.

## Mark scheme (4 marks)

1. As the ambulance approaches, successive wave fronts are compressed (the wavelength of sound reaching the observer is shorter than the emitted wavelength) because the source moves towards the observer between each compression.
2. Since the speed of sound in air is constant, a shorter wavelength means a higher frequency (f = v/λ), so the observed frequency is greater than the emitted frequency as the ambulance approaches.
3. As the ambulance recedes, the source moves away from the observer between each compression, stretching the wavefronts so the received wavelength is longer than the emitted wavelength.
4. The observed frequency is therefore lower than the emitted frequency as the ambulance recedes, so the observer hears a sudden drop in pitch as the ambulance passes.

## Key terms

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

## 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-a-stationary-observer-hears-db8b1952) · Published by Druglandscape Ltd.