# 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

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

## 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

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

## 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)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

---
Source: [GradeNine](https://www.gradenine.co.uk/q/explain-why-an-observer-stationary-beside-3cf828cc) · Published by Druglandscape Ltd.