# A student claps two wooden boards together once while standing 85 m from a large flat wall. She hears an echo shortly after the original clap. Describe how the student could use this observation to determine the speed of sound in air, and explain why the wave she hears as an echo is the same type of wave as the original sound wave.

> AQA A-Level Physics (7408) — 3.3.1 Progressive and Stationary Waves · Describe and Explain · 5 marks

## Mark scheme (5 marks)

1. Measure the time between the original clap and the echo being heard
2. Total distance travelled by the sound is 2 × 85 m = 170 m (sound travels to the wall and back)
3. Use speed = distance ÷ time (wave speed equation) to calculate the speed of sound
4. Sound waves are longitudinal waves — oscillations/vibrations are parallel to the direction of travel/energy transfer
5. Reflection does not change the type of wave — the echo is produced by the sound wave bouncing off the wall, so it remains a longitudinal wave

## Key terms

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

## Related

- [Revision notes for AQA A-Level Physics (7408)](https://www.gradenine.co.uk/learn)
- [How to answer "Describe and 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/a-student-claps-two-wooden-boards-49dd4f30) · Published by Druglandscape Ltd.