Echo

Class 9 Science: sound (echo and speed of sound)

Clap in front of a wall and time the echo. Work out the distance from the speed of sound.

Words to know

Echo: A sound heard again after it bounces off a surface.
Speed of sound: About 343 m/s in air; faster in water and much faster in steel.
Reflection of sound: Sound bouncing back from a hard surface.
Persistence of hearing: Sounds less than 0.1 s apart blend together.

Answer 3 questions to check what you learned.

1. The echo returns after 1 s in air (343 m/s). How far is the wall?

2. In which does sound travel fastest?

3. To hear a clear echo in air, the wall must be at least about…

Stand about 50 big steps from a tall wall and clap. Can you hear the echo? Try clapping in rhythm with the echo and count claps in 10 seconds.