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Echoes

Waves, sound and light · level 1, lesson 4 of 6

A question to start

Shout "hello!" in a big empty hall, or towards a cliff, and a moment later you hear "hello!" coming back. Who is answering?

The idea

Nobody is answering. It is an echo: your own sound, bouncing back to you.

Sound travels through the air. When it hits something big and hard, like a wall or a cliff, it bounces off, just as a ball bounces off a wall. If the wall is far enough away, you hear the bounced sound a little after your shout.

your shoutthe echo comes backcliff

Soft things like curtains, cushions and carpets soak up sound instead of bouncing it back. That is why a room full of furniture has less echo than an empty one.

Worked examples

1. An empty room. Clap in an empty room and it sounds "boomy", because the sound bounces around the bare walls.

2. A tunnel. Shout in a tunnel and the echoes go on and on, bouncing between the walls.

3. Bats make squeaks and listen for the echoes, to find their way in the dark.

Try it

  1. What is an echo?
  2. What kind of things make good echoes?
  3. Why is there less echo in a room with curtains and carpets?
  4. How do bats use echoes?
Answers
  1. A sound bouncing back to you.
  2. Big, hard things, like walls and cliffs.
  3. Soft things soak up the sound instead of bouncing it back.
  4. They listen for the echoes of their squeaks to find things in the dark.

Common mistake

"The echo is a different sound." An echo is your own sound, bounced back. That is why it sounds just like you.

Where it leads