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Sound is a vibration

Waves, sound and light · level 3, lesson 1 of 6

A question to start

Sprinkle some rice on a drum and tap it. The rice jumps. Now hold a speaker playing music close to a candle flame, and the flame flickers. What is the sound doing to the air?

The idea

Air is made of tiny particles. When a drum skin moves out, it squashes the air particles in front of it together. When it moves back, it leaves a stretched-out patch where they are spread apart.

Each squashed patch pushes on the next air along, and that pushes on the next. So a pattern of squashes and stretches travels out from the drum. This travelling pattern is a sound wave.

The air particles themselves only jiggle back and forth a tiny bit. It is the pattern, and the energy, that travels.

squashstretchthe wave moves on

Worked examples

1. The rice. The drum skin shakes up and down, and the rice is thrown into the air.

2. The candle. The squashes and stretches from the speaker push the air near the flame back and forth, and it flickers.

3. A spring. Push and pull one end of a long, loose spring. You can see squashed bunches travelling along it, just like a sound wave.

Try it

  1. What does a vibrating drum do to the air next to it?
  2. What travels along in a sound wave?
  3. Do the air particles travel all the way from the drum to your ear?
  4. How can you show sound waves with a long spring?
Answers
  1. It squashes and stretches it, over and over.
  2. A pattern of squashes and stretches, carrying energy.
  3. No. They only jiggle back and forth; the pattern travels.
  4. Push and pull one end, and squashed bunches travel along the spring.

Common mistake

"Sound is air blowing from the drum to you." No air rushes across the room. Each bit of air only jiggles a tiny amount and passes the push on.

Where it leads