Sound carries energy

Sound carries energy. Stretch a sheet tightly over a container and sprinkle a few grains on it. Make a loud sound nearby, without touching it, and the grains jump. The sound made the sheet vibrate, so sound must carry energy.

The vibrating source gives energy to the medium; the vibrating particles bump into their neighbours and pass it on. What reaches your ear is the energy, not the air from the source.

Two devices that change sound energy:

• a microphone turns sound into electrical energy: a thin sheet (diaphragm) inside it vibrates;
• a speaker does the opposite: an electrical signal makes a cone vibrate, giving sound.

In fact, particles are never completely still; they always jiggle about randomly because of heat. A sound wave just makes them vibrate more, for a short while.

Drawing a sound wave as a graph. Put distance along the bottom (x-axis) and density up the side (y-axis). Draw the average density as a dashed line.

• In a compression, the density rises above average. Its highest point is the crest.
• In a rarefaction, it falls below average. Its lowest point is the trough.

Careful: this graph is not a picture of particles moving up and down. Sound particles move back and forth along the wave. The graph just shows how crowded the air is at each place.