Making and hearing sound

1Sound is a vibration

Every sound is made by something vibrating: a guitar string, a drum skin, your vocal cords. Rice on a drum jumps when the skin vibrates.

vibrating

2Compressions and rarefactions

A vibrating loudspeaker pushes and pulls on the air, making compressions (particles squashed together) and rarefactions (particles spread out). They travel out as a sound wave.

compressionrarefaction

3Sound needs a medium

Sound needs a medium (a solid, liquid or gas) to travel through. Pump the air out of a jar and a ringing bell goes quiet: sound can't cross a vacuum.

mediumvacuum

4The speed of sound

Sound travels at about 340 m/s in air, 1500 m/s in water and 6000 m/s in steel. It is fastest in solids, where the particles are close together and strongly linked.

5Light is faster

Light travels about a million times faster than sound. You see lightning at once, but the thunder takes about 3 seconds to travel each kilometre.

6Hearing sound

Sound makes your eardrum vibrate. Three tiny bones pass the vibrations to the cochlea, which turns them into electrical signals that a nerve carries to the brain.

eardrumcochlea

7Microphones and loudspeakers

In a microphone, sound makes a thin diaphragm vibrate, which makes an electrical signal. A loudspeaker does the reverse: the signal makes its cone vibrate.

microphonediaphragm