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Sound is made when something vibrates. A vibrating guitar string pushes the air next to it, squashing it together (a compression) and then letting it spread out (a rarefaction). These squashes and stretches travel outwards as a sound wave. When they reach your ear, they make your eardrum vibrate.

Sound needs something to travel through: a gas, a liquid or a solid. In air at 20 °C it travels at about 343 metres per second. In water it is much faster, about 1480 m/s, and in steel about 5960 m/s.

Most of space is almost completely empty, a vacuum. With no air or anything else to carry the vibrations, sound cannot travel there.

Experiment: The string telephone

1. Make a small hole in the bottom of each cup (ask an adult).
2. Thread the string through both holes and tie each end to a paper clip inside the cup, so it cannot slip out.
3. Walk apart until the string is tight. One person talks quietly into a cup; the other holds the other cup to their ear.
4. Now let the string go slack. Can you still hear?
5. Try again with the string touching a door frame or a hand. What happens?

Test	Could you hear? (yes/no)	How clear? (1–5)
Tight string
Slack string
String touching something
No string, same distance, quiet voice

Questions

1. Explain how a sound gets from a drum to your ear.

2. Put in order from slowest to fastest for sound: water, air, steel.

3. Why can astronauts on a spacewalk not hear each other without a radio?

4. What happened when the string telephone's string was slack? Why?

Sources for every fact on this sheet: echoesof.science/school/sound/sources/

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