How Sound Is Made and How It Travels

1Sound is made by vibrations

A vibration is a very fast movement to and fro. All sounds are made by something vibrating.

• When a drummer hits the drum, its skin vibrates. Put a few grains of rice on a drum and tap it: the rice jumps!
• When you pluck the string of a guitar or an inanga, the string vibrates.
• When you speak or sing, your vocal cords in your throat vibrate. Put your fingers on your throat and hum to feel them.

The vibrating object makes the air particles next to it vibrate too. They pass the vibration on to the next particles, and so on, like people in a queue passing on a push. These travelling vibrations are called sound waves. When they reach your ear, your eardrum vibrates in the same way, and you hear the sound.

2Sound needs something to travel through

Sound can travel through gases (like air), liquids (like water) and solids (like wood, stone and metal).

• Put your ear on a desk and ask a friend to scratch the other end gently. You hear it clearly through the wood.
• Sound travels fastest through solids, because their particles are packed close together.
• Sound cannot travel through empty space (a vacuum), because there is nothing to vibrate. That is why astronauts in space need radios to talk to each other.

3Sound bounces: the echo

When sound hits a hard surface, such as a hillside, a cliff or the wall of a big empty hall, it bounces back. This is called the reflection of sound. If the reflected sound reaches you a moment after the first sound, you hear it again: that is an echo. Shout "Muraho!" across a quiet valley in Rwanda's hills and the hills may shout it back.

Soft things, like curtains, mats and clothes, soak up sound instead of reflecting it, so a room full of people and curtains echoes less than an empty one.

4Explore it

Open the Sound Waves simulation. Make a speaker vibrate and watch the waves spread out through the air. Then try taking the air away!

Try it yourself

Try each one first, then tap Show answer to check.