Sound: how it is made and how it travels

A school bell rings, a dog barks, a friend calls your name. All of these are sounds, and all sounds begin in the same way: with a vibration.

Sound is produced by vibration

A vibration is a rapid to-and-fro movement. Pluck a stretched rubber band and you can see it blur as it vibrates and hear it hum. Touch a ringing bell gently and you feel it tremble; hold it firmly and the sound stops. In a tabla or a drum the stretched skin vibrates. In a sitar or guitar the strings vibrate. In a flute the air inside vibrates. When we speak, two folds in the throat called the vocal cords vibrate as air from the lungs passes between them. Put your fingers on your throat and say "aaah" to feel them.

Sound needs a medium

The vibrating object pushes the air next to it. That air pushes the air beyond it, and so the disturbance travels outwards as a sound wave until it reaches our ears. The substance that carries the sound is called the medium.

Sound can travel through gases, liquids and solids. In fact, it travels fastest through solids and slowest through gases. Put your ear to a long table while a friend scratches the far end: the sound is clearer through the wood than through the air. But sound cannot travel through a vacuum, a space with no air at all. That is why there is no sound on the moon, and astronauts talk by radio.

Sound is much slower than light. In air it covers about 340 metres in a second, while light is almost instant. So we see lightning first and hear the thunder later.

How we hear

The outer ear collects the sound waves and leads them to a thin stretched skin called the eardrum. The eardrum vibrates, tiny bones pass the vibration to the inner ear, and nerves carry the message to the brain. Never put sharp things in your ear; a damaged eardrum can cause deafness.

Loudness and pitch

The amplitude of a vibration is how far the object moves to each side. A bigger amplitude gives a louder sound; hit a drum harder and it is louder. Loudness is measured in decibels (dB).

The frequency is the number of vibrations in one second, measured in hertz (Hz). A higher frequency gives a higher pitch, a shriller sound. A whistle has a high pitch and a drum a low one. Human ears can hear sounds from about 20 Hz to 20,000 Hz. Dogs and bats can hear sounds above this range.

Echo and noise

When sound bounces back from a hard surface such as a cliff or the wall of an empty hall, we may hear it again. This repeated sound is an echo. Bats and ships use echoes to find their way.

Unpleasant, unwanted sound is noise. Loud horns, loudspeakers and machines cause noise pollution, which can lead to headaches, lack of sleep and loss of hearing. Speaking softly, keeping the volume low and planting trees along roads all help.