Sound spreads out
Have you thrown a pebble in a pond? The chances are that you have thrown several pebbles or even large stones - just to see the splash they made. You may have noticed that, when the pebble hit the surface then sank below it, a circular wave formed on the surface around the place where the pebble went in. A moment later another wave formed outside the first one, then another and another. Each wave formed a circle that was larger than the last. If you looked very closely you would see that the sizes of the waves changed too. The first wave, made nearest the place where the pebble went in, was the highest and, as the waves spread out across the surface of the pond, they became lower and lower. Perhaps some waves never formed near the shore. When a large stone is thrown in, much larger waves are made and some waves forming near the shore wet your feet.

When you throw a pebble in a pond the waves seem to race across the surface of the water. However, something different is happening. If the waves really raced across the water then anything floating on it would be carried along and dumped on the side of the shore. If you put a boat on the pond and throw a pebble close by, the boat does not shoot across the water surface with the wave. The boat simply bobs up and down as the waves pass. The reason for this is that the wave is not made by water near the surface rushing outwards towards the shore. It is made by water particles simply moving up and down in turn.

When a pebble enters the pond it pushes down on the water and this makes the water next to it rise up and make the crest of the wave. The rising of the water to the crest makes the particles nearby sink down and form the hollow or trough of the wave. In turn this makes other particles further away rise up and make another crest. However as the movement energy which causes the rising and falling of the particles passes through the water, it becomes used up. This means that as you move further away from the place where the pebble went in the waves become smaller and smaller.

Sound waves are also produced by moving particles but the particles move in a different way from those which make waves on a pond. The particles in water moved up and down while the wave moved across them or at right angles to their movement. A sound wave is produced by particles, which move in the same direction as the sound wave. However the particles do not move all the way with each wave. They move backwards and forwards, just as the particles making the water wave moved up and down.

A sound wave is not made by plunging something into the air like plunging a pebble into a pond. It is made by making an object move very quickly to and fro. This movement is called vibrating. If you took an elastic band, stretched it between your finger and thumb then plucked it, you would see the elastic band move to and fro as it vibrated. You would also hear the sound produced by the vibration. As the elastic band moves out it pushes on air particles and makes them move forwards. These push on other air particles and make them move forwards too. As the elastic band moves in, it leaves space into which the air particles swing back. This creates space further from the vibrating object into which other air particles swing back. When the elastic band moves outwards again the air particles are moved forwards again and the swinging of the air particles continues as long as the object is vibrating.

You can think of the movement of the swinging of the air particles as being similar to the movement of the water particles, only the air particles move to and fro instead of up and down. When the particles are pushed together this is similar to the particles bobbing up in the water and making the crest of the wave. When the particles swing back into the space left by the vibrating object this is similar to the particles bobbing down in the water and making the trough of the wave.

The sound wave is produced by movement energy passing between the air particles. As this energy passes between the particles it also becomes used up and sound waves, like those on water, become smaller and smaller and smaller until there is no energy left to move the particles. Try this little experiment to see how the sound waves produced by the fan of your computer become smaller as they travel across the room. Get up and walk slowly away. How far do you get before you can no longer hear your computer? You have found the distance at which the particles of air ran out of energy to carry the sound any further.

Why do people shout when they want their voice to travel a long way?
When a person shouts they use a great deal of energy. This is passed onto the vocal cords, which vibrate strongly and make the air particles vibrate strongly too. The strongly vibrating air particles carry the sound wave much further than if the person simply spoke or whispered. You can think of the sound waves made when a person shouts as having high crests and deep troughs, like the waves on a sea in a storm.

Why do people cup their hands round their mouth when they shout?
If you shout without cupping your hands, the sound waves spread out in a half circle from your mouth in all directions. The energy from your voice box is spread out equally in front of your face. When you cup your hands the waves cannot spread out but are forced to travel directly away from the front of your mouth. All the energy is concentrated forwards which makes the waves remain louder and travel further than the waves from an uncupped mouth.

Why do people sometimes cup their ears when they are listening to something?
When the sound waves reach the ear they strike its surface and bounce off. You can see a similar kind of reflection in water waves in a pond. If you throw in a pebble close to a rock sticking out of the water, the ripples strike the rock and new ripples are set up in the opposite direction. When a person cups their ears they make a larger surface for the reflection of sound waves. The sound waves bounce around the hand on the cupped ear and enter the earhole. Cupping the ear helps to collect sound waves that may otherwise pass the earhole without entering it.

What is a wavelength?
You may have heard of the term 'wavelength' in connection with radio stations. Radio waves carry energy that is made into the sounds that you hear on a radio. A wavelength is made up of two parts of a wave. It is made up from a crest and a trough. Think of the crest of the wave being an upside-down U and the trough as a normal U connected to it on the right but just below it. The two Us joined together make an S, which is tilted to one side.

Are all sounds the same wavelength?
No. There are many different wavelengths. The longest sound waves make the lowest sounds. The shortest sound waves make the highest pitched notes.

Can you give some examples of wavelengths?
A deep note made by a singer with a bass voice may have a wavelength of one metre while the high note made by a soprano singer may have a wavelength of about twenty six centimetres. You may like to look at these measurements on a metre rule and imagine a wave with a crest and trough filling the distances you are looking at.