How sounds travel
If you watch someone walking on a road you see them move from one place to another. When a car goes past them you see it move from one place to another too. If it is a quiet evening, you may be able to hear the person's shoes as they strike the pavement and you would certainly hear the car. You would be aware that sound had travelled from the person and the car but you would not have seen it, so how does sound get from one place to another? It needs the help of tiny particles from which all materials are made.

There are three kinds of materials. They are solids, liquids and gases. All these materials have one thing in common. They are all made from tiny particles, which are so small that they can only be seen by the most powerful microscopes. In solids, the particles are held quite firmly together but can move just a little. In liquids, the particles are not held as tightly together and they can slide around each other. In gases the particles are not held close together. In fact they are free to move anywhere.

Let us think about how sound travels from a vibrating object like a ruler hanging over the edge of a desk. When you strike the end of the ruler, it moves up and down very quickly. Around the end of the ruler are particles of gas. Before you strike or twang the ruler, the particles are spread out evenly but as soon as the end of the ruler starts to vibrate changes take place. When the ruler moves upwards, it pushes a group of particles together. When the ruler moves back, some of the particles find they have more space to move in and they move backwards. A moment later the end of the ruler is back again, pushing on the particles then moving away again. The end of the vibrating ruler sets up rows of vibrating gas particles. They swing backwards and forwards in the air and carry a wave of sound energy away from the ruler. Although you cannot see a sound wave you can think of it looking like a ripple moving across the surface of a puddle.

If you tap the water at the edge of a puddle, you create ripples, which gradually become smaller as they move away from your foot. If the puddle is large, the ripple may not reach the far side. The same thing happens with sound waves. They become fainter as they move away from the sound source. This is due to sound being a form of energy. The sound can only move because some of its energy is changed to movement energy to allow the particles to vibrate. As the sound wave moves outwards, more and more particles take some of its energy to help it move. Eventually there is not enough energy for the particles to vibrate so the sound wave stops.

The speed at which a sound can travel through a material depends on how the particles are arranged. In a solid, although the particles are held close together, they can vibrate a little. Only a small amount of energy is needed to make a particle in a solid vibrate, compared to a particle in a gas. The energy in the sound wave therefore lasts longer than it does in a gas. This means that the sound wave in a solid can travel much further than it does in gas. Also as the particles are much closer together, they can pass on their vibrations much more quickly. This means that the sound travels much faster in a solid than in a gas. The particles in liquids are closer together than the particles in gas. This means that sound travels faster through a liquid than a gas. However, the particles are not held as firmly as they are in a solid. This means that sounds in a liquid do not travel as fast or as far as they do in a solid.

You may wonder what happens out in space. You will have seen films of space ships roaring through space and may think that space could be a noisy place too. You would be wrong. In space there are few particles of matter. These are particles from the gases from stars or from the atmospheres of planets. These particles are spread out so much that if you could twang a ruler in space it may just push on one particle which would be too far from the next one to make it vibrate. The sound of the ruler or in fact anything in space cannot be heard because there are no particles to carry the sound waves. Sound is put onto films about space ships to make the film exciting. A space ship moving in silence would make the film too dull even though it was more accurate.

Listen to the sounds around you. Most are coming directly to you through the air. Some however do pass through solids. Listen to the hum of your computer. The sound is travelling through air from the motor fan, then through the solid casing, before travelling through the air again to reach your ears. If you can hear a sound outside and the windows are shut, the sound is also travelling in a similar way - through air, then a solid - that is through the glass and the wall, then air again. What sounds can you hear now that are travelling in this way?

Are sound waves just like ripples on a pond?
No. They show how a wave moves out from its source and how the wave becomes smaller as it moves away from its source but the waves are not made by particles of matter moving in the same way. In a sound wave the particles move backwards and forwards in the same direction that the wave moves. In a ripple the particle moves up and down at right angles to the way the wave moves. You can think of it this way. If you put a boat in the puddle and made ripples the boat would move backwards and forwards as a wave ripple went by. In fact the boat bobs up and down because that is what the particles do when a ripple passes by.

Do sound waves in water move like ripples?
No. They move in the same way as they do through solids and gases. When a sound is made underwater the particles of water next to the vibrating object move to and fro in the direction of the sound wave. They do not move up and down.

Can you hear sounds if your head is underwater?
Yes, you can. The sound travels from its source to your ear. It may then travel through the small amount of air inside your ear to where the sound detectors are.

What are the sound detectors in your ear?
The part of the ear which is sensitive to sound waves is the eardrum. This is a thin piece of skin which stretches across the hole in your ear. It is some distance from the surface of your head but you should not put things in your ear as they could touch the eardrum and damage it. When a sound wave reaches the eardrum it makes the eardrum vibrate. There are three bones attached to the eardrum. They are the smallest bones in your body and they are attached to each other in a special way. Each bone acts as a lever. It makes the small force of the vibrating eardrum into a larger force. One of the bones knocks on the side of the second sound detector in the ear with this much stronger force. Inside this second detector is a liquid. The force of the tapping bone sets up sound waves in the liquid, which flow over the ends of thousands of nerves. The sound waves cause the nerves to send messages to the brain, telling it that a sound has been detected.

Can animals hear sounds underwater?
Yes, they can. Whales such as the hump back whale make a long series of sounds which scientists have called songs. When a whale sings a song, the sound travels for hundreds of kilometres through the water of the oceans. It is believed that whales keep in touch with each other over these huge distances by singing their songs.

Dolphins make sounds in their head, which spread out through the water and bounce back off any object they reach. The dolphin listens to the echoes of these sounds and uses them to catch fish and avoid obstacles and predators such as sharks.

Does sound travel through all solids equally well?
No. Some hard solids, like metals, carry sound very well. Soft solids, such as foam, which contain air spaces actually absorb the sound and stop it travelling. These materials are used in soundproofing rooms. The conditions of the solid is also important. If you make a string telephone but keep the string slack you will not hear any messages passing through it. The string needs to be pulled tight so the particles in it can vibrate efficiently to transport the sound waves.

Do sound waves travel down ordinary telephone wires?
No. In the telephone is a microphone. This is a device which converts the energy in sound waves into pulses of electrical energy. It is these pulses of electrical energy which travel along the wire. In the earpiece of a telephone is a loudspeaker. This is a device which converts the energy in the electrical pulses back into sound waves. These travel from the loudspeaker into the ear.