How loud is a sound?
Can you whisper very quietly? Just try. You will probably find that you are just mouthing the words and that there is very little sound. Perhaps you make just a few small clicks as you change the shape of your lips. You may also make some slight noises as your tongue smacks lightly against your teeth as you make "t" sounds.

Now try and talk a little louder and you should be able to hear all the words in your whisper. Change from whispering to talking. You may give yourself a surprise as the loudness of your voice increases. If there is no one else around, raise your voice even more, perhaps even shout. Finally think about what it would sound like to scream but don't try it or other people in the building may come to see what is wrong.

Each time you raised the sound level of your voice, you should have felt as if you were using more energy. Sound is a form of energy and the more energy in a sound, the louder it is. Sounds are caused by vibrations. These are rapid to and fro movements. When you speak you use your vocal cords. These are flaps of flesh in your voice box. They vibrate to produce your voice. The sound that comes out of your mouth is a combination of the vibration of your vocal cords and the amount of air you breathe out. If you don't believe it, just whisper again and feel how little air leaves your mouth. Now think about shouting and how much you breathe in before you make the loudest sound you can. All the air you inhaled before your shout is released to make your shout as powerful as possible.

The air is full of tiny particles that can only be seen by using the most powerful microscopes. When an object vibrates, it makes the particles move in a certain way. As the vibrating object moves in one direction, it pushes all the particles around it into a line. Then, as the object moves in the other direction, it allows them to move back. The vibrating object starts the particles swinging to and fro. The size of the swing of the particles is due entirely to the size of the vibration of the sound-making object. If the object covers a great distance as it moves to and fro, this means that it possesses a large amount of moving energy and it will pass this on to the air particles around it. The air particles then swing a large distance to and fro and make other air particles, further away, swing with similar strength. The sound wave made by these movements is a large one and the sound itself is loud.

If an object only moves a small distance as it vibrates, this means that the air particles around it only move a small distance too. The sound wave made by these movements is only small and the sound itself is only quiet or soft.

You may like to think about the sound waves of a quiet sound and loud sound in the following way. Small sound waves are like the small waves that break on a beach on a hot summer afternoon while loud sound waves are like the huge, foaming waves that break on the beach in a winter's gale.

When an object vibrates it can cause other objects to vibrate and increase the loudness of the sound. This can be shown well with a tuning fork and a table. A tuning fork is a piece of metal with two long prongs. When the prongs are struck on a piece of cork, they start to vibrate. If you hold the tuning fork up to your ear, you can hear a quiet sound coming from them. If you strike the tuning fork again but this time hold it so the metal handle touches the table, you will hear a much louder sound. This is due to the tuning fork passing on its vibrations to the table and the table vibrating with it. As the table is much larger than the tuning fork it makes much larger sound waves when it vibrates and these in turn make a much louder sound. The process of increasing a sound by using some other device besides the vibrating object is called amplification.

You may not think that a table can produce a noise or indeed that it could vibrate. Just try this simple experiment to prove to yourself that it does both. Put a few small coins on a table (not the one that this computer is on) then hit the table with your fist. You should hear a bang and see the coins rise up off the table. Your fist will have given the table enough energy to vibrate and as it did so it flung the coins into the air and made a loud sound wave pass through the air and enter your ears.

Why do loud sounds travel further than quiet sounds?
A sound is made when an object vibrates. Some of the movement energy of the vibrating object passes to the particles in the air around it. This makes the particles vibrate too and form a sound wave, which passes through the air. As the particles vibrate they use up movement energy. If the vibrating object is making only small vibrations it has only a little energy to pass on to the particles around it. They soon use it up so the sound wave does not get far from the object before it cannot be heard. If the vibrating object is making large vibrations it has a large amount of energy to pass on to the surrounding particles. They use this energy to make large vibrations over a much longer distance so a loud sound can be heard far away from the vibrating object.

Why do string instruments have a box with a hole in behind their strings?
If you look at a guitar you can see the strings arranged across a hollow wooden body. There is a hole in this body just under the strings that points outwards from the front of the instrument. When a guitar string is plucked it vibrates. The air around it vibrates and some of this air enters the hole of the body and makes the wooden back and sides vibrate too. As they have a much larger surface than the string, they push and pull on the air particles with greater force and this makes a louder sound. The waves of loud sound then pass out through the hole in the front of the guitar body to the listeners. Other string instruments such as the violin and cello have a wooden body, which amplifies the sound.

Does the amplifier on an electric guitar work in the same way?
No. In an electric guitar there is a microphone called a pick up under the guitar strings. When a string is plucked the vibrations in the air pass to the microphone. The movement energy of the particles is converted into pulses of electrical energy and these travel along a wire to an amplifier. This device makes the pulses of electricity much stronger and then passes them to a loudspeaker. In the loudspeaker the strong pulses of electrical energy are changed to movement energy which sends large sound waves carrying a loud sound to the listeners.

Are loud sounds harmful?
Yes. Some people think that they can only enjoy music if it is very loud. They play their CD players as loud as they can. When they use a portable CD player they turn up the speakers in the head set so that everyone standing close by can hear the music. When these people go to a pop concert they stand near the huge columns of speakers which send out the loud sounds. People who behave in this way are slowly but surely doing damage to their ears. When a sound wave enters the ear it meet a thin piece of skin called the eardrum. This is stretched across the ear hole a little way inside the head. If the sound is very loud it can tear the eardrum and make a hole in it. This prevents the eardrum from sending the vibrations further into the ear so they can be heard and the person becomes deaf. Further inside the ear are thousands of nerves, which carry messages to the brain. If the ear receives large quantities of loud sounds over a long time many of the nerves become damaged and cannot send messages to the brain. This means that the brain can no longer hear certain sounds.

Can ear damage be repaired?
A hole in an eardrum can be repaired but the damage to the nerves cannot be repaired and the person simply cannot hear certain sounds anymore.

Some people can sing and shatter a glass. How does that happen?
The number of times an object vibrates in a second is called the frequency of the vibrations or simply the frequency. Every object has a frequency at which it will vibrate. Usually, objects receive sound waves, which have frequencies, which do not make them vibrate. Very occasionally an object may receive a sound wave which has the same frequency at which the object vibrates. When this happens the vibrations build up in the object and make it produce a loud sound. You may find this happens when you turn up the music on a CD player. At some point a door may start to vibrate in its doorframe. This happens because the frequency of the sound makes the door vibrate strongly. When some people sing very loud, high-pitched notes they may have the same frequency at which glass vibrates. The sound of the note builds up the vibrations in the glass until they shake the glass to pieces and it shatters.