Feeling and seeing sounds
Just listen! What can you hear? You should be able to hear the hum of the computer. What else can you hear? You may be able to hear other people around you or music in the next room. If you are near an open window, you can probably hear sounds from outside. What are they? Perhaps you can hear the sound of traffic, the sounds of birds or an aircraft passing overhead. Although the sounds are all different, they have something in common. They are all caused by vibrations.

What is a vibration?

It is a movement in one direction followed by a movement in the opposite direction. You may think from this that if you walk in one direction and then in another that you are vibrating. However, there is more to vibrating than this. When an object vibrates, it moves very rapidly. A ruler is an object which can be made to vibrate easily. First, take a ruler and put it on a table. Second, slide the rule along the table until part of it hangs over the edge. Third, clamp the other part of the ruler to the tabletop by pressing down on it with your hand. Finally, push down a little on the end of the ruler which is hanging over the table, then let it go. The ruler springs up and down rapidly. It is vibrating and making a rattling noise.

If you watch a ruler vibrating you will not be able to see the vibrating end clearly. The reason for this is that it is moving many times in a second. The eye sends pictures of what it sees to the brain at a certain rate every second too. However, the rate at which the eye sends pictures to the brain is slower than the vibrations at the end of the ruler. This means that the brain cannot see clear pictures of the end of the ruler but sees a blur. You can only see the end of the vibrating ruler for a fraction of a second as it slows down just before it stops.

How do other objects vibrate to make sounds?
The fan of the computer is powered by an electric motor. Inside the motor is a part which spins round. This part causes the whole motor to vibrate and make a humming sound. The sound of our voice is made by vibrations too. You can feel them easily. Just slide your fingers down from under your chin until they press on a hard lump in your throat. This lump is called your voice box. If you press on your voice box and sing the sounds "eeeeee", "oooooo" and "arrrrr" you should feel your voice box vibrate. Your voice box is at the top of your windpipe and just below your mouth. Air passes through it as you breathe in and out. Inside your voice box are two folds of skin. They are called vocal cords. When you speak or sing the cords move out into the centre of the voice box and vibrate to make the sounds.

If you can hear music in the next room it may be coming from a musical instrument or from a radio or CD player. If the sound is coming from a musical instrument, it is produced by the way the instrument vibrates. Each instrument has a part, which is set vibrating to make its sound. For example, a drum skin is beaten to cause vibrations while a violin string is plucked or scraped to cause a vibration. If the music is coming from a radio or a CD player, it is the loudspeaker which vibrates to make the sounds.

The main sound of traffic comes from the roar of the engines although you may sometimes hear the toot of a horn or the screech of brakes. Inside a car engine petrol is changed into a gas then exploded. The exploding gases provide the power to turn the wheels and make the car move. The exploding gases also set off vibrations which create the roar of the engine. Exploding gases also provide the power to move an aeroplane through the air and their vibrations create the deafening noise you hear if you are close to them.

By contrast, birds - one of nature's flying machines - make much less noise. You may hear them flutter their wings and set up vibrations as they take off and you may hear their calls and songs but none of them is so loud and harmful as aircraft noise.

Sounds are all around us. We do not listen to most of them. Just try this simple sound experiment to see how many sounds we really do not listen to. Stand up, walk towards a door, open it and close it. All the time listen for any sound you make. On your journey, your chair may have made a sound as you pushed it back, your feet may have made a sound on the floor and even the floor may have creaked. At the door you may have heard a sound as you turned the handle, another when you opened the door and yet one more when you closed the door. All these sounds were made by vibrating objects.

Do our vocal cords work when we breathe in and out?
If you try to speak when you breathe out and then breathe in you will find that you cannot speak properly when you breathe in. That is why people sometimes gasp for breath if they are telling you something quickly.

Do people have to stop talking between breaths because their vocal cords are tired?
No. They stop because all the air that they can use has been exhaled or breathed out from their lungs. They stop to breathe in so they can have another supply of air to breathe out slowly as they speak.

A bird seems to be able to sing for a long time without gasping for breath. Why is this?
The bird's lungs have also got large air spaces attached to them which run into many places inside the bird's body. When a bird sings it not only uses up air from its lungs but also from these air spaces too. That is why it can sing for much longer than we can.

Does a snake have vocal cords to make a hissing sound?
No. This sound is made by the snake forcing air through an opening in the end of its windpipe into the back of its mouth.

Is thunder made by gases in the air?
Yes, it is. When a flash of lightning passes through the air it produces a great deal of heat. This makes the gases in the air expand rapidly and set up vibrations that cause the roaring sound called thunder.

Why do we see a flash of light from the lightning then hear the thunder later?
You may think that we should hear the thunder at the same time as we see the lightning flash because the flash has caused the thunder. The reason for the difference is that light travels much faster than sound. Usually we see lightning some distance away. The light from the flash covers this distance almost immediately but sound takes longer and we hear it later.

Why don't we see and hear other things differently like the lightning and thunder?
You may think that if, for example, you put down a spoon you should see it rest on the table then hear the thud afterwards. The reason you see the spoon strike the table and hear it at the same time is that you are close to the spoon. You only notice the difference if you are watching something, such as lightning, make a sound from a great distance.

Why does a loudspeaker vibrate?
A loudspeaker has a cone made of paper or plastic. This is attached to a coil of wire, which sits inside a circular magnet. When a musical programme is on the radio or a piece of music is played on a CD player, pulses of electricity pass through the coil of wire. This makes the coil of wire behave like a magnet. The circular magnet around the coil then pushes and pulls on the coil as if it were a magnet. This pushing and pulling causes the card or plastic part of the loudspeaker to vibrate and make a musical sound.