Wind instruments
In most studies on sound we think of the air as something which carries sound waves. It is made of tiny particles that can only be seen by the most powerful microscopes and these particles swing to and fro to transport the sound wave through the air. Sometimes air can be made to behave differently. It can be made to produce the vibrations, which make the sound waves too. We use this behaviour of air when we play a wind instrument.

It may seem strange that vibrating air can make musical notes but you may have made a simple instrument that worked by vibrating air and not realised it. Have you ever arranged a row of glass bottles and filled them with different amounts of water? You may then have taken a spoon and tapped them gently on the side. When you did this you would have found that the bottle with the least water in it gave a low-pitched sound while the bottle with the most water in it gave the highest-pitched sound. When some people try this investigation they believe that the change in pitch is due to the amount of water in the bottle. They ignore the fact that the bottles contain different amounts of air.

You can turn the row of bottles from a percussion instrument to a wind instrument simply by blowing over the top of each bottle. When you do this, you get the same result as when you tapped the bottles with a spoon. The bottle with the least water (and the most amount of air) makes a low-pitched note and the bottle with the most water in it (and the least amount of air) makes a high-pitched note.

When a bottle containing air is tapped or has its mouth blown over, the air inside the bottle vibrates. It produces a wavelength of sound, which is related to the length of the column of air in the bottle. This means that the bottle with only a small amount of water in it has a long column of air and this can produce a vibration with a long wavelength. When this vibration is passed through the sides of the bottle and into the air, it makes sound waves with a long wavelength. This means that the number of wavelengths made in a second, called the frequency, is low and in turn this produces a note with a low pitch. A bottle with a large amount of water in it only has a short column of air so when this vibrates it produces sound waves with a shorter wavelength. This means that a larger number of waves are made in a second so the frequency and the pitch are higher.

You can confirm these observations by trying another activity. This time take a straw and dip the end into a bottle full of water. Blow across the top of the straw and you should hear a low-pitched sound. This is due to the long column of air that is vibrating in the straw. If you keep blowing and lower the straw into the bottle of water you will hear the pitch of the note rise. This is because the column of air becomes shorter and so produces sound waves with a shorter wavelength and higher frequency.

If you play a recorder you find that you can create notes of different pitch from it without dipping it in water. How can notes of different pitch be made? The holes in the side of the recorder provide a clue. When you cover all the holes with your fingers and blow down the mouthpiece the recorder makes the lowest-pitched note it can. This happens because by blocking up all the holes you have made a very long column of air in the recorder, which can vibrate and produce a note. When you remove fingers from holes lower down on the recorder and blow, you hear that the pitch of the notes begins to rise. This is due to the column of vibrating air becoming shorter. As you remove more and more fingers, the length of the vibrating air column becomes shorter and this means that the wavelength of the sound that is produced also decreases. This change continues to raise the pitch of the note. If you place more fingers on the holes you lengthen the vibrating air column and produce lower-pitched notes. By opening and closing the holes on the recorder you control the length of the column of vibrating air and control the notes you play.

You may not be interested in playing tunes on a recorder but do try and have a go on one. Ask someone who plays to show you how to place your fingers. Cover up all the holes. Blow gently and steadily and open the holes, starting at the bottom and moving up to the top. Not only will you get the feeling of playing a wind instrument but also you will appreciate the sounds that you can make with a simple column of air.

What starts the air vibrating in a recorder?
When you blow into the mouthpiece of a recorder, the air strikes a sharp edge in the mouthpiece. This makes the air vibrate and set up a vibration in the air inside the recorder.

Are there any differences between a recorder and flute?
Yes. The column of air in the flute is made to vibrate by blowing over the top of the mouthpiece as you might blow over the top of the bottle to get a sound. The flute has keypads over the holes, which are raised above the holes on springs. When you wish to close the holes in a flute, you push on the keypads and they sink over the holes. The keypads are lined with felt to make an air-tight seal around the hole. Some of the holes on the flute are so spread out that the fingers cannot reach them all. However, there are a number of levers, which can be pushed by the fingers to make the more distant holes close when needed.

How do you make a sound for a brass instrument?
Most brass instruments have a mouthpiece, which is shaped like a cup. The player purses their lips, applies them to the cup and blows a 'raspberry' into the instrument. This sound is made by the vibrating lips. It sets up a vibration in the column of air in the instrument.

Are there holes in a trumpet like there are in a recorder?
No. The length of the column of vibrating air is controlled by valves. These are metal buttons, which stick up on the top of the trumpet. When they are pressed down they connect the various tubes in the trumpet together to make a longer column of air. By pressing down on different valves, or different combinations of valves, the length of the vibrating air column can be controlled and a wide range of notes can be played. The trumpeter also uses his or her lips to make different notes. This is done by changing the tension in the lips.

How do you get different notes out of a trombone?
A trombone has a mouthpiece like a trumpet and the trombone player blows into it in a similar way to a trumpet player. A trombone does not have valves. It has a long U-shaped tube called a slider. This is pushed out and pulled in to change the length of the column of vibrating air in the instrument.

What is a reed instrument?
This is an instrument which has a reed to set the air inside it vibrating. The reed in an instrument is actually a piece of cane. When the instrument is blown the reed vibrates and sets up a vibration in the column of air in the instrument. The clarinet has one reed. The oboe and the bassoon have two reeds. These three instruments are not brass instruments. They are called woodwind instruments.

If the sounds of each wind instrument are produced by air why don't they all sound alike?
Once the column of air has started vibrating inside the instrument it passes its vibrations onto the rest of the instrument. Different instruments are made from different materials and have different shapes. These affect the way the instrument vibrates and give each instrument its particular sound.