Making a musical sound
Have you seen a tuning fork? It is made from a piece of metal which is shaped into two long prongs with a short bar, which serves as a handle. The prongs are called tines and when they are struck they give out a musical note. They do this because they vibrate in a regular way. After the tines have been struck they vibrate a certain number of times every second. This sets up sound waves in the air, which also vibrate at the same rate. The number of times an object vibrates in a second is called its frequency and the vibrations set up a sound wave, which has the same frequency too.

Imagine for a moment that a tuning fork was struck and vibrated 524 times in a second. The tines would push on the air particles around them and send out 524 sound waves every second. If this regular number of sound waves reached your ear it would make the musical note 'doh'. There are other tuning forks which vibrate at even higher frequencies and they give you the range of musical notes 'ray', 'me', 'fah', 'soh', 'lah', 'tee'. Try singing these notes slowly. You should find that you can feel your voice box vibrating as it makes the sounds.

Imagine that you have a metal tray and you drop it on the ground. Although the object is made of metal, like the tuning fork, the noise it makes is completely different and far from musical. When the tray hit the ground part of it vibrated at a certain frequency and other parts vibrated at other frequencies. The parts that hit the ground first struck it more strongly and a louder noise was made while parts which struck the ground moments later struck the ground less strongly and made quieter sounds. This mixture of sound waves of different frequencies and loudness produces a very complicated sound, which we find unpleasant. A musical note, then, has a much simpler sound, which the ear finds pleasant.

When people talk about musical sounds, they often say that a sound has either a low or a high pitch. An example of a low-pitched sound is the note 'doh'. An example of high-pitched sound is the note 'tee'. When people are talking about low- and high-pitched sounds they are really talking about sounds with low or high frequencies.

This link between pitch and frequency may be easier to understand if we think about a very simple musical instrument. A very simple musical instrument can be made from just a few pieces of wood. It should be made from the same wood with pieces the same width and thickness but cut to different lengths. The pieces of wood should be arranged in order starting with the shortest and ending with the longest. This instrument is a simple xylophone. When the longest piece of wood is struck with a stick, it makes a low-pitched note. When the shortest piece of wood is struck it makes a high-pitched note. The difference is due to the amount of material in each piece of wood. When the longest piece of wood is struck, the energy it receives has to vibrate a large amount of material. When the shortest piece of wood is struck, the energy it receives has to vibrate a small amount of material. As the energy that the long piece of wood receives has to shake a large number of particles, it shakes them at a low frequency and this makes a low-pitched note. As the energy that the short piece of wood receives has to shake a smaller number of particles, it shakes them at a high frequency and this makes a high-pitched note.

The two major groups of musical instruments are wind instruments and stringed instruments. The instruments in both groups make musical notes by causing regular vibrations in the air. In the wind instruments the vibrations are actually set up by the air vibrating inside the instrument. This vibration passes through the body of the instrument to produce sound waves of certain frequencies, which travel to our ears and we hear them as musical notes. In stringed instruments the vibrations are set up by plucking or scraping a string. These vibrations pass to the rest of the instrument then also produce sound waves we hear as musical notes.

We must not forget that the voice is a musical instrument. Try and make a tune from the notes 'doh', 'ray', 'me', 'fah', 'soh', 'lah', 'tee'. If you have learned the basics of drumming on the page about damping sound you could sing your tune and give it a rhythm with your hands and feet.

If you hit a piece of wood softly and then loudly would it change its pitch?
No, pieces of wood, and all objects, have a natural frequency at which they vibrate. The extra energy just makes them vibrate more strongly but not more frequently. When an object vibrates more strongly it produces more powerful sound waves and these make a louder sound.

So pitch and loudness are not the same thing?
No. People often confuse the two. The pitch of a sound is due to the number of waves that reach your ear in a second. The loudness of a sound is due to the size of the wave that reaches your ear at any time.

When studying sound waves can you think of them like waves on a pond?
Yes, it sometimes helps, but you must remember that the waves on a pond are caused by particles of water moving up and down and the wave moving across them or at right angles to them. In a sound wave the particles of air swing to and fro in the same direction as the particles swinging away from the vibrating object.

You can think of sound waves as wiggly lines moving through air, water or solids. One wave is made up of one wiggle that is a wave crest and a wave trough. The crest is made by an upside-down U shape and the trough is made by a normal U shape.

Is there a link between the wavelength of a sound and its pitch?
If a wavelength is short, a large number of waves can be produced by a vibrating object in a second. This means that the sound has a high frequency and has a high pitch. If a wavelength is long only a small number of waves can be produced by a vibrating object in a second. This means that the sound has a low frequency and has a low pitch.

How can you play sounds of different frequency and pitch on a musical instrument?
By changing the size of the vibrating part of the instrument. In the xylophone you change the pitch by hitting a different piece of wood. In a wind instrument, you close some of the holes in its side to alter the amount of air vibrating inside it. In a stringed instrument you change the string you pluck or scrape because each string is a different size.

When you hear the siren of a police car or an ambulance it seems to change pitch as it passes you. How does this happen?
When the vehicle with the siren is stationary the sound waves travel through the air to your ear and are a certain wavelength. This gives you the pitch of the siren's note. If the vehicle is coming quickly towards you the sound waves leaving the siren and moving towards you become squashed together. This shortens their wavelength and raises their frequency. As a consequence, you hear a high-pitched note. When the vehicle passes you and moves away the sound waves moving towards you become stretched. This lengthens their wavelength and lowers their frequency. As a consequence you hear a lower-pitched note. The siren itself does not change the pitch of the note. It is the way the sound waves are squashed and stretched that makes the note change pitch.