Look at a set of tuning forks. There are usually eight forks in a set, and they are all of different sizes. The fork is ''played'' by striking it against a hard surface, which makes the prongs vibrate - rather like plucking a string. The same rules also apply to tuning forks - the lower notes are made by forks that are longer and thicker, so that they do not vibrate so fast.
A set of tuning forks make the notes C, D, E, F G, A, B & C. The first and largest fork produces middle C. Then they go down in length and thickness, making the notes D to B. The smallest (eighth) fork is a C again, but this note is one octave higher than middle C.
They are very carefully made, so that each fork makes a pure sound, with a regular wave that has a fixed number of vibrations per second.

The sound from a tuning fork is pure and has a smooth, regular sound-wave pattern. The hammer's sound has an irregular, ''spiky'' wave, which is typical of a noise.

Noise occurs when a sound wave has no smooth, regular pattern. It consists of a mixture of different vibrations. The wave is all mixed up. For example, the hissing made by air escaping from a bicycle tire is not a pure sound. It is a noise. Its sound-wave pattern is irregular

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