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Sound waves
Sound waves travel through the air by the air molecules vibrating backwards and forwards.
The faster the vibration the higher the pitch of the note and the bigger the vibration the louder the note.

A pure note will be a smooth wave with no quick changes. The size and shape of the wave produced by a sound wave will depend on the pitch of the note, what produces it and how loud it is.
The range of human hearing will be from the lowest audible note at about 50Hz to the highest at about 20 000Hz. A man can sing down to about 64Hz and up to about 500Hz, while a woman normally has a range from 200Hz to about 900Hz. Middle C on a piano is 256Hz. The older you get the less well you will hear the high pitch notes.
© Keith Gibbs 2009