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Sound is produced when the air is disturbed in some way, for example by a
vibrating object. A speaker cone from a hi-fi system serves as a good
illustration. It may be possible to see the movement of a bass speaker
cone, providing it is producing very low frequency sound. As the cone
moves forward the air immediately in front is compressed causing a slight
increase in air pressure, it then moves back past its rest position and
causes a reduction in the air pressure (rarefaction). The process
continues so that a wave of alternating high and low pressure is radiated
away from the speaker cone at the speed of sound.
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