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When the rapid variations in pressure occur between about 20 and 20,000
times per second (ie at a frequency between 20Hz and 20kHz) sound is
potentially audible even though the pressure variation can sometimes be as
low as only a few millionths of a Pascal. Movements of the ear drum as
small as the diameter of a hydrogen atom can be audible! Louder sounds are
caused by greater variation in pressure - 1 Pascal, for example, will
sound quite loud, provided that most of the acoustic energy is in the
mid-frequencies (1kHz - 4kHz) where the ear is most sensitive.
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