How small and rapid are the changes of air pressure which cause sound?
When the rapid variations in pressure occur between about 20 and 20,000 times per second (i.e. 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 tens of 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. A sound wave of one Pascal amplitude, for example, will sound quite loud, provided that most of the acoustic energy is in the mid frequencies (1kHz – 4kHz) where the human ear is most sensitive. It is commonly accepted that the threshold of human hearing for a 1 kHz sound wave is about 20 micro-Pascals.
