What is sound ?

Sound is the quickly varying pressure wave within a medium.
We usually mean audible sound, which is the sensation (as detected by
the ear) of very small rapid changes in the air pressure above and
below a static value. This “static” value is atmospheric pressure
(about 100,000 Pascals) which does nevertheless vary slowly, as shown
on a barometer. Associated with the sound pressure wave is a flow of
energy. Sound is often represented diagrammatically as a sine wave, but
physically sound (in air) is a longitudinal wave where the wave motion
is in the direction of the movement of energy. The wave crests can be
considered as the pressure maxima whilst the troughs represent the
pressure minima.

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 (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.