waves are succesive or fluctuating levels of pressure,
in some elastic medium, usually air. These fluctuations consist of a series
of compressions and rarefactions of the molecules of the medium or in other
terms a rapid succession of raisings and lowerings of air pressure from
its static norm.
waves which are the result of greater swings of pressure
or compressions/rarefactions are
said to exhibit greater amplitude than those with smaller changes in pressure.
There is a corresponding subjective perception of loudness with amplitude,
where sounds of larger amplitude are generally perceived as being louder
than those softer sounds of smaller amplitude.
waves fluctuate at some rate of frequency
between 20 and 20,000 times each second. This is the
of human hearing with the lower frequencies corresponding
roughly to lower perceived pitch
and the higher frequencies to higher perceived pitch.