The human hearing mechanism requires that the variations occur at least 20 times a second, but not more frequent than 20 000
times a second (often written as 20 kHz).
The number of variations per second is called the frequency of the sound and is expressed in Hertz (Hz).
Pressure variations with frequencies lower than 20 Hz should also be regarded as sound. They are normally referred to as
infrasound. Sounds with frequencies higher than 20 kHz lie above the audible region and are referred to as ultrasound.
The pressure variation can be either very small or very large, or anything in between. The pressure is measured in Pascal which equals
Newton per square meter, which is a derived SI unit. It is not so practical to measure Newton and present it to "the people"
since it seldom gives a feeling or understanding of what the number really represents. If we, however, referes the sound level
on a decibel scale to the normal, human threshold of hearing, we get reasonable number that is easily related to the perceived
hearing level.
dB calculation:
P
L = 10 x log(----)
P0
wher P is the current pressure in Pascal and P0 is the threshold of hearing 0.00002 Pascal.