The strength of a sound wave is most conveniently described as sound pressure.
Sound pressure is the extremely small alternating deviation above and below atmospheric pressure due to the propagated wave of compression and rarefaction.
Because the ear operates over a range of sound pressures (its dynamic range) which is greater than a million to one, stimulus strength is usually expressed in logarithmic units known as decibels (dB).
The decibel notation always expresses the ratio between two intensities.
For studies of hearing, P2 is taken as the sound pressure at the threshold hearing of a normal listener.
If, for example, the sound pressure from one source (P1) is is ten times greater than that from a second (P2), the difference is 20 dB.
The sound pressure of a very loudsound, such as a jet plane, may be one million times (106) the pressure of the weakest sound that can be detected by someone with normal hearing; these two sounds differ by 120 dB: