Sound pressure amplitude

The amplitude of a sound wave is the whole change in pressure which occurs during a single cycle. Since of the huge range in sound wave amplitudes, P, it is expressed in logarithmic scale as a ratio of a reference pressure, P_ref

                               Sound pressure level (SPL) = 20 log₁₀ P/P_ref

Pref is 2 x 10^-5 Pa, a sound pressure that is at the threshold of human hearing. The unit of SPL is decibel (dB). Each 10-fold rise in SPL is equal to 20 dB. Sound pressure levels in surplus of 100 dB can results in damage to hearing, and at 120 dB auditory pain occurs. Dissimilarities in sound pressure level are perceived as differences in loudness that varies with frequency in a way that is determined by the sensitivity of the ear.