In acoustics, the decibel is quantified relative to a reference that has been set at a sound pressure level of 20 micro pascals and is called a 0 dB. The following equation is used to calculate the sound pressure level or amplitude:

Amplitude [dB] = 20 log₁₀ s/s_o, so is the reference pressure which is 20 micro pascals or 0 dB, and s is the observed sound pressure.

The human ear has a standard sound threshold of 120 dB, which expressed logarithmically is around 1012.

This is a standard threshold, but it also depends on frequency. Loudness is a measure of sound intensity taking frequency into account and is called an A-weighted decibel, dB(A), or a phon.