Level (logarithmic quantity)
In the International System of Quantities, level is the logarithm of the ratio of a quantity Q to a reference value of that quantity, Q0. Examples are sound power level (literally, the level of the sound power) and sound exposure level (level of the sound exposure, abbreviated SEL). Another example is sound pressure level, which is the level of the (root-mean-square) sound pressure, and is abbreviated SPL.
In equation form: LQ = log(Q/Q0).
The neper, bel, and decibel (one tenth of a bel) are units of level that are often applied to such things as power, intensity, or gain. According to ANSI S1.1-2013 Acoustical Terminology, the octave and semitone (one twelfth of an octave) are also units of level, specifically "frequency level", though that concept is seldom seen outside of the standard.
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