"Level quantity" redirects here. For other uses, seeLevel measurement.
Inscience and engineering, apower level and afield level (also called aroot-power level) arelogarithmic magnitudes of certain quantities referenced to a standard reference value of the same type.
Apower level is a logarithmic quantity used to measure power, power density or sometimes energy, with commonly used unitdecibel (dB).
Afield level (orroot-power level) is a logarithmic quantity used to measure quantities of which the square is typically proportional to power (for instance, the square of voltage is proportional to power by the inverse of the conductor's resistance), etc., with commonly used unitsneper (Np) ordecibel (dB).
The type of level and choice of units indicate the scaling of the logarithm of theratio between the quantity and its reference value, though a logarithm may be considered to be a dimensionless quantity.[1][2][3] The reference values for each type of quantity are often specified by international standards.
The level of aroot-power quantity (also known as afield quantity), denotedLF, is defined by[5]
where
F is the root-power quantity, proportional to the square root of power quantity;
F0 is the reference value ofF.
If the power quantityP is proportional toF2, and if the reference value of the power quantity,P0, is in the same proportion toF02, the levelsLF andLP are equal.
Theneper,bel, anddecibel (one tenth of a bel) are units of level that are often applied to such quantities as power, intensity, or gain.[6] The neper, bel, and decibel are related by[7]
The ISO standard defines each of the quantities power level and field level to be dimensionless, with1 Np = 1. This is motivated by simplifying the expressions involved, as in systems ofnatural units.
Thelogarithmic frequency ratio (also known asfrequency level) of two frequencies is the logarithm of their ratio, and may be expressed using the unitoctave (symbol: oct) corresponding to the ratio 2 or the unitdecade (symbol: dec) corresponding to the ratio 10:[7]
Inmusic theory, theoctave is a unit used with logarithm base 2 (calledinterval).[10] Asemitone is one twelfth of an octave. Acent is one hundredth of a semitone. In this context, the reference frequency is taken to beC0, four octaves belowmiddle C.[11]
