Apower quantity is a power or a quantity directly proportional topower, e.g.,energy density,acoustic intensity, andluminous intensity.[1] Energy quantities may also be labelled as power quantities in this context.[2]
Aroot-power quantity is a quantity such asvoltage,current,sound pressure,electric field strength,speed, orcharge density, the square of which, in linear systems, is proportional to power.[3] The termroot-power quantity refers to thesquare root that relates these quantities to power. The term was introduced inISO 80000-1 § Annex C; it replaces and deprecates the termfield quantity.
It is essential to know which category a measurement belongs to when usingdecibels (dB) for comparing thelevels of such quantities. A change of one bel in the level corresponds to a 10× change inpower, so when comparing power quantitiesx andy, the difference is defined to be 10×log10(y/x) decibel. With root-power quantities, however the difference is defined as 20×log10(y/x) dB.[3]
In the analysis of signals and systems using sinusoids, field quantities and root-power quantities may becomplex-valued,[4][5][6][disputed –discuss] as in thepropagation constant.
In justifying the deprecation of the term "field quantity" and instead using "root-power quantity" in the context of levels, ISO 80000 draws attention to the conflicting use of the former term to mean a quantity that depends on the position,[7] which in physics is called afield. Such a field is often called afield quantity in the literature,[citation needed] but is called afield here for clarity. Several types of field (such as theelectromagnetic field) meet the definition of a root-power quantity, whereas others (such as thePoynting vector andtemperature) do not. Conversely, not every root-power quantity is a field (such as the voltage on aloudspeaker).[citation needed]