[2] A root-power quantity is a quantity such as voltage, current, sound pressure, electric field strength, speed, or charge density, the square of which, in linear systems, is proportional to power.
It is essential to know which category a measurement belongs to when using decibels (dB) for comparing the levels of such quantities.
[3] In the analysis of signals and systems using sinusoids, field quantities and root-power quantities may be complex-valued,[4][5][6][disputed – discuss] as in the propagation 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 a field.
Conversely, not every root-power quantity is a field (such as the voltage on a loudspeaker).