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Field, power, and root-power quantities
A power quantity is a power or a quantity directly proportional to power, e.g., energy density, acoustic intensity, and luminous intensity.〔Ainslie, M. A. (A Century of Sonar: Planetary Oceanography, Underwater Noise Monitoring, and the Terminology of Underwater Sound ). Acoustics Today (2015)〕
A field 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. The term root-power quantity was introduced in the ISO 80000-1#Annex C, defined as the square root of a power quantity; it replaces and deprecates the term ''field quantity''.
If a field quantity is a complex value (e.g., a phasor), then its magnitude is implied in the logarithm calculations involved. The phrase "root-power quantity" avoids this ambiguity by definition, as power is always positive, and its square root is always real-valued.
The distinctions here described have implications for the definition of decibel, which depends on whether the measured property is a power quantity or a field quantity. So does the level of a root-power quantity or of a power quantity.
==See also==
* Fresnel reflection field and power equations
* Sound level, defined in different ways for the pressure or power of sound
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