power, root-power, and field quantities

A power quantity is a power or a quantity directly proportional to power, e.g., energy density, acoustic intensity, and luminous intensity. Energy quantities may also be labelled as power quantities in this context.

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. The term ''root-power quantity'' refers to the square root that relates these quantities to power. The term was introduced in ; it replaces and deprecates the term field quantity.

Implications

It is essential to know which category a measurement belongs to when using decibels (dB) for comparing the levels of such quantities. A change of one bel in the level corresponds to a 10× change in ''power'', so when comparing power quantities ''x'' and ''y'', 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.

In the analysis of signals and systems using sinusoids, field quantities and root-power quantities may be complex-valued, as in the propagation constant.

"Root-power quantity" vs. "field quantity"

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,ISO 80000-1:2009 § C.2 which in physics is called a '' field''. Such a field is often called a ''field quantity'' in the literature, but is called a ''field'' here for clarity. Several types of field (such as the electromagnetic field) meet the definition of a root-power quantity, whereas others (such as the Poynting vector and temperature) do not. Conversely, not every root-power quantity is a field (such as the voltage on a loudspeaker).

See also

* Level (logarithmic quantity)
* Fresnel reflection field and power equations
* Sound level, defined for each of several quantities associated with sound

References

{{reflist

Mathematical terminology