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.{{cite journal |last=Ainslie |first=Michael A. |url=http://acousticstoday.org/wp-content/uploads/2015/02/A-Century-of-Sonar.pdf |title=A Century of Sonar: Planetary Oceanography, Underwater Noise Monitoring, and the Terminology of Underwater Sound |journal=Acoustics Today |volume=11 |issue=1 |pages=12–19 |date=Winter 2015}} Energy quantities may also be labelled as power quantities in this context.ISO 80000:1-2009 § C.3
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.{{cite book|author=Brian C.J. Moore |title=Hearing |date=1995 |publisher=Academic Press |isbn=978-0-08-053386-5 |page=11 |url={{Google books |plainurl=yes |id=OywDx9pxCMYC |page=11 }} }} The term root-power quantity refers to the square root that relates these quantities to power. The term was introduced in {{section link|ISO 80000-1|Annex C}}; 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,ISO 80000-1:2009 § C.2ISO 80000-3:2006 § 0.5IEC 60027-3:2002{{disputed inline|discuss=Talk:Level_(logarithmic_quantity)#"root-power_quantity"_vs._"field_quantity"|date=December 2017}} 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,{{fact|date=January 2024}} 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).{{fact|date=January 2024}}
See also
- Level (logarithmic quantity)
- Fresnel reflection field and power equations
- Sound level, defined for each of several quantities associated with sound
References
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