Power, root-power, and field quantities: Difference between revisions

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A '''power quantity''' is a power or a quantity directly proportional to [[Power (physics)|power]], e.g., [[energy density]], [[acoustic intensity]], and [[luminous intensity]].<ref>Ainslie, M. A. [http://acousticstoday.org/wp-content/uploads/2015/02/A-Century-of-Sonar.pdf A Century of Sonar: Planetary Oceanography, Underwater Noise Monitoring, and the Terminology of Underwater Sound]. Acoustics Today (2015)</ref>
A '''power quantity''' is a power or a quantity directly proportional to [[Power (physics)|power]], e.g., [[energy density]], [[acoustic intensity]], and [[luminous intensity]].<ref>{{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}}</ref> Energy quantities may also be labelled as power quantities in this context.<ref>ISO 80000:1-2009 §&nbsp;C.3</ref>


A '''field quantity''' is a quantity such as [[voltage]], [[Electric current|current]], [[sound pressure]], [[electric field strength]], [[speed]], or [[charge density]], the square of which, in linear systems, is proportional to power.<ref>{{cite book|author=Brian C.J. Moore|title=Hearing|date=1995|publisher=Academic Press|isbn=9780080533865|page=11 |url={{Google books |plainurl=yes |id=OywDx9pxCMYC |page=11 }} }}</ref> 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''.
A '''root-power quantity''' is a quantity such as [[voltage]], [[Electric current|current]], [[sound pressure]], [[electric field strength]], [[speed]], or [[charge density]], the square of which, in linear systems, is proportional to power.<ref name=Hearing>{{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 }} }}</ref> 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 [[decibel]]s (dB) for comparing the [[Level (logarithmic quantity)|level]]s 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×log<sub>10</sub>(''y''/''x'') decibel. With root-power quantities, however the difference is defined as 20×log<sub>10</sub>(''y''/''x'') dB.<ref name=Hearing/>


In the analysis of signals and systems using sinusoids, field quantities and root-power quantities may be [[complex number|complex]]-valued.<ref>ISO 80000-1:2009 §&nbsp;C.2</ref><ref>ISO 80000-3:2006 §&nbsp;0.5</ref><ref>IEC 60027-3:2002</ref>{{disputed inline|discuss=Talk:Level_(logarithmic_quantity)#"root-power_quantity"_vs._"field_quantity"|date=December 2017}}

=="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&nbsp;80000 draws attention to the conflicting use of the former term to mean a quantity that depends on the position,<ref>ISO 80000-1:2009 §&nbsp;C.2</ref> which in physics is called a ''[[Field (physics)|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==
==See also==
* [[Level (logarithmic quantity)]]
* [[Fresnel reflection]] [[Fresnel equations#Amplitude or field equations|field]] and [[Fresnel equations#Power or intensity equations|power]] equations
* [[Fresnel reflection]] [[Fresnel equations#Amplitude or field equations|field]] and [[Fresnel equations#Power or intensity equations|power]] equations
* [[Sound level (disambiguation)|Sound level]], defined in different ways for the pressure or power of sound
* [[Sound level (disambiguation)|Sound level]], defined for each of several quantities associated with sound


==References==
==References==

Revision as of 14:06, 22 April 2021

A power quantity is a power or a quantity directly proportional to power, e.g., energy density, acoustic intensity, and luminous intensity.[1] Energy quantities may also be labelled as power quantities in this context.[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.[3] The term root-power quantity refers to the square root that relates these quantities to power. The term was introduced in 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.[3]

In the analysis of signals and systems using sinusoids, field quantities and root-power quantities may be complex-valued.[4][5][6][disputed ]

"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,[7] 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

References

  1. ^ Ainslie, Michael A. (Winter 2015). "A Century of Sonar: Planetary Oceanography, Underwater Noise Monitoring, and the Terminology of Underwater Sound" (PDF). Acoustics Today. 11 (1): 12–19.
  2. ^ ISO 80000:1-2009 § C.3
  3. ^ a b Brian C.J. Moore (1995). Hearing. Academic Press. p. 11. ISBN 978-0-08-053386-5.
  4. ^ ISO 80000-1:2009 § C.2
  5. ^ ISO 80000-3:2006 § 0.5
  6. ^ IEC 60027-3:2002
  7. ^ ISO 80000-1:2009 § C.2