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.[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 was introduced in the ISO 80000-1 § Annex C; it replaces and deprecates the term field quantity.
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] These definitions allow the distinction between root-power quantities and power quantities to be ignored when specifying changes as levels: an amplifier can be described as having "3 dB" of gain without needing to specify whether voltage or power are being compared; for a given linear load (e.g. an 8 Ω speaker), such an increase will similarly result in a 3 dB increase in both the sound pressure level and the sound power level at a given location near the speaker. Conversely, when ratios cannot be identified as either power or root-power quantities, the units neper (Np) and decibel (dB) cannot be sensibly used.
In the analysis of signals and systems using sinusoids, field quantities and root-power quantities may be complex valued.[4][5][6]
See also[edit]
- Level (logarithmic quantity)
- Fresnel reflection field and power equations
- Sound level, defined in different ways for the pressure or power of sound
References[edit]
- ↑ 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.
- ↑ ISO 80000:1-2009 § C.3
- ↑ 3.0 3.1 Brian C.J. Moore (1995). Hearing. Academic Press. p. 11. ISBN 978-0-08-053386-5. Search this book on
- ↑ ISO 80000-1:2009
- ↑ ISO 80000-3:2006
- ↑ IEC 60027-3:2002
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