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]

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.[2] 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.

It is essential to know which category a measurement belongs to when using decibels (dB) for comparing such quantities. One bel is a 10× change in power, so when comparing power quantities x and y, the difference is described as 10×log10(y/x). With field quantities, however the difference is 20×log10(y/x).[2] This apparent inconsistency actually results in consistent meaning within the intended field of application. An amplifier can be described as having "3 dB" of gain without needing to specify whether volts or watts are being compared; for a given load (e.g. an 8 Ω speaker), it makes no difference, and will produce a 3 dB increase in the resultant sound volume.

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. 1 2 Brian C.J. Moore (1995). Hearing. Academic Press. p. 11. ISBN 978-0-08-053386-5.
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