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Power, root-power, and field quantities

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er Noise Monitorn linear sys and deprecates the term .

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.[1] In linear systems, 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 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.[2][3][4][disputed – discuss]

"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,[5] 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. ^ Cite error: The named reference Hearing was invoked but never defined (see the help page).
  2. ^ ISO 80000-1:2009 § C.2
  3. ^ ISO 80000-3:2006 § 0.5
  4. ^ IEC 60027-3:2002
  5. ^ ISO 80000-1:2009 § C.2
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