Performance size

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In different contexts, especially of electrical engineering and acoustics (eg. As audio level , voltage gain , shielding effectiveness ) are physical quantities not stated directly, but only as a ratio to a second variable or fixed size of the same type. Preferably, the two sizes the ratio of which is specified, in each case by performance quantities or performance root quantities .

If the ratios extend over several powers of ten , it makes sense to specify them as a logarithmic quantity .

Performance size

A performance quantity is a quantity that is proportional to a performance .

Examples: electrical power , electromagnetic and acoustic power and associated power densities

In this context, energy quantities, i.e. quantities that are related to an energy , are also referred to as power quantities .

Examples: electrical energy , electromagnetic and acoustic energy and associated energy densities ( sound power , sound intensity , sound energy density )

Achievement root size

A performance root quantity is a quantity whose square is proportional to a performance quantity. Power root quantities were previously referred to as field quantities .

Examples: electrical voltage , electrical current strength , electrical and magnetic field strength , electrical and magnetic flux density , sound pressure , sound velocity

Power root quantities are usually effective values ; for a sine- shaped alternating quantity their can amplitude , complex amplitude or its complex RMS may be used.

Logarithmic ratios

Logarithmic ratio with field sizes

Logarithmic ratio with performance quantities

Example for the gain of a two-port with the real voltages at the output and at the input:
or with the complex sizes :


  • Horst Clausert, Gunther Wiesemann, Volker Hinrichsen , Jürgen Stenzel: Basic areas of electrical engineering. Volume 2: Alternating currents, three-phase currents, cables, applications of Fourier, Laplace and Z transformation. 11th, corrected edition. Oldenbourg, Munich et al. 2011, ISBN 978-3-486-59719-6 .

Individual evidence

  1. a b c DIN 5493: 2013-10: Logarithmic quantities and units
  2. a b c DIN EN 60027-3: 2007-11: Symbols for electrical engineering - Part 3: Logarithmic and related quantities and their units