The Sound power reference article from the English Wikipedia on 24-Apr-2004 (provided by Fixed Reference: snapshots of Wikipedia from wikipedia.org)

Sound power

Sound power or acoustic power is a measure of sonic energy E per time t unit. It is measured in watts,or sound intensity J times area A:

The measure of a ratio of two sound powers is

where W₁ and W₀ are the powers.

The sound power level PWL or L_W of a source is expressed in decibels (dB) and is equal to 10 times the logarithm to the base 10 of the ratio of the sound power of the source to a reference sound power.

The reference sound power in air is normally taken to be 10^-12 watt = 0 dB SWL.

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