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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:
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P_{\mathrm{acoustic}} = J \cdot A
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The measure of a ratio of two sound powers is
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L_w=10\, \log_{10}\left(\frac{W_1}{W_0}\right)dB
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where
W1 and W0 are the powers.
The sound power level PWL or LW 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. It is thus a logarithmic measure.
The reference sound power in air is normally taken to be 10−12 watt = 0 dB SWL.
Notice: Usable music sound (trumpet) and noise sound (excavator) have both the same sound power of 0.3 watts, but will be judged psycho-acousticly totally different.
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