The Sound intensity level reference article from the English Wikipedia on 24-Apr-2004
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Sound intensity level

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Sound intensity level or acoustic intensity level is a logarithmic measure of the sound intensity in comparison to the reference level of 0 dB (decibels).

The measure of a ratio of two sound intensities is

where J1 and J0 are the intensities.

The sound intensity level is given the letter "LJ" and is measured in "dBW". dB is dimensionless.

If J0 is the standard reference sound intensity, where

(W = watt), then instead of "dB" we use "dB SWL". (SWL = sound intensity level).

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