"of a quantity related to power: the ratio, expressed in decibels, of the magnitude of the quantity to a specified reference magnitude.
Note 1: the word level is frequently used in compound terms such as "intensity level" and "sound pressure level" to relate a magnitude to a common reference magnitude or "zero level".
Note 2: in the absence of any statement to the contrary, the reference magnitude for stating sound pressure levels in air shall be 0.0002 dyne/sq.cm, and that for sound intensity levels 10-16watt/sq.cm." 
- . Glossarial Index. Dans: Acoustic Behavior of Animals. Acoustic Behavior of Animals. Elsevier; 1963.
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