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1938 Bell Lab. Rec. 16 213/1 The intensity level of the 1000-cycle reference tone was defined as the loudness level of this tone, and any other sound which is judged by listeners to be equally loud is said to have an equal value of loudness level.

1938 Bell Lab. Rec. 16 214/2 The unit to be used for intensity level measurements is the decibel, but the unit to be used for the equivalent loudness level measurements is the phon.