The unit on the subjective scale of equivalent loudness, proposed by Barkhausen and adopted by the First International Acoustical Conference (Paris, July 1937).¹ One phon is the loudness level of a sound wave which is judged by a person with normal hearing, under standard listening conditions, to have the same loudness as a sound wave having a sound pressure level of 1 decibel (reference level of 20 millipascals) at 1 kilohertz.²

1. The First International Acoustical Conference.
Nature, volume 140, page 370 (August 28, 1937).

2. Quantitities and Units in Acoustics. (ISO 31/VII)
American National Standards Institute, New York, 1978.

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