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A unit of loudness level; the loudness level, in phons, of a sound is numerically equal to the sound pressure level, in decibels, of a 1000-hertz reference tone which is judged by listeners to be equally loud to the sound under evaluation.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.
The following article is from The Great Soviet Encyclopedia (1979). It might be outdated or ideologically biased.



the unit of loudness level. Since sounds of differing intensity—that is, of differing sound pressure—may have the same loudness at different frequencies, the loudness of a sound is evaluated by comparing it with the loudness of a pure reference tone, usually a tone with a frequency of 1,000 hertz (Hz). One phon is the difference between the loudness levels of two sounds of a given frequency whose intensities, or sound levels, at a frequency of 1,000 Hz differ by 1 decibel (dB). For pure tones with a frequency of 1,000 Hz, a difference of 1 phon in loudness level is equal to a difference of 1 dB in sound level.

The Great Soviet Encyclopedia, 3rd Edition (1970-1979). © 2010 The Gale Group, Inc. All rights reserved.

loudness level

Of a sound, the sound-pressure level of an equally loud 1,000-Hz pure tone, expressed in units called phons.
McGraw-Hill Dictionary of Architecture and Construction. Copyright © 2003 by McGraw-Hill Companies, Inc.