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A unit of apparent loudness, equal in number to the intensity in decibels of a 1,000-hertz tone perceived to be as loud as the sound being measured.
[German, from Greek phōnē, sound; see phone1.]
(Units) a unit of loudness that measures the intensity of a sound by the number of decibels it is above a reference tone having a frequency of 1000 hertz and a root-mean-square sound pressure of 20 × 10–6 pascal
[C20: via German from Greek phōnē sound, voice]