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A unit of loudness of sound. The intensity of sound to be measured is compared by the human ear to a reference tone of 2 × 10-5 pascal sound pressure and 1,000 hertz frequency. The intensity of the reference tone is increased until it appears to be equal in loudness to the sound being measured; the loudness of this sound in phons is then equal to the number of decibels by which the reference tone has had to be increased.

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