A unit of loudness of sound. The intensity of a sound to be measured is compared by the human ear to a reference tone of 2 X 10⁻⁵ pascal sound pressure and 1000 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.