bel

The bel is a unit used to express power level relative to an arbitrary reference level defined as the common logarithm of the ratio of the powers. Named for Alexander Graham Bell, the bel is seldom used, the decibel (1 dB = 0.1 bel) and neper being preferred by acoustics and telecommunication engineers.

 

phon

The phon is 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.