The decibel (dB) is a logarithmic unit of measurement of the ratio of two powers. It is a
dimensionless outside the international system. It is used especially in the fields of acoustics, physics, electronics and is widely used in all fields of engineering.
The bel (symbol B) is used in telecommunications, electronics, acoustics as well as
mathematics. Invented by Bell Labs engineers to measure the signal attenuation
audio over a distance of one mile (1.6 km), longueurstandard a telephone cable, he was called
unit "transmission" in the beginning, or TU ((en) Transmission unit), but was renamed in 1923 or
1924 in honor of the founder of the laboratory and pioneer in telecom, Alexander Graham Bell. For more information, here is a pdf File Decibel