Bel (B), the SI unit of a logarithmic relative quantity (the decimal logarithm of the ratio of the values of two physical quantities of the same kind). Named in honour of the American scientist A. G. Bell. Usually the 0.1 fraction of the bel — the decibel — is used. 1B = lg (P2/P1) at P2=10 P1, where P1 and P2— powers, energies and other energy quantities, or 1B = 2lg(F2/F1) at F2= √10·F1 where F1 and F2— voltages, currents and other analogous quantities. The unit bel is used in many fields of physics and engineering (acoustics, radio engineering and others).