- A dimensionless unit which expresses the ratio of two powers or intensities, such as voltages, currents, or sound intensities. The number of bels represents the logarithm of the ratio of the quantities measured. A bel is equal to approximately 1.1513 nepers. The decibel, which is 0.1 bels, is a much more frequently encountered unit, as the bel is too large for most applications. Its symbol is b, or B.