Decibel

  One tenth of a unit called the bel, after A.G. Bell, the decibel (dB) denotes the tenfold logarithm to base 10 of the ratio of two amounts of power, . The dB is a convenient way to define attenuation and gain in a system; according to the above definition, 20dB describes a ratio of 100:1, -3dB is close to a factor of 1/2, -20dB stand for a factor of 0.01. One decibel in dynamic range corresponds to 0.3322 bits.

The same measure is often used by engineers with a factor of 2 applied to denote the ratios of voltages (or currents) in the form , as power is proportional to the square of the voltage. Note that for a voltage or current ratio the system impedance must be constant.

In the frequent use of the unit in the domain of audible noise one often (mistakenly) thinks of dB as an absolute unit; in reality, decibel is a unit to express ratios of sound pressure p₁/p₀, with the above definition, where p₀ is the ``smallest audible noise''. Audio engineers also use dB in the above sense of voltage ratios, and write dBV if they scale by setting 0dBV = 1V, or dBu if the scale is given by 0dBu = 0.775V.