dB

The decibel (dB) is a logarithmic unit used to express the ratio between two quantities, most often power or a field quantity such as voltage or sound pressure. It is not an SI unit, but a convenient way to compare magnitudes by referencing a chosen standard.

For power ratios, the dB value is defined as Lp = 10 log10(P1/Pref), where Pref is the reference power. For quantities proportional to the square of a quantity (such as voltage or current with constant impedance), the dB value is often written as L = 20 log10(Q1/Qref), since power scales with the square of the quantity.

Common references include dBm (power relative to 1 milliwatt) and dBW (relative to 1 watt). In electronics, dBV expresses a voltage ratio relative to 1 volt. In acoustics, dB SPL (sound pressure level) uses p0 = 20 micropascals in air as the reference.

Interpreting changes: a 3 dB increase roughly doubles the power; a 10 dB increase corresponds to a tenfold increase in power. In voltage terms, a 20 dB change represents a tenfold change in voltage for a fixed impedance. Because the decibel is a logarithmic scale, large ranges of ratios can be described compactly, and dB values are additive for cascaded systems.

Applications are widespread, including audio engineering, telecommunications, signal processing, and acoustics, where dB conveys gains, losses, and dynamic range in a consistent, relative framework.