Audio levels are expressed in rms volts or millivolts but more often in one of the several logarithmic dB (decibel) systems. Decibel units are much easier to work with as system gain and loss factors are simply additive or subtractive. In dB systems, levels are expressed as a ratio with respect to a chosen reference. In the dBV system, for example, the value is the absolute voltage level with respect to 1 volt. The use of dBV has pretty much fallen out of favor in the U.S.

For many years the standard system used in audio work has been the dBm in which the voltage reference is the voltage across a 600 W terminating resistance when a power of 1 mW is dissipated.

Although dBm is loosely called dB, the correct designation specifies the impedance and is 600 W dBm. The unit that has come into general use in the broadcast industry is the dBu. The u is not the Greek letter m used for micro but is a lower case U and stands for unterminated. This system uses the same voltage reference, 0.775 V but works on the principle that all loads have a high input impedance and do not load the source. Zero dBu stands for 0.775 V from a voltage source with much lower impedance than the loads. VU (volume unit) meters are also calibrated in dB and indicate the 0.775 V reference at zero dBu and dBm.