dBu

dBu is a unit of level used in audio engineering to express voltage levels relative to a fixed electrical reference. The reference voltage for dBu is 0.775 volts RMS, and 0 dBu corresponds to that voltage. Values higher or lower than zero indicate how many decibels the signal is above or below 0.775 V RMS. The quantity is an absolute level measurement in decibels, but it is defined in terms of voltage rather than power.

Definition and calculation: dBu = 20 log10(V / 0.775 V), where V is the root-mean-square voltage of the signal. The 0.775 V reference traces back to the historical standard of delivering 1 milliwatt into a 600-ohm load, which yields approximately 0.775 V RMS. Because dBu describes voltage, it does not by itself specify a particular impedance or the resulting power; the actual power depends on the load driving the signal.

Common values and usage: In professional audio, +4 dBu is a common nominal line level, corresponding to about 1.23 V RMS. 0 dBu equals 0.775 V RMS, and -10 dBu is about 0.245 V RMS, a level more typical of some consumer gear but defined against the same 0.775 V reference. The dBu scale is widely used to specify operating levels for analog equipment such as mixers, processors, and interfaces, allowing consistent headroom and gain staging across devices.

Relation to other scales: dBu is voltage-based and differs from dBV, which uses 1 V as reference. It is also distinct from digital scales like dBFS, which relate to full-scale digital samples.