dBu (a.k.a. dBv (lower case v))

Abbreviation for decibels as referenced to voltage. In this case the zero reference (0 dBu) is .775 volts, but does not assume any load impedance. At an impedance of 600 ohms dBu is equal to dBm (meaning the .775 volts produces one milliwatt of power). Since most modern equipment has input impedances much higher than the old 600 ohm reference it is safe to say that at .775 volts the actual power being transferred is much less than one milliwatt, but here we are only concerned with the voltage because at high impedances very little power has to be transferred in order to pass a signal. The dBu is commonly used in specifications to denote input and output levels for audio equipment. A rating of +4 dBu in equipment specifications implies a reference or nominal voltage of 1.23 volts. Keep in mind that voltage does not relate to decibels linearly. It is a logarithmic function. The formula for calculating dBu is 20log (V2/V1) where V1 and V2 are your two voltages (remember dB is always a relative difference between two values). With this you can see that 20log (1.23/0.775) = +4 dBu. The dBu used to be listed as dBv (lower case v), but to avoid confusion with the dBV (upper case V) the “u” was adopted.