dBVdBu

dBV and dBu are decibel measures of voltage used in audio engineering. They express a voltage level relative to a reference voltage. Specifically, dBV is defined as 20 times the base-10 logarithm of the ratio of a voltage to 1 volt RMS, so dBV = 20 log10(V/1 V). dBu uses a reference of 0.775 volts RMS, so dBu = 20 log10(V/0.775 V). By definition, 0 dBV equals 1 V RMS, while 0 dBu equals 0.775 V RMS. Because the references differ, the same voltage yields different dB values on the two scales.

Relation between the scales is straightforward: dBu ≈ dBV + 2.21 dB, since 20 log10(1/0.775) ≈ 2.21. Conversely, dBV ≈ dBu − 2.21 dB. This makes it easy to convert between the two when comparing equipment or specifications. For example, a voltage of 1.23 V RMS corresponds to about +4 dBu and roughly +1.81 dBV.

Common practice in audio distinguishes nominal levels: consumer equipment often uses around −10 dBV as line level, while professional audio commonly specifies +4 dBu (about 1.23 V RMS, ≈ +1.8 dBV). It is important to note that these values refer to RMS voltage into a defined load; actual voltage can vary with impedance and loading. Peak levels and crest factors differ from RMS values, so appropriate conversions should be used when relating to AC waveforms and power.