dBu

dBu is a decibel unit for measuring audio signal voltage, referenced to 0.775 V RMS.

The reference voltage (0.775 V) is derived from the voltage required to dissipate 1 milliwatt of power across a 600 Ω resistive load - a legacy of telephone engineering. Today, the 600 Ω load is rarely used in audio equipment design, but the voltage reference (0 dBu = 0.775 V RMS) has been retained as the professional audio standard.

Conversion

Relationship	Formula
dBu from voltage	dBu = 20 × log₁₀(V / 0.775)
Voltage from dBu	V = 0.775 × 10^(dBu/20)
dBu to dBV	dBV = dBu − 2.218

Common Reference Points

Level	dBu	Voltage	Use
+4 dBu	+4	1.228 V RMS	Pro audio nominal
0 dBu	0	0.775 V RMS	Reference voltage
−10 dBu	−10	0.245 V RMS	Consumer nominal

In Practice

The professional AV industry uses +4 dBu as the nominal operating level for line-level signals (see SANE-001). Headroom of at least 20 dB above nominal is standard, placing the clip point at a minimum of +24 dBu.

Do Not Confuse With

• dBV: Referenced to 1.0 V RMS, used in consumer equipment (−10 dBV is consumer nominal)
• dBFS: Referenced to digital full scale, used in DAWs and digital audio equipment
• dBSPL: Acoustic sound pressure level - an entirely different physical quantity