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Collaboration, Voice & AV · Intermediate

Decibel Unit dBu

A decibel unit for audio voltage referenced to 0.775 volts RMS, used for professional line levels.

Definition

dBu = 20 log10(V / 0.775 V). The 0.775 V reference comes from the voltage that dissipates 1 mW in 600 ohms, but dBu is defined independent of load. dBV, by contrast, is referenced to 1 V. Professional nominal level is +4 dBu.

Example

1.228 V RMS equals +4 dBu, and 0.775 V equals 0 dBu.

Exam tip

Voltage decibels use 20 log; power decibels use 10 log. Know which formula applies.

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