Last Updated 6 hours ago
Filmmaking Dictionary: Number 0 / 0 dBu
Definition
0 dBu is an audio signal level equal to approximately 0.775 volts RMS. The dBu scale expresses voltage relative to that reference. It is used in specifications for microphone preamps, mixers, wireless receivers, recorders and other audio equipment.
It does not describe acoustic loudness or a digital file’s level by itself. A voltage at an input becomes a digital level only through the equipment’s gain and calibration settings.
Reading a dBu Specification
A positive dBu value is above the 0.775-volt reference; a negative value is below it. Neither sign is automatically good or bad. The important comparison is between the signal being sent and the level the receiving input is designed to accept.
| Level | Approximate RMS voltage |
|---|---|
| −20 dBu | 0.0775 V |
| 0 dBu | 0.775 V |
| +4 dBu | 1.23 V |
| +20 dBu | 7.75 V |
These are voltage conversions, not recommended recording targets. For example, an input’s maximum accepted level and its nominal operating level are different specifications.
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Why It Matters on Set
A sound mixer may send an analogue output to a camera for reference audio. If the output is set for line level while the camera input expects a small microphone signal, the receiving stage can overload. A low-looking meter later in the path does not necessarily reveal where that distortion occurred.
Check the camera’s input mode, the mixer’s output setting and the intended reference tone level. When a transmitter or recorder specification lists a maximum input in dBu, compare it with the output of the actual connected device rather than relying on the connector type.
dBu, dBV and dBFS
- dBu: Voltage relative to approximately 0.775 V RMS.
- dBV: Voltage relative to 1 V RMS.
- dBFS: Level relative to digital full scale.
- VU: A meter indication whose reference depends on alignment.
For the same voltage, the numerical dBu value is approximately 2.2 dB higher than the dBV value. Thus, 0 dBu is approximately −2.2 dBV. There is no single equivalent dBFS value without knowing the converter’s calibration and gain.
Nominal Level Versus Maximum Level
+4 dBu is a common professional nominal line level. It is not the maximum level of every professional output, and 0 dBu is not a universal clipping threshold. Equipment normally needs room above its nominal level for programme peaks, but the available margin varies.
For an illustrative device with a +4 dBu nominal level and a +24 dBu maximum, the difference is 20 dB. Those example values should not be assumed for an unfamiliar mixer or camera.
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Frequently Asked Questions
Does dBu require a 600-ohm load?
No. It is a voltage reference. Its historical connection to power measurement in a 600-ohm system does not impose that load on modern dBu specifications.
Does an XLR connector guarantee a particular dBu level?
No. The connector can carry different signal types and levels. Check the device and its settings.
Related Terms
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