Why audio uses so many decibels
A decibel is a ratio, so every "dB" needs a reference. In audio the common references are:
- dBu: relative to 0.775 V RMS – the voltage that produces 1 mW in 600 Ω. Used for professional gear.
- dBV: relative to 1 V RMS. Used for consumer gear.
- dBFS: relative to digital full scale – the maximum value a converter can represent. Always 0 or negative.
- dBm: relative to 1 milliwatt of power, originally into 600 Ω.
- dBW: relative to 1 watt, useful for amplifiers.
dBu = 20 × log10(V ÷ 0.7746) · dBV = 20 × log10(V) · Watts = V² ÷ impedance
Common reference levels
| Level | Volts RMS | Where |
|---|
| −60 to −40 dBu | 0.8 – 7.8 mV | Microphone outputs |
| −10 dBV (−7.8 dBu) | 0.316 V | Consumer line level (RCA) |
| 0 dBu | 0.775 V | Reference |
| +4 dBu | 1.228 V | Professional line level |
| +18 to +24 dBu | 6.2 – 12.3 V | Typical 0 dBFS / clip level of pro gear |
| 2 V RMS (+8.2 dBu) | 2.0 V | CD player / DAC full-scale output |
dBFS alignment
A digital recorder or interface maps its full scale (0 dBFS) to a certain analog voltage. In Europe the EBU alignment sets 0 dBFS at +18 dBu, so +4 dBu reads −14 dBFS; US film and broadcast often use +20 or +24 dBu. Knowing the alignment helps you set gain staging so typical levels sit well below clipping.
Connecting pro and consumer gear
Feeding a −10 dBV consumer output into a +4 dBu pro input leaves about 12 dB less level, which may add noise. Going the other way can overload consumer inputs. Many devices have a level switch; otherwise use an attenuator or a line-level converter box.
Amplifier power
An amplifier delivering 28.3 V RMS into 8 Ω produces 100 W. The same voltage into 4 Ω would be 200 W – if the amplifier can supply the current. Use the watts option to see the voltage swing an amplifier needs for a given power.
Frequently asked questions
What is the difference between dBu and dBV?
dBu is referenced to 0.775 V and dBV to 1 V. 0 dBV equals +2.2 dBu.
How many volts is +4 dBu?
About 1.228 V RMS.
What is −10 dBV in volts?
About 0.316 V RMS – the consumer line level.
What does 0 dBFS mean?
The maximum level a digital system can represent. Anything above it clips.
How do I convert volts to watts for speakers?
Watts = volts² ÷ impedance. 20 V RMS into 8 Ω is 50 W.
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