Audio Units & Levels
Units
A decibel (dB) is a logarithmic ratio between two values, it is dimensionless, meaning it carries no unit on its own. A value in dB is always relative to a reference point. Saying “10 dB” without a reference is like saying “50 km” when asked about your speed: the number only makes sense with a denominator.
By convention, the reference value defines 0 dB for that unit, not 1 dB. This follows directly from
the logarithmic nature of the decibel: a ratio of 1 (signal equal to reference) gives log(1) = 0.
So, for example, 0 dBu means the signal is exactly at the reference level (0.775 V RMS), not zero volts.
The common audio-specific dB units each define a different reference:
| Unit | Reference value | Domain | Notes |
|---|---|---|---|
| dBV | 1 V RMS | Voltage | Independent of load impedance. |
| dBu | 0.775 V RMS | Voltage | Unterminated load. Replaced dBv to avoid confusion with dBV. |
| dBv | 0.775 V RMS | Voltage | 600 Ω load. Deprecated in favour of dBu. |
| dBm | 1 mW | Power | 600 Ω load (corresponds to 0.775 V RMS). |
| dBFS | Full scale | Digital | Maximum representable level in a digital system. All practical levels are negative. |
| dB/oct | - | - | Expresses filter roll-off steepness. Higher value = sharper filter. |
Analog levels
Analog audio levels are measured as voltage ratios, typically in dBu or dBV. Since these describe continuous AC voltages, levels are expressed as RMS averages.
Two standards are widely used in the music industry:
| Application | Nominal level | V RMS | V peak | V peak-to-peak |
|---|---|---|---|---|
| Professional / studio | +4 dBu | 1.228 | 1.736 | 3.472 |
| Consumer / home | −10 dBV | 0.316 | 0.447 | 0.894 |
The difference is not subtle: +4 dBu is approximately 12 dB higher than −10 dBV, which corresponds to a voltage ratio of about 4:1. Connecting consumer and professional equipment without level matching will result in either a very weak signal or significant clipping.
Line level
Line level refers to the nominal signal strength used to interconnect audio components, CD players, mixers, amplifiers, audio interfaces, and similar equipment. The two nominal levels in the table above are the line level standards in common use.
Digital levels
In the digital domain, audio is represented as fixed-precision integer or floating-point values bounded by the converter’s bit depth. The concept of RMS voltage no longer applies directly, so a different reference is needed: dBFS (decibels relative to full scale).
0 dBFS is the maximum level a digital system can represent. All practical signal levels are negative dBFS values. Sufficient headroom below 0 dBFS should be maintained to avoid clipping.
Each bit doubles the amplitude, so each additional bit extends the dynamic range by 20 × log10(2) ≈ 6.02 dB. The smallest step a converter can resolve (its least significant bit) sits at −N × 6.02 dBFS below full scale:
| Bit depth | LSB level | Dynamic range |
|---|---|---|
| 16 bit | −96.3 dBFS | 96.3 dB |
| 24 bit | −144.5 dBFS | 144.5 dB |
| 32 bit | −192.7 dBFS | 192.7 dB |
| 96 bit | −578.0 dBFS | 578.0 dB |
dBu vs dBFS
dBu indicate voltage ratios in RMS levels, while dBFS have a fixed scale and state peak levels.
The two cannot be compared directly. dBu expresses RMS voltage in the analog domain; dBFS expresses peak level relative to a converter’s (ADC or DAC) bit-depth ceiling. 0 dBFS is the point where the converter has used its full resolution: an ADC can encode nothing higher, a DAC can output nothing higher. Any signal above this clips.
Beyond that, the mapping between them is not fixed: it depends on the converter’s internal reference voltage, which varies from device to device. There is no universal conversion formula; the only way to know the relationship for a specific converter is to measure it empirically.
Remember: 0 dBu is not 0 dBFS, and vice versa.