Level References

Canonical Definition

A level reference identifies what a decibel value or voltage measurement is measured against, allowing the number to have a defined physical or digital meaning.

House rule: state the reference before the number.

+6 dB describes a change. +6 dBu, -6 dBFS sample peak, and 2.0 Vrms describe different kinds of level.

The Main Scales

Label	Reference	What it normally describes
dB	A stated ratio or previous value	Gain, attenuation, response change, or comparison
dBFS	Digital full scale or overload	Digital samples, DSP meters, numerical signal level
dBu	0.775 Vrms	Analog line level
dBV	1.000 Vrms	Analog line level
Vrms	Physical RMS voltage	Measured source, processor, amplifier input, or amplifier output voltage
dB SPL	20 µPa in air	Acoustic sound-pressure level

dB: A Ratio, Not A Complete Level

Decibels express a logarithmic ratio. The same number can describe a voltage change, power change, acoustic change, or digital change only when the reference and quantity are known.

Voltage or pressure ratio:  dB = 20 log10(V2 / V1)
Power ratio:                dB = 10 log10(P2 / P1)

Useful anchors:

+6.02 dB is approximately double voltage.
-6.02 dB is approximately half voltage.
+3.01 dB is double power when impedance is unchanged.
-3.01 dB is half power when impedance is unchanged.

dBFS: Name The Convention And Meter Type

dBFS describes digital level relative to the overload or full-scale boundary. It does not tell you an analog voltage until the converter and analog circuitry are calibrated.

Different RMS conventions exist. In a conventional sample-peak display, the largest stored sample is 0 dBFS peak. Some measurement standards instead assign a full-scale sine wave to 0 dBFS RMS; in that convention the sine's peak is approximately +3.01 dBFS. Other standards reference RMS to a full-scale square wave. Therefore, a responsible report says whether the number is sample peak, RMS, or another detector and states the convention when ambiguity matters.

Do not compare a DSP meter reading directly with an RCA voltage specification.

The relationship depends on ADC/DAC calibration, input and output scaling, analog gain, and the exact meter location.

dBu, dBV, And Vrms

dBu = 20 log10(Vrms / 0.775)
dBV = 20 log10(Vrms / 1.000)

Vrms from dBu = 0.775 × 10^(dBu / 20)
Vrms from dBV = 1.000 × 10^(dBV / 20)

Reference value	Equivalent voltage
0 dBu	0.775 Vrms
+4 dBu	about 1.228 Vrms
0 dBV	1.000 Vrms
-10 dBV	about 0.316 Vrms
2.0 Vrms	about +8.23 dBu or +6.02 dBV
4.0 Vrms	about +14.25 dBu or +12.04 dBV

RMS, Peak, Sample Peak, And True Peak

RMS voltage describes the effective value of a waveform over time and is commonly used for steady sine-wave voltage measurements.
Analog peak is the instantaneous maximum voltage of the continuous waveform.
Sample peak is the highest stored digital sample value.
True peak estimates the highest reconstructed waveform between stored samples.

These measurements answer different questions. A true-RMS DMM can report steady sine voltage but does not prove waveform cleanliness. A sample-peak meter does not describe perceived loudness. A LUFS meter does not replace a clip detector.

Car-Audio Example

A complete setup note might read:

Source: 1 kHz, -5 dBFS sine; source volume 32; measured output 2.1 Vrms.

DSP: input range configured for the measured source; post-processing peak recorded at -4 dBFS on the named meter; analog output 3.5 Vrms clean.

Amplifier: gain adjusted until 20 Vrms at the speaker output, with waveform verified clean at the selected electrical-system voltage.

Every number names its domain. None of the digital values is treated as though it were automatically an analog voltage.

Reporting Checklist

Name the device and signal point.
Name the reference: dBFS, dBu, dBV, Vrms, dB SPL, or a relative baseline.
Name the detector: RMS, sample peak, true peak, average, or another defined method.
Record frequency, channel, test-signal level, processing state, and load where relevant.
Do not compare measurements gathered under incompatible references or methods.

Related Resources

Standards Notes

ITU publications explicitly recognize multiple dBFS RMS conventions. ITU-R BS.1770 defines algorithms for program loudness and true-peak measurement. These standards are measurement contexts, not universal car-audio hardware-calibration rules.