In the audio world the decibel shows up everywhere: on DAW meters, microphone spec sheets, interface knobs and platform specifications. The problem is that it is almost always a different decibel. Understanding what each one refers to prevents level mistakes and lets you read any specification correctly.
The decibel is a ratio
The decibel does not measure a quantity in an absolute sense: it expresses the ratio between two values on a logarithmic scale. For amplitude quantities, such as voltage or pressure, the formula is:
dB = 20 × log10(value / reference)
A few handy rules follow from this formula:
- +6 dB is roughly double the amplitude;
- -6 dB roughly half;
- +20 dB ten times the amplitude;
- 0 dB means the value equals the reference.
For power quantities the factor is 10 instead of 20: doubling the power is worth about +3 dB.
An important consequence: when a specification just says "dB", with no suffix, it refers to a relative change (for example a gain of +10 dB), not an absolute level.
dBFS: the digital world
dBFS (decibels relative to full scale) is the reference of digital audio. The highest representable value is 0 dBFS; everything else is negative. There are no positive values in an integer system: above zero the signal gets cut off (clipping).
Typical examples:
- peaks of a well-recorded voice: between -12 and -6 dBFS;
- average reference level while recording: around -18 dBFS;
- noise floor of a good home recording: well below -60 dBFS.
dBu and dBV: the analog world
Analog equipment works with voltages, and the reference changes:
- dBu: referenced to 0.775 volts RMS. It is the professional standard; the typical nominal level is +4 dBu.
- dBV: referenced to 1 volt RMS. It is used for consumer equipment; the typical nominal level is -10 dBV.
The difference between the two nominal levels is about 12 dB. That is why connecting a consumer output to a professional input can give a signal that is too low, and the reverse a signal that is too hot. Many interfaces have a +4/-10 switch for exactly this reason.
The bridge between analog and digital
Each converter defines how many dBu correspond to 0 dBFS. This value varies from model to model and is listed in the specifications as "maximum input level". If an interface has a maximum of +18 dBu, a nominal +4 dBu signal will arrive at about -14 dBFS: there is headroom for peaks.
dB SPL: sound in the air
dB SPL (Sound Pressure Level) measures sound pressure in the air, referenced to 20 micropascals, a value close to the threshold of hearing.
| Indicative situation | Approximate dB SPL |
|---|---|
| Very quiet room at night | 25-30 |
| Normal conversation at one meter | about 60 |
| Heavy street traffic | 80-85 |
| Rock concert near the stage | over 100 |
Microphone spec sheets use dB SPL to state the maximum level they can handle (Max SPL) and their self-noise (often A-weighted, in dBA). A microphone with low self-noise suits quiet sources; one with a high Max SPL can handle a drum kit or a guitar amp.
Weightings: dBA and dBC
dBA measurements apply a filter that mimics the ear's lower sensitivity to lows and highs at moderate volumes. dBC measurements are flatter and include more of the low frequencies. A sound level meter set to dBA underestimates the bass content of a powerful system.
How to read a complete chain
Let's follow a voice from the singer to the file:
- The voice produces about 80-90 dB SPL a few centimeters from the microphone.
- The microphone converts it into a few millivolts, according to its sensitivity.
- The preamp adds gain (in relative dB) and brings the signal close to line level, measured in dBu.
- The converter turns it into samples, measured in dBFS.
In my work, when a level doesn't add up, I walk through this chain one step at a time: the problem is almost always a wrong gain setting or a mismatch between +4 dBu and -10 dBV.
In short
- A decibel expresses a ratio: with no suffix it means a change, not a level.
- dBFS applies in digital, with 0 dBFS as the absolute maximum.
- dBu and dBV measure analog voltages; +4 dBu and -10 dBV are the typical nominal levels.
- dB SPL measures sound pressure in the air and appears in microphone specs.
- Following the chain from sound to file helps locate the source of level problems.
