Decibels, demystified
The logarithmic unit behind audio levels, gain, loss and signal-to-noise ratio.
The decibel (dB) shows up everywhere in AV: microphone sensitivity, amplifier gain, cable loss, loudspeaker output, signal-to-noise ratio, network transceiver budgets. It intimidates people because it's logarithmic — but that is exactly what makes it useful.
A decibel is a ratio
A decibel on its own isn't an amount of anything. It describes how much bigger or smaller one value is compared with another. “+6 dB” means “this is larger than that by a certain factor,” whatever the starting point.
The scale is logarithmic because AV deals with enormous ranges. The loudest sound you can tolerate carries roughly a trillion times the power of the quietest one you can hear. In decibels that range is about 120 — a number a person can reason with.
Two formulas, depending on what you measure
Power is proportional to the square of voltage, which is where the 20 comes from. The practical upshot: the same decibel figure means a different ratio depending on whether you're talking power or voltage.
3.98×
Amplifier watts, acoustic power.
2.00×
Signal voltage, sound pressure (SPL).
log scale · centre line = 1× (0 dB)
In ratios that would be 100× × 0.50× × … — in decibels it's plain addition.
Numbers worth memorising
| Change | Power | Voltage / SPL | How it feels |
|---|---|---|---|
| +3 dB | 2× | 1.41× | Just noticeable |
| +6 dB | 4× | 2× | Clearly louder |
| +10 dB | 10× | 3.16× | Roughly twice as loud |
| +20 dB | 100× | 10× | Roughly four times as loud |
- +3 dB
- Power2×
- Voltage / SPL1.41×
- How it feelsJust noticeable
- +6 dB
- Power4×
- Voltage / SPL2×
- How it feelsClearly louder
- +10 dB
- Power10×
- Voltage / SPL3.16×
- How it feelsRoughly twice as loud
- +20 dB
- Power100×
- Voltage / SPL10×
- How it feelsRoughly four times as loud
Perception is approximate — loudness depends on frequency and level — but the “+10 dB ≈ twice as loud” rule is how audio professionals talk. It also explains a common surprise: doubling an amplifier's power gives only +3 dB, a barely noticeable change.
When dB becomes an absolute level
Attach a reference value and a decibel becomes a real measurement. The suffix tells you the reference:
| Unit | 0 dB equals | Used for |
|---|---|---|
| dB SPL | 20 µPa (threshold of hearing) | Sound pressure level in a room |
| dBu | 0.775 V | Professional audio signal levels |
| dBV | 1 V | Consumer audio signal levels |
| dBFS | Digital full scale (the maximum) | Digital audio — everything is ≤ 0 dBFS |
| dBm | 1 mW | RF and fibre-optic power |
- dB SPL
- 0 dB equals20 µPa (threshold of hearing)
- Used forSound pressure level in a room
- dBu
- 0 dB equals0.775 V
- Used forProfessional audio signal levels
- dBV
- 0 dB equals1 V
- Used forConsumer audio signal levels
- dBFS
- 0 dB equalsDigital full scale (the maximum)
- Used forDigital audio — everything is ≤ 0 dBFS
- dBm
- 0 dB equals1 mW
- Used forRF and fibre-optic power
Signal-to-noise ratio
SNR is the difference, in dB, between your wanted signal and the noise floor beneath it. Because everything is in decibels, it's simple subtraction: a signal at +4 dBu over a noise floor of −86 dBu has an SNR of 90 dB. The higher the better — and every stage in a chain is a chance to lose some of it, which is the whole subject of gain structure later in the course.
Key takeaways
- 01A decibel is a ratio, not a quantity — it says how much bigger or smaller one value is than another.
- 02Use 10·log for power and 20·log for voltage and sound pressure.
- 03+3 dB doubles power, +6 dB doubles voltage, +10 dB sounds about twice as loud.
- 04Suffixes like dBu, dBV, dB SPL and dBFS attach a reference and turn dB into an absolute level.
- 05Gains and losses in a chain simply add in decibels.
Check your understanding
Q1An amplifier's output is doubled from 100 W to 200 W. How much louder is it?
Q2A preamp adds 30 dB, a cable loses 2 dB and an output stage cuts 8 dB. What's the net gain?
Q3What does 0 dBFS represent?