Almost every specification in audio is quoted in decibels, and the scale behaves in ways that surprise people the first time they meet it. Understanding it removes most of the confusion around power ratings, sensitivity and how loud a system will actually be.
A decibel is a ratio, not an absolute quantity. Doubling amplifier power adds 3 dB. Doubling distance from a point source loses 6 dB. A 10 dB increase is perceived as roughly twice as loud, and it takes ten times the power to achieve it.
| Change | Cause | Perceived effect |
|---|---|---|
| +3 dB | Double the amplifier power | Just noticeably louder |
| +6 dB | Double the voltage, or halve distance | Clearly louder |
| +10 dB | Ten times the power | About twice as loud |
| -6 dB | Double the distance from source | Clearly quieter |
Why the scale is logarithmic
Human hearing spans an enormous range, from the threshold of audibility to the threshold of pain, a ratio of roughly one to ten million in sound pressure. A linear scale covering that range would be unusable.
The decibel compresses it into manageable numbers by expressing ratios logarithmically. That is why decibel values add rather than multiply: combining two gains of 6 dB gives 12 dB, not 36.
It also means a small numerical change can represent a large physical one. The difference between 100 dB and 110 dB is ten times the power, even though it reads as a modest increase.
dB SPL, dBu and dBV: three different references
dB SPL measures sound pressure in air, referenced to the quietest sound a healthy ear can detect. This is the figure quoted for speaker output and for noise exposure limits.
dBu and dBV measure electrical signal level. dBu is referenced to 0.775 volts and is standard in professional audio. dBV is referenced to 1 volt and appears more often on consumer equipment.
This is why connecting consumer equipment to professional inputs often produces a quiet signal. The two references differ by about 12 dB, which is why line-level mismatches are so common.
The 3 dB and 6 dB rules in practice
Doubling amplifier power adds 3 dB. Going from a 250 watt amplifier to a 500 watt amplifier gains you 3 dB, which is audible but modest. This is why buying a much larger amplifier rarely transforms perceived loudness.
Doubling distance from a point source loses 6 dB. Someone twenty metres away hears 6 dB less than someone at ten metres, which is a substantial difference and the entire reason distributed systems and line arrays exist.
Choosing a speaker with 3 dB higher sensitivity gains the same level as doubling amplifier power, usually at lower cost.
Headroom and why peaks matter more than averages
Speech and music have peaks well above their average level. A programme with an average of 90 dB may contain transients at 105 dB or higher.
If the system can only reach 100 dB cleanly, those peaks clip even though the average sits comfortably below the limit. Design for the peaks, not the average.
As a working target, allow at least 10 dB of headroom above your intended average operating level for speech and considerably more for music with strong dynamics.