Decibels — the same word for two different things
A 10 dB increase is 10× the power — or 3.16× the pressure. Whether you double or 10× depends on whether it is a power quantity or a field quantity.
The decibel is a ratio. The confusion is which ratio: power ratios use 10 log, field-amplitude ratios (voltage, pressure) use 20 log. Both give the same 'per bel of ratio' meaning because power is proportional to amplitude squared, so 10 log(P²/P₀²) = 20 log(P/P₀).
The formulas
Power dB: L = 10 × log₁₀(P/Pref) > Field dB: L = 20 × log₁₀(A/Aref) > Sound pressure level (SPL, dB re 20 µPa): 20 × log₁₀(p / 20 µPa) > +3 dB power = 2× power. +10 dB power = 10× power. > +6 dB SPL = 2× pressure. +20 dB SPL = 10× pressure.
SPL reference — 20 µPa
20 microPascals is the human hearing threshold at 1 kHz. Every dB SPL number is referenced to it. 0 dB SPL = 20 µPa (threshold). 60 dB = conversation. 85 dB = OSHA action level. 100 dB = jackhammer. 120 dB = pain threshold. 194 dB = maximum possible in air (limited by atmospheric pressure).
A, B, C weighting
The human ear is not equally sensitive at all frequencies. dBA applies a frequency filter matching low-level hearing (used for OSHA and traffic-noise regulation). dBC is flatter (used for peak measurements). dBZ is unweighted. A jet engine at 130 dBC may be 125 dBA — the two are not interchangeable.
Worked example — noise doubling
Two identical machines each producing 80 dBA at 1 m. Combined SPL is not 160 dBA (never — dB is not a linear quantity to add). Instead: pressures add in quadrature. Each contributes 0.2 Pa; two contribute √(0.2² + 0.2²) = 0.283 Pa. That is 20 × log(0.283/20e-6) = 83 dBA — +3 dB above one machine. Doubling identical noise sources adds 3 dB, always.
The five-second recap
Power dB: 10 log. Field/pressure dB: 20 log. SPL reference: 20 µPa. Doubling identical sources = +3 dB. 10× power = +10 dB = 3.16× pressure.
Try the converters mentioned in this article
FAQ
Is 40 dB twice as loud as 20 dB?+
10× the pressure, 100× the power, but only about 4× as loud perceptually. Human loudness roughly doubles every 10 dB.
Can dB be negative?+
Yes — anything below the reference. −10 dBm = 0.1 mW. Anechoic chambers reach −20 dBA SPL.
Why is dB SPL capped at 194?+
At 194 dB SPL (~101 kPa peak), sound-pressure oscillation equals atmospheric pressure. Above that, the wave becomes a shock wave, not a sound wave.
This article was written by The Turbo Unit Converter engineering desk (Acoustics & signal processing) and last reviewed on 2026-06-30 against NIST SP 811 and the BIPM SI Brochure. Read our full editorial policy.