Convert between amplitude ratios and decibels, and find the combined level of two sources — handy for gain staging, live sound and acoustics.
Decibel (dB) Calculator
Amplitude ratio ↔ decibels
Combine two levels (dB SPL)
For amplitude/voltage, dB = 20 × log₁₀(ratio). Two equal sources sum to +3 dB (10 × log₁₀ of the summed power), not double the dB — a common surprise when stacking sources or speakers.
How it works
For amplitude or voltage, decibels are 20 × log₁₀(ratio); for power it’s 10 × log₁₀. Combining two equal sources adds 3 dB (not 6), because you’re summing power. The tool handles both the ratio conversion and the level-summing.
FAQ
Why do two equal sources only add 3 dB?
Decibels are logarithmic and level sums by power, so doubling the power is +3 dB. Doubling perceived loudness actually takes about +10 dB.
Is it 10 or 20 times log?
Use 20 × log₁₀ for amplitude/voltage ratios and 10 × log₁₀ for power ratios — they describe the same dB change.