Decibel Calculator - dB Ratios, Adding Sound Levels and Distance (Inverse Square)
Decibel Calculator
dB ratios, combined sound levels and level against distance
Ratios Both Ways
Turn a power ratio into decibels, use 20 log for voltage, current and pressure, and turn decibels back into a ratio.
Adding Sound Levels
Independent sources add as power, so two equal sources are only 3 dB louder than one.
Distance
See how the level changes when you move away from or toward a source: 6 dB for each doubling of distance in free space.
Private
Nothing you enter is stored.
Why Decibels Work the Way They Do
A decibel is ten times the base-10 logarithm of a ratio of two powers. The logarithm squeezes the enormous range of everyday quantities, from a whisper to a jet engine, into numbers from 0 to about 140. A ratio of 2 in power is about 3 dB, a ratio of 10 is 10 dB, and a ratio of 100 is 20 dB. For voltage, current and sound pressure the power depends on the square of the quantity, so the factor is 20 instead of 10: doubling the voltage adds 6 dB.
Because decibels are logarithmic you cannot add them directly. Two 60 dB sounds do not make 120 dB; they make about 63 dB, because their powers add and the logarithm is taken afterwards. In open space the sound intensity from a point source spreads over a sphere, so every doubling of the distance reduces the level by about 6 dB. Indoors, reflections make the real drop smaller.
Key Takeaways
- Reference matters: dB alone is a ratio. dB SPL, dBm and dBV each state what the ratio is measured against.
- 3 dB and 6 dB: About 3 dB is double the power and about 6 dB is double the voltage or pressure.
- Related calculators: See also the Ohm's Law Calculator and the Electrical Power Calculator.