Decibel Sum Calculator
Add multiple SPL (sound pressure level) values correctly. Decibels are logarithmic units, so they cannot be added arithmetically. This calculator performs the correct logarithmic addition for combining sound sources.
dB
dB
Quick Reference Table
When combining two equal sound sources:
| Level Difference | Add to Higher Level |
|---|---|
| 0 dB (equal levels) | +3.0 dB |
| 1 dB difference | +2.5 dB |
| 2 dB difference | +2.1 dB |
| 3 dB difference | +1.8 dB |
| 4 dB difference | +1.5 dB |
| 5 dB difference | +1.2 dB |
| 6 dB difference | +1.0 dB |
| 7 dB difference | +0.8 dB |
| 8 dB difference | +0.6 dB |
| 9 dB difference | +0.5 dB |
| 10+ dB difference | +0.4 dB or less |
When two levels differ by 10 dB or more, the lower level has negligible contribution to the total.
Formula
To add decibels, convert each dB value to intensity (power), sum the intensities, then convert back to dB:
Ltotal = 10 × log₁₀(10^(L1/10) + 10^(L2/10) + ... + 10^(Ln/10)) Where L1, L2, ... Ln are the individual sound pressure levels in dB.
Important: This calculation assumes the sound sources are uncorrelated (random phase). If sources are coherent (same phase), the result can be up to 6 dB higher for two equal sources.