Work out decibel addition instantly with clear inputs, formula shown and shareable results.
Decibels cannot be added arithmetically - the intensities add, so each level must be converted back to a power ratio first. 85 dB and 82 dB combine to 86.8 dB, not 167 dB. Two identical sources always give exactly 3 dB more, and a source 10 dB quieter adds only 0.4 dB.
Logarithmic addition
L = 10 log10(10^(L1/10) + 10^(L2/10))
Equal sources
two equal levels add 3.01 dB
Because the intensity doubles, and 10 log10(2) is 3.01 dB. Doubling power is a 3 dB step, not a doubling of the number.
Once it is more than 10 dB below, it contributes under 0.5 dB - usually inside measurement uncertainty.