Decibels do not add. Two equal sounds together are 3 dB louder, four are 6, ten are 10 — and a sound 10 dB quieter than another adds less than half a decibel to it.
A decibel is a logarithm of a RATIO, so adding two of them multiplies the things they describe. Unrelated sounds do not line their pressure peaks up, which means their energies add rather than their pressures, and doubling energy is 3 dB by definition. The consequence is the useful half: in any room the loudest single thing is very nearly the whole answer. Silencing the second-loudest machine on a factory floor changes the total by a few tenths of a decibel, which is why noise control money goes to the worst offender and nowhere else.
It assumes the sources are uncorrelated. Two loudspeakers fed the SAME signal can add coherently — up to 6 dB where they reinforce, and cancel entirely where they do not, which is what makes stereo phase problems audible. And it is about level, not annoyance: a 40 dB dripping tap at 3am beats 60 dB of steady traffic for keeping people awake.
Estimate with the rule, then check it against the calculator that models it properly.
Open Sound Levels & Decibels →Decibels do not add. Two equal sounds together are 3 dB louder, four are 6, ten are 10 — and a sound 10 dB quieter than another adds less than half a decibel to it. A decibel is a logarithm of a RATIO, so adding two of them multiplies the things they describe.