How to add noise sources in decibels
Decibels compress sound pressure onto a logarithmic scale, so two 90 dB machines running together do not produce 180 dB. Each level has to be converted back to an energy ratio, summed, and converted to a level again. That result is the combined sound level a worker actually hears at the point of measurement.
The formula is combined level = 10 × log₁₀(10^(L₁/10) + 10^(L₂/10) + …). Two equal sources add about 3 dB, four add about 6 dB, and ten add 10 dB. The handy rule is that every doubling of identical sources costs another 3 dB.
When levels are far apart the quieter source almost disappears. A 6 dB gap adds roughly 1 dB and a 10 dB gap only about 0.4 dB, which is why noise control money is best spent on the single loudest machine rather than on several quiet ones.
This calculation combines sound pressure levels measured at the same location. It does not model distance attenuation, room reflections, barriers or exposure time weighting, so hearing conservation decisions should still rest on measured dosimetry and a review by an industrial hygienist.
Frequently asked questions
No. Decibels are logarithmic and cannot be added directly. Doubling an equal noise source adds about 3 dB, so two 90 dB machines together read roughly 93 dB.
A source 10 dB quieter adds only about 0.4 dB. That is why noise control has to start with the loudest machine before quieter equipment is worth treating.
Not necessarily. This combines sound pressure levels at one point and ignores distance attenuation, reflections, barriers and exposure time, so dosimetry results still need real measurement.