Work with decibels
Two 80 dB machines are not 160 dB — decibel math is logarithmic, and this keeps it honest.
Tools used in this guide
The job
Decibels compress huge ratios into small numbers — but they don't add. The everyday questions are: what are two equal sources together, what is a power or amplitude ratio in dB, and how loud is the source at a different distance.
Three operations
Sum up to 64 sources by energy; convert a power ratio with 10·log10 or an amplitude ratio with 20·log10; move an SPL reading to a new distance with the inverse-square law.
Worked example
Two identical 80 dB sources sum to 83.0 dB — doubling energy adds just 3 dB. Moving from 2 m to 10 m (5× distance) drops a level by 20·log10(5) ≈ 14 dB.
Where it stops
Incoherent summing assumes the sounds aren't phase-locked; coherent sources can add 6 dB instead of 3. Distance assumes a free field — walls, floors, and barriers change real rooms.