Adding Sound
Two identical machines do not produce twice the decibels. The arithmetic is unintuitive and it decides which noise control measures are worth doing.
When employers review noise dose alongside working patterns, this resource can provide operational context, while exposure limits still need to be assessed from sound level and duration.
The rule
Doubling the number of identical sources adds 3 dB.
One machine at 90 dBA. Two at 93. Four at 96. Eight at 99.
Readers comparing the discussion here with outside material can use OSHA's occupational noise overview.
Going from one to two costs 3 dB. Going from eight to sixteen also costs 3 dB. Each doubling is the same increment because each doubles the energy.
The consequence that surprises people
Removing one machine from ten changes almost nothing.
Ten sources to nine is a reduction of about 0.5 dB — inaudible, and it does not move the dose meaningfully.
Halving the count buys 3 dB. Going from ten to five is a real reduction and is enough to halve the permitted exposure calculation, which is a substantial result for a large intervention.
Which is why partial noise control disappoints. The effort is proportional to how many sources you address and the benefit is logarithmic.
Unequal sources
When sources differ, the loudest dominates.
A source 10 dB below another contributes about 0.4 dB to the total. Silencing it entirely is imperceptible.
Two sources within 1 dB of each other combine to about 3 dB above the louder.
The practical rule: find the loudest thing and work on that. Anything more than 10 dB below it is not the problem, however irritating it sounds.
Where this changes decisions
Buying quieter equipment. A replacement 6 dB quieter than the unit it replaces removes three quarters of that source's energy — but only if it was the dominant source.
Enclosure and distance. Both act on individual sources and both are subject to the same domination rule.
And complaints. A single intermittent noise can be the whole subjective problem while contributing nothing measurable to the total, which is a genuine conflict between the measurement and the experience rather than somebody being unreasonable.
Distance
In free field, doubling the distance from a point source reduces level by about 6 dB.
Indoors this holds only close to the source. Beyond a certain distance the reflected field dominates and moving further away stops helping — which is why a large room can be uniformly loud and why distance is unreliable as the only control.
The short version
- Doubling the number of identical sources adds 3 dB, and every subsequent doubling adds the same 3 dB
- Removing one machine from ten reduces the level by about half a decibel — inaudible and not worth counting
- Halving the number of sources buys 3 dB, which is a real reduction for a large intervention
- When sources differ, the loudest dominates: anything more than 10 dB below it contributes almost nothing
- Find the loudest source and work on that; quieter equipment only helps if that unit was dominant
- Doubling distance gives about 6 dB outdoors, and indoors it stops helping once the reflected field dominates