Knowledge Centre · Understanding Noise
Adding decibels is not ordinary addition.
Sound levels are measured on a logarithmic scale. This means two noise sources measuring 60 dB each do not produce 120 dB. Together, they produce 63 dB. Enter up to four sound levels below to calculate their combined level.
Read time
2 minutes
Relevant to
Anyone reading a noise report or condition
This calculator combines simultaneous sound levels on an energetic basis. Only combine values that use the same weighting, acoustic descriptor and time period. For example, do not directly combine an LAeq with an LAmax, or a dB(A) value with a dB(C) value.
Two equal sound levels of 60.0 dB(A) combine to produce 63.0 dB(A), an increase of 3.0 dB.
Illustrative combination diagram, not to scale.
How decibel addition works
Decibels are a logarithmic representation of sound energy, so combining two or more simultaneous sources means adding their underlying energy, not their decibel values. A few results worth remembering:
- Two equal sound levels produce an increase of 3 dB over one alone.
- Three equal sound levels produce an increase of approximately 4.8 dB.
- Four equal sound levels produce an increase of 6 dB.
- A source 10 dB below another adds only about 0.4 dB to the higher source’s level.
- When the difference between two levels is much larger still, the quieter source has very little effect on the combined result.
Only combine like with like
Sound levels can only be meaningfully combined when they use the same weighting, the same time period and the same acoustic descriptor. Do not directly combine an LAeq with an LAmax, or a dB(A) reading with a dB(C) reading — the calculator above assumes every value you enter already meets this condition.
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