How decibel addition works
Decibels are a logarithmic scale, so two noise sources don't add by simple arithmetic — 70 dB + 70 dB is not 140 dB. Each source's sound intensity is converted back to a linear value, the intensities are summed, and the result is converted back to decibels.
- L₁ — sound level of source 1 (dB)
- L₂ — sound level of source 2 (dB)
- L — combined total sound level (dB)
More detail
Why two equal sources always add +3 dB
If L₁ = L₂ = L, the formula reduces to L + 10·log₁₀(2) ≈ L + 3.01 dB. Doubling the sound intensity (running two identical machines instead of one) raises the level by about 3 dB — not double the number. That's why 70 dB + 70 dB ≈ 73 dB, not 140 dB.
A much quieter second source barely matters
When one source is 10 dB or more below the other, its contribution to the total is under 0.5 dB — effectively inaudible against the louder source. The gap only matters when the two levels are close.
Practical tip. To combine more than two sources, apply this calculator pairwise: sum the first two, then sum that result with the third source, and so on — the logarithmic addition is associative.
Frequently asked questions
If two 70 dB machines run together, is the noise 140 dB?
No — decibels are logarithmic, so they don't add arithmetically. With the default 70 dB + 70 dB inputs above, the combined level is about 73.0 dB, not 140 dB.
Why does adding two equal noise sources always give +3 dB?
When L₁ = L₂, the formula simplifies to L + 10·log₁₀(2) ≈ L + 3.01 dB, regardless of the starting level. Doubling sound intensity always raises the decibel reading by about 3 dB, whether the sources are 50 dB or 90 dB.
What if one noise source is much quieter than the other?
It barely changes the total. For example 70 dB + 60 dB gives about 70.4 dB — the note under the result shows how many dB the combined level exceeds the louder source alone (+0.4 dB here).
What happens if I enter 0 or a negative dB value?
0 dB is a valid reading (the threshold of hearing), and 0 dB + 0 dB gives about 3.0 dB — the same +3 dB rule as any other equal pair. Negative or non-numeric entries show "—" instead.