Decibel addition calculator
Two 90 dB sources don't make 180 dB. Add levels properly, or take the background noise out of a measurement.
Combined level
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Increase over the loudest
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Subtract background noise
The source on its own
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How it works
Ltotal = 10 × log10(10L1/10 + 10L2/10 + …)
Lsource = 10 × log10(10Ltotal/10 − 10Lbackground/10)
Lsource = 10 × log10(10Ltotal/10 − 10Lbackground/10)
Decibels are logarithmic, so you add the powers, not the numbers. Two equal sources add 3 dB, four add 6 dB, ten add 10 dB. A source 10 dB quieter than another adds less than half a decibel.
This is the sum for uncorrelated sources: different instruments, separate noise sources, or speakers playing to a listener at different distances. Two identical, in-phase signals arriving at the same moment can add up to 6 dB instead.
Quick reference
| Difference between two levels | Add to the louder one |
|---|---|
| 0 dB (equal) | +3.0 dB |
| 1 dB | +2.5 dB |
| 3 dB | +1.8 dB |
| 6 dB | +1.0 dB |
| 10 dB | +0.4 dB |
| 20 dB | +0.04 dB |
Field notes
- Measuring a quiet source in a noisy room? If the source only raises the meter by 3 dB or less, the background is doing most of the work and the subtraction gets unreliable. Measure when it's quieter if you can.
- Doubling the PA adds 3 dB for most of the audience, and more in the low end where the boxes couple. It's a smaller step than most people expect.
- Find the loudest noise source first. Quieting a source that's 10 dB below the loudest one barely changes the total.