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Decibel Combine Calculator

Add two decibel levels together.

How it is calculated

Decibels are logarithmic, so the tool converts each level to a relative power, adds them, and converts back: L = 10 × log10(10L1/10 + 10L2/10). L1 and L2 are the two sound levels in dB, and L is the combined level in dB, rounded to two decimals. The formula treats the sources as incoherent and uncorrelated (ordinary noise from separate machines), measured at the same point. The combined level is always at least as high as the louder source and at most about 3 dB above it. Only two levels are combined per run; for more, combine the result with the next level.

Worked example

Two machines measure 85 dB and 82 dB at the same spot. Convert: 108.5 ≈ 316,228,000 and 108.2 ≈ 158,489,000. Add them: about 474,717,000. Take log10, which is about 8.6764, and multiply by 10. The tool shows 86.76 dB, only about 1.8 dB above the louder machine.

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Common mistakes

  • Adding the decibel numbers directly (85 + 82 = 167 dB). The scale is logarithmic and this is wrong by a huge margin.
  • Expecting two equal sources to double the number. Two identical 70 dB sources give about 73 dB.
  • Combining a very quiet source with a loud one and expecting a change. A source 10 dB or more below the other adds well under half a decibel.
  • Mixing different weightings, such as dB(A) with dB(C), or levels measured at different distances.
  • Using the formula to combine coherent sources such as two identical tones in phase, where the levels do not add this way.

About this calculator

Sound levels in decibels (dB) are logarithmic, so they can't simply be added together — combining two sources requires converting each back to sound intensity, adding the intensities, then converting back to dB: L = 10 · log₁₀(10^(L₁/10) + 10^(L₂/10)). Two equal sound sources don't double the decibel reading; they only add about 3 dB (e.g. two 70 dB machines running together produce about 73 dB, not 140 dB), because decibels measure intensity on a logarithmic power scale.

Acoustic engineers, audio engineers and occupational health/safety professionals use decibel combination constantly: predicting the combined noise level of multiple machines on a factory floor, calculating total sound pressure when several loudspeakers play simultaneously, or estimating cumulative environmental noise from multiple sources (traffic, HVAC units, industrial equipment) for regulatory compliance and hearing-protection planning.

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Comments (1)

  • Editorial team

    Decibels add on an energy basis, which gives two handy checks: two equal sources are 3 dB louder than one, and a source 10 dB or more below another adds under half a decibel. For three or more machines, combine two, then feed that result in with the next one. This assumes unrelated noise sources measured at the same spot with the same weighting, so do not mix dB(A) with dB(C). How many sources are you trying to combine?

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