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Decibel Addition and Background Correction Calculator

Add up to six sound levels on an energy basis, or take a background level out of a measured total to find what the source produces on its own.

  • Answers as you type
  • Every formula cited
  • Calculated in your browser
SettingsSettings for this calculationUS
Market
Imperial · sales tax
Adding sources together, or removing the background from a measurement.

Combining is for predicting the total from several machines or several contributions. Background correction is the reverse: a measurement near a source includes everything else audible there, and taking out the background measured with the source switched off leaves the source's own level.

One source's level, in the same weighting as all the others.

All levels must be the same kind of figure — all A-weighted, all C-weighted, or all sound power levels — and at the same position. A level of zero leaves the row out.

One source's level, in the same weighting as all the others.

All levels must be the same kind of figure — all A-weighted, all C-weighted, or all sound power levels — and at the same position. A level of zero leaves the row out.

One source's level, in the same weighting as all the others.

All levels must be the same kind of figure — all A-weighted, all C-weighted, or all sound power levels — and at the same position. A level of zero leaves the row out.

One source's level, in the same weighting as all the others.

All levels must be the same kind of figure — all A-weighted, all C-weighted, or all sound power levels — and at the same position. A level of zero leaves the row out.

One source's level, in the same weighting as all the others.

All levels must be the same kind of figure — all A-weighted, all C-weighted, or all sound power levels — and at the same position. A level of zero leaves the row out.

One source's level, in the same weighting as all the others.

All levels must be the same kind of figure — all A-weighted, all C-weighted, or all sound power levels — and at the same position. A level of zero leaves the row out.

Resulting level

87.44 dB (combined)

High confidence

3 levels combine to 87.4 dB, 2.4 dB above the loudest of them on its own.

Level 1's share of the total energy
57.07 %
Level 2's share of the total energy
28.6 %
Level 3's share of the total energy
14.33 %
Rise over the loudest single level
2.44 dB
Then change the inputs to see how far the answer moves.

Show calculation logic

How this was calculated

Formula source(s)

  • Sound levels combine on energy: L = 10 log10(10^(L1 ÷ 10) + 10^(L2 ÷ 10) + … + 10^(Ln ÷ 10)), so two equal levels are 3.01 dB above either and a level 10 dB below another adds 0.41 dB to it
  • The residual-sound correction used in ISO 1996-2 and BS 4142: the source alone is Ls = 10 log10(10^(Lt ÷ 10) − 10^(Lb ÷ 10)), the same energy arithmetic in reverse, which depends on the background being measured at the same position with the source off

Inputs used

What to Work Out
Combine several levels into one
Level 1 (dB, 0 to leave out)
85
Level 2 (dB, 0 to leave out)
82
Level 3 (dB, 0 to leave out)
79
Level 4 (dB, 0 to leave out)
0
Level 5 (dB, 0 to leave out)
0
Level 6 (dB, 0 to leave out)
0
Measured Total Level (dB)
88
Background Level (dB)
82

Intermediate steps

Level 1's share of the total energy
57.07 %
Level 2's share of the total energy
28.6 %
Level 3's share of the total energy
14.33 %
Rise over the loudest single level
2.44 dB
Final result87.44 dB (combined)

Confidence note: 3 levels combine to 87.4 dB, 2.4 dB above the loudest of them on its own.

What this calculation does not cover

  • Every level must use the same weighting and the same position. Adding an A-weighted level to a C-weighted one, or a sound power level to a sound pressure level, produces a number that means nothing.
  • The energy sum assumes the sources are unrelated, which is true of machines, voices and traffic. Two identical pure tones in step with each other can add or cancel quite differently, and that case needs a specialist.
  • A background correction is only as good as the two measurements behind it: the background has to be taken at the same place and in the same conditions, with the source off, and it changes through the day.

Computed in your browser — nothing you enter is uploaded. Presented in US customary units and US trade terminology. Where a formula follows a published standard, that standard and its edition are cited beside it on this page; where none governs, the page says so. Local amendments override model codes — verify against the code in force where you build.

Sources checked 2026-09-21 · v1.0.0

Regulatory standards & verification citations2
  1. Sound levels combine on energy: L = 10 log10(10^(L1 ÷ 10) + 10^(L2 ÷ 10) + … + 10^(Ln ÷ 10)), so two equal levels are 3.01 dB above either and a level 10 dB below another adds 0.41 dB to it
  2. The residual-sound correction used in ISO 1996-2 and BS 4142: the source alone is Ls = 10 log10(10^(Lt ÷ 10) − 10^(Lb ÷ 10)), the same energy arithmetic in reverse, which depends on the background being measured at the same position with the source off

Which documents these citations point at

Standards referenced: ISO 1996-2 (International Organization for Standardization, International).

Cite this page

Your workspace

Most jobs need more than one number. Add the calculators you need next and they open right here, underneath this one — your figures stay on screen and nothing is lost to a page change.

How to calculate decibel addition and background correction in 10 steps

  1. What to Work OutAdding sources together, or removing the background from a measurement.
  2. Level 1 (dB, 0 to leave out)One source's level, in the same weighting as all the others.
  3. Level 2 (dB, 0 to leave out)One source's level, in the same weighting as all the others.
  4. Level 3 (dB, 0 to leave out)One source's level, in the same weighting as all the others.
  5. Level 4 (dB, 0 to leave out)One source's level, in the same weighting as all the others.
  6. Level 5 (dB, 0 to leave out)One source's level, in the same weighting as all the others.
  7. Level 6 (dB, 0 to leave out)One source's level, in the same weighting as all the others.
  8. Measured Total Level (dB)The level measured with the source running.
  9. Background Level (dB)The level at the same position with the source switched off.
  10. Resulting levelThe tool computes the resulting level from those figures and shows the formula, its sources, and a confidence rating alongside it.

Resulting level by level 1 (dB, 0 to leave out)

Page defaults, not your figures above.

Level 1 (dB, 0 to leave out)Resulting level (dB (combined))
1083.8
2083.8
5083.8
100100
200200

Frequently asked questions

Why do two 85 dB machines make 88 dB and not 170?
Decibels are a logarithm of sound energy, so they cannot be added directly. Two machines give twice the energy of one, and twice the energy is 3 dB more on the logarithmic scale — 88 dB. Ten identical machines are 10 dB more than one. It works the other way too: switching off one of two equal machines only takes 3 dB off, which is why noise control starts with the loudest source rather than with the number of sources.
Why does a much quieter source hardly change the total?
Because its energy is a small fraction of the loud one's. A source 10 dB quieter than another carries a tenth of its energy and adds 0.4 dB to it; 20 dB quieter, it carries a hundredth and adds less than 0.05 dB. In a combination, the loudest level dominates so completely that quieter contributions can often be ignored — and silencing them achieves almost nothing.
When is a background correction worth making?
When the measured total is between about 3 and 10 dB above the background. Closer than 3 dB and a small error in either reading moves the answer a long way, so such a difference is commonly treated as too close to separate the source reliably. More than 10 dB and the correction is under half a decibel, so the total already describes the source. The background must be measured at the same position, with the same weighting and with the source switched off.
Preliminary estimate, not certified engineering. This tool produces an indicative quantity calculation for planning purposes only — it is not a certified structural analysis, a guaranteed material takeoff, or a substitute for building department approval. Always verify measurements on-site and have a licensed contractor or structural engineer review any load-bearing, code-sensitive, or safety-critical work before purchasing materials or starting construction. Spotted an arithmetic or standards error? Report it to contact@craftquantities.com with your inputs — a confirmed fix gets a permanent check of its own, so the same mistake cannot come back.