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Decibel Addition and Distance Calculator

Combine sound levels from several sources — such as two condensers side by side — and estimate how loud a source is at a different distance.

Method reviewed October 26, 2026. Version 1.0.0. How it works

Your inputs

Unit system

Result

Combined sound level
93dB

Combined about 93 dB.

  • Sound levels add logarithmically: two equal sources are 3 dB louder than one, not twice as loud. Use the same weighting (e.g. dBA) for all sources.
Increase over the loudest source
3.01dB
Show the working
  1. L = 10 × log₁₀(Σ 10^(Lᵢ/10))
    10 × log₁₀(10^(90/10) + 10^(90/10)) = 93.01 dB

Adding decibels

Decibels are logarithmic, so they cannot be added arithmetically. Each level is converted back to its energy, the energies are added, and the total is converted back to decibels: L = 10 × log₁₀(Σ 10^(Lᵢ ÷ 10)).

Two equal sources are 3 dB louder than one; the OSHA Technical Manual’s example combines two 90 dB sources into 93 dB. A source 10 dB quieter than another adds less than half a decibel.

Distance

For a small source outdoors with nothing to reflect the sound, the level falls by 6 dB each time the distance doubles: L₂ = L₁ − 20 × log₁₀(d₂ ÷ d₁). Walls, the ground, fences and the source’s own shape change this, so treat it as a first estimate.

Worked example

These are the example values the calculator opens with, and the result the engine calculates for them.

Inputs

Calculate
Combined level of several sources
Number of sources
2 sources
Source 1
90 dB
Source 2
90 dB

Result

Combined sound level
93 dB
Increase over the loudest source
3.01 dB

Combined about 93 dB.

Assumptions

  • Incoherent sources add on an energy basis; a point source in a free field loses 6 dB per doubling of distance (20 log₁₀).

Limitations

  • Reflections, barriers, ground effects and source directivity change real levels; local noise rules are not checked here.

Frequently asked questions

How much louder are two air conditioners than one?

If they are equally loud and in the same place, 3 dB louder — noticeable, but not twice as loud to the ear.

How much quieter is it twice as far away?

About 6 dB in open air. Near walls or buildings, reflections make it drop more slowly.

Should I use dB or dBA?

Use the same weighting for every source. Equipment sound ratings are often given in dBA, which weights frequencies as the ear hears them.

Sources

  1. OSHA Technical Manual, Section III Chapter 5: Noise — U.S. Department of Labor, OSHA. Sound levels add logarithmically (two 90 dB sources → 93 dB); a point source in a free field loses 6 dB per doubling of distance, L₂ = L₁ + 20 log(d₁/d₂) (85 dBA at 100 ft → 97 dBA at 25 ft).

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