Decibel (Logarithmic) Addition Calculator

Q: How do you add decibels?

Decibels are logarithmic, so you cannot add them arithmetically. Convert each level to acoustic energy, add the energies, and convert back: L_total = 10·log10(Σ 10^(Li/10)). For example, 85 dB combined with 88 dB gives 89.8 dB, not 173.

Q: Why doesn't 90 dB plus 90 dB equal 180 dB?

Because the decibel scale is logarithmic. Two equal sources double the acoustic energy, and doubling energy adds only 3 dB. So 90 dB + 90 dB = 93 dB. Likewise 10 dB + 10 dB = 13 dB, not 20.

Q: How much louder is two of the same source?

N equal sources of level L combine to L + 10·log10(N). Two equal sources add 3 dB, four add 6 dB, and ten add 10 dB. That is why the single loudest source usually dominates a combined total.

Combine two or more sound levels the correct way. Because decibels are logarithmic, levels never simply add — this tool sums the acoustic energy per ANSI S1.4, or subtracts a known background from a measured total, and shows the result live.

Method reviewed by CSTI's INCE Board-Certified engineers

Sound levels

Enter levels in decibels (dB). Use the same weighting and reference for every value you combine.

Result

Energy sum of all sources (ANSI S1.4)

Combined level

—dB

Above loudest source

—dB

The method (ANSI S1.4 — energy summation)

Add L_total = 10 · log₁₀ ( Σ 10^(Lᵢ / 10) ) (N equal sources: L + 10·log₁₀ N)

Subtract L = 10 · log₁₀ ( 10^(L_total / 10) − 10^(L_bg / 10) ) (requires L_bg < L_total)

Worked example: 85 dB + 88 dB = 10·log₁₀(10^8.5 + 10^8.8) = 10·log₁₀(9.472×10⁸) = 89.8 dB — only 1.8 dB above the louder source, because the quieter one adds little energy. Two equal 80 dB sources give 80 + 10·log₁₀2 = 83 dB. Decibels are logarithmic, so they never simply add: 10 dB + 10 dB = 13 dB, not 20. When many sources overlap or the background is close to the total, a measured, source-resolved survey is the defensible number. Energy summation follows the sound-level definition in ANSI/ASA S1.4.

Frequently asked

How do you add decibels?

You cannot add them arithmetically, because the decibel scale is logarithmic. Convert each level to acoustic energy, add the energies, then convert back: L_total = 10·log₁₀(Σ 10^(Lᵢ/10)). For example, an add decibels calculator combines 85 dB and 88 dB into 89.8 dB — not 173.

Why doesn't 90 dB + 90 dB equal 180 dB?

When you combine sound levels, you are combining energy, not the decibel numbers. Two equal sources double the acoustic energy, and doubling energy adds exactly 3 dB. So 90 dB + 90 dB = 93 dB, and the classic gotcha holds: 10 dB + 10 dB = 13 dB, not 20.

How much louder is two of the same source?

Knowing how to add dB for equal sources is simple: N identical sources of level L combine to L + 10·log₁₀(N). Two add 3 dB, four add 6 dB, ten add 10 dB. That is why the single loudest source usually dominates the total, and why removing one quiet source often changes little.