Decibel Sum Calculator
Enter two sound levels in decibels and the calculator combines them by energy addition — and shows why two equal sources add up to only about 3 dB more, not double.
How the decibel sum calculator works
Adding two sound levels
The decibel sum calculator combines two sound pressure levels into a single total level. Because decibels are a logarithmic scale, you cannot simply add the numbers — two 60 dB sources do not make 120 dB. Instead the levels are converted back to linear energy ratios, added, and converted to decibels again. Two equal sources rise by only about 3 dB, while a source that is 10 dB louder almost completely dominates the result.
Enter two sound levels in decibels to get the combined level instantly — two equal sources add up to roughly 3 dB more, not twice the number.
Each level is converted from decibels to a linear energy ratio with 10^(L/10), the ratios are added, and the sum is converted back with 10 × log10.
L = 10 × log₁₀(10^(L₁/10) + 10^(L₂/10))The logarithm is the key: decibels measure ratios on a logarithmic scale, so doubling the acoustic energy adds a fixed 3 dB rather than doubling the number. That is why two identical machines, two equal speakers, or two people talking at the same level produce only about 3 dB more than one alone.
Suppose two machines each measure 60 dB at your listening position.
Convert each level to energy
10^(60/10) = 10^6 = 1,000,000 for each of the two sources.
Add the energy ratios
1,000,000 + 1,000,000 = 2,000,000 — double the energy of one source.
Convert back to decibels
10 × log₁₀(2,000,000) ≈ 63.01 dB — only about 3 dB above a single 60 dB source.
The combined level tells you how loud two sources are together, and the surprise is how little louder the pair is than the louder one alone. Two equal sources add about 3 dB — doubling the energy is only a 3 dB step, not a doubling of the number, because the decibel scale is logarithmic. The gap between the two levels matters far more than their sum. When one source is 10 dB louder than the other, the quieter one contributes only about 0.4 dB and is effectively drowned out, so the total is barely above the louder source. This is why a single loud machine dominates a workshop full of quieter ones, and why silencing the loudest source helps far more than removing several quiet ones.
The energy-sum formula is exact for independent sources, but a couple of points are worth keeping in mind.
Assumes uncorrelated sound pressure levels
This calculator adds two sound pressure levels assuming the sources are uncorrelated (incoherent) — independent noises such as separate machines or voices. Correlated sources at the same frequency can interfere and add differently, up to 6 dB for perfectly in-phase tones. Make sure both inputs are levels in decibels referenced to the same quantity, and remember the result is a level, not a simple arithmetic total.