Decibel (dB) Addition Calculator

1. Enter the first sound level in decibels.
2. Enter the second sound level in decibels.
3. Read the combined level of the two sources together.
4. See the difference between them and how much the total rises above the louder one.

Decibel addition is the correct way to combine two sound levels. Because decibels are a logarithmic scale, you cannot simply add the numbers: two 80 decibel sources do not make 160 decibels, they make about 83.

The decibel compares a sound to a reference using a logarithm, so equal steps represent equal ratios, not equal amounts. That is why combining sounds needs a logarithmic formula rather than ordinary arithmetic. The same logic governs why a room with ten people talking is nowhere near ten times as loud as one person, and why turning up an amplifier a few decibels can feel like a modest rather than a huge change.

1. Convert each level back to a power ratio by raising ten to the level over ten.
2. Add the two power ratios together.
3. Take the base-ten logarithm of the sum.
4. Multiply by ten to return to decibels.

The key rule that falls out of this: doubling the sound power, such as adding a second identical source, always raises the level by about 3 decibels.

Two machines each measure 80 decibels. Converting to power ratios and adding gives twice the power of one, and ten times the log of two is about 3. So the pair reaches roughly 83 decibels, only 3 more than one alone. If instead the levels were 90 and 75 decibels, the quieter source barely matters: the total stays close to 90, because a 15 decibel gap means the quieter sound adds almost nothing. This is a genuinely useful shortcut on site: when one source clearly dominates the others, you can often treat its level as the total and save yourself the arithmetic, knowing the error is a fraction of a decibel.

The Gap Between Sources

When two levels are equal the total rises by 3 decibels; the wider the gap, the less the quieter source adds.

Number of Sources

Each doubling of identical sources adds another 3 decibels, so four equal sources are 6 decibels above one.

Power Versus Pressure

This addition works on sound power or intensity; combining voltages or pressures directly uses a different factor.

Correlation

The formula assumes uncorrelated sources, such as separate machines, rather than identical signals played in phase.

A few reference points make the scale intuitive.

This is why a perceived doubling of loudness needs about 10 decibels, not the 3 that doubles the actual power.

Combining Noise Sources

Work out the total level from two machines, speakers or fans running at once.

Studio and Live Sound

Estimate how stacking sources or adding a subwoofer changes the overall level.

Noise Assessments

Add measured levels correctly when checking exposure against a limit.

1. Adding Decibels Directly

Summing 80 and 80 to get 160 is wrong; the true total is about 83 decibels.

2. Ignoring the Gap

A source more than 10 decibels quieter adds almost nothing, so it is often safe to ignore.

3. Confusing Power and Loudness

Plus 3 decibels doubles the power, but the ear needs about plus 10 to sound twice as loud.

4. Mixing Power and Pressure Formulas

Use the power form here; adding pressures or voltages uses a factor of 20, not 10.

The logarithmic addition is exact for uncorrelated power levels, so the combined figure is reliable. It does not judge loudness perception or frequency weighting.

Convert each level to a power ratio, add them, take the base-ten log and multiply by ten. Two 80 dB sources combine to about 83 dB, not 160.

Because decibels are logarithmic, doubling the sound power adds ten times the log of two, which is about 3 decibels.

The quieter source adds very little. A gap of 10 decibels adds only about 0.4 decibels to the louder one.

About 10 decibels. Perceived loudness doubling needs roughly ten times the power, unlike the 3 decibels that doubles actual power.

Yes, in stages: combine two, then add the next to that total. Each doubling of identical sources adds another 3 decibels.

Power or intensity levels use a factor of 10 in the formula, while sound pressure or voltage ratios use a factor of 20.

No. It gives the physical combined level. Perceived loudness also depends on frequency and duration, which this tool does not weight.

Yes. A level below the reference is negative, and the calculator handles negative inputs correctly.

No. The calculator runs entirely in your browser and nothing you enter is stored or sent anywhere.

• The decibel (Wikipedia).
• Sound pressure and levels (Wikipedia).
• Sound intensity (Wikipedia).