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Decibel calculator
Decibels are logarithmic, so they do not add the ordinary way: two sources of 60 dB together make 63 dB, not 120 dB. This calculator combines levels, removes a background reading from a measurement, and converts decibels into the ratios, pressures and powers they stand for.
Combine sound levels
Two sources running together. Enter each level in decibels; add a row for a third or fourth.
- Combined level
- 63.0 dB
- Energy average (Leq)
- 60.0 dB
The loudest source is 60.0 dB, so everything else together adds 3.01 dB. Two equal sources always add 3 dB, ten equal sources add 10 dB.
Subtract background noise
You measured the machine running and then the room without it. This removes the background energy.
- The source on its own
- 64.0 dB
A gap of 3 to 10 dB is the band where a background correction is worth making, and this is it.
Convert decibels and ratios
A decibel is a ratio, not a quantity. Power and intensity use ten times the logarithm; amplitude, pressure and voltage use twenty.
Sound pressure level is referenced to 20 micropascals, the nominal quietest audible pressure at 1 kHz. One pascal is 94 dB, which is why acoustic calibrators are built to produce exactly that.
Arithmetic only: nothing here uses your microphone, and combining levels assumes separate, unrelated sources. Two copies of the same signal can add up to 6 dB instead of 3, and can also cancel.
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How much to add to the louder source
The field shortcut, and the arithmetic behind the first calculator above. Find the gap between your two levels, then add the figure in the second column to the louder one.
| Difference between them | Add to the louder level | What it means in practice |
|---|---|---|
| 0 dB | +3.0 dB | Two equal sources. The most any single addition can do. |
| 1 dB | +2.5 dB | Both sources matter; neither can be ignored. |
| 2 dB | +2.1 dB | Both sources matter; neither can be ignored. |
| 3 dB | +1.8 dB | Both sources matter; neither can be ignored. |
| 4 dB | +1.5 dB | The quieter one still shows, but only just. |
| 5 dB | +1.2 dB | The quieter one still shows, but only just. |
| 6 dB | +1.0 dB | The quieter one still shows, but only just. |
| 8 dB | +0.6 dB | Under half a decibel — at the edge of what a meter resolves. |
| 10 dB | +0.4 dB | Under half a decibel — at the edge of what a meter resolves. |
| 15 dB | +0.1 dB | The quieter source has stopped mattering entirely. |
| 20 dB | +0.0 dB | The quieter source has stopped mattering entirely. |
This is arithmetic on the definition of the decibel, not a measurement. It assumes the sources are unrelated to one another, which is the normal case for two machines or a machine and traffic. Two copies of the same signal are a different problem: in phase they add 6 dB, out of phase they can cancel.
Why decibels work this way
Human hearing covers an enormous range of pressures, from about 20 micropascals at the threshold to 20 pascals at the threshold of pain — a factor of a million. A linear scale would be unusable, so sound is expressed as the logarithm of a ratio against a reference. That compression is what makes the everyday world fit between 0 and 140 on the decibel chart, and it is also why the arithmetic on this page is necessary at all. If you want the idea rather than the calculation, start with what a decibel actually is.
Three rules worth memorising
- 3 dB is double the sound energy. Two identical machines, or twice the traffic.
- 6 dB is double the sound pressure, and also what you lose each time the distance from a point source doubles. The distance calculator works that one out.
- 10 dB is roughly twice as loud to a listener, and takes ten identical sources.
Converting between decibels and what they measure
A decibel is always a ratio to something. Power, intensity and energy use ten times the logarithm; amplitude, pressure and voltage use twenty, because power goes as the square of pressure. That single fact explains why the same physical doubling is 3 dB in one column and 6 dB in the other. The third card above converts in both directions, including sound pressure in pascals to dB SPL: one pascal is 94 dB, which is why acoustic calibrators are built to produce exactly 94 dB at 1 kHz.
Why is 60 dB plus 60 dB not 120 dB?
Because decibels are a logarithmic ratio, not a quantity you can add. Two sources of equal level carry twice the sound energy between them, and twice the energy is an increase of 3 dB, so 60 dB and 60 dB together make 63 dB. Getting to 120 dB from 60 dB would take a million times the energy, which is a million identical machines.
How do I subtract background noise from a measurement?
Convert both readings back to energy, subtract, and convert to decibels again, which is what the calculator above does. The rule that matters is the gap between them: more than 10 dB and the background changes almost nothing, between 3 and 10 dB the correction is worth making, and under 3 dB no correction is valid because the background is as loud as the thing you are measuring.
What is the difference between dB, dBA and dB SPL?
dB on its own is only a ratio and means nothing until you name the reference. dB SPL is that ratio against 20 micropascals, the quietest audible pressure at 1 kHz, which is what a sound level meter reads. dBA is dB SPL after A-weighting, a filter that discounts the low frequencies your ear is less sensitive to, and it is the number nearly every noise regulation is written in.
What is dBFS and why is it negative?
dBFS is decibels relative to digital full scale, used inside audio software. Full scale is the loudest sample a file can hold, so it is 0 and everything below it is negative. There is no fixed relationship between dBFS and dB SPL in the room: -20 dBFS could be a whisper or a rock concert depending on the gain of the microphone and the volume of the playback.
Can I just average decibel readings?
Not arithmetically. Averaging 60 dB and 80 dB the ordinary way gives 70 dB, but by energy the answer is 77 dB, because the loud moment dominates. Sound exposure standards use that energy average, called Leq, for exactly this reason, and the calculator above shows both so the difference is visible.
Does doubling the number of machines double the noise?
No. Doubling the number of equal sources adds 3 dB, which most people describe as slightly louder rather than twice as loud. Perceived loudness roughly doubles every 10 dB, and that takes ten identical machines, not two.
Measure a level instead of calculating one
These are numbers you type in. To get a reading from the room you are in, the free online decibel meter uses your microphone with A-weighting and a saved report, and the spectrum analyser shows which frequencies the sound is actually made of.