Decibel Adder

How to use

1. 1Choose Combine levels to add sound sources, or Subtract background to recover one source from a measurement.
2. 2In combine mode, enter each source level in decibels and set a count for several identical sources, then use Add source for more rows.
3. 3Read the combined level, the increase over the loudest source, and the energy share each source contributes.
4. 4In subtract mode, enter the combined measurement and the background level; the tool returns the source level and warns when the two are too close.
5. 5Copy the summary for your notes, or load an example to see how equal sources add only about 3 dB.

About this tool

Decibel Adder combines sound levels the way they actually combine, which is by energy and not by ordinary arithmetic. Because a decibel is a logarithmic ratio, two equal sources do not add to double the number: two fans at 60 dB together read about 63 dB, not 120 dB. The reason is that every 3 dB rise represents a doubling of sound energy, so adding a second identical source doubles the energy and adds 3 dB. To combine any set of levels the tool converts each one to a relative power with ten raised to the level divided by ten, adds those powers, and converts the total back with ten times the base ten logarithm, which is the standard method for uncorrelated sources such as separate machines, appliances, fans, or vehicles. A count field lets you enter several identical sources at once, for example four pumps at 85 dB, without typing the same value four times. The result panel shows the combined level, how much louder it is than the single loudest source, and an energy share bar for each source so you can see at a glance which ones matter; a source more than ten decibels below the loudest contributes only a fraction of a decibel, and one twenty decibels below is effectively negligible, which is why the loudest source usually dominates a mix. The second mode does the inverse and subtracts a background level from a combined measurement to recover the level of a single source on its own. This is the background noise correction used in occupational noise and acoustics work: you measure the level with the source running, measure the background with it switched off, and the tool subtracts the powers to give the source level. That subtraction is only defined when the combined reading is higher than the background, and it becomes uncertain when the two are within about three decibels, so the tool flags both cases clearly rather than returning a misleading number. A difference lookup table shows the classic rule that two levels differing by zero decibels add three, by ten add about four tenths, and by twenty add almost nothing, and an everyday reference table lists typical sound pressure levels from a quiet bedroom to a jet takeoff so you can sanity check a result. The tool assumes a single frequency weighting, so mix only levels measured the same way, for example all A weighted dBA, and it treats sources as uncorrelated, which is correct for most real world noise but not for pure tones that can interfere. All calculations run in your browser with plain logarithms, and the levels you enter are never uploaded, logged, or stored.

Free to use. Works in your browser. No signup, no login.