---
title: "Decibel Calculator: Add, Average & Distance | City Hum Daily"
description: "Add, subtract and average decibel levels the logarithmic way, remove background noise and find the level at another distance (inverse square law)."
url: https://cityhumdaily.buzz/decibel-calculator/
---

Logarithmic sums

# Decibel calculator

Decibels do not add like ordinary numbers: two 80 dB machines make 83 dB, not 160. Four calculators for the sums that come up most.

## The formulas behind the four panels

A decibel level is 10 × log10 of an energy ratio, so the sums happen on the energies. **Adding**: convert each level L to 10^(L/10), add, and convert back with 10 × log10. Two equal sources give +3 dB, ten give +10 dB, and a source 10 dB quieter than another adds only 0.4 dB.

**Subtracting** works the same way backwards: 10 × log10(10^(Lt/10) − 10^(Lb/10)). When the total is at least 10 dB above the background the correction is under half a decibel. Under 3 dB, the source is quieter than the background and the answer is too sensitive to trust; measure closer to the source or at a quieter time.

The **energy average** is what a sound level meter's Leq does: the loud moments dominate, so 90 and 60 dB average to 87 dB, not 75. Use it for readings of equal duration.

**Distance**: in open air, sound from a small source spreads over a sphere and drops 20 × log10(r2/r1), about 6 dB each time the distance doubles (the inverse square law). A long line such as a road or railway spreads over a cylinder and drops about 3 dB per doubling. Walls, ground, wind and air absorption change real results, often by several decibels.

Mix only levels with the same weighting: dBA with dBA, dBC with dBC.

## Questions

**How many decibels is twice as loud?**

Doubling the sound energy adds 3 dB, which is a clear but modest change. Most people judge a sound about 10 dB higher as roughly twice as loud, and that takes ten times the energy: ten identical machines instead of one.

**Can I add dBA readings from different machines?**

Yes, as long as each was measured at the same spot and all are A-weighted. The sum estimates the level with everything running together. Adding a dBA reading to a dBC or unweighted one gives a number with no clear meaning.

**Why does my measured level not drop 6 dB per doubling?**

The rule needs open air, a small source and a listener far from reflecting surfaces. Indoors, reflections from walls and ceilings keep the level up away from the source, and close to a large machine you are in its near field, where the level changes less predictably.