How loud will it be at a distance?
Add noise sources heard from different distances, or find how far you must be to get down to a target level.
This is a screening tool, not a safety assessment. It models a point source in open air; real sites have walls, ground and reflections. It does not replace measurement by a qualified person. How we check our calculators.
For each source enter the level you measured, the distance you measured it at, and how far the listener is from it.
Sources that sit close together act like one louder source: each doubling adds 3 dB.
What this calculator is for
The decibel calculator adds levels at one spot and moves one level to another distance. This page is for planning: you have measured several machines at known distances and want the level at a worker, a neighbor or a property line, or you have a limit and want to know how far back to stand. Use A-weighted readings (dBA) if you will compare the answer with hearing limits, which are stated in dBA. The distance rule is exact only for an idealized source in open air, so the result is an estimate.
The formulas
Moving a level: L₂ = L₁ − 20 × log₁₀(r₂ ÷ r₁), where L₁ is the level measured at distance r₁ and L₂ is the level at r₂. The 20 comes from intensity falling with the square of the distance, so each doubling removes 20 × log₁₀ 2, about 6 dB. Adding sources: L = 10 × log₁₀(Σ 10^(Lᵢ ÷ 10)). Several identical sources together add 10 × log₁₀ n. Distance for a target: r₂ = r₁ × 10^((L₁ − L_target) ÷ 20). Only the ratio of the two distances counts, so feet and meters give the same level for the same layout.
Worked example
Three identical compressors each measure 85 dB at 10 ft. Together they are 85 + 4.8 = 89.8 dB at 10 ft. A neighbor wants the noise at the fence down to 55 dB: r = 10 × 10^((89.8 − 55) ÷ 20) = 548 ft. A single compressor would need 316 ft. This is a lot of distance, and a good reminder of why barriers, enclosures and quieter machines are used instead. The calculator opens with this case in the second mode. The first mode opens with two sources: a 95 dB machine measured at 3 ft and a listener at 50 ft (70.6 dB), and an 88 dB machine measured at 3 ft and a listener at 20 ft (71.5 dB). Together they total 74.1 dB at the listener; the background box is left empty, and a 50 dB background would change nothing noticeable.
Limits of this method
- Open air only. OSHA's Technical Manual says the 6 dB per doubling holds in a free field. In a room, reflected sound makes the level fall by less, and most real places are in between.
- A point source. A long pipeline, a road or a line of machines falls off more slowly than a single point does. The formula treats every source as small compared with the distance.
- No air, ground or weather effects. Absorption by the air at high frequencies, ground cover, barriers, wind and temperature gradients can add or remove many decibels over long distances. This page includes none of them.
- The measurement distance must be in the far field. Readings taken inside the near field are unreliable (see the questions below).
- Averages versus peaks. A meter reading at a moment is not an exposure. Limits are written for time-weighted averages, and impulse noise has its own peak limit of 140 dB (OSHA and NIOSH).
Common questions
How much does sound drop with distance?
From a small source in open air, by 6 dB every time the distance doubles. OSHA's Technical Manual gives the example of 90 dB at 1 meter: 84 dB at 2 meters, 78 dB at 4 meters. The formula is L₂ = L₁ − 20 × log₁₀(r₂ ÷ r₁). Ten times the distance is 20 dB quieter, so 85 dB measured at 10 ft is 65 dB at 100 ft, if nothing reflects the sound and the air is calm.
How do I add several noise sources?
Move each one to the listener's position with the distance formula first, then add the levels as energy: 10 × log₁₀(10^(L₁÷10) + 10^(L₂÷10) + …). Equal sources add little: two at 85 dB make 88.0 dB, and three identical sources add 4.8 dB to one. Do not add the decibel numbers. In the default example, a 95 dB machine measured at 3 ft and heard from 50 ft is 70.6 dB, a second at 88 dB heard from 20 ft is 71.5 dB, and together they are 74.1 dB.
Does background noise matter?
Yes, but it matters least when it is much quieter than the source. The level of a source ten decibels below the background adds only about 0.4 dB to it; a 60 dB source with a 50 dB background reads 60.4 dB. In the default example the two machines total 74.1 dB; a 50 dB background changes that by less than 0.1 dB, while a background of 72 dB lifts it to 76.2 dB. When the source you are checking is well below the background, you cannot hear or measure it separately, and a limit on its own contribution cannot be checked by a meter at that spot.
How far away is the noise safe?
There is no distance that is safe for everything, because it depends on how loud the source is, for how long and the limit you use. NIOSH recommends no more than 85 dBA averaged over 8 hours, halving the time for each further 3 dB (8 hours at 85 dBA); OSHA's action level, where a hearing conservation program starts, is also 85 dBA as an 8-hour average. The EPA's 1974 Levels Document names 70 dB averaged over 24 hours as a level that protects hearing from environmental noise, and 55 dB outdoors as a day-night average that avoids interference with activities. For time at a given level, see the <a href="/decibel-calculator/">decibel calculator</a> and the <a href="/noise-dose-calculator/">noise dose calculator</a>.
Why do I measure less of a drop indoors?
In a room the sound reflects from walls, floor and ceiling. Close to the source you hear mostly the direct sound; farther away the reflected sound dominates, and the level stops falling at 6 dB per doubling. OSHA's Technical Manual describes this as the reverberant field, in which the level falls by less than 6 dB per doubling, and says real rooms are somewhere between the free field and the reverberant field. Adding absorbing material helps with the reflected part but does nothing for the direct sound along the line of sight.
How close can I measure for the formula to work?
Not too close. The near field, the space within about one wavelength of the lowest frequency in the sound, is where readings jump with small changes in position. OSHA's Technical Manual puts it at a little more than 4 ft for a 25 Hz tone and about 1 ft for a 1,000 Hz tone. Measure a few feet or more from the source for broadband machinery, and write down the distance, because the whole calculation depends on it.
This is a screening tool, not a safety assessment. Hearing damage is permanent; measure real noise with a calibrated meter or dosimeter, and ask a qualified person for advice on controls and protection.
Sources: OSHA Technical Manual, section III, chapter 5, Noise (sound fields, inverse-square law, near and far field, reverberant field; osha.gov/otm, read October 27, 2026); U.S. EPA, Information on Levels of Environmental Noise Requisite to Protect Public Health and Welfare with an Adequate Margin of Safety, EPA 550/9-74-004 (March 1974), table 1; NIOSH, Criteria for a Recommended Standard: Occupational Noise Exposure, publication 98-126. Related: decibel calculator, noise dose calculator, speed of sound calculator, lightning distance calculator. How we check our calculators.