Noise Attenuation by Distance Calculator
Sound falls off with distance: in the open, a point source loses about 6 dB every time you double the distance from it. Enter the level you measured and where you measured it, and this calculator gives the level at any other distance, the distance at which the source drops to 85 or 90 dBA, and the combined level when more than one source reaches the same spot.
Level at the new distance
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Results are estimates for planning purposes and do not replace a competent person's evaluation or an engineer's design. Follow the OSHA standards that apply to your work and your employer's written program.
How to use it
- 1Measure the source once, close in. Note the level and the exact distance from the microphone to the source. A tool level quoted without a distance is unusable — 100 dBA at 3 ft and 100 dBA at 50 ft are two very different machines.
- 2Enter the distance you are considering. The generator you want to push to the far side of the slab, the spot where the laborers will be working, the property line.
- 3Add the other noise at that spot. Distance only helps against the source you moved. A second machine, or the plant floor itself, is added logarithmically and often decides the answer.
- 4Read the two distances. The page tells you how far that source alone has to be to reach 90 dBA (the permissible exposure limit) and 85 dBA (the action level that triggers a hearing conservation program).
Distance, doublings and decibels
| Distance multiplied by | Doublings | Change in level | Sound energy at the ear |
|---|---|---|---|
| × 0.5 (half as far) | −1 | +6 dB | 4 × more |
| × 1 | 0 | — | reference |
| × 2 | 1 | −6 dB | 1/4 |
| × 4 | 2 | −12 dB | 1/16 |
| × 8 | 3 | −18 dB | 1/64 |
| × 10 | 3.32 | −20 dB | 1/100 |
| × 100 | 6.64 | −40 dB | 1/10,000 |
The exact figure is 6.02 dB per doubling. OSHA's Technical Manual gives the round number: “the sound pressure level decreases 6 dB … each time the distance from the point source is doubled.”
Adding sound levels without a calculator
OSHA publishes a field shortcut for combining two levels — accurate to about 1 dB, and enough to settle an argument on a jobsite. Take the two levels, find the difference, and add the amount below to the higher of the two. For three or more sources, combine two at a time.
| Difference between the two levels | Add this to the higher level |
|---|---|
| 0–1 dB | 3 dB |
| 2–4 dB | 2 dB |
| 5–9 dB | 1 dB |
| 10 dB or more | 0 dB |
OSHA's own example: three sources of 95 dB each. First two — difference 0 dB, so 95 + 3 = 98 dB. Then 98 against the third 95 — difference 3 dB, so 98 + 2 = 100 dB. The exact logarithmic sum is 99.8 dB.
The practical consequence: quieting one of two equal machines by 10 dB only buys you about 2.5 dB at the worker, because the other machine is still running. Controls have to reach the loudest sources, and usually more than one of them.
Where this arithmetic stops being true
- Indoors. Reflected sound dominates away from the source, so the level drops by less than 6 dB per doubling and eventually flattens out. The free-field number is a best case, not a prediction.
- Very close to the source. Inside the near field — roughly within one wavelength of the lowest important frequency — readings are unstable and the inverse-square law does not apply.
- Sources that are not points. A production line, a highway or a long conveyor behaves as a line source and falls off more slowly than a single machine.
- Impulse noise. Nail guns, powder-actuated tools and presses produce peaks that a slow-response A-weighted reading understates. Exposure to impulsive or impact noise should not exceed a 140 dB peak sound pressure level (29 CFR 1910.95(a), Table G-16 footnote).
- Barriers, cabs and enclosures. Real attenuation from plywood barriers, equipment cabs or enclosures is not a distance calculation — it has to be measured.
How it's calculated
- Sound level at a new distance (point source, free field)
L₂ = L₁ + 20 × log₁₀(d₁ / d₂)L₁ is the level measured at distance d₁, L₂ the level at distance d₂. Any unit of length works as long as both distances use the same one — only the ratio matters. Doubling the distance gives 20 × log₁₀(0.5) = −6.02 dB.- Number of doublings
n = log₂(d₂ / d₁) · drop ≈ 6.02 × n dBThe same result stated the way OSHA states it: −6 dB per doubling of distance, +6 dB per halving.- Combining several sound levels at one position
L_total = 10 × log₁₀(10^(L₁/10) + 10^(L₂/10) + … + 10^(Lₙ/10))Decibels are logarithmic and cannot be added arithmetically. Two equal sources give +3 dB, not double the number: 90 dB and 90 dB combine to 93 dB.- Distance at which a source reaches a target level
d_target = d₁ × 10^((L₁ − L_target) / 20)The distance form of the same equation, solved for d₂. Used here for the 85 dBA action level and the 90 dBA PEL. It describes the source alone — other noise at that spot has to be added back.- Permissible and allowable durations at the resulting level
OSHA: T = 8 / 2^((L − 90)/5) · NIOSH: T = 8 / 2^((L − 85)/3)OSHA reference duration from Table G-16 and Appendix B.4 of the Technical Manual; the NIOSH allowable duration uses the 85 dBA criterion and 3 dB exchange rate of NIOSH 98-126 and is shown for comparison only.
Frequently asked questions
About 6 dB, for a point source outdoors with nothing to reflect off. OSHA's Technical Manual puts it this way: a source that reads 90 dB at 1 meter reads 84 dB at 2 meters and 78 dB at 4 meters. Halving the distance adds the same 6 dB back.
No, and assuming it does is the classic mistake. Inside, sound reflects off walls, floors, ceilings and equipment, so the level drops by less than 6 dB per doubling and eventually stops dropping at all — the reverberant field. OSHA's Technical Manual states it plainly: away from the source, reflected sound dominates and the level decreases by less than 6 dB each time the distance doubles. Treat any indoor number from this calculator as a best case and measure the real position.
Logarithmically, not arithmetically. L_total = 10 log10(10^(L1/10) + 10^(L2/10) + …). Two 90 dB machines side by side give 93 dB, not 180 dB. OSHA also publishes a field shortcut: if two levels are within 1 dB add 3 dB to the higher one, 2 to 4 dB apart add 2, 5 to 9 dB apart add 1, and 10 dB or more apart add nothing.
Your dose falls faster than that. Six fewer decibels means the sound energy at your ear is one quarter of what it was, and under the OSHA 5 dB exchange rate the permitted time at that level goes up by a factor of about 2.3. That is why OSHA's construction noise guide lists Move It — extension cords, longer air hoses, welding leads — as a control in its own right, right next to Reduce It and Block It.
If that 100 dBA was measured at 5 ft, the free-field answer is about 28 ft: d = 5 × 10^((100−85)/20). Two things then matter. Distance protects the person who moves, not the operator holding the tool, and any other noise reaching that spot has to be added back logarithmically.
Sometimes for bystanders, never as a paper exercise. OSHA requires feasible engineering and administrative controls when exposures exceed the 90 dBA permissible exposure limit (1910.95(b)(1), 1926.52(b)), and moving the source or the worker is one of those controls. But the decision rests on measured exposure over the whole shift, not on a single calculated level, and hearing protection stays required wherever the exposure still reaches the limits.
Usually for one of four reasons: you are in the near field, closer to the source than one wavelength of its lowest important frequency, where readings are unreliable; the space is reverberant; the source is not a point source, like a whole production line or a road; or the meter was not field-calibrated that day. Compliance measurements under 1910.95(d) need a calibrated instrument and a representative sampling strategy.
No. It reproduces two published equations from the OSHA Technical Manual so you can plan a layout, place a generator, or sanity-check a reading. A determination of exposure — the 8-hour TWA that decides whether a hearing conservation program is required — needs a calibrated dosimeter or sound level meter, used by someone competent in noise measurement. Use the noise exposure TWA calculator for the arithmetic once you have real measurements.
Sources & references
- OSHA Technical Manual, Section III Chapter 5, Appendix B.6 — Calculating sound pressure level at a distance (Lpd2 = Lpd1 + 20 log(d1/d2); 6 dB per doubling in a free field)
- OSHA Technical Manual, Section III Chapter 5, Appendix B.3 — Combining and averaging sound levels (Total L = 10 log10 Σ 10^(Ln/10), and the 0-1/2-4/5-9 dB rule of thumb)
- OSHA Technical Manual, Section III Chapter 5, Appendix B.4 — Permissible durations and Table G-16 (T = 8 / 2^((L-90)/5))
- 29 CFR 1910.95 — Occupational noise exposure: Table G-16, 85 dBA action level (c)(1), engineering controls (b)(1)
- 29 CFR 1926.52 — Occupational noise exposure (construction): Table D-2 and (b) on feasible administrative or engineering controls
- OSHA 3498 — Protecting Yourself from Noise in Construction (the Reduce It / Move It / Block It control process and the job site sound level chart)
- NIOSH Publication 98-126 — Criteria for a Recommended Standard: Occupational Noise Exposure (85 dBA REL, 3 dB exchange rate)
Content checked against these sources — last reviewed August 28, 2026.