Move twice as far from a single noise source in the open and the level drops by about 6 dB. Move four times as far and you lose about 12 dB. NIOSH lists the same rule among its administrative noise controls: doubling the distance from the noise source can decrease noise levels by as much as 6 dB.
That 6 dB figure applies to one compact source with nothing in the way. Indoors, next to a highway, or behind a wall, the drop is usually smaller, and sometimes there’s barely any drop at all.
Why sound drops 6 dB per doubling of distance #
Sound energy from a small source spreads out over a sphere. Double the radius and the surface area of that sphere quadruples, so the same energy is spread over four times the area. A quarter of the intensity works out to 10 × log10(1/4), which is about -6 dB.
Starting from 100 dB measured one meter away:
| Distance from source | Level | Drop from 1 m |
|---|---|---|
| 1 m | 100 dB | 0 dB |
| 2 m | 94 dB | 6 dB |
| 4 m | 88 dB | 12 dB |
| 8 m | 82 dB | 18 dB |
| 16 m | 76 dB | 24 dB |
| 32 m | 70 dB | 30 dB |
The general form: the drop equals 20 × log10(d2 / d1). So going from 2 meters to 3 meters costs you about 3.5 dB, not a round number. To lose a specific amount, multiply your distance by 2 raised to the power of (dB wanted ÷ 6). Cutting 10 dB means standing about 3.2 times further away.
When the 6 dB rule fails #
Line sources. A busy highway or a passing train isn’t a point, it’s a long line of sources. Levels from a line source fall roughly 3 dB per doubling instead of 6, which is why traffic noise carries so much further than you’d expect. Moving from 30 meters to 60 meters from a freeway may only buy you a few decibels.
Rooms. Indoors, you hear the source directly plus every reflection off walls, ceiling and furniture. Close to the source the direct sound dominates and the 6 dB rule roughly holds. Past a certain distance the reflected sound takes over and the level barely changes no matter how far back you go. In a small hard-surfaced room, that point can be a meter or two from the source.
Barriers. A solid barrier that blocks the line of sight helps, but not unlimited amounts. The Federal Highway Administration reports that effective noise barriers typically reduce noise levels by 5 to 10 decibels, that a barrier gets about 5 dB once it’s tall enough to break the line of sight from the highway to the home, and roughly 1.5 dB more for each additional meter of height. Barriers work best within about 61 meters (200 feet) of the road.
Weather and air. Over long distances, air absorbs high frequencies faster than low ones, which is why a distant concert is all bass. Wind blowing from the source toward you, and temperature inversions at night, can bend sound back down and make a far-off source louder than the math predicts.
How far do you need to be to reach a safe level? #
If you can measure the level at a known distance, you can work out where the level lands. This table assumes a point source outdoors with nothing in the way, so treat it as the best case.
| Level at 1 m | Distance to reach 85 dB | Distance to reach 70 dB |
|---|---|---|
| 95 dB | about 3 m | about 18 m |
| 100 dB | about 6 m | about 32 m |
| 105 dB | about 11 m | about 56 m |
| 110 dB | about 18 m | about 100 m |
Two cautions. Distance helps with one machine in the open, not with a room full of noise or a crowd at a concert, where you’re inside the sound rather than away from it. And the level at your ear is what counts: a mower is loud at the handle because your ears are a meter from the engine, whatever the reading is at the far end of the yard.
Using distance to cut your exposure #
Distance is one of three levers, along with level and time. NIOSH’s hierarchy puts engineering controls first, then administrative controls like distance and scheduling, then hearing protection. Its engineering guidance also notes that even a 3 dB decrease in noise exposure can reduce a worker’s risk of hearing loss.
Practical versions of that idea:
- Stand further back while a machine runs. Start the shop vac or the compressor and step away instead of standing over it.
- Put the source away from corners. NIOSH notes that the worst placement for a noise source is in a corner, where sound reflects from three surfaces, and the best is away from walls.
- Move the listener, not just the source. A desk on the far side of the room from the HVAC return is a free few decibels.
- Combine distance with time. Under the NIOSH exposure guideline, every 3 dB you shave doubles how long the level is safe for. See how loud 85 dB is for the time table.
Distance stops helping once you’re in the reverberant part of a room or in a crowd. That’s when hearing protection becomes the right tool.
How to measure the drop yourself #
You don’t need calibrated equipment to check how a level falls off, because you’re measuring a difference rather than an absolute value. The same phone in the same orientation will track a 9 dB drop accurately even if its absolute readings are off by a few dB.
- Pick a steady source: a running appliance, a fan, a machine that doesn’t change.
- Measure at a known distance, say 1 meter, holding the phone at arm’s length with the microphone facing the source.
- Let it run 30 to 60 seconds and read the energy average rather than the jumping instantaneous number. What is Leq? explains why the average is the number to write down.
- Repeat at 2 m, 4 m and 8 m without changing phone, orientation or settings.
- Compare. A steady 6 dB per doubling outdoors means the source behaves like a point. Much less than that indoors means reflections are filling the room.
Sound Meter dB suits this job because it gives you Leq, min and max for each session and saves sessions of ten seconds or more on the phone, so you can line up four measurements afterwards instead of scribbling numbers. Each session can be exported as CSV. It reads the phone’s microphone with A-weighting and works offline, and like every phone meter it’s an estimate rather than a certified measurement, which is fine for comparing one spot to another. Our guide to how accurate phone sound meters are covers the size of the error.
Frequently asked questions #
Does sound really drop 6 dB every time you double the distance? #
For a single compact source outdoors with a clear path, yes, that’s close. For a highway, a long conveyor or any other line of sources, expect closer to 3 dB per doubling. Indoors, reflections flatten the curve and the level can stop dropping altogether after a few meters.
How far away from a concert speaker is safe? #
There’s no distance that makes a loud venue safe, because you’re hearing many speakers plus the room, not one point source. Moving back from the stacks and out of the direct beam usually helps by several decibels, but at concert levels earplugs matter more than position.
Why can I hear a highway from so far away? #
A highway acts as a line source, so it loses only about 3 dB per doubling of distance. Low-frequency sound also travels further because air absorbs it less, and at night temperature inversions can bend sound back toward the ground.
How much does a wall or a closed window reduce noise? #
It depends entirely on the construction, so measure rather than guess. Take a reading in the same spot with the window open and closed and compare the two averages. That difference is reliable even on an uncalibrated phone, since both readings come from the same microphone.
Is 6 dB quieter half as loud? #
Not quite. A 6 dB drop is a quarter of the sound energy, but perceived loudness roughly halves for every 10 dB. A 6 dB reduction sounds clearly quieter without sounding half as loud.