How far from a road do I need to be to not hear the traffic?

I work in an office that's inside a forest, and about 800 m away from a busy motorway (2x2 lanes, 120 km/h speed limit), with noise barriers along the motorway. I go for a walk in the forest every day. At 800 m, the motorway is inaudible. At ~400 m, it starts to become audible if the wind is from the motorway toward me. At ~300 m, it's audible in any weather.

posted over 7 years ago

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Hobbes‭

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Adding to what @strongbad had said, you also have other complex issues to deal with as well... For example, if it is hot outside, you can be as little as 50-100 yards from the road (or less) and not hear anything from the road because the heat carries the sound waves upwards. Meteorology plays a significant factor in the science of sound. That's why a few battles during the Civil War were won by those armies. The enemy never heard their cannon fire.

Edit: administrators, please feel free to delete this answer. My SE app failed to load part of strongbad's answer, so I missed half of what they said. My apologies.

posted over 7 years ago

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PiGuy88‭

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It is not just the distance, but depends on the barriers. The barrier may be highway installations, landscapes, forest etc.

Refer the picture below. The place behind the barrier has less noise than the one on the top of the hill, even though it is further away in distance

Image source: nonoise.org

posted over 7 years ago

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spideringweb‭

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The answer is it depends. Highway noise is typically 70-80 dB at 15 m. To a first order approximation, sound waves obey the inverse square law where there is a 6 dB drop every doubling of the distance. This means a sound source that has a level of 72 dB at 15 m would have a level of 0 dB at 60 km. While absolute threshold is typically described as 0 dB, highway noise probably becomes imperceptible from the forest background noise at 30 dB, or about 3.8 km from the highway under ideal conditions.

The first order approximation is not very good. This book chapter seems to touch on the key factors meteorological conditions (e.g., temperature and humidity), terrain (e.g., hills and what is on the ground) and obstructions that will influence the propagation of the sound. Some of these things cause sound to get attenuated quicker while others can cause it to propagate further (e.g., a forest can act as a wave guide). If the vegetation is thick enough, it can provide substantial attenuation (10 dB per 61 m). Further, the inverse square law is for point sources and, as the linked chapter points out, highway noise is potentially better modeled as multiple point sources 9or maybe even a line source if there is lots of traffic). As I said, the answer depends.

posted over 7 years ago

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StrongBad‭

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This depends enormously on landscape, weather, and infrastructure.

I grew up in The Netherlands several hundred meter from a motorway, but with good noise barriers, we heard almost nothing. My friend lives less than 100 metre from a noise-barriered motorway and it's nearly inaudible. Now I live several km from a motorway in England, without noise barriers. Depending on the weather, I either hear nothing at all or it's so annoying I can not stand being in my garden, and hear it even with the windows closed. I've been considering to buy a sound pressure level meter to compare my impression to something measurable and try to relate it to my circumstances. The quietest weather tends to be dense fog, when traffic is either crawling or stationary, both of which are sufficiently quiet several kms away.

If there's a hill between you and the road, or if the road is underground, you will hear (nearly) nothing even very close. But if the road is in a valley and you're up on the slope, you might hear it many kms away. I've been 10 km from roads and still found some heavy powerful lorries going uphill disturbing, but that on a mostly open landscape with at most small sub-Arctic mountain birch forest type trees.

In some countries there are interactive noise maps, such as this one for England. I found the value of this quite limited, as my personal experience is worse than this map claims for my area.

It also depends on the road, of course. Some roads have traffic all night, others become completely quiet.

Personally, my strategy if I'm near a road, is to seek out a sound I find pleasant rather than annoying: specifically, a stream or river. Those may drown out the road sound such that I can pretend to be in the wilderness rather than right next to a road.

posted over 7 years ago

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gerrit‭

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