Post #453: Noise abatement, addendum: Plantings don’t reduce noise.

Posted on November 10, 2019

Today I have the joy of doing a posting that nobody is going to believe.  To the best of my understanding, the following statements are true.

With one possible exception, plantings of any sort, of a scale that can be included in the MAC streetscape, will have no material effect on traffic noise as heard at the sidewalk.  There are psychological advantages, and certainly aesthetic advantages, and possibly some modest health advantages to greenery adjacent to a city street.  But noise reduction is not one of them.

As far as I can tell, this is something that most experts on noise abatement agree with.  And that every expert on landscaping disagrees with.

The one exception is the type of green wall where the entire wall is covered with a sound-absorbing medium, on which plants may grow.  And even there, the noise reduction is due to the sound-absorbing growing medium, and only trivially to the plants.

It takes a forest

First, to understand this topic, you need to know one factoid about decibels:  It takes a 5-decibel reduction in sound intensity before the average person can perceive any difference in the sound level.  Anything less than that, and the average person would not perceive one situation as quieter than another.

Second, if you see a claim that plants reduce noise level, you have to nail down the exact circumstances.  You have to ignore the claims that are just made in general, find the literature where somebody actually measured the noise level, and look at what they were measuring.  In the context of trees and hedges, noise reduction will typically refer to belts of trees that are hundreds of feet thick, or hedges that are tens of feet thick.  None of that applies to what can be done along Maple Avenue.

Here’s an example from the Commonwealth of Virginia (.pdf), citing the scholarly literature.  They cite three studies finding noise reductions of 5 to 10 decibels for belts of trees 15 to 30 meters (50 to 100 feet) thick.  But as part of that study (pages iii and iv), when engineers from Virginia Tech actually measured road noise reduction from existing stands of pines, they found that 20 meters (65 feet) of standing pine forest provided absolutely no reduction in noise levels.

So, can trees reduce noise?  Sure, maybe, if we’re talking about a 100′ deep stand of trees.  And the right kind of trees.  And even then, if you read on in that document, you’ll find that the noise reduction is largely due to sound-absorbing quality of the loose soil under the trees, and only secondarily to the trees themselves.  A stand of trees, planted on an otherwise paved surface, would have little effect at reducing noise.

(I should point out that once you’re 100 feet off Maple, the traffic noise isn’t an issue anyway, due to the inverse-square-law for sound propagation.  For example, the open dining at Tequila Grande is 100′ from Maple, and from there, the Maple traffic noise is audible, but purely as some modest background noise.)

The same goes for hedges.  Here’s a scholarly study (.pdf) that found an average 2 decibel reduction, at standing height, from the hedges they studied.  That would be too small to be perceived as quieter.  And the median (average) hedge studied was about 6 feet thick and 9 feet tall.

And bamboo?  Sure, closely-grown bamboo can be an effective noise barrier (.pdf), when grown in stands that are 6 meters (20 feet) thick.

So you get the drift, I think.  There are plenty of reasonably well-done studies out there, and they all indicate that you need a large stand of greenery to have a material reduction in the level of transmitted sound.  And, sure, I bet if you look long enough, you might find some outlier measurement that says otherwise.  But when you see such clear agreement, across so many sources, based on such a simple and flat-footed experimental method — chances are that this is a real finding.  A forest can attenuate noise pretty well.  Fair bet that the Hogwarts Triwizard hedge knocked a few decibels off transmitted sound.  But a handful of widely-spaced shade trees, or a planter with some flowers — not so.

What does work, then, in a narrow streetscape?

First, greenery has a great psychological benefit.  Roughly 90 percent of the population believes that greenery makes a space quieter.  As long as you don’t have to do anything functional (e.g., hold a conversation), greenery will make most people think that a space is quieter, relative to a space without greenery.  (This is my reference for that .pdf, where they literally used an EEG to track individuals’ responses to scenes .  I think this was the main study behind this more comprehensive literature review .pdf, which says the same thing).

So, greenery makes you feel better about the noise level.  Even if it does not, empirically, reduce the noise level.

Weirdly, in the streetscape, trees actually make the noise levels slightly worse.  (Although the effect is too little to be perceived, I think.)  For the range of frequencies where most traffic noise lies, the leaf canopy, on net, actually reflects noise town toward the street, noise that would otherwise simply radiate upward.  Again, this effect is too slight to be perceived by ear.

If I had to give one citation explaining green walls and roofs, I would look at this one (.pdf).  I thought it was pretty easy to understand, and they made it clear at the sound-absorbing substrate was what drove most of the sound reduction from green walls and such (page 10).  And they demonstrate that even a low curbside noise barrier — similar to my 3′ planter (Post #451) — could significantly reduce street noise, if covered with a plant-growing sound-absorbing substrate (page 12 ff).  They show a 9 decibel sound reduction behind the barrier, not too different from what the 7 decibel reduction I calculated for a concrete barrier of the same size (Post #451).

The key point with this last bit is that you now have two independent sources — one amateur (me) and one pro (them) — saying that low (3′) curbside barriers would help materially with the noise on Maple Avenue.  If the Town can ever acknowledge that the noise is an issue, then there’s at least one potential abatement measure just waiting to be used.  It shouldn’t be all that difficult to arrange for a real-world test, or hire an expert to tell you whether or not this might work.