Post #451: Noise abatement measures for the MAC streetscape

Posted on November 9, 2019

I sometimes feel that I’m the only one who cares about the noise level adjacent to Maple.  But, hey, I get to pick the topic.  And today’s topic is noise abatement measures in the urban streetscape.  Not because I think the Town of Vienna is going to implement any of this, but because I’d like to know more.  And I think the Town should factor the high level of traffic noise into its decisions moving forward.

Why am I doing this?

Here’s how I view the current situation.  Under MAC, we’ve ended up with   government-mandated outdoor seating where no private entity* in Vienna would willingly place it — directly adjacent to a noisy arterial highway.  Well, that’s where our “gathering space” is at Chick-fil-a-car-wash, Marco Polo/Vienna Market, and Tequila Grande/444 Maple West.

* I’m dead serious about the private entity/market test part of that.  Just think about it for a second.  It’s not just that we don’t observe anybody doing that currently. In addition, suppose somebody came to you and said: “I have a great idea for a business.  I’m going to put an outdoor cafe with all the seating 20′ or less from Maple Avenue.”  Would you be willing to invest in that business?  I wouldn’t.

That’s not by design, necessarily.  I think the Town was focused on how the MAC streetscape would look, and I don’t think the Town realized quite how loud Maple Avenue traffic is.  Maybe the developers realized that the streetside space was unpleasant due to the noise level, and were OK with giving it to the public.  But I’m pretty sure I was the first person who ever took a decibel meter and measured the noise level there, as summarized in the table above.   (You can see the entire analysis in this posting.)

But now, in my opinion, the Town can’t start a conversation about making noise abatement measures part of redevelopment, because the folks involved can’t bring themselves to say “oops”.  Or the just don’t believe me.  Or they’ve never tried to hold a conversation, walking down Maple at rush hour.

Which is why I keep harping on the noise issue because it may be partly fixable, but it ain’t going to fix itself. I mean, even Herndon has the good sense to mention noise abatement in its streetscape guidelines (.pdf).  Herndon can recognize that, but we can’t?  What is this world coming to.

In short, I think this is a mistake that might be partly fixable.  But I see no move to fix it.  Instead, the Town seems intent on creating more of this pretty-but-unusable space, and ignoring the noise issue.

I wish that somebody in authority would stop and ask the simple question “Is there anything we can do about it?”. 

And since they won’t, I guess I will.  At the very least, I’m going to take something that seems feasible and plausible, and see how much noise reduction we could get.

The sound wall calculation redux

Scientific analysis of traffic noise recognizes several independent components, including tire noise, wind noise, engine noise, exhaust noise, and brake noise.*

* Sadly, science ignores a##hole-with-stereo-cranked-and-windows-down noise.

Purely by ear, I think that tire noise is the largest single component of Maple Avenue traffic noise.  That’s a guess, because I have no way to demonstrate that for typical Maple Avenue traffic conditions.  But, typically, tire noise is known to be the largest component of road noise at speeds above 50 KPH (~ 30 MPH).  While 30 MPH is often more of a hope than a reality on Maple, let me me start with tire noise.

Because tire noise occurs at pavement level, it’s plausible that a continuous low sound wall (e.g., a concrete planter box) could block much of that for a person sitting nearby.  Working in favor of that is that noise follows an inverse-square law of sorts.  Noise sources close to you sound much louder than noise sources far away.  That makes it much more critical to block the noise of cars directly adjacent to the sidewalk.  And that’s what a concrete planter, directly adjacent to the road, will do best.

Some time ago I looked up the math showing how sound drops off over a distance, and how well sound walls work at blocking sound.  So I’m now going to redo that sound wall calculation for a sound source at ground level, and a person’s ear 4′ off the ground (typical for a seated person).

This turns out to be a surprisingly tricky calculation.  As explained on the page referenced just above, sound power follows an inverse-square law.  Decibels are a log scale.  And your perception of loudness follows a power law — every 10 decibel increase is perceived as “twice as loud”.

Maple has four lanes of traffic, at different distances from the listener.  To cut to the chase, the entire calculation has to be done in the aggregate sound power hitting the ear, from those four lanes.  Then converted to decibels.  And then converted to the perceived difference in sound level.

I don’t think it’s worth explaining the full detail, as most people perceive math about the same way they perceive, say, alchemy or witchcraft.  Surely someone would have to check my calculation before anyone acted on it.  And I have only modeled the sound traveling directly from road surface to ear, not any reflected sound.  And recall that this is only for tire noise.  And I only did this for one frequency (500 Hertz), not for a range of frequencies.

But I think the results have a reasonable degree of face validity.

First, the figure in yellow shows just how important the car directly adjacent to the sidewalk is.  From the perspective of someone seated 15′ from the Maple Avenue curb, if the total sound power of that one car is set to 1.0, then the full ensemble of four cars across four lanes only has a sound power of 1.62.  The upshot is that, by far, the most important thing you can do is to take care of the noise from the car that’s in the curb lane.

Next, the middle set of columns, in decibels, comes from the complex calculation that I used in my sound wall post, cited above.  That’s from, and can be found on this web page.

But the bottom line is in orange.  Even a low concrete planter (2′) would result in a roughly 5 decibel reduction in tire noise reaching the seated person.  That would translate to a 30% reduction in perceived tire noise.  That’s just about the threshold at which people would typically notice that it’s not quite so loud (and that’s the reason that charts like the one at the top of this page are typically shown in 5 decibel increments.)

So even a 2′ planter would produce a perceptible reduction in tire noise. And, of course, the taller the planter-cum-sound-wall, the greater the perceived reduction in tire noise.  A four foot tall planter would cut perceived tire noise roughly in half.

Of potential interest, some European cities have gone so far as to install glass sound barriers at the street side.  Presumably, topping a (cheap) 2′ concrete planter with an (expensive) glass sound barrier would get you a level of perceived tire noise reduction somewhere between 30% and 55%.


My main point is not that the Town should require planters.  My point is that the Town could address this noise issue as part of the rewrite of the MAC and commercial codes.  Anyone who spends any time walking along Maple on a Saturday afternoon, and tries to carry on a conversation, knows that it’s noisy.  But, in my opinion, because they can’t say “oops” regarding all the Maple-adjacent “gathering space”, they are ignoring it.  And that’s a shame.

As demonstrated above, curbside planters of sufficient height will result in some noise reduction.  Clearly, if you were doing that, you’d angle the road-facing side of the planter back slightly, so that the noise was reflected up, away from the opposite side of the street.

But planters are not the only possible noise reduction strategy.  Maybe if you pile on enough noise abatement strategies, you can make this noise problem go away.  And so end up with “gathering space” adjacent to Maple that won’t repel people due to the noise level.

For starters, we are responsible for paving Maple (using VDOT funds), and we could at least look at using a quiet pavement for the next repaving.  The choice of pavement can have a significant impact on urban road noise.

European roadways and streets sometimes use glass noise barriers.  Large ones, though, tend to kill a lot of birds.  And that gives a very urbanized look to the streetscape.  And requires maintenance.  But they are a product that is currently available.

Fountains and other water features produce white noise that masks the traffic noise.  But those don’t solve the problem of trying to talk to people in the “gathering space”.  They just add a layer of less-unpleasant noise to the existing traffic noise.  (Try holding a conversation in front of the fountain on the Town Green, during rush hour, and you’ll immediately get it.)

Hilariously enough, the number one technique that is recommended for streetside noise abatement is putting distance between the street and the listener.  And the number one way to do that is  …. wait for it … by putting your parking lots next to the street.  (“placing nonresidential land uses such as parking lots, maintenance facilities, and utility areas between the source and the receiver;”)  Which, of course, is forbidden under MAC.

Surely there are other noise abatement techniques that I haven’t yet run across.  So my point is not that this is an exhaustive list, but there are things that the Town could do to reduce this problem.  If they are wedded to the idea of “gathering space” directly adjacent to Maple, they need to address the noise issue head-on.