Post #241: Wawa comes through.

Posted on April 15, 2019

I know what convenience store I’m going to use, henceforth, in the Town of Vienna.  I’m a Wawa convert. 

Read the back story in this post, and this one.

Briefly, thanks to an odd 1974 ruling by the Town of Vienna, there’s no masonry wall at the back of the Coldwell Banker property (corner of Maple and Nutley), where it adjoins the residential neighborhood.  This is now a problem for the family living behind that property, because the sleepy little Coldwell Banker training facility is going to be replaced by what may well be a very busy Wawa convenience store.

And, although they were under no legal obligation, Wawa has now proposed to put up a 6′ vinyl privacy fence, at the back of the lot.  Just to be a good neighbor.  And they’ll put it up in such as way as to spare the large silver maples currently on or near the lot line.

There’s still a question about details.  There are sound-blocking vinyl fences that are designed to stop noise transmission.  But these are fairly exotic.  And there’s standard vinyl fencing, which blocks light, but is only modestly better than tightly-constructed wooden fencing, in terms of blocking sound.

I certainly hope they go all the way, and put in a sound-blocking vinyl privacy fence.  Assuming it’s durable, that would be the most cost-effective way to provide the best protection to the neighbors, compared to either a concrete-panel fence or a block wall.

No matter what the details are, this is a really nice thing for them to do.  I certainly hope this sail through the Board of Architectural Review (BAR) review, this Thursday 4/18/2018, at 8 PM in Town Hall.   You can see the details at this link.

Addendum:  A brief note on fence/wall accoustics.  In an earlier post, I went through the mathematics of sound propagation.  Here I’m going to clarify sound transmission and sound diffraction , the ways in which sound gets from one side of a sound barrier to the other.

Sound diffraction depends only on wall height/geometry.  For a barrier like a 6′ cincerblock wall, most of what you hear is sound that diffracts (bends) around the top of it.  If you were in an enclosed space —  two rooms inside a building, say — you could barely hear someone talking on the other side of a solid cinderblock wall.  (Read these charts on Wikipedia.)  The cinderblock wall transmits very little sound in the frequency range of speech.  And in an enclosed space, the sound can’t diffract (bend) around the wall.  In an open space, by contrast, you will be able to hear someone talking on the other side of a 6′ block wall quite clearly.  The sound isn’t going through the wall.  Mostly it’s bending around the top of the wall.  It’s diffracting.

The amount of sound that diffracts over a wall is determined entirely by geometry (and the frequency of the sound in question).  To a close approximation, any 6′ wall will allow the same amount of sound to diffract over the top.

And it’s the geometry that makes it important for Wawa to build the wall, not the neighbors.  A 6′ wall is better at blocking sounds that occur close to the wall.  In effect, those sounds have to bend more to diffract over the top of the wall and so be heard.  A sound wall at the Wawa property line will be more effective at blocking sound than would a wall at the neighbor’s property line.

So, for diffracted noise, all 6′ walls or fences are equal, more or less.  And the closer to the noise source, the better.

But in addition to diffraction, walls or fences made of thinner materials can  transmit a significant amount of sound, compared to a block wall.  How much they transmit depends on the details, so it is very hard to get any numbers.  In general, construction with no air gaps provides vastly better resistance to sound transmission than any sort of construction that has (even tiny) gaps in the barrier.  So, e.g., board fences, even fairly tight board fences, generally perform poorly when it comes to blocking sound.

This manufacturer claims a Sound Transmission Class (STC) of 33 for their vinyl barrier.  At that level, from sound conduction alone, you would not be able to make out loud speech on the other side of the wall.   That’s not quite as good as cinderblock (STC of 42, per Wikipedia charts cited above), but it’s pretty good.

Mass-loaded vinyl products (e.g., this one) typically claim an STC of 32 or so, which is also quite good for a sound blocker.  These are sheets of thick, heavy vinyl manufactured to absorb sound.  They also claim to be particularly good about blocking low-frequency sounds.  In this instance, these are often sold as an aftermarket solution for reducing sound transmission through existing fencing.

But standard vinyl fencing — if there is such a thing — I have yet to be able to find much information on STC.  Most vinyl fence manufacturers do not stress sound control.  Certainteed’s Sim-Tek line has an STC of 26 — not perfect, but they describe it as 98%, on a scale where a concrete wall is 100%.  I have not been able to find STC ratings for vinyl fence by other manufacturers.