Q: I am putting a ground-level deck on a client’s home that’s located next to a busy road. Besides plantings, what is the best strategy for reducing the road noise on the deck?
A: Dr. Bonnie Schnitta, president of Sound Sense, a company that specializes in acoustic engineering and consulting, responds: It takes roughly 200 feet of dense foliage to equal the sound reduction of an acoustic barrier that can be as thin as 1/8 inch. Plants don’t stop sound, but they can help to diffuse it and prevent a fence or wall from becoming a reflective surface—particularly important on the client’s side of the fence, because any noise that goes over the fence will hit the house, bounce back, and then reflect off the fence, amplifying the sound. So while plants themselves aren’t great at reducing noise, they can help to make your solution work.
When you’re building a barrier to reduce noise, whatever is built needs to have the right transmission loss in the frequencies (such as car exhaust noise) that are disturbing. For an outdoor wall or fence, I recommend a minimum sound transmission class (STC) of 30. (STC is the rating of an object’s ability to block sound.) The next goal is to make certain that the barrier provides a level of reduction that is perceivable to the client.
A barrier or fence that’s intended to reduce outdoor noise should be placed as close to the noise source (the road) or to the receiver (the client on the deck) as possible. Placing the fence close to the road would be preferable, but setback requirements might reduce the effectiveness of a roadside barrier. Having a barrier in both locations would provide even greater noise reduction, whereas a fence placed halfway between the deck and the road would be far less effective, if at all.
Taller fences provide greater noise reduction, but if you are building a fence next to the road, zoning requirements or visibility issues may restrict its height, making it less effective. To provide perceivable noise reduction, an acoustic fence placed next to the road would need to be at least 8 feet tall.
A wooden fence does not have sufficient STC by itself, so it should be lined with an acoustic barrier having an STC of at least 30. Typically, we use a mass-loaded vinyl barrier, such as the SoundSense NoiseOut2 or the equivalent. (If the barrier is not UV-resistant, be sure to keep direct sunlight off it.) A mass-loaded vinyl barrier works by providing a dense, resilient layer as well as a surface free of voids to block the sound. This approach lets the client and contractor choose the fence style they like and then line the fence with a vinyl barrier (usually black). In order to have sufficient transmission loss in the lower frequency of car exhaust, use a mass-loaded vinyl with a weight of 1 1/2 pounds per square foot or greater. Most loaded vinyl products are lighter, so be sure to choose the heavier product (for more about this material, see “Retrofit Soundproofing,” Jan/15).
If an 8-foot fence is not possible, then I’d locate the barrier beside the deck or on the deck. For a fence that doesn’t block the light, use Plexiglas, laminated glass, or clear mass-loaded vinyl intended for outdoor applications, with an STC rating of 27 or greater. One strategy we’ve used is building a lattice planter with a clear acoustic material attached. The vines that grow in the planter and on the lattice prevent the acoustic barrier from becoming too reflective, while the transparent material allows dappled light through.
Finally, when a client voices concerns about noise in a finished product, we recommend that the contractor hire an acoustic engineer to review the situation and recommend a solution. An expert’s opinion can be a reality check and adjust the client’s expectations for whatever solution they decide to pursue.