Green Scene

Green Scene: Ways to dampen traffic noise on your property

By Joel M. Lerner
Special to The Washington Post
Saturday, May 15, 2010

If it seems like your neighborhood has become noisier in recent years, that's because it probably has. With increased suburban development, the Capital Beltway and other major arteries such as Interstate 270 and Route 50 in Maryland, Interstate 66 in Virginia have been widened to accommodate more traffic. Add to that the construction of the Fairfax County Parkway and one of the latest highway projects, the Intercounty Connector between Montgomery and Prince George's counties, and road expansion has touched most of the Washington area.

Today, massive sound barrier walls accompany highway construction to muffle road noise and reduce its effect on people living nearby. The Federal Highway Administration acknowledges that outside noise can affect our physical and psychological well-being. They are working on a healthy acoustical balance between residential and commercial development to maintain quiet neighborhoods in growing metropolitan areas.

But if you live near one of these noisy highways, what can you do on your side of the wall?

Begin with a barrier on your property. The most important guideline is that the fence or wall must have no openings. A continuous mound of soil eight to 10 feet high would serve the purpose. It would have to extend as far as possible to diffuse the noise. Any opening in walls, fences or soil barriers will let sound seep through.

To understand this, consider the following example:

While driving along the highway, open the car window slightly and notice the roar. By the same token, it doesn't take much of an opening on your property for the noise of trucks, cars or motorcycles to penetrate the space.

I have not found that plants qualify as complete noise-screening elements. Sometimes in woodland settings, noise -- even talking -- seems amplified. Any openings between mixed plantings will allow sound to penetrate. Plants offer only implied screening by camouflaging the source of the sound.

An officer from the Montgomery County Office of Environmental Policy and Compliance once demonstrated for me the difficulty of protecting areas from "noise pollution" by using plants. He checked decibel levels from behind a wide band of flora and at a clearing in the vegetation and found that highway noise did not change significantly between the two locations. Many considerations affect noise transmission, including the distance from the origin of the noise, the types and heights of different noise, grade changes and the presence of other structures.

I have read several texts that support the theory that plantings have impacts on noise, but they are simply not practical fixes for most residential properties. For example, the Federal Highway Administration suggests that an area at least 100 feet wide covered with mixed plants of every foliage size and texture can absorb and deflect sound waves. The mix of plants is important, because different types of leaves reduce noises of different frequencies.

Unbroken barriers work best to reduce noise, but to acquire some semblance of year-round noise reduction from plants, use mixes of evergreens, including arborvitaes, spruces, pines and hollies. How well they control noise depends on the intensity, frequency and direction of the sound as well as the health, height and density of the planting. To be effective sound barriers, flora must have foliage that reaches the ground. And include lawn or shade-tolerant ground cover in areas with low sunlight. Turf or low-growing vegetation has a muffling effect, compared with bare soil or paving materials.

Noticing noise can be as much psychological as physical. Try using the soothing sound of fountains, streams, waterfalls or other man-made water features, or outdoor speakers for music. Music in the garden can have a profound effect on the ambiance of any outdoor space. Combinations work well, and they can mask and take attention away from highways, airplanes, trains or other noise sources. This implied screening could turn what was annoying background sound into white noise.

The most effective measures you can take against noise using plants depend more on the configuration of the soil than the tree or shrub you're planting. Establish a soil berm for your flora. Large mounds of soil, thickly planted as described above, enhance sound shielding. Make your berm as high as possible, at least eight feet tall and 20 feet wide, and at least as long as your property line. According to one well-respected text, "Plants in the Landscape," by Philip L. Carpenter, Theodore D. Walker and Frederick O. Lanphear, a well-planted solid berm can cut auto and truck noise by 70 to 80 percent and substantially reduce it from other sources, such as playgrounds, sporting activities and industry.

You can reduce noise for small townhouses or tiny properties with tall walls or fences that have no openings in them. It'll work just like the barriers you see along the highway. These types of barriers are far more expensive than typical fencing, because they have to be completely sealed. A tongue-and-groove wooden fence constructed of unfinished two-by-10-inch lumber built as tall as allowed would serve this purpose. Architects will have more data. This could be a custom-order item, and there are many local fence companies, carpenters or masons to choose from. Be sure to check local codes and permitting requirements for fences and walls before proceeding.

Joel M. Lerner is president of Environmental Design in Capitol View Park, Md.

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