A reader's good question was the impetus for this week's column. The problem she writes about is not only tough to control, but is becoming steadily worse and widely shared. If you live near the Beltway or any other major thoroughfare in this area, you too might have noticed how the decibel levels have increased in recent years.
A buffer of mixed plants can absorb and deflect sound waves. The mix of plants is important because different types of leaves reduce different types of noises. How much noise control they provide depends on the intensity, frequency and direction of the sound, and the location, height, width and density of the planting.
Mixed broadleaf plantings at least 25 feet thick and conifers 50 to 100 feet thick can drop noise levels by up to 10 decibels. For year-round noise reduction, plant a mix of evergreens such as arborvitaes, spruces, pines and hollies. To be effective sound barriers, these trees must have foliage that reaches to the ground.
Deciduous plants are also effective for noise abatement, but only when foliage is present. Like evergreens, these must also have foliage from the ground up to really do the job. Thickets of sassafras and paw paw have been found to be relatively effective for this purpose.
Include lawn or some other ground cover in shady areas. Turf grass or other low vegetation has a muffling effect on sound, compared with surface areas of bare soil or various paving materials, which are more likely to bounce sounds off their surfaces.
But noticing noise might be as much psychological as physical. When you don't see the source of the sound, there's an implied screening that makes it less apparent. So the use of plantings between you and the noise at any width is valuable for most home landscapes. That's also a good reason to install something to try to camouflage noise. Installing a fountain, music and screening might further contribute to a quieter yard.
Flowing water can be a wonderful foil for noise, especially if it has a cascading flow and makes a splashing sound. There are free-standing, tiered water features that offer some degree of noise screening.
Music in the garden -- classical, country, jazz or whatever you prefer -- can have a profoundly soothing effect on your surroundings, and make the world around you seem to fade away. Some weatherproof speakers specially designed to be used outdoors have a very good sound. I even have seen high-quality speakers in housings designed to look like ordinary garden rocks.
Despite these measures, however, noise control is most effective when a solid barrier is used. When the Montgomery County Office of Environmental Policy and Compliance checked decibel levels from behind a wide band of plants, highway noise didn't change significantly from summer to winter.
So does foliage account for more than psychological noise screening? The jury is out on the issue because so much depends on how far you are from the source of the sound, plant height differences and the presence of other noise barriers, such as soil, concrete or wood.
Consider the following example: Next time you're driving down the highway, note the surge of noise that fills the car when you crack open the window just a fraction of an inch. It doesn't take a large opening for noise to get through. In just this way, any opening in mixed plantings will allow lots of noise through. This illustrates the difficulty of protecting your landscape from undesirable sounds exclusively with plants.
The most effective measures you can take against noise with plants depends more on the configuration of the soil than the tree or shrub you're putting into it.
The best way to reduce noise is to establish a soil berm for your plantings: Large mounds of soil thickly planted, as described above, do a much better job of blocking sound than plants alone. Make your berm as high as possible, at least eight feet tall and 20 feet wide, and as long as your property line. A solid, well-planted berm can cut auto and truck noise by 70 to 80 percent and substantially reduce sounds from playgrounds, sporting activities or factories.
You can also effectively dampen noise for a small townhouse or postage-stamp-sized property with a fence or wall. Install a fence or wall with no openings that is tall and dense enough to shield outside clamor. It will work just like the barriers you see along the highway. These types of barriers are far more expensive than your typical garden-variety fencing because they have to be completely sealed.
If you can't get your local transportation department to do the job, get as close as you can to building that type of barrier. It must be solid, with no spaces to let sound through. A tongue-in-groove style of wooden fence constructed of unfinished 2-by-10-inch lumber built to be as tall as possible would serve this purpose. Architects will have more . This might be a special-order item for residential use, but you could have one built by a local custom fence company, carpenter or mason. Be sure to check local codes and permitting requirements for fences and walls before proceeding.
As increasing urbanization, particularly vehicular traffic, has added to the clamor in our environment, the field of noise-abatement engineering has grown rapidly. Look at any highway with adjacent residential neighborhoods and you see more miles of sound walls being erected every day to protect the ears of nearby residents. They really work as long as they're constructed along both sides of the road.
It is the government's responsibility to erect these tall, dense barriers to help abate noise in our communities, but property owners can take the smaller steps of installing dense plantings, berms, fences and walls.
Joel M. Lerner is president of Environmental Design in Capitol View Park, Md. E-mail or contact him through his Web site, www.gardenlerner.com.