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.
