YOU LIVE in a relatively quiet residential area for years, then a truck stop is established about 1,000 feet from your home. The noise volume rises, sometimes to annoying levels, especially at night when other background noises are lower and atmospheric conditions favor the transmission of sound. What can you do to lower the noise level, short of moving out of the area?
Someone in the community has heard that trees and other forms of vegetation are known to have some effect as sound barriers. You investigate and find the proper authorities that can offer advice about alleviating the situation.
A solution is recommended. It calls for a 75-foot-wide belt of trees to be planted between the residential area and the truck stop, with the trees as close as possible to the truck stop. Suitable evergreens will be planted with a minimum spacing, and a soft ground cover of taller grasses or other vegetation will be maintained between the truck stop and the residences. A solid wall close to the noise source and high enough to screen the trucks could be a possible temporary measure, removable when the trees reach a height of 15 to 20 feet.
The noise level would be decreased substantially, except during occasional periods of unfavorable atmospheric conditions. Though still audible outdoors most of the time, it should not be objectionable to most people.
This is a hypothetical case, but research is proving the effectiveness of trees and shrubs as noise abaters—research prompted by the growing awareness that excessive noise is a form of environmental pollution. The average community noise level has risen fourfold in the past 20 years with jet aircraft, the heavy vehicular traffic, and domestic power equipment contributing to the problem. It is likely to go higher if it is not checked.
Noise is a subjective quantity and therefore difficult to measure. Sound, however, which is caused by a variation in air pressure, may be measured accurately and is usually expressed in decibels. A device often used to measure loudness is the decibel A-weighted scale (dBA) of the precision sound-level meter, which approximates human response to loudness. This scale gives a relatively high correlation with subjective loudness estimates of broadband noises, such as vehicular traffic.
A zero decibel level corresponds to the threshold of hearing. Most ordinary sounds we hear fall in the range of 25 decibels, as in a quiet library, to about 80 decibels at a noisy street corner. A difference of one decibel is the smallest change in loudness that can be easily detected by the ear. An increase of 10 decibels corresponds to approximately doubling the apparent loudness of a sound. In residential areas, a level of 55 to 60 dBA is desirable during daytime, and 50 to 57 dBA during the evening hours.
Although trees and other forms of vegetation have some effect on the transmission of sound, precise information on their use as noise screens is somewhat meager. A recent cooperative study made by the Forest Service and the University of Nebraska attempted to derive accurate useful information on such usage and to add to the knowledge about sound propagation. Actual plantings of trees and shrubs in the form of shelterbelts on the Nebraska plains were studied, as were screen-plantings of shrub-tree combinations in urban areas.
Traffic noises produced by trucks, cars, and city buses were recorded on magnetic tape to provide the sound source. These prerecorded sounds were played back through tree and shrub barriers, and the sound level was measured behind the barriers at varying distances. This procedure was repeated at nearby locations, but without the trees, to evaluate the effectiveness of trees in reducing the noise level.
Many of the shelterbelts had been planted during the Dust Bowl days of the late 1930’s and early 1940’s under the Prairie States Forestry Project directed by the Forest Service. They had been established to reduce loss of topsoil from wind erosion, and to provide man, animals, and crops with protection from the wind. Now they were being used for noise tests in rural areas where trucks account for much of the vehicular traffic.
The potential value of vegetation as noise abaters, as determined by the study, was deemed very good. Findings showed that reduction of sound values in the order of 5 to 10 decibels are not unusual for wide belts of tall, dense trees. Species did not appear to differ greatly in their ability to reduce noise levels, provided the deciduous varieties were in full leaf. However, evergreens are favored for year-round noise screening. A supplementary study of various surfaces indicated that, from a noise-reduction standpoint, surfaces covered with trees were the best.
Screening of urban residential property was effective with a single row of dense shrubs backed by a row of taller trees, totaling a depth of 20 feet. Screening for rural areas or freeways where truck traffic is heavy requires wider belts consisting of several rows of tall trees in dense plantings.
Distances of 100 feet or more between the noise source and the area to be protected were found desirable.
Recommendations arising from the study that may be applied to some current noise problems include:
—To reduce noise from high-speed car and truck traffic in rural areas, plant 65- to 100-foot-wide belts of trees and shrubs, with the edge of the belt within 50 to 80 feet of the center of the nearest traffic lane. Center trees should be at least 45 feet tall. Consult local nurserymen and landscape architects for specific varieties at a given locality.
—To reduce noise from moderate-speed car traffic in urban areas where the interaction of tires and roadway is the principal cause of noise, plant 20- to 50-foot-wide belts of trees and shrubs, with the edge of the belt from 20 to 50 feet from the center of the nearest traffic lane. Use shrubs 6 to 8 feet tall next to the traffic lane, with backup rows of trees 15 to 30 feet tall.
—For best results, trees and shrubs should be planted close to the noise source rather than to the area that needs protection.
—Where possible, use taller varieties of trees that have dense foliage and relatively uniform vertical foliage distribution, or combinations of shorter shrubs and taller trees to give this effect. Where the use of tall trees is restricted, use combinations of shorter shrubs and tall grass or similar soft ground cover in preference to paved, crushed rock, or gravel surfaces.
—Trees and shrubs should be planted as close together as practical to form a continuous, dense barrier. The spacing should conform to the established local practices for each species.
—Where year-round noise screening is desired, evergreens or deciduous varieties that retain their leaves throughout most of the year are recommended.
—The planted belt should be approximately twice as long as the distance from the noise source to the receiver. When used as a noise screen parallel to a roadway, it should extend equal distances along the roadway on both sides of the protected area.
Screening is most effective when trees and shrubs are combined with soft rather than hard surfaces, such as pavement or gravel. This can result in a 50 percent or more reduction in the apparent noise.
Certain natural and practical considerations limit the use of trees and shrubs as noise screens. Due to the physical nature of sound and the extreme sensitivity of the human ear, sound cannot be brought below the threshold of hearing, no matter how extensive the natural vegetation. Very thinly planted trees, or trees in poor condition as a result of neglect or of an unfavorable growth environment, offer little resistance to the passage of sound. Ground forms are frequently limiting, as when elevated highways are above the treetops so that there is relatively minor sound absorption from below. Also, a right-of-way or land use requirements may prevent an effective noise screening, especially where belts of 75- to 100-foot widths are needed.
Although the limitations are formidable, trees and shrubs can effectively reduce noise levels. However, they will not do so in all situations. A knowledge of out-of-door sound propagation, aided by experience, is necessary to make valid judgments on the use of trees and other plants as sound barriers.