Overstory #60 - Trees as Noise Buffers

Editor's Note

This edition of The Overstory is adapted with permission from the USDA National Agroforestry Center's (NAC) article, "Leaf the Noise Out" (Inside Agroforestry Spring 1998), and other materials provided by the NAC. It introduces some guidelines for planting trees and shrubs to reduce noise.

Introduction

Noise can cause anxiety, tension, or even illness. Prolonged exposure to high levels of noise can cause hearing loss. Today we regard noise as a form of environmental pollution, and in some circles noise is considered an international health concern.

Trees to the rescue? Planting "noise buffers" composed of trees and shrubs can reduce noise five to ten decibels (reduces noise approximately 50% to the human ear). To achieve this effect, the species and the planting design must be chosen carefully.

Generalized Recommendations for Noise Buffers

Generalized recommendations to reduce noise with rows of trees and shrubs include:

1. For best results, plant the noise buffer close to the noise source (rather than close to the area to be protected).
2. Plant trees/shrubs as close together as the species will allow and not be overly inhibited.
3. When possible use plants with dense foliage. A diversity tree species, with a range of foliage shapes and sizes within the noise buffer may also improve noise reduction.
4. Foliage of the plants should persist from the ground up. A combination of shrubs and trees may be necessary to achieve this effect.
5. Evergreen varieties that retain their leaves will give better year-round protection.
6. When possible use taller plants. Where the use of tall trees is restricted, use combinations of shorter shrubs and tall grass or similar soft ground cover as opposed to harder paved surfaces.

Example: Designing Buffers Against Neighboring Engine Noise

To reduce neighboring noise from power mowers, generators, or other small engines, plant one to two continuous rows of heavy dense shrubs, having maximum density near ground level, and one to two continuous rows of medium height dense trees. Placement of the buffer is usually governed by practical considerations--normally it is placed near the lot boundary, as close to the source of noise as possible. If the noise problem is severe, the noise buffer may also include rock or masonry walls, or solid wooden fences at least 1.5 m (4.5 ft) high, with the trees and shrubs planted on the near side of these barriers.

Example: Designing Buffers Against Road or Highway Noise

Persistent and troublesome noise pollution often comes from roads and highways. In fact, this problem was first recorded in the first century A.D., when Julius Caesar banned chariot traffic from the streets of Rome at night because citizens couldn't sleep!

To reduce noise of moderate traffic in communities:

• plant belts of trees 7-17 m (20-50 ft) wide along roadsides
• plant the nearest edge of the belt within 7-17 m (20-50 ft) of the center of the nearest traffic lane
• use 2-3 m (6-8 ft) shrubs next to the road and back up tree rows a minimum of 3-7 m (15-20 ft) tall when mature
• the length of the tree belt should be twice as long as the distance from the road to the recipient of the noise
• the buffer should also extend equal distance in both directions parallel to the road

To reduce heavy vehicle noise in suburban or rural areas:

• plant belts of trees 20-35 m (65-100 ft) wide along roadsides
• plant the nearest edge of the belt within 20-25 m (60-80 ft) of the center of the nearest traffic lane
• use 2-3 m (6-8 ft) foot shrubs next to the road, and back-up tree rows a minimum of 15 m (45 ft) tall at the center row
• the length of the tree belt should be twice as long as the distance from the road to the recipient of the noise
• the buffer should also extend equal distance in both directions parallel to the road

When planning roadside developments or when locating schools or residences adjacent to roads, natural terrain such as hills, ridges, and depressed highways can help to serve as noise screens. Existing trees, shrubs, and grass should be left undisturbed when possible, rather than replacing them with harder surfaces detrimental to noise control. Noise may also be further ameliorated by constructing land-forms, earthen dikes, masonry walls, or solid wooden fences, and planting the trees and shrubs on the near side of these barriers.

Multiple uses

Noise buffers can also be planned to provide multiple products or uses. For example, a noise buffer oriented properly may also serve as a windbreak. When they contain native plants or habitat, noise buffers can provide wildlife corridors. Species can also be selected to provide useful products such as fuelwood, fruits, or nuts.

References

Special thanks to the staff of the USDA National Agroforestry Center for providing the material for this edition of The Overstory. This Overstory was adapted from the article "Leaf the Noise Out," which first appeared in the Spring 1998 edition of the NAC's quarterly newsletter, Inside Agroforestry.

Cook, D.I. and D.F. Van Haverbeke. 1977. Suburban Noise Control with Plant Materials and Solid Barriers: A Report of a Study Conducted by the University of Nebraska and the U.S. Forest Service. Research Bulletin EM 100. Rocky Mtn. Forest and Range Experiment Station, USDA Forest Service, University of Nebraska, Lincoln, NE, USA.

Cook, D.I. and D.F. Van Haverbeke. 1974. Tree-Covered Land-Forms for Noise Control. Research Bulletin 263. Rocky Mtn. Forest and Range Experiment Station, USDA Forest Service, University of Nebraska, Lincoln, NE, USA.

Cook, D.I. and D.F. Van Haverbeke. 1972. Trees, shrubs, and land-forms for noise control. Journal of Soil and Water Conservation, November-December 1972, Ankeny, Iowa, USA.

Related Editions to The Overstory

• The Overstory #73--Buffers, Common-Sense Conservation
• The Overstory #46--Human Health and Agroecosystems
• The Overstory #38--Live Fences
• The Overstory #32--Multipurpose Windbreaks
• The Overstory #24--Sustaining Your Physical Health