Sound Barriers : The Use of Trees and Shrubs to Reduce Noise
Noise is defined as unwanted sound.
Noise reduction can he achieved by various methods, including:
- Reducing the noise at its source
- Increasing the distance between the source and the hearer
- Erecting a solid barrier
- Planting trees and shrubs
What is Sound?
Sound is created by vibrations in the air which cause variations in air pressure. The result is waves which radiate from the source like the ripples on a pond caused by a stone.
How is Sound Measured?
Sound pressure levels are used to measure the intensity of sound and are described in terms of decibels. The decibel (dB) is a logarithmic unit which expresses the ratio of the sound pressure level being measured to a standard reference level.
A zero decibel level corresponds to the threshold of human hearing. An increase of 1 decibel is roughly equivalent to the smallest difference in loudness perceptible to the human ear and an increase of 10 decibels roughly corresponds to a doubling in the apparent loudness of a sound. Thus 20dB is twice as loud as 10dB, and 30dB is four times louder than 10dB, and so on.
The decibel levels of some common sounds.
- Threshold of Hearing 0dB
- Whisper 20dB
- Normal Speech 48dB
- Busy Dual Carriageway 75dB
- Dog Barking 92dB
- Passing Train 100dB
- Jet Aircraft 110db
Solid barriers such as fences or mounds of earth are frequently used as sound barriers, but where space permits, trees and shrubs can also be effective in reducing noise.
Alternatively, trees and shrubs may be used in conjunction with solid barriers to achieve the best of all scenarios.
Sound is greatest nearest to the source and diminishes with distance.
Sound travels differently over various kinds of surfaces. Hard surfaces reflect virtually all incident sound whereas soft surfaces absorb sound.
Trees reduce the perception of noise by creating a visual barrier between the source and the hearer. It has been suggested that people are less conscious of noise if they cannot see the source.
Trees and shrubs produce a masking effect through the rustling of leaves, the movement of branches in the wind, the sounds of birds, insects and other animals.
Published results on the effectiveness of tree and shrub barriers vary enormously, however, a review by Huddurt (1990) shows that in some instances noise can he reduced by 6dB over a distance of 30 metres where planting is particularly dense. Leonard and Parr (1970) and Reethof (1973) found that a dense belt of trees and shrubs between 15-30m wide could reduce sound levels by as much as 6-8dB. Cook and Van Haverheke (1972) also found reductions in noise level of 5-10dB for belts of trees between 15-30m wide.
A dense belt of trees and shrubs should provide a reduction in noise of several decibels although reductions will be significantly less than a purpose-built noise barrier of the same height and length.
How Trees Reduce Noise
The reduction of sound by vegetation is commonly attributed to the processes of reflection, deflection and absorption.
Foliage appears to be the most efficient part of a tree for scattering sound and it seems that large leaves are more effective than small leaves. Low shrubs and/or hedges along the edge of a group of trees can improve sound reduction, particularly those on the side nearest the sound source.
Noise reduction tends to increase with tree height up to 10-12m after which it tends to decrease. This is probably a result of lower branches dying and allowing sound to travel more easily.
Noise reduction is correlated with the width of a belt of trees, i.e. the wider it is, the greater the noise reduction.
The length that a tree and shrub belt extends will influence its effectiveness in noise reduction. Actual prescriptions are difficult, however, as they will depend on the dimensions of the noise source.
A screen placed relatively close to a noise source is more effective than one placed close to the area to be protected. However, at midway between the source and receiver, noise reduction is least.
Large-leaved deciduous species may be more effective at reducing noise during spring and summer but evergreens will provide better year-round attenuation.
There are several factors to be considered before deciding to create a tree and shrub barrier against noise.
Noise is more effectively reduced by completely screening the source from view. A noise barrier should be planted as close to the noise source as possible. Wide belts of high-density trees and shrubs are required to achieve significant noise reductions. Effectiveness of noise reduction is closely related to the density of stems, branches and leaves. For all year-round noise reduction use broadleaved evergreens or a combination of conifers and broadleaved evergreen species. Soft ground is an efficient noise absorber. Cultivating ground before planting and the addition of well-rotted organic matter to the soil surface may also help to reduce noise whilst vegetation becomes established.
APN6 Trees and Shrubs for Noise Control
HIGHWAY TRAFFIC NOISE ANALYSIS AND ABATEMENT POLICY AND GUIDANCE by the Dept of Transportation et al