Sound Barriers-The use of Trees and Shrubs to Reduce Noise

Introduction

Noise is defined as unwanted sound. Noise levels can be reduced by;

  • reducing the noise at it’s source
  • increasing the distance between the source and the hearer
  • erecting a solid barrier
  • planting trees and shrubs

How is Sound Measured?

 Sound pressure levels are used to measure the intensity of sound and are described in 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 10 dB and 30dB is four times louder than 10dB, and so on.

Sound pressure levels of some common sounds. Threshold of Hearing    0 dB

Whisper                                   20 dB

Normal Speech                       48 dB Busy Dual Carriageway 75 dB Dog Barking   92 dB

Passing Train                        100 dB

Jet Aircraft                              110dB

Reducing Noise

 Solid barriers such as fences or mounds of earth are frequently used as sound barriers, but where space permits, trees and shrubs can be effective in reducing noise.

Alternatively trees and shrubs may be used in conjunction with solid barriers to achieve the best of all worlds.

Trees reduce perception of noise is 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 from study to study, however, it is generally agreed that a belt of trees and shrubs will significantly reduce noise levels.

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 Can Trees Reduce Noise

 The reduction of sound by vegetation is usually 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.

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.

There is a positive correlation between noise reduction and the width of the belt of trees; the wider the belt the greater the reduction.

The length the tree 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 tree belt placed relatively close to the source of the noise 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.

Conclusion

 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 sound barrier should be planted as close to the source of the noise 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.

References:

 APN6 Trees and Shrubs for Noise Control

Highway Traffic Noise Analysis and Abatement Policy and Guidance by U.S. Dept of Transport et al