SOUND PROPAGATION THROUGH FORESTS AND TREE BELTS

SOUND PROPAGATION THROUGH FORESTS AND TREE
BELTS
K Attenborough Engineering and Innovation. The Open University. Milton Keynes MK7 6AA
S Taherzadeh Engineering and Innovation. The Open University. Milton Keynes MKT 6AA
1 INTRODUCTION
The potential of forests or narrow belts of trees alongside surface transport corridors for reducing
noise is often dismissed. Partly this is a consequence of conflicting experimental evidence. But also
it is the result of incomplete understanding of the various attenuation mechanisms involved and.
therefore. of how they could be exploited and used together in tree planting schemes. First
experimental evidence supporting the use of forests or tree belts for noise abatement is presented.
Subsequently the principal attenuation mechanisms and models for them are reviewed and
resulting predictions are compared with data. Finally numerical simulations showing the potential for
trafc noise reduction by narrow tree belts are outlined.
The United States Department of Agriculture National Agroforestry Center has published guidelines
for the lanting of trees and bushes for noise control based on extensive data collected in the
19705" . Another study in the USA found a reduction of 3 dB in the A-weighted Lee. from road trafc
due to propagation through 100 m of red pine forest compared with open grassland☜. in this study.
the edge of the forest was 10 m from the edge of the highway and the trees occupied a gradual
downward slope from the roadway extending about 325 rn in each direction along the highway from
the measurement site. A TRRL study in the UK found an extra 6 dB reduction in A-weighted L10
index of trafc noise propagating through 30 m of dense spruce compared with the same depth of
grassland☜. Also this study found that the effectiveness of the vegetation was greatest closest to the
road. Measurements made with broadband source 2m high and receiver height 1.5 m through 500m
of coniferous woodland have shown signicant extra attenuation compared with CONCAWE
predictions for propagation over acoustically-soft ground particularly in 63 Hz (3.2 dB). 125 Hz (9.7
dB), 2 kHz (21.7 dB) and 4 kHz (24.7 dB) octave bandss. A relative reduction of 5 dB in the A
weighted L☁o index was found after transmission through 10 m of vegetation. In an investigation of
the attenuation of sound by 35 different tree belts☝. a point source was placed in front of the tree
belts and sound pressure levels inside the tree belts were measured at different positions.
Attenuation was found to depend on the width. height. length and density of tree belts. Large shrubs
and densely populated tree belts were found to give more than 6 dBA attenuation. medium size
shrubs and tree belts attenuated the sound by between 3 and 6 dBA and sparsely distributed tree
belts and shrubs attenuated the sound by less than 3 dBA. The depth of vegetation was found to be
the most important factor; the greater the depth. the greater the pathway of sound through the
vegetation resulting in higher sound absorption and diffusion. The shrubs were considered to be the
most effective in reducing noise due to scattering from dense foliage and branches at lower source
receiver heights. It was concluded that tree belts and shrubs should be planted together to provide
best attenuation performance.
The propagation of sound through forests and tree belts involves ☁soft' ground effect due to
decaying leaf litter, reverberant scattering out of the direct source-to-receiver path by trunks and
branches, absorption by tree bark. loss of coherence between ground-reected and direct sound
due to scattering. acoustically-induced vibrations of leaves and visco-thermal scattering by foliage.
On the basis of calculations made in this paper and elsewhere. acoustically-induced leaf vibration
and bark absorption do not contribute much to the overall attenuation whereas ground effect. the
inuence on ground effect of scattering by trunks and branches and attenuation by foliage appear to
be relatively important. Ways of modelling these more important contributions are considered in the
next section.