Introduction to Acoustics

130 Part A Propagation of Sound
Part A 4.7
tenuation with frequency and linear fits to the foliage
attenuation.
Often the insertion loss of tree belts alongside highways
is considered relative to that over open grassland.
An unfortunate consequence of the lower-frequency
ground effect observed in mature tree stands is that the
low-frequency destructive interference resulting from
the relatively soft ground between the trees is associated
with a constructive interference maximum at
important frequencies (near 1 kHz) for traffic noise. Consequently
many narrow tree belts alongside roads do
not offer much additional attenuation of traffic noise
compared with the same distances over open grassland.
A Danish study found relative attenuation of 3 dB in
the A-weighted Leq due to traffic noise for tree belts
15–41 m wide [4.86]. Data obtained in the UK [4.87]
indicates a maximum reduction of the A-weighted L10
level due to traffic noise of 6 dB through 30 m of dense
spruce compared with the same depth of grassland. This
study also found that the effectiveness of the vegetation
was greatest closest to the road. A relative reduction of
5 dB in the A-weighted L10 level was found after 10 m of
vegetation. For a narrow tree belt to be effective against
traffic noise it is important that (a) the ground effect
is similar to that for grassland, (b) there is substantial
reduction of coherence between ground-reflected and direct
sound at frequencies of 1 kHz and above, and that
the attenuation through scattering is significant.
If the belt is sufficiently wide then the resulting
greater extent of the ground-effect dip can compensate
for its low frequency. Through 100 m of red pine forest,
Heisler et al. [4.88] have found 8 dB reduction in the
A-weighted Leq due to road traffic compared with open
grassland. 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 m in
each direction along the highway from the study site.
Compared with open grassland Huisman [4.89] has predicted
an extra 10 dB(A) attenuation of road traffic noise
through 100 m of pine forest. He has remarked also that,
whereas downward-refracting conditions lead to higher
sound levels over grassland, the levels in woodland are
comparatively unaffected. This suggests that extra attenuation
obtained through the use of trees should be
relatively robust to changing meteorology.
Defrance et al. [4.90] have compared results from
both numerical calculations and outdoor measurements
for different meteorological situations. A numerical
parabolic equation code has been developed [4.91] and
adapted to road traffic noise situations [4.92] where
road line sources are modeled as series of equivalent
point sources of height 0.5 m. The data showed a reductioninA-weightedLeq
due to the trees of 3 dB during
downward-refracting conditions, 2 dB during homogeneous
conditions and 1 dB during upward-refracting
conditions. The numerical predictions suggest that in
downward-refracting conditions the extra attenuation
due to the forest is 2–6 dB(A) with the receiver at least
100 m away from the road. In upward-refracting conditions,
the numerical model predicts that the forest may
increase the received sound levels somewhat at large
distances but this is of less importance since levels at
larger distances tend to be relatively low anyway. In
homogeneous conditions, it is predicted that sound propagation
through the forest is affected only by scattering
by trunks and foliage. Defrance et al. [4.90] have concluded
that a forest strip of at least 100 m wide appears to
be a useful natural acoustical barrier. Nevertheless both
the data and numerical simulations were compared to
sound levels without the trees present, i. e., over ground
from which the trees had simply been removed. This
means that the ground effect both with and without trees
would have been similar. This is rarely likely to be the
case.
A similar numerical model has been developed recently
[4.93] including allowance for ground effect,
wind speed gradient through foliage and assuming
effective wave numbers deduced from multiple scattering
theory for the scattering effects of trunks,
branches and foliage. Again, the model predicts that
the large wind speed gradient in the foliage tends
to refract sound towards the ground and has an important
effect particularly during upwind conditions.
However, neither of the PE models [4.91, 93] include
back-scatter or turbulence effects. The neglect of the
back-scatter is inherent to the PE, which is a oneway
prediction method. While this is not a serious
problem for propagation over flat ground because the
back-scatter is small, nor over an impermeable barrier
because the back-scattered field, though strong,
does not propagate through the barrier, back scatter
is likely to be significant for a forest. Indeed acoustic
reflections from the edges of forests are readily
detectable.