PROCEEDINGS of the
23rd International Congress on Acoustics
9 to 13 September 2019 in Aachen, Germany

A review of natural means for noise abatement
Keith ATTENBOROUGH1;
1

The Open University, UK

ABSTRACT
Natural means to reduce surface transport noise during propagation between source and receiver outdoors
include tree belts, crops, earth berms and ‘soft’ ground effects. They can be exploited through landscaping
and design. Although there are few full-scale demonstrations of what can be achieved, the noise reducing
capabilities of the proposed arrangements have been tested in the laboratory and/or numerically evaluated.
Well-designed, natural arrangements offer ways of abating surface transport noise, either to complement
conventional noise barriers or provide useful alternatives in cases where noise barriers are neither practicalnor desirable. While meteorological effects, such as downward refraction and turbulence, reduce theefficiency of usual methods of outdoor noise mitigation, judicious arrangements of vegetation andlandscaping can counteract these effects as well as provide non-auditory benefits.
Keywords: Outdoor Sound, Ground effect, Attenuation

1. INTRODUCTION
A comparative study of noise levels in eight UK and six EU cities (1) has shown that overall noiselevels depend on the proportions and distributions of green areas and porous ground. Sound levels
tend to be higher in UK cities with higher building densities fewer green areas within 30 km 2 of thecity centre. The balance and dispersion between green space surfaces and built -up surfaces turns out
to be a more meaningful indicator than green space coverage alone. Noise levels will tend to be lower
if more areas involve naturally porous ground since this will mean, in turn, that housing developments
and road networks have lower densities and there will be larger distances between noise sources anddwellings. The usual decrease of noise levels with distance can be augmented by extra reductions
from 'soft' ground effects, tree belts, crops, bushes and hedges. While acknowledging that there is
growing evidence that ‘greening’ the environment has a positive influence on noise perception (2),this paper reviews measured and predicted noise reductions from certain types of ‘greening’, thephysical mechanisms that contribute to these reductions, and ways in which they might be exploited.

2. GROUND EFFECTS
2.1 Porous ground
When the sound source is located above a ground surface, sound waves reflected from the ground combinewith those travelling directly from the source. If the sound waves arriving along the two paths are in phasewhich is true at some frequencies whatever the surface, there are increased levels. When they are out of phase,at other frequencies, the total sound is less. These interactions are known as ground effect. Complicating
factors such as multiple sources including engine and exhaust in the case of road traffic, atmosphericturbulence and naturally uneven and non-uniform ground mean that the increases in level due to acousticallyhard ground corresponds more nearly to energy doubling. If the ground is porous then not only is themagnitude of sound reduced on reflection but the phase change due to the finite (complex) ground impedancecombines with the phase change due to path length difference with the consequence that, for a given sourcereceiver geometry, the first destructive interference occurs at a lower frequency than over hard ground. The
resulting reduction in sound levels, sometimes called ‘ground absorption’, is important for surface transportnoise. It depends on the type of ground surface and the locations of the sources and receivers.
According to the common methodological framework for strategic noise mapping under theEnvironmental Noise Directive (2002/49/EC) (3), "the acoustic absorption properties of ground are mainly
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keith.attenborough@open.ac.uk

linked to its porosity". While porosity is a factor, the acoustical properties of porous ground are affected mostby the ease with which air can move in and out of the ground surface. This is indicated by the flow resistivity
which represents the ratio of the applied pressure gradient to the induced steady volume flow rate of air
through the surface of the ground. The porosity of naturally-occurring ground surfaces does not vary as muchas their flow resistivity. If the ground surface has a high flow resistivity, it means that it is difficult for air toflow through the surface. This can result from very low or negligible surface porosity. Hot-rolled asphalt andnon-porous concrete have near zero porosity and a very high flow resistivity whereas many forest floors andfreshly-fallen snow have very much lower flow resistivity and a high porosity
The Calculation of Road Traffic Noise (CRTN) scheme (4), used for assessing eligibility for soundinsulation of dwellings near new or improved roads in the UK, includes an allowance for the extra reductionin levels " If the ground surface between the edge of the nearside carriageway of the road or road segment
and the reception point is totally or partially of an absorbent nature, (e.g. grassland, cultivated fields orplantations)". The extra reduction depends on the mean height of the sound travelling from the road source
to the locations of interest and distance travelled over soft ground. It is noted that " To avoid the difficulty ofdefining adequately the many other more absorbent types of ground cover, the correction shown ... is to be
used for all predominantly absorbent surfaces. Thus the calculations will slightly underestimate attenuation
effects, particularly where the intervening ground is intensively cultivated or planted." A similar correction
and the same qualifying statement appear in Calculation of Railway Noise (CRN) (5).
For a mean path height of 1.5 m, CRTN and CRN predict ‘soft ground’ reductions of 4 dB and 1 dB
respectively at 50 m from the nearest side of a road or railway. The lower reduction for railway noise isconsistent with the fact that wheel/rail sources are at a height of 0.4 m on the tops of tracks above sleepers
and ballast and so are higher than tyre/road sources (assumed to be 0.01 m high).
HARMONOISE and NORD2000 prediction schemes (6, 7) identify eight categories of ground for the
purposes of predicting the extra sound attenuation associated with porous ground effect. Three of them referto various types of grass-covered ground. The categories are used to specify an effective flow resistivity that,in turn, can be used to predict the corresponding ground effect using an impedance model. More accurate
allowance can be made for a specific soft ground if its surface impedance is known. Deduction of ground
impedance from complex pressure ratios gives impedance-model-independent information but it might notalways be convenient to make such measurements. To date relatively few deductions of ground impedancefrom complex pressure ratio measurements have been reported. It is more common to deduce parametervalues for impedance models by fitting short range level difference spectra using ‘template’ methods (8, 9).
Subsequently these models and parameter values can be employed in prediction schemes.
A one parameter semi-empirical model, used widely for outdoor sound prediction, is unphysical (10). For
example, at low frequencies it predicts negative real parts of surface impedance and complex density.
Comparisons of predictions with data obtained using signals from a point source at vertically separated
microphones, have shown that, for many grasslands, physically admissible two parameter models lead tobetter agreement (11).
Although prediction schemes allow for ground effect, they are concerned with accounting for the
existing environment rather than ways of exploiting ground effects for noise control. CRN includes
an additional reduction of 1.5 dB as a result of the porous (sound absorbing) nature of ballast. For thesame reason the use of porous rather than non-porous slab track is to be preferred when constructing
high speed rail links. Porous road surfaces and road surfaces with embedded resonators are to bepreferred to non-porous hot-rolled asphalt (12).
Measurements of the acoustical properties of ground surfaces together with numerical calculationsof noise levels due to a 2-lane urban road (5% heavy vehicles, 95 % light vehicles, travelling at 50km/h), suggest a ‘soft’ ground reduction of 5 dB at a 1.5 m high receiver located 50 m from the roadover compacted grassland. But for less compacted grassland (meadow) and the same receiver location,
8 dB reduction is predicted; significantly greater than the 4 dB predicted by CRTN for any grassland.
Predictions of urban road traffic noise spectra at 1.5 m high receiver 50 m from the road withintervening hard ground or either of two types of soft ground are compared in Fig. 1. The ‘softer’ground is predicted to give 10 dB lower levels at peak traffic noise frequencies th an the hard ground.
Some predicted 'ground cover' reductions increase with distance faster than predicted by CRTN (13).
The introduction of grassy areas between and alongside tramways has been found to give
perceptible noise reduction (12). Specifically, the introduction of grass between and alongside thetracks reduced tram noise levels at a 1.5 m high receiver 4 m from the nearest track by between 1 and10 dB(A) with an average reduction of 3 dB(A). Moreover, a survey of subjective reactions suggested
that the associated reduction in annoyance is equivalent to a further 1 - 2 dB reduction (see Fig.2).

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Hard surface
Compacted grass
Meadow

Figure 1 Calculated traffic noise spectra (5% heavy vehicles, 95 % light vehicles, travelling at 50km/h) at a 1.5 m high receiver 50 m from the nearside lane with intervening hard ground or two
types of grassland (13).

Figure 2 Measured tram noise levels in Grenoble without (black dots) and with grassy areas(green dots) plotted against perceived annoyance. The reduction in annoyance is equivalent to a 1-2
dB greater reduction than the actual 2 – 3 dBA reduction in levels.
2.2 Rough groundIf ground is otherwise acoustically hard, roughness, even at scales smaller than the shortest incident
wavelength, affects outdoor sound propagation. The presence of small-scale roughness makes asurface that is acoustically hard at normal incidence appear acoustically soft near grazing incidence.
Part of the incident sound energy is reflected in the specular direction and the remainder is scattereddiffusely. The distribution of scattered sound depends on roughness topology, roughness dimensions
compared to the incident wavelength, and the relative locations of source and receiver. If a sufficiently
large fraction of the reflected sound retains a phase relationship with the incident sound so there issignificant coherent scattering and specular reflection, there can be a significant roughness inducedchange in ground effect near grazing incidence. One method of deliberately introducing roughness
outdoors is to construct an array of low parallel walls. Low walls with heights of 0.3 m or less (0.3 m
is approximately the wavelength of sound in air at 1 kHz), can be considered as a form of artificiallyarranged ground roughness. The potential usefulness of regularly spaced low parallel walls for roadtraffic noise reduction was suggested and demonstrated in 1982 (14). An array of sixteen 0.21 m highparallel brick walls with edge-to-edge spacings of about 20 cm placed on compacted grassland wasfound to give a broadband (between 100 Hz and 12,500 Hz) insertion loss (IL) of slightly more than
4 dB(A) including insertion losses of up to 20 dB(A) in the 1/3 octave bands between 400 a nd 1000
Hz. The creation and subsequent attenuation of surface waves was considered as the main mechanism
for noise reduction. Although surface wave creation is one of the consequences of placing a low
parallel wall array on an acoustically hard ground, the array has a significant influence on ground552

effect over a wider range of frequencies than those affected by the surface wave . This is demonstrated
by the results of drive-by tests conducted with the brick configurations shown in Figure 3. Figure 4
shows spectra measured at a 1.5 m high receiver 10 m from the nearside wheels of car 1 and car 2
without and with the low parallel wall or lattice configurations respectively. The overall broadband
insertion loss of about 3 dB due to a 2.30 m wide, 0.2 m high, 16 m long nine parallel wall array was
found to be more or less the same as that of the 1.1 m wide square cell lattice wall configuration of
the same height even though the latter occupies significantly less land area.

Figure 3 Parallel low wall and lattice brick arrangements used in drive-by tests

Figure 4 1/3 octave band SPL spectra measured by a 1.5 m high receiver 10 m from the nearside wheels ofcars at their closest point of approach without and with the 2.30 m wide array of nine parallel brick walls
using car 1 and a 1.1 m wide brick lattice using car 2 (15).

Height m

Insertion Loss colour scale

In contrast to the insertion loss due to 'soft' ground, the insertion losses predicted for the latticeand parallel wall configurations are not much affected if the receiver he ight is increased from 1.5 m
to 4 m. This is a consequence of the predicted insertion loss 'beam' shown in Figure 5 which is a
contour plot of overall insertion loss for a frequency range 178 Hz to 4.44 kHz due to a 0.2 m high 6
m wide lattice with 0.065 m thick walls and centre-to-centre spacing of 0.26 m with the nearside 2.5
m from a 0.05 m high line source emitting a spectrum corresponding to 70 km/h light vehicular traffic.

Distance m
Figure 5 A plot of overall BEM predicted insertion loss contours from a 0.05 m high line source emitting aspectrum corresponding to light vehicles travelling at 70 km/h due to a 0.2 m high 6 m wide lattice.

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A 'beam' of higher insertion loss (a6 dB) compared with hard smooth ground extends from 0.2 m
near the source to about 5 m height at 100 m. The slightly lower insertion loss ( a5 dB) predicted near
the ground is a consequence of the roughness-induced surface wave.

3. NOISE REDUCTION BY CROPS, HEDGES AND TREE BELTS
3.1 Influence of vegetation on ground impedance
Growing plants create root zones and, thereby, change the near-surface flow resistivity and porosity.
In the presence of plants, the ground surface has a different surface impedance to that for the sameground with little or no vegetation. The best fit impedance model parameters obtained from shortrange vertical level difference measurements over ground containing winter wheat crops (16) are aneffective flow resistivity of 170 kPa s m -2 and a porosity of 0.2 using a two-parameter slit pore model(11). These values can be compared with those obtained over nearby bare ground, i.e. the same typeof soil but on which no crops were growing or had been grown. In comparison, the best fit flow
resistivity and porosity values over the bare unplanted ground are 2000 kPa s m -2 and 0.2 respectively
which suggests that growing winter wheat reduced the effective flow resistivity of a soil surface by atleast a factor of 10.
3.2 Visco-thermal losses in foliage
Foliage and stems in vegetation scatter sound and sound energy is converted into heat by viscousand thermal processes at leaf surfaces. In hedges and bushes there will be scattering by trunks andbranches also. As a result of measuring sound transmission loss through dense corn, hemlock, red pinetrees, hardwood brush and densely planted reeds in water, Aylor (17) suggested a relationship between
normalised excess attenuation, i.e. the attenuation in excess of that due to ground effect divided by
the square root of the product of foliage area per unit volume, F, and path length, L, and the scatteringparameter (the product of wave-number, k, and a characteristic leaf dimension, a). Figure 6 compares
Aylor’s data for normalized excess attenuation through corn and reeds with predictions of eqn (1).
‫ܣܧ‬ሺ݀‫ܤ‬ሻ
ξ‫ܮܨ‬

ൌ

ξ݇ܽ
ͲǤͳͶ͸
൬
൰ ൅ ͲǤ͹͸ξ݇ܽ(1)Normalized excess attenuation

scattering parameter (ka)Figure 6 Aylor's data for the normalized sound attenuation through corn (boxes F = 3/m; diamonds F =6.3/m) with mean leaf width 0.074 m and reeds (open circles) with mean leaf width 0.032 m compared withpredictions of equation (1) [solid line].
3.3 Propagation through crops
Measurements of sound transmission through winter wheat crops (average stem diameter 2.63 mm)have been compared with predictions including ground effect based on short range fitting of level
difference spectra, multiple scattering by stems with known distribution of diameters, l oss ofcoherence due to scattering and foliage attenuation by known leaf size and area density (16). Figures
7 compares spectra of the difference in levels at 0.3 m high receivers 1 m and at (a) 2.5 m (b) 7.5 m
and (c) 10 m from a 0.3 m high loudspeaker source (continuous lines), and calculations of (i) groundeffect alone (broken lines), using the 2-parameter slit pore model (flow resistivity 100 kPa s m -2 and554

porosity 0.27), (ii) ground effect plus multiple scattering, and loss of coherence due to scatter ing using
equivalent turbulence (18), and (iii) these effects plus visco-thermal attenuation in foliage. The totalof the predicted effects agrees well with the data. A simpler way of predicting propagation over or
through crops is to add ground effect and foliage effect only. This requires larger ‘effective’ values
for foliage per unit area and mean leaf size but avoids the need for the detailed calculations of multiple
scattering and of the loss of coherence (16).
g
10Level Difference dB

5
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-10
-15
-20
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-30

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Frequency Hz

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Figure 7 Spectrum of the level difference between 0.3 m high reference receivers at 1 m and 10 m from a
0.3 m high omnidirectional source measured in 0.45 m to 0.5 m high winter wheat (solid [blue] line). Also
shown (16) are predictions of ground effect alone (broken [red] line); ground effect plus incoherence plus
multiple scattering by stems (broken [black] line); ground effect plus incoherence plus multiple scatteringby stems plus visco-thermal attenuation (solid [black] line)
3.4 Propagation through hedges
Predictions of the simplified method have been compared also with data for transmission of soundfrom different single vehicles passing by a 2 m thick hornbeam' hedge near a tennis court (19). Figure
7 compares predictions with data during pass-bys with two different vehicles. Overall pass-by
experiments at two hedges gave median insertion loss values of between 1.5 and 3.6 dB(A).
65(a)

A-weighted SPL dB

60

(b)

70

(b)

65

(j)

60

55

55

50

50
45

45

40

No Hedge
Hedge
Aylor + Ground prediction

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35
10

2

10

3

Frequency Hz

No Hedge
Hedge
Aylor + Ground prediction

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30
10

2

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Frequency Hz

Figure 8 (a) and (b) Sound level spectra measured during different car pass-bys without (continuous blacklines) and with (broken blue lines) a 2 m thick hornbeam hedge and predictions (broken red lines) which
sum the attenuations due to ground and foliage effects (19).
3.5 Propagation through trees
An important contribution to sound attenuation through a belt of trees is the destructive
interference from ‘soft’ ground effect associated with the leaf litter and humus layers that form beneaththe trees. This is reduced by scattering from tree trunks but supplemented at high frequencies byfoliage attenuation. Figure 9 compares third octave band excess attenuation spectra (joined circles),obtained from measurements in a pine forest with an 0.8 m high loudspeaker source and 1.2 m highreceivers at 10 m and 80 m respectively from the source in a pine forest, with predictions (brokenlines) obtained by summing ground effect modified by scattering-induced incoherence and foliage
attenuation (18). An analytical 3D scattering model for propagation in forests has been developed (20)which includes tree height as a parameter. But further work is needed to improve the ‘absorption crosssection’ used to account for visco-thermal effects in foliage. Also, since the 3D scattering theory
assumes random spacing, it does not account for the predicted potential additional attenuation
resulting from regular spacing i.e. ‘sonic crystal’ effects (12).

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80 m

Sound Level refree field dB

10 m

Frequency Hz

Frequency Hz

Figure 9 Sound level re free field spectra measured (joined circles) and predicted (broken blue lines) in apine forest

4. MITIGATION OF ADVERSE METEOROGICAL EFFECTS
4.1 Downwind performance of noise barriers
A significant problem for conventional noise barriers outdoors is their interaction withmeteorological conditions. The presence of the barrier disturbs the airflow during wind. The alteredwind profile in the vicinity of a barrier causes deterioration in acoustical performance, since windspeed gradients and intensity of turbulence become larger near the barrier. The consequent increased
refraction of sound during downwind sound propagation (sometimes called REfraction of Sound by
WINd-induced Gradients (RESWING)), reduces the shadow region behind a barrier. As well as the
additional refraction near the barrier, turbulence, including screen-induced turbulence, results inincreased noise levels behind the barrier. But for thin rectangular barriers, broadband traffic noise andlimited distances behind the noise barrier, turbulence effects are small compared with screen-induced
refraction.
The effect of a row of trees (in leaf) behind a noise barrier, i.e. on the receiver side, on windgradient-induced refraction has been investigated. Measurements made continuously at a location withand without a row of trees behind a noise barrier from the middle of the summer until fall have been
compared. For downwind locations and for an orthogonal incident wind, the efficiency of the noisebarrier with trees was better than that of the noise barrier without trees and the downwind
improvement increases with increasing wind speed (21).
4.2 Berms v barriers
Predictions indicate that roughening an acoustically hard berm can improve its insertion loss (12).
Vegetated berms retain the soft ground effect that might be lost with the erection of a conventional
barrier. Whereas the shielding by a conventional noise barrier can almost disappear for receivers
downwind, predictions indicate that the insertion loss provided by a berm or bund of similar height
can remain relatively unaffected (22). Where land use permits, in considering long-term equivalentnoise levels, including periods with wind, acoustically soft and shallow berms are to be preferred tovertical barriers, since not only do they mitigate downwind effects and reduce potential nuisancecaused by source side reflections, they offer non-acoustical, i.e. aesthetic, benefits.

5. CONCLUSIONSOther ‘natural’ means for reducing noise include vegetative coverings on the faces of noise barriers
and buildings and barriers made from cage-supported piles of stones (gabions). While it is difficult touse natural means to achieve reductions comparable say with a 2 m high conventional noise barrier,vegetation in urban spaces can contribute 'positive' sounds as well as reduce undesirable ones. Treebelts increase wind sounds and form corridors that attract animals and their associated calls.
Vegetation on façades can improve the visual quality of environments. The extent to which such visualchanges also influence auditory perception of noise is debatable. Nevertheless, the effect on theoverall environment is important, and noise-mitigation methods that also improve aesthetic aspects
are obviously better than methods that do not.
The HOSANNA project (12) looked also into the costs and benefits of the various methods fornoise reduction. The most robustly cost-efficient of the environmentally friendly methods, after
accounting for non-acoustical benefits such as air pollution reduction and bio-fuel generation, was556

found to be the provision of dense tree belts alongside surface transport corridors .
More work is needed to find optimum combinations of soil, vegetation and landscape. Only a starthas been made in modelling, and, thereby, designing natural means of noise reduction. Exploration ofthese possibilities needs more accurate modelling than achievable with current prediction schemes(23). In any case, the use of non-physical models for describing acoustical properties of soils(6,7,24,25) should be avoided. Physically admissible models give at least as good predictions (10,11).

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