Download Treball presentat

European Drying Conference - EuroDrying'2011
Palma. Balearic Island, Spain, 26-28 October 2011
THE EFFECT OF DRYING ON THE ACOUSTIC ABSORPTION OF NOVEL
GREEN NOISE INSULATION
H.Benkreira1, K.V. Horoshenkov 1, A. Khan 1, A. Mandon2, R. Rohr2
1
School of Engineering, Design & Technology
University of Bradford, Bradford, BD7 1DP, UK
Tel.:+441274233721, E-mail: [email protected]
2
Canevaflor,
24 Rue du Docteur Guffon, 69170 Tarare, France
Abstract: This paper presents acoustic absorption data of plant-soil systems that can be
used as green noise insulation on building facades. They show that absorption varies
significantly depending on the type of plant and soil which in turn depend on the drying
they experience in a natural environment.
Keywords: soil, plants, acoustic absorption, moisture.
INTRODUCTION
In recent years and with the advent of global
warming, the world has awakened to the importance
of green technology with the consequence that
governments throughout the world are undertaking
major plans to implement green initiatives
particularly for energy generation (see Germany
recent decision) and the control of CO2 emission (see
the Montreal Protocol). In parallel with these
developments, there are new initiatives, particularly
in Europe of greening cities, i.e. embellishing them
with as much greenery as it is possible and desirable
to play a part in the sequestration of CO2 and the fight
against global warming. Greening the cities initiative
has given also an opportunity to explore the
possibility of using plants on building facades as a
means of absorbing noise from traffic and other
sources that plague cities which are now increasingly
densely built. These also provide a pleasing green
contrast to build form, seasonal events such as spring
leaves, flowers, autumn leaf fall, and the provision of
shade in summer. The end gain then becomes one of
controlling both CO2 and noise pollution but also
providing pleasing scenery in an urban environment.
This paper falls precisely within this ultimate
objective by studying the acoustic absorption of
plants, potted in various soils and arranged as
insulation on building facades (see Figure 1).
Fig.1: A typical green wall
With regard to drying, the theme of this conference,
this paper examines the effect of moisture content on
the acoustic absorption of these systems (their soil
and foliage). This is clearly an innovative study in
term of the particular system but well rooted in the
concept of porous media and how these media absorb
and/or transmit noise depending on the porosity.
With these green systems, the soils are clearly porous
media whose porosity depends on the moisture
trapped and which varies upon drying in time. The
foliage can also be viewed as porous media in that
void exist between the leaves, although of course the
scales are much different but can be “normal” for
certain types of plants (see Figure 2).
and tortuosity were measured semi-theoretically.
Two other soils were investigated, one clay based
and typical of soil used by local authorities in the UK
(on account they are plentiful and typical of the
country) and one specially designed by our
collaborating company Canevaflor to retain moisturea mix of perlite, coconut fibres and polymer gel.
Table 1: Characteristics of the plants tested.
Plant
species
Pieris
Japonica
Green
Ivy
Primrose
Average
thickness
of single
leaf
(mm)
Weight
of
single
leaf
(g)
Area
of
single
leaf
(m2)
Number
of leafs
on plant
0.41
0.4
0.0012
166
0.23
0.2
0.0021
44
0.74
1.9
0.004
17
Fig. 2: Leaves of the three plants studied.
Although limited and predominantly based on the
effect trees, there is supporting evidence from
previous research [1-2] that vegetation provide
acoustic absorption properties that depend on size
and orientation of leaves. In this research we
approach the problem by considering both the soil
and plant as the acoustic absorption system. We
however investigate separately the soil from the
leaves in an effort to quantify the relative
contribution and the effect of moisture which is the
changing variable in time. Both the soils and the
leaves are chosen to cover a range of soil types, leaf
types, leaf sizes and foliage density. We carry out
this quantification through acoustic measurements
using standard techniques and underpin the
measurements with theoretical models.
EXPERIMENTAL
The experimental programme was carried using three
types of plants species which can be grown in a
living wall system and three types of soil. These
systems were fully characterized as shown below
prior to acoustic absorption measurements.
The Plants: These were Pieris Japonica, Green Ivy
and Primrose, shown in Figure 2 and characterized
using a number of parameters that describe the
thickness, area and weight of their single leaf; the
number of leaves and the height and equivalent
volume occupied by the plants. Such parameters are
presented in Table 1.
The Soils: These were chosen according to the plants
grown and were: Pieris Japonica soil, Green Ivy soil
and Primrose soil. These soil-plant systems were
used as received (from a typical garden centre) but as
explained later; their soil porosity, air flow resistivity
Compared to clay-based soil, little or no aggregation
occurs in the low-density soil substratum which is
able to retain water at least three times its own
weight due to the presence of hydrophilic polymer
gel. The texture of the two soils determines the pore
size distribution which controls their acoustical
properties. The presence of fibres, large particles of
perlite and polymer gel in the substratum give rise to
larger pores and, therefore have a major influence on
its acoustic absorption coefficient. Table 2 shows the
density and porosity of these soils, measured in the
case of porosity using image analysis.
Table 2: Characteristics of the soils used.
Water added to soils
(cm3)
Substratum
Clay soil
Density
(kg/m3)
250
1255
Porosity
0.76
0.39
Acoustic Absorption Measuring Technique: A
standard technique was used- a Bruel and Kjaer
impedance tube fitted with two microphones. The
method is described in ISO 10534-2, 1998 [3] and
illustrated in Figure 3.
Fig. 3: Plant and Soil placed inside impedance tube
holders
0.9
Absorption coefficient
0.8
0.7
0.6
0.5
Japonica - Soil +
leaves
0.4
0.3
0.2
Soil
0.1
0
0
500
1000
Frequency (Hz)
1500
1
0.9
0.8
Absorption coefficient
RESULTS & DISCUSSION
Performance of Plant-Soil Systems: For ease of
comparison, the acoustic absorption of the three
plants systems are shown in Figure 4 which shows
the acoustic absorption of the soil, the soil and plant
and the leaves separately.
The data show a significant difference in
both the soils and the plants and indicate that the
Primrose soil and plants are ideally suited as acoustic
absorbers. Interestingly, with Primrose, the leaves
contribution to acoustic absorption is very significant
and this can be attributed to the larger surface area of
their leaves which act as membrane. As indicated in
Table 1, this large surface area (0.004 m2, compared
to 0.0012 m2 for the Japonica and 0.0021m2 for the
Ivy) coupled with a large the thickness (0.74mm,
compared to 0.41 for the Japonica and 0.23mm for
the Ivy) enable them to cushion and absorb more
effectively noise (pressure) energy.
In order to understand further the relative
influence of soil on the measured absorption, it is
desirable to measure the structural characteristics of
the soil such as porosity, air flow resistivity,
tortuosity and the standard deviation, the intrinsic
material properties controlling absorption. Rather
than measuring these directly, we employ here a well
established model [4-5] that uses acoustic absorption
data to infer them. Essentially the method is based
on minimising the discrepancy between the measured
absorption coefficient spectrum and the spectrum
predicted by the model. Table 3 presents the four
non-acoustical parameters for the three soils deduced
from the optimisation analysis. On account of its
high porosity, a tortuosity =1 and a relatively high air
flow resistivity, Primrose soil is the better acoustic
absorber. It is also the soil that exhibit a smaller
standard deviation suggesting that in comparison
with the other soil, the pore size distribution is less
wider albeit that all these soils exhibit a wide pore
size distribution.
The difference in acoustic
absorption and structural parameter reflect the fact
that these soils sustain different moisture levels
dictated by the plant s root system and moisture
feeding characteristics.
1
Green Ivy - Soil +
leaves
0.7
0.6
0.5
0.4
Soil
0.3
0.2
0.1
0
0
500
1000
1500
Frequency (Hz)
1
0.9
0.8
Absorption coefficient
The spacing between the two microphones was
100mm which enabled to determine the acoustic
absorption spectra in the frequency range between 50
and 1600 Hz. Using this technique, the soil and
leaves acoustic absorptions were measured separately
and this was followed by inserting a complete potted
plant in the impedance tube and measuring the
acoustic absorption of the whole system.
Primrose - Soil +
leaves
0.7
Soil
0.6
0.5
0.4
0.3
0.2
0.1
0
0
500
1000
1500
Frequency (Hz)
Fig.4: Acoustic absorption of the 3 plant-soil systems
with and without foliage.
Table 3: Structural-Acoustic Parameters of the three
soils tested.
Plant
species
Pieris
Japonica
Green
Ivy
Primrose
Flow
Porosity, Tortuosity, Standard
φ
α∞
resistivity,
deviation,
2
σ ,(Pa.s/m )
s
30,551
0.17
1.00
1.95
1,843
0.28
1.20
1.44
3,550
0.3
1.00
0.96
rubber granulates.”, Appl. Acoust. 62: 665-690
(2001).
[5] K. V. Horoshenkov and M. J. Swift, “The
acoustic properties of granular materials with pore
size distribution close to log-normal.”, J. Acoust.
Soc. Am. 104 (3): 1198-1208 (2001).
Performance of the Clay and Substratum Soils:
Figure 5 compares the measured acoustic absorption
performance of bag, dry and saturated substratum
with clay based soil. The absorption data presented
suggest that low density bag and dry substratum are
high acoustic absorbers whereas saturated substratum
and clay based soils have a limited acoustic
absorption. The effect of moisture saturation on
acoustic performance is shown in Figure 6 for the
two soils. These data indicate clearly the very good
performance of the substratum.
CONCLUSIONS
Plants, if chosen appropriately, i.e. with the right
porosity, air flow resistivity and porosity can be used
as effective screens to absorb noise. Soil play an
important part in the absorption process but the
plants add to it and in some cases significantly.
Performance depends crucially on soil types which in
turns depend on how the soil absorbs moisture. The
benefit of this work is that plants can be arranged on
building facades to provide as well as noise
absorption, also CO2 absorption and of course a
pleasing natural environment in densely built urban
cities. In the context of drying, the data show that
performance is crucially related to the moisture
content of the soil which in turns depends on the
natural drying the plants experience.
Fig.5: Acoustic absorption performance of the clay
and substratum soils at different moisture levels.
ACKNOWLEDGEMENTS
We acknowledge the financial support from the
European Commission under the EU Framework 7
project HOSANNA (http://www.greener-cities.eu/)
and are grateful for the helpful support we receive
from all our partners in this project.
REFERENCES
[1] S. H. Burns, “The absorption of sound by pine
trees.”, J. Acoust. Soc. Am., 65(3):658–61 (1979).
[2] M. J. M. Martens, “Foliage as a low pass filter:
Experiments with model forests in an anechoic
chamber.”, J. Acoust. Soc. Am., 67(1): 66-72 (1980).
[3] ISO 10534-2. 1998. Determination of sound
absorption coefficient and impedance in impedance
tubes - Part 2: Transfer-function method.
[4] K. V. Horoshenkov and M. J. Swift, “The effect
of consolidation on the acoustic properties of loose
Fig.6: Comparison on the effect of saturation levels
on acoustic absorption performance of the soils.