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ICSV21, Beijing, China, 13-17 July 2014 1
The 21st International Congress on Sound and Vibration
13-17 July, 2014, Beijing/China
Francesco Asdrubali, Francesco D’Alessandro and Nicholas Mencarelli
Department of Engineering, University of Perugia, Italy 06125
Kirill V. Horoshenkov
Department of Mechanical Engineering, University of Sheffield, United Kingdom S1 1WB
The paper presents the experimental results of sound absorption coefficient measurements of
several tropical plants, including fern, baby tears, begonia, maidenhair fern and ivy. All these
plants live in tropical underbrush in conditions of low lighting, warm temperature and high
relative humidity, conditions that can be often found inside common buildings. A vertically
mounted impedance tube with a diameter of 100 mm is used in accordance with ISO 10534-2
method to determine the absorption coefficient spectra. A substrate made of coconut and per-
lite soil for hydroponics is used with all the samples. It is chosen because of its high porosity
and, as a result, good sound absorption properties. The paper also presents the random inci-
dence absorption coefficient spectra which were measured in a reverberating room in accord-
ance with ISO 354. The aim of this research is to investigate the feasibility of introducing ab-
sorptive panels made of living plants as a replacement for conventional man-made acoustic
treatment of surface used to improve the acoustic quality of indoor spaces.
1. Introduction
The integration of green systems in architecture has been used for centuries. The first (proba-
bly fortuitous) examples date back to the 11th century BC, when Vikings used to build their huts
with wood and peat bricks1. These components allowed the development of low growing vegeta-
tion, whose roots created a monolithic structure with the peat bricks stiffening the structure. Later,
the use of green elements in architecture gains a purely aesthetic role, as demonstrated by the Hang-
ing Gardens of Babylon (7th 6th century BC), the Hadrian’s mausoleum (now Castel Sant’Angelo,
2th century BC) in Rome or the hanging gardens in the roman villas, as the one in Ercolano2. In the
Middle Age hanging gardens loses their aesthetic role in favour of functionality: they are used as
vegetable gardens in convent's cloister or on the top of city walls (as in the Italian city of Lucca) to
increase the defence efficiency against the opposing armies. In the Renaissance age green elements
are again used as ornament of palaces: examples can be found in Giovanni dei Medici’s villa in
Fiesole (1451) or in Piccolomini Palace in Pienza (1460), both in Italy3. The birth of modern green
walls dates to the first patent deposited by Stanley Hart White in 19384: also in this case, the pro-
posed solution represents an ornamental solution for architects, unlike green roofs, whose positive
environmental and hygienic impacts were stressed in the first half of the 20th century by famous
architects, such as Alvar Aalto, Frank Lloyd Wright and Le Corbusier.
21st International Congress on Sound and Vibration (ICSV21), Beijing, China, 13-17 July 2014
ICSV21, Beijing, China, 13-17 July 2014 2
Coming to our days, the revolution of green walls happens in the ‘80s thanks to the ideas of
the French botanist Patrick Blanc5, who developed lightweight and modular structures adaptable to
every façade. This is made possible by the absence of a soil substrate: the plants can indeed live
only with water, oxygen and carbon dioxide and in this configuration the roots develop only in the
upper layer of the structure, without damaging the structure underneath. Some of his most famous
green walls are those covering the facades of the Museé du Quai Branley in Paris and the Caixa
Forum in Madrid.
Green roofs have been intensively studied in the last decades, showing that they can be very
effective in reducing the Urban Heat Island (UHI) effect, in improving air quality and in giving an
extra protection to buildings in shielding heat fluxes and electromagnetic waves; also the acoustic
performance have been evaluated6; on the contrary green walls have often been seen as pure decora-
tions, with the unique functional application represented by green road noise barriers.
Only in the last years researchers have been focusing their attention on green walls; also the
acoustic performance is examined, as done in the EU funded Hosanna Project7 which focused on
outdoor noise control. Unlike many other works, this paper studies the use of green systems for the
acoustic treatment of indoor spaces. It is believed that these systems can potentially replace tradi-
tional or even sustainable man-made acoustic absorbers8 because of their good aesthetic and re-
storative effects9,10.
2. Sound absorption of greenery
Even if the general opinion is that greenery is not a good sound absorber, it was demonstrated
that vegetation surfaces are able to absorb up to 50% of incident sound energy11. A review of the
interaction between sound and vegetation can be found in the outcomes of the Hosanna Project7.
The viscous absorption effects in crops were studied by Aylor12. The absorption effects related to
leaves vibration were studied by Martens13 and by Tang et al14 who showed that this phenomenon
contributes to the absorption at frequencies higher than 1 kHz. More recently Horoshenkov et al15
measured the acoustic absorption of 5 different types of low growing pants in the presence and ab-
sence of soil substrates. Their results showed that morphological parameters, such as leaf area den-
sity and dominant leaf angle, are the main factors which affect the visco-thermal acoustic absorp-
tion by plants in the frequency range of 50 1200 Hz. Ding et al16 studied the influence of leaves
presence on the acoustic absorption of a porous substrate. The impedance tube measurements and
theoretical predictions for the set up their adopted suggested that there is no influence in the low
frequency range (below 250 Hz). On the contrary, it is demonstrated that there is an increase in the
absorption coefficient at middle frequencies (500-2000 Hz) and a decrease at higher frequency
(above 2000 Hz) due to the vibration and shielding effect by the leaves.
The purpose of this paper is to carry out laboratory experiments to understand better the abil-
ity of different plants to absorb noise in the presence of soil.
3. Description of the tested plants
The species selection is based on three fundamental parameters which were identified with the
help from the Department of Agriculture of University of Perugia. These are: (i) ability of a plant to
grow in indoor climate conditions, (ii) ability of a plant to survive without direct sunlight, (iii) abil-
ity of a plant to branch in horizontal conditions. The first two parameters are fulfilled in thermopile
plants which are common in tropical climates and in the warmer zones of temperate climates. These
climates have temperature and humidity conditions very close to the indoor spaces conditions. Fur-
thermore, it can be possible to find habitats in which plants species grow naturally in horizontal
position, as in waterfall sides or river blanks (Figure 1).
21st International Congress on Sound and Vibration (ICSV21), Beijing, China, 13-17 July 2014
ICSV21, Beijing, China, 13-17 July 2014 3
Figure 1. Mae Ya Waterfall in Doi Inthanon National Park, Chiang Mai, Thailand.
The selected thermopile species are Nephroepis Exaltata (Fern), Helxine soleirolii (baby
tears), Begonia Rex (Begonia) and Adiantum capillus-veneris (Maidenhair Fern) (Figure 2). The
species Hedera helix (Green Ivy), which is not a thermopile but fulfils all the selection parameters,
was also selected. It is widespread in several EU countries and is able to survive in different climate
conditions. The investigated soil was a low-density substrate supplied by Perlite Italiana s.r.l. which
is used in hydroponics cultivations. The substrate is made of perlite (30%) and of coconut fibres
Figure 2. The five selected species of plants.
4. Measurement of normal incidence absorption coefficient
4.1 Method
The absorption coefficient of the five different samples were obtained using a standard im-
pedance tube in accordance with ISO 10534-2 method17. The 100 mm diameter samples holder al-
lows to investigate the frequency range 50 1600 Hz18. Higher frequencies were not analysed be-
cause of the samples dimensions that did not fit in a 29 mm diameter holder. The impedance tube
was hung on the wall to keep the specimen (substrates and plant) upright during the measurements.
The samples leaves were trimmed to allow the plant to fit in the 100 mm tube, also the roots were
cut from the plants. Each of the five plant species was measured three times in the impedance tube,
extracting and replacing the sample in the tube taking care of not compressing the substrate at the
bottom. The absorption coefficient of a 10 cm layer of low-density soil substrate (Perlite Italiana
s.r.l. substrate) was measured in a separate experiment to provide a reference set of data.
21st International Congress on Sound and Vibration (ICSV21), Beijing, China, 13-17 July 2014
ICSV21, Beijing, China, 13-17 July 2014 4
4.2 Results
The results suggest that all the species analyzed are able to absorb a relatively high proportion
of the incident acoustic energy. However, it can be also observed that the main absorber is the layer
of substrate which is able to absorb up to 80% of incident sound energy. The presence of foliage
helps to improve the low (below 500 Hz) and higher (above 1000 Hz) absorption performance of
the layer of soil substrate by 10 20%. As the graph shows, some species are more efficient than
others, in fact some tested plants contribute to the acoustic absorption for the whole frequency
range, other worsen the acoustic absorption in several frequencies (Figure 3). In particularly Begon-
ia shows a pejorative behavior in the frequency range 650 1100 Hz, despite it is able to absorb up
to 97% of incident acoustic energy at high frequencies (1600 Hz). The Green Ivy worsens the ab-
sorption spectra at frequencies above 850 Hz which is consistent with the findings of Horoshenkov
et al15. The Fern (Nephroepis Exaltata) specimen is able to absorb up to 98% of incident acoustic
energy at frequencies close to 1600 Hz. Also the baby tears provides a very good absorption spec-
tra, increasing the soil substrate acoustic absorption coefficient for the whole investigated frequency
range. Despite the very small thickness of the sample (about 3 cm) the baby tears is able to absorb
up to 90% of incident acoustic energy.
Figure 3. Acoustic absorption coefficients of the five species of plants tested.
The most efficient species among tested samples are certainly the fern, which is able to absorb
up to 98% of the incident acoustic energy, and the baby tears, which does not show the highest ab-
sorption but provides an important contribute in the absorption for the whole frequency range. In
order to characterize acoustically in a complete way these two plant types, further measurements
were carried out in diffuse field conditions in a reverberation chamber.
5. Measurement of diffuse field absorption coefficient
5.1 Method
After performing measurements on small samples using an impedance tube, the diffused field
absorption coefficient of larger samples was measured in the reverberation chamber of the Universi-
ty of Perugia in compliance with ISO 354 method18, 19. The chamber dimensions are 4.6 m depth, 4
m width, and 2.9 m height. Therefore the chamber volume is 53.36 m3, lower than the one requested
by ISO 354. The acoustic set-up consisted in calibrated microphones, amplifier, omnidirectional
source, data recording and processing system and a thermohygrometer. The measurement positions
of microphones and omnidirectional source were chosen in accordance with ISO 354 prescriptions.
50 250 450 650 850 1050 1250 1450 1650
Absorption Coefficient (a)
Frequency (Hz)
Soil Substrates
Soil and Baby tears
Soil and Fern
Soil and Green Ivy
Soil and Maidenhair Fern
21st International Congress on Sound and Vibration (ICSV21), Beijing, China, 13-17 July 2014
ICSV21, Beijing, China, 13-17 July 2014 5
Each measurement configuration had an omnidirectional source position with three different micro-
phone positions. Despite of the low volume of the chamber, a variable number of diffusers were
installed in the chamber at various positions in order to achieve satisfactory diffusion field charac-
teristics as suggested in the Annex A of ISO 354. Using a suitable test specimen, the different dif-
fusers configurations were tested in accordance with previous works of the Authors20. It was ob-
served that the best conditions of diffuse sound field were achieved with 12 diffusers arranged at the
four room corner. In order to fix the samples to the wall and support the substrate and the plants, a
wood structure was built. The side perimeter of the wood element was covered with a thin Plexiglas
layer to avoid any influence of the wood in the acoustic absorption. The tested configurations were
three: (i) Soil and Baby tears in different greenery coverage densities (25, 50 and 90 species in-
stalled on the sample; Fig. 4); (ii) Soil and Fern in different greenery coverage densities (25, 50 and
90 species installed on the sample; Fig. 5); (iii) Soil with Baby tears and Fern (45 baby tears species
and 45 Fern species installed on the sample). The dimension of the tested samples was 1.06 x 1.06
m and 12.5cm in thickness.
Figure 4. Baby tears configurations. Figure 5. Fern configurations.
5.2 Results
The measured diffused field absorption coefficient can be higher than the unit, which is phys-
ically unrealistic. This relates to the smaller dimensions of the tested samples and smaller volume of
the reverberation chamber than those recommended in the ISO 354. Nevertheless, the obtained data
enable us to observe the relative trend in the absorption coefficient of the soil substrate with and
without the plants. Because of the small size of the room the results are significant above 200 Hz.
The results confirm that the main absorber is the substrate soil, which provides a very high
acoustic absorption in the frequency range of 500 600 Hz. The greenery coverage improves the
acoustic absorption only in high density conditions, i.e. when a large number of plants is installed
on the sample. Specifically, the configurations with 25 and 50 baby tears samples show a fluctuat-
ing behavior (Figure 6). The plants foliage, in such densities, improves the substrate acoustic ab-
sorption at some frequencies, while is pejorative at others. These variations are not significant (an
increase/decrease of substrate acoustic absorption of 5-8%). The highest increase in the acoustic
absorption occurs in the frequency range of 800 1600 Hz. This trend occurs again in the last con-
figuration with 90 baby tears installed. In this case, plants improve the substrate absorption coeffi-
cient for the whole frequency range analyzed. It was observed an increase of 11% of the substrate
sound energy absorption.
21st International Congress on Sound and Vibration (ICSV21), Beijing, China, 13-17 July 2014
ICSV21, Beijing, China, 13-17 July 2014 6
Figure 6. Acoustic absorption coefficients of Baby tears configurations.
Figure 7. Acoustic absorption coefficients of Fern configurations.
The configurations with 25 and 50 fern samples do not show a noticeable change in the spec-
trum of the diffused field absorption coefficient (see Figure 7). The greatest increase in the acoustic
absorption coefficient is observed in the frequency range of 800 1600 Hz. The 90 fern configura-
tion improves the substrate acoustic absorption up to 25% of acoustic energy absorbed, within the
frequency range 800 1600 Hz.
Figure 8 shows the absorption coefficient for a soil sample with and without 45 specimens of
baby tears and 45 specimens of fern samples. The highest acoustic absorption occurs within the
frequency range 800 1600 Hz, where the plants species in combination are able to improve the
substrate’s acoustic absorption by up to 12%.
21st International Congress on Sound and Vibration (ICSV21), Beijing, China, 13-17 July 2014
ICSV21, Beijing, China, 13-17 July 2014 7
Figure 8. Acoustic absorption coefficients of Baby tears and Fern configurations.
According to Wong et al11, the RT drop at lower frequencies is due to substrate soil (200-500
Hz), while at higher frequencies the reduction is due to the foliage. Also the acoustic absorption
coefficient behaviors are very similar.
6. Conclusions
The measurements, both in normal incidence and in diffuse field, showed that the main ab-
sorber is the substrate soil, which is able to capture a high quantity of acoustic energy. The presence
of the plants becomes useful only when a large number of them is installed on the sample, otherwise
is even pejorative within some frequency ranges. Using the impedance tube, fern and baby tears
were found as being the most efficient plants among tested species. The baby tears with soil demon-
strated a capability to absorb up to 90% of incident acoustic energy, while the fern demonstrated the
capability to absorb up to 98%. In acoustic diffuse field conditions, in the reverberation chamber of
Perugia, these two species were tested with soil, using a wood structure as support (1,06 x 1,06 x
0,125 m). The most efficient plant is the fern which improves the substrate acoustic absorption by
the 25%. Future developments of this work will take into account the influence of substrate humidi-
ty on the absorption coefficient and the influence of morphological changes of the plants on the
absorption coefficient such as the leaf area density and angle of leaf orientation. Larger samples will
also be tested a bigger reverberation room, to avoid the physical inconsistency of Figures 6, 7 and 8.
The Authors are indebted to Prof. Marco Maovaz of the Department of Agriculture of the
University of Perugia for the support in the selection of the plants and with Perlite Italiana srl for
providing the soil substrate for the measurements.
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3 Pittaluga, I., Experimental characterization of the acoustical performances of different green
roof system, PhD dissertation PhD in Mechanical Engineering, University of Genova, Italy
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and breadth of canopy, J. Acoust. Soc. Am. 51, 411414 (1972).
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- Determination of sound absorption coefficient and impedance in impedance tubes - Part 2:
Transfer-function method, (1998).
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of rubber crumbs, Proceedings of the European Conference on Noise Control EURONOISE
2008, Paris, France; 29 June 4 July (2008).
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... The study states that "the leaf area density and dominant angle of leaf orientation are two key morphological characteristics that can be used to predict accurately the effective flow resistivity and tortuosity of plants" [33]. Two other studies obtained similar results [34,35]. The main conclusion of Asdrubali et al. [34] was that "the main absorber is the substrate soil ( … ). ...
... Two other studies obtained similar results [34,35]. The main conclusion of Asdrubali et al. [34] was that "the main absorber is the substrate soil ( … ). The presence of the plants becomes useful only when a large number of them is installed on the sample, otherwise is even pejorative within some frequency ranges." ...
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Noise control refers to a set of methods, techniques, and technologies that allows obtaining acceptable noise levels in a given place, according to economic and operational considerations. The question of “acceptance” is for what or for whom. Generally, there is no single answer to this question, nor is there a single solution to any given problem, as long as regulatory compliance is achieved. Noise control does not necessarily imply the reduction of noise emissions—it refers to making acceptable sound pressure levels of immission (i.e., the signal reaching the receiver). This chapter aims to present the basis of noise control techniques, both in emission and propagation, to finally achieve the most current protection techniques for the receivers, when there are no more alternatives in the previous steps.
... Typical tropical plants which can survive without direct sunlight and grow in indoor climate conditions are found not only suitable for indoor decoration, but also helps in absorbing sound. Asdrubali et al. [15] tested the acoustic properties of tropical plants such as fern, baby tears, begonia, maidenhair fern and ivy in reverberation chamber. High density of fern shows the best absorption coefficient which is above 1.4 at frequency range 800 Hz-1.6 kHz. ...
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