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ICSV22, Florence (Italy) 12-16 July 2015 1
CHARACTERIZATION OF NATURAL FIBERS FOR SOUND ABSORPTION
Gino Iannace
Dipartimento di Architettura e Disegno Industriale, Seconda Universita’ di Napoli, Italy
e-mail: [email protected]
Umberto Berardi
Department of Architectural Science, Ryerson University, Toronto, Canada.
e-mail:[email protected]
Green materials, either natural or from recycled materials, are becoming a valid alternative to
traditional synthetic materials for sound absorption treatments. In particular, plant fibers are
often considered a valid raw material for making sound absorbing panels at a reduced cost.
Moreover, plant fibers often show good thermal insulation properties and have not harmful
effects over human health. After a literature review about existing green acoustic materials,
this paper reports the acoustical characterization of several natural fibers: kenaf, wood fiber,
mineralized wood fiber, hemp, palm date fiber, cork, cane, cardboard, and sheep wool. The
absorption coefficient of samples of the above-mentioned materials with different thicknesses
was measured in the frequency range from 250 Hz to 2000 Hz. The flow resistance was also
measured, and by using these physical properties, some preliminary considerations of the rea-
sons which justify the acoustic behavior of the different materials are included.
1. Introduction
Sound absorption panels for room acoustic applications are generally composed of porous syn-
thetic materials, such as rock wool, glass wool, polyurethane or polyester, which are expensive to
produce and often based on petrochemicals. The growing awareness towards the environmental im-
plications and human health issues of these materials has increased the attention towards natural ma-
terials [1-4]. These are generally defined according to the low level of environmental pollution during
their production or at their end-of-life, or to the low embodied energy.
A growing attention for thermal and acoustic purposes has been received by natural fibers [5-7].
A variety of natural fibers that can be used in buildings exist; although some fibers have started being
commercialized already, the majority of the products investigated in this paper is still at a research
stage. Fibers are a class of hair-like material that appears as continuous filaments or in discrete elon-
gated pieces, similar to pieces of thread; they can be spun into filaments, thread or rope, or they can
be used as a component for composites materials.
Natural fiber can be very cost effective material in the building sector, thanks to their low density,
acceptable mechanical properties, favorable processing properties (for instance low wear on tools,
etc.), good accident performance (high stability, less splintering), occupational health benefits during
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ICSV22, Florence, Italy, 12-16 July 2015 2
production, reduced fogging behavior, price advantages, high quantity availability, and reduced pol-
lutions for the production [8]. Several authors have shown that natural fibers oppose to climate change
thanks to CO2 absorption during the growth of the plant [9,10]. Others have sometimes questioned
about their real sustainability, especially in regard to the toxicity of the chemical products used for
their cultivation or during their transformation. Moreover, during preliminary evaluations of possible
materials for this study, it emerged that fiber plants are subject to fungi and parasites and are less
resistant to fire than typical mineral fibers. Therefore, they need special treatments before being used
in buildings. These treatments may reduce the inherent sustainability of the materials. Finally, atten-
tion must be paid to the environmental impact of the process and to the toxicity of the products used
during the entire process of panel production.
The materials selected for this paper are mainly vegetable, and derive from a plant process consisting
of treatments plus plant crushing finalized to obtain basic fibers. These are then compressed for sound
absorbing panels. Among the materials with a fibrous structure investigated in this paper, there are
kenaf, hemp, wood, cane, and coconut; among the materials with cellular structure, there is the cork.
2. Green materials and acoustics
The literature review about acoustics studies of natural materials show a certain variety of experi-
ence, generally focused on the production and characterization of a single material. Reversely, in this
paper, the characterization of several fibres is reported. Absorption measurements have been meas-
ured following the procedure described in the standard ISO 10534-2 [11], using a Kundt tube, with
an internal diameter of 10 cm, a length of 56 cm and mounting two 1/4 microphones at a distance of
5 cm. The measurements were hence valid in a frequency range from 250 Hz to 2000 Hz. The meas-
urements were repeated for four samples of each material, and only the average values among meas-
urements are reported below. The effect of the irregularities in the samples and in particular at the
edges of them was taken into high consideration. Precision, limitations, and sources of error or un-
certainty of the measurements may be found in the standard. The resistance to air flow was measured
with the method of the alternating flow in accordance with the ISO 9053 [12], using the cam system
that generates a sinusoidal air flow at the frequency of 2 Hz. As suggested in the ISO 9053, for each
of the samples ten measurements were performed for different air velocities in order to determine the
airflow resistance. To obtain flow resistivity, it was necessary to divide the airflow resistance by the
sample thickness. It is important to state that that the uncertainty associated with the flow resistivity
measurement is generally very large due to density change or inaccuracy in thickness determination
[13]. The air flow resistivity was then correlated to the porosity of the material. Table 1 reports the
values of the density, thickness, and air flow resistivity of the selected materials. The repeatability of
the sound absorption measurements was high and the standard deviation was always below 0.1.
Table 1. Main properties of the materials studied in the present paper.
material density
[kg/m3]
thickness
[m]
flow resistivity
[Rayl/m]
kenaf light/ dense 50/ 100 0.06/ 0.04 2700 (±290)/ 3500 (±240)
wood fibers/ mineralized 100/ 260 0.04/ 0.03 1600 (±300)/ 1800 (±450)
hemp 50 0.03 1400 (±170)
coconut 60 0.04/0.06 1500 (±200)
cork 100 0.03 1000 (±150)
cane
(mixed)
(only wooden)
(only bark)
400
470
145
0.04
0.04
0.04
850 (±130)
1000 (±125)
800 (± 40)
cardboard 140 0.115 250 (± 50)
sheep wool 40 0.04/0.06 2100 (±150)
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3. Acoustic characterization
3.1 Kenaf
The kenaf fiber, scientifically known as hibiscus cannabinus, is obtained from the stem of the
plant. Kenaf has many similarities with the hemp, and it is widespread in Africa, Asia and India,
where it is generally used for making bags. The British imported kenaf into Europe before it diffused
in North America. Kenaf panels are commercially available as semi-rigid panels of different density
and thickness. During the fabrication process, the kenaf fibers are mixed together with polyester fibers
to increase their rigidity.
The measured values of the sound absorption coefficient were high at medium and high frequen-
cies, but at low frequency values, the absorption coefficient assumed low values (Fig.1). An increase
in the density from 50 to 100 kg/m3 allowed an increase in the absorption coefficient up to 0.9 in the
mid frequency range. The tests also showed that doubling the density of the material has comparable
effects than a doubling of the thickness. These results agree with similar investigations [14].
Figure 1. Absorption coefficient of kenaf with different density and thickness (4 and 6 cm).
3.2 Wood fiber
Wood fiber panels are produced by processing the waste of wood works. The waste of the row
material is generally then cut and weakened before being mixed with a binder. The results of acoustic
measurements showed high values of the sound absorption above 500 Hz with a pick close to 1 at
1650 Hz (Fig.2).
Figure 2. Absorption coefficient of wood fiber (6 cm thick).
0.0
0.2
0.4
0.6
0.8
1.0
0 500 1000 1500 2000
Ab
sorp
tio
n c
oef
fici
ent
Frequency [Hz]
6cm, 50kg/m3
4cm,100kg/m3
6cm,100kg/m3
0.0
0.2
0.4
0.6
0.8
1.0
0 500 1000 1500 2000
Ab
sorp
tio
n c
oef
fici
ent
Frequency [Hz]
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3.3 Mineralized wood fibre
Mineralized wood fibers are commercialized in different types depending on the binder and the
manufacturing process. Semi-rigid panels are often made with crushed fibers impregnated with ce-
ment. The final product is robust, ease to transport and to install, but rarely light (the density of the
panel described below was 2.6 times higher than the density of the wood described in section 3.2.
The results of the absorption coefficient measurements revealed very low values at low and me-
dium frequencies (0.1 at 500 Hz), and modest values at higher frequencies (Fig.3). These results
discourage the use of this material for sound absorption although it may be a valued option for noise
insulation application.
Figure 3. Absorption coefficient of mineralized wood fiber (3 cm thick).
3.4 Hemp fiber
The hemp fiber, scientifically known as cannabis sativa, derives from the textile hemp whose low
quality fibers cannot be used for textile applications. The plant grows in temperate zones and it is
generally available in large quantities. The insulating panels are produced by treating the hemp fibers
with soda or boron salts to improve the fire behavior; then the material is subjected to heat setting to
form a more robust panel. Processed hemp fibers contain no toxic substances, and do not entail risks
for the health neither during the stages of processing or during the useful life.
The hemp fiber panels have good thermal and acoustic properties. The values of the absorption
coefficient at medium and high frequencies were discrete, but at low frequency, the coefficients re-
sulted negligible (Fig.4). The results of the sound absorption coefficient of the hemp was generally
lower from those find by Oldham et al. [15], who found for a sample 5 cm thick absorption values
above 0.4 at 500 Hz, 0.75 at 1000 Hz, and 0.95 at 2000 Hz.
Figure 4. Absorption coefficient of hemp fiber (3 cm thick).
0.0
0.2
0.4
0.6
0.8
1.0
0 500 1000 1500 2000
Ab
sorp
tio
n c
oef
fici
ent
Frequency, Hz
0.0
0.2
0.4
0.6
0.8
1.0
0 500 1000 1500 2000
Ab
sorp
tio
n c
oef
fici
ent
Frequency [Hz]
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3.5 Coconut fiber
The palm plant is typical in tropical regions, and its fruit fiber (coconut) has generally been used
in applications where robustness was demanded. Coconut fiber is obtained from the mesocarp, the
thick fibrous layer that covers the shell of the nut. It is produced as a waste of an agricultural produc-
tion. It has excellent characteristics of thermal insulation and sound absorption [7,16]. Various appli-
cations of the coconut fiber have been investigated. For panels in the building industry, the fiber is
mixed with binders to improve the characteristics of rigidity, anti-fungus, and flammability.
The raw material has a good sound absorption coefficient at both low and medium frequencies,
although in thicker layers, the behavior was much more remarkable (Fig.5). The results are compara-
ble with those in Fouladi et al. [7], who studied a coir-based samples produced using fresh coconut
husk with or without the addition of a binder. That study proved that the sample with the binder had
a significant reduction in the absorption coefficient. Moreover, the sample without the binder closely
followed the Biot-Allard model, while the sample with the binder was better predicted by using the
Delany-Bazley model.
Figure 5. Absorption coefficient of palm date fiber (5 and 10 cm thick).
3.6 Cork
The cork oak an evergreen oak species reaching a height of 20 m, is the raw material for the
production of cork stoppers. Nowadays, cork oaks are grown mainly in Portugal, Spain, Tunisia,
Morocco, Italy, France and Algeria. The European cork industry produces about 340.000 tons of cork
per year. Portugal is number one of the cork producing countries with a share of 31 % of the cork
oaks; 51 % of the world production is produced in Portugal’s cork forests [18]. Cork stoppers pro-
duced from the bark of the cork oak are used for tapping wine bottles.
The cork is obtained from the bark of oak. The cork is composed of spherical granules containing
air. This property allows the panel to be light and elastic, and being a good material for acoustic and
thermal insulation. The panel is commercialized in semi-rigid panels of different size and thickness.
To obtain a cork panel with good sound absorption characteristics, it is generally necessary to
adopt large thicknesses (Fig.6). The absorption coefficient of cork at low and medium frequencies is
negligible due to the size of the cork grains. On the contrary, results showed values up to 0.9 at 1600
Hz.
0.0
0.2
0.4
0.6
0.8
1.0
0 500 1000 1500 2000
Ab
sorp
tio
n c
oe
ffic
ien
t
Frequency [Hz]
5 cm thick
10 cm thick
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Figure 6. Absorption coefficient of cork (3 cm thick).
3.7 Cane
The cane, scientifically known as arundo donax, is a widespread plant that grows near water
courses. It has a very fast growing process, which often creates conflicts with the agriculture, and for
this reason, the canes are cut and the row material is widely available. The giant reeds usually reach
6 m in height and a diameter of 2–3 cm, the leaves are 30–60 cm long and 2–6 cm wide. The canes
are cut and crushed in order to have a granular material, with an average size of 4 cm in length, 1 cm
in width, and 0.3 cm thick.
The material is generally made from both the wood and the bark, although the excessive presence
of bark reduces the acoustic characteristics. In this study the shredded material represents three types:
(1) solely wooden parts (average length 40 mm, width 10 mm, and thickness 3.0 mm); (2) mixed
composed of wooden parts and the cortex with varying dimensions (the bark comes from the outer
coating of the giant reed tuber and for the speedy shredding operation will not be separated); (3) only
bark parts. The absorption coefficient for the cortex has a low value at low frequencies (Fig.7), while
the panel obtained with only the wooden part presents a more regular absorption. Finally, the panel
obtained by mixing the wood part and the bark one has an intermediate behavior.
Figure 7. Absorption coefficient of bark, wood and mix of water canes (4 and 8 cm thick).
0.0
0.2
0.4
0.6
0.8
1.0
0 500 1000 1500 2000
Ab
sorp
tio
n c
oef
fici
ent
Frequency [Hz]
0.0
0.2
0.4
0.6
0.8
1.0
0 500 1000 1500 2000
Ab
sorp
tio
n c
oef
fici
ent
Frequency [Hz]
bark 4 cm
bark 8 cm
0.0
0.2
0.4
0.6
0.8
1.0
0 500 1000 1500 2000
Ab
sorp
tio
n c
oef
fici
ent
Frequency [Hz]
wood 4 cm
wood 8 cm
0.0
0.2
0.4
0.6
0.8
1.0
0 500 1000 1500 2000
Ab
sorp
tio
n c
oef
fici
ent
Frequency [Hz]
mix 4 cm
mix 8 cm
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3.8 Cardboard
Recycled cardboard is produced by recycling used papers. The material is generally mounted so
that the veins are arranged in parallel tubes in which the sound waves propagate. The material is light
and available in great quantities, which makes it effective for temporary structures. The main limit of
this material is its poor fire performance which obliges treatments with fire retardant sprays, reducing
the inherent sustainability. Recycled materials have recently gained significant attention for the pos-
sibility of creating panels with high acoustic performance, while guaranteeing a second life to the
material [2,19].
Results of the sound absorption coefficient of recycled cardboard samples show that the material
has a good absorption coefficient at medium and high frequencies, and only below 400 Hz its behavior
is negligible (Fig.8).
Figure 8. Absorption coefficient of recycled cardboard (10 cm thick).
3.9 Sheep wool
Since ancient times, the sheep wool has been used for clothing thanks to its excellent thermal
insulation properties. However, in the last years, there has been a scarce use of wool and the product
in excess is often burnt or buried. In case of fire, the wool is self-extinguishing and do not emit toxic
substances, but can be attacked by moths or parasites.
The sheep wool is an excellent sound-absorbing material, thanks to micro-cavities of which it is
compose. The value of the absorption coefficient resulted high at medium and high frequencies, with
a fairly homogenous behavior (Fig. 9). An increase in the thickness of the panel leads to a significant
increase of the sound absorption mostly at middle frequency.
Figure 9. Absorption coefficient of sheep wool (4 or 6 cm thick).
0.0
0.2
0.4
0.6
0.8
1.0
0 500 1000 1500 2000
Ab
sorp
tio
n c
oef
fici
ent
Frequency [Hz]
0.0
0.2
0.4
0.6
0.8
1.0
0 500 1000 1500 2000
Ab
sorp
tio
n c
oe
ffic
ien
t
Frequency [Hz]
4cm thick
6cm thick
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4. Conclusions
The measurements carried out on samples of natural fibers have shown that these materials have
often good values of the absorption coefficient, especially at medium and high frequencies. Only by
a significant increase in their thickness it was possible to increase their absorption values at low fre-
quencies. This behavior reflects that of traditional porous materials. It is important to note that the
materials were manufactured in the research laboratory. Further research will focus on standardize
the manufacturing process and investigate the behavior of these fibers with different binders.
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