Textiles 2021, 1, 55–85. https://doi.org/10.3390/textiles1010005 www.mdpi.com/journal/textiles
Factors Affecting Acoustic Properties of Natural-Fiber-Based
Materials and Composites: A Review
, Hafsa Jamshaid
, Rajesh Mishra
, Muhammad Qamar Khan
, Michal Petru
, Martin Tichy
and Miroslav Muller
Protective Textile Group, National Textiles University, Faculty of Textile Engineering, Department of Fabric
Manufacturing, Faisalabad 37610, Pakistan; firstname.lastname@example.org (T.H.); email@example.com (H.J.)
Faculty of Engineering, Czech University of Life Sciences Prague, Kamýcká 129,
165 00 Praha-Suchdol, Czech Republic; firstname.lastname@example.org (M.T.); email@example.com (M.M.)
Department of Textile and Clothing, National Textile University Karachi Campus, Karachi 74900, Pakistan;
Faculty of Mechanical Engineering, Technical University of Liberec, Studentska 2,
46117 Liberec, Czech Republic; firstname.lastname@example.org
* Correspondence: email@example.com
† These authors contributed equally.
Abstract: Recently, very rapid growth has been observed in the innovations and use of natu-
ral-fiber-based materials and composites for acoustic applications due to their environmentally
friendly nature, low cost, and good acoustic absorption capability. However, there are still chal-
lenges for researchers to improve the mechanical and acoustic properties of natural fiber compo-
sites. In contrast, synthetic fiber-based composites have good mechanical properties and can be
used in a wide range of structural and automotive applications. This review aims to provide a short
overview of the different factors that affect the acoustic properties of natural-fiber-based materials
and composites. The various factors that influence acoustic performance are fiber type, fineness,
length, orientation, density, volume fraction in the composite, thickness, level of compression, and
design. The details of various factors affecting the acoustic behavior of the fiber-based composites
are described. Natural-fiber-based composites exhibit relatively good sound absorption capability
due to their porous structure. Surface modification by alkali treatment can enhance the sound
absorption performance. These materials can be used in buildings and interiors for efficient sound
Keywords: acoustic; natural fibers; composites; sound absorption coefficient; noise attenuation
Recent advancement in controlling noise through sound absorption provides an
opportunity to investigate various porous materials including fiber-based composites.
Commercially available sound absorption materials are of three types, i.e., fibrous,
granular, and cellular. Fibrous sound absorption materials are further divided into two
categories, natural and synthetic, based on fiber origin. Interest in fiber-based composites
is growing very rapidly due to their lightweight and high performance in several appli-
cations. Such composite materials receive great attention because of their wide range of
applications in automotive, wind energy, sports, aerospace, and civil engineering appli-
cations. Fiber-based composite is the combination of fibers and matrix and the resultant
material with improved mechanical properties compared with individual fibers and
matrix [1,2]. Further, the natural-fiber-based composites are gaining importance because
of their environmentally friendly nature. Natural-fiber-based composites have ad-
vantages such as high abrasive resistance, low emission of toxic fumes with heat, high
specific strength, light weight, low cost, and eco-friendliness [3,4]. There are different
Citation: Hassan, T.; Jamshaid, H.;
Mishra, R.; Khan, M.Q.; Petru, M.;
Tichy, M.; Muller, M. Factors
Affecting Acoustic Properties of
Natural Fiber Based Materials and
Composites: A Review. Textiles 2021,
1, 55–85. https://doi.org/10.3390/
Academic Editors: Philippe Boisse
and Stepan Lomov
Received: 16 March 2021
Accepted: 26 May 2021
Published: 31 May 2021
Publisher’s Note: MDPI stays
neutral with regard to jurisdictional
claims in published maps and
Copyright: © 2021 by the authors.
Licensee MDPI, Basel, Switzerland.
This article is an open access article
distributed under the terms and
conditions of the Creative Commons
Attribution (CC BY) license
Textiles 2021, 1, 1 56
types of sound absorbers, which include hollow resonant structures, porous structures,
and composites, which also have a unique sound absorbing capability while interacting
with different intensities and frequencies. In the beginning, asbestos-based materials
were used as a sound absorber, but later, they were replaced with advanced alternatives.
Asbestos was the material earlier used for soundproofing and sound insulation in 1800s
in the US during the industrial revolution. Some properties of asbestos are
non-flammability, non-corrosiveness, and good electrical insulation. It was one of the
most widely used materials for soundproof applications in roofs, offices, houses, and roof
ceilings in schools, etc. Asbestos consists of mineral fibers like anthophyllite, tremolite,
crocidolite, amosite, actinolite, and chrysotile .
At the initial stage, there was a lack of understanding about the harmful effects of
asbestos on animal and human health. Later, researchers found that asbestos is carcino-
genic and hazardous to humans as well as animals. Since then, most countries around the
world have banned the use of asbestos. Especially the European Union took very strict
action and banned the use, import, and export of asbestos. Some industries use synthetic
fibers as an alternative to asbestos fibers [6,7]. For synthetic fibers, often the starting ma-
terials used are cellulose or natural polymers. It was found that synthetic fibers are also
hazardous for human health. Inhalation of synthetic fiber can cause lung injury, which
leads to cancer [8–10].
Researchers are also working on the addition of granular materials to the fiber based
composites, which significantly enhance flow resistivity and bulk density of the compo-
site for increasing the chances of low-frequency sound absorption. However, the incor-
poration of such materials causes increased environmental pollution and CO
which cause global warming. Many sustainable natural fibers such as coir, banana, sug-
arcane, jute, and sisal are available for designing potential sound absorbers .
2. Acoustic Properties of Fibrous Materials and Composites
The sound absorption coefficient (SAC) can be calculated by measuring the total
amount of sound energy absorbed by the materials. The range of SAC lies between 0 to 1,
in which 1 represents the highest absorption, while 0 shows no absorption at all. Ab-
sorption of low-frequency sound waves, e.g., 500 Hz, is very difficult as compared to
high-frequency sound waves. Propagation of sound waves through a medium without
any absorption and loss of frequency is known as transmission [12,13]. The transmission
coefficient (t) is the fraction of incident energy that is not reflected or absorbed. The
transmission loss can be defined as 10log(t) dB. When the sound waves strike surfaces,
some part is absorbed, while the rest is reflected [14,15].
There are mainly two methods of measurement reported for sound absorption: the
reverberation chamber method and the impedance tube method. The reverberation
chamber method is widely used for a bigger sample size in order to determine the sound
absorption coefficient . The sample is mounted inside a sound insulated reverbera-
tion room/chamber. The walls, roof, floor, etc., are highly reflective. The sound in dif-
ferent frequencies is generated by a source and is allowed to propagate in all directions.
The sample absorbs part of the sound energy, and the rest is reflected or diffused or even
transmitted. This method involves a random incidence sound absorption, and the coeffi-
cient is termed the random incidence sound absorption coefficient.
In the case of a smaller sample size, the normal incidence impedance tube method is
preferred [17,18]. The sound absorption performance of porous materials can be tested by
the two-microphone impedance tube method, as shown in Figure 1. A two-microphone
transfer function impedance tube (ISO-10534-2) in the frequency range of 100–2500 Hz is
used during acoustic testing. It requires relatively small circular samples, either 29 or 100
mm in diameter according to the frequency range (500 to 6.4 kHz or 50 to 500 Hz, re-
spectively). This method avoids the need to fabricate a large test sample with lateral di-
mensions several times the acoustic wavelength. The sound absorption coefficient is
Textiles 2021, 1, 1 57
calculated for low as well as high frequency ranges. Average sound absorption coeffi-
cient values are reported.
Figure 1. Impedance tube method for measurement of sound absorption .
2.1. Classification of Sound in Terms of SAC
The various classes of sound absorption as per ASTM C423—17 are given in Table 1.
Table 1. Classes of the sound absorption coefficient .
Range SAC Class
0.90, 0.95, 1.00 A
0.80, 0.85 B
0.60, 0.65, 0.70, 0.75
0.30, 0.35, 0.40, 0.50, 0.55 D
0.15, 0.20, 0.25 E
0.00, 0.05, 0.10 F
Based on SAC, the sound absorption performance is classified into 6 classes, A, B, C,
D, E, and F, as shown in Table 1. Category “A” is the most efficient class with the highest
sound absorption coefficient, while category “F” denotes the minimum sound absorption
2.2. Sound-Absorbing Materials
Sound absorbers are divided into resonators, porous absorbers, panel absorbers, and
membranes. Some examples of porous absorbers are open-cell foams, mineral wool, and
carpet. Porous absorbers allow sound and airwaves to pass inside the materials contain-
ing channels and cavities. As per the literature, sound absorbers are further categorized
into fibrous, cellular, and granular types. Studies on sound absorption are focused on fi-
brous composite materials . A comparative account of sound absorption in recycled
polyurethane foam using various models and experiments is shown in Figure 2.
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Figure 2. Sound absorption behavior based on various models .
The normal incidence sound absorption coefficient was measured for samples of 2
cm thickness using the impedance tube method. Experimental results were compared
with two established theoretical models. The Dunn and Davern model is a
semi-empirical model used to describe the acoustic behavior. This model determines the
real and imaginary parts of the characteristic propagation constant and the characteristic
wave impedance. It takes into account the air flow resistivity (N s/m
) and frequency (Hz)
The Voronina model uses simple analytical functions that depend on the porosity of
the material, the frequency, and the average pore diameter. A quantitative estimation of
sound absorption in porous material is determined using structural characteristics. Using
this model, the sound wave impedance and propagation constant are calculated from the
porosity and average pore diameter for a material .
2.3. Factors Affecting Acoustic Properties of Fibrous Sound-Absorbing Materials and Composites
Materials with the ability to significantly absorb sound energy are known as sound
absorbers. Sound absorption occurs while sound waves pass through a porous material,
and a reduction in sound energy takes place due to friction with the pore walls and
thermal exchange. There are certain factors like fiber size, temperature, porosity, and
flow resistivity, density, thickness, compression, and design or placement that signifi-
cantly affect the acoustic properties of fibrous materials and their composites.
2.3.1. Effect of Different Fiber Types
Researchers investigated the acoustic properties of lignocellulosic fibers based
composites. Three types of thermoplastic binders, e.g., zein, polylactic acid, and poly-
propylene, were used. Additionally, two types of thermoset binders, epoxy and unsatu-
rated polyester (UP), were used for impregnation. Further, five types of fibers, betel nuts,
oil palm, rice straw, sisal, and luffa fibers, were used as reinforcement. The results re-
vealed that physically and chemically treated fibers show a higher sound absorption co-
efficient than untreated fibers. As fiber volume fraction is increased, the sound absorp-
tion coefficient increases as well. Among all samples, the polypropylene/rice straw
composite showed the highest sound absorption coefficient. Surface modification causes
further enhancement of the interfacial adhesion, which significantly enhances mechanical
properties. Mihai Bratu et al. investigated the acoustic behavior of different composites
based on waste fibers and other wastes. Formaldehyde was used as a matrix along with
Textiles 2021, 1, 1 59
steelworks slag, fiberglass waste, wood waste, and waste ash from burning shells of
plants as filler. The best result was obtained for wood waste and glass fiber waste.
Therefore, it can be used as an absorbing panel in industries and automotive, etc. .
Yang and Li investigated the acoustic behavior of natural-fiber-based composites.
They used jute, flax, and ramie fibers as a reinforcement and Epoxy resin as a matrix. It
was found that, at a frequency range of 256–2000 Hz, ramie, jute, flax, glass, and carbon
fibers show SAC of 0.6, 0.65, 0.65, 0.35, and 0.45 respectively. The jute fiber-based com-
posite materials show a sound absorption coefficient of 0.9 at 10,000 Hz frequency, as
shown in Figure 3 .
Elammaran Jayamani et al. investigated the acoustic behavior of lignocellulosic ag-
ricultural fibers and their composites. Kenaf and rice straws (RS) were used along with
urea-formaldehyde (UF) and polypropylene (PP) as a matrix. It was found that, by in-
creasing the frequency of incident sound waves, the absorption coefficient also increases.
Kenaf-fiber-based urea-formaldehyde composites with a thickness of 1.8 cm showed a
higher sound absorption coefficient as compared to PP-based samples of similar thick-
ness. It is stated that polymer composites have an average SAC ranging between 0.008–
0.065 . Zhang et al. investigated the sound absorption of natural fibers and sandwich
composites structures. Flax fabric was used as reinforcement and epoxy as the matrix. It
was found that flax-fiber-based composites show superior acoustic absorption compared
to glass-fabric-based composites, as shown in Figure 4. Flax fiber composites show rela-
tively better sound absorption at a wider frequency range. It is due to their fibrous mi-
crostructure and multi-scale micro-morphology .
Figure 3. SAC of (a) different fibers, (b) epoxy resin, (c) fiber composites .
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Figure 4. SAC of flax-fiber-based composite and glass-fiber-based composite .
Zhang et al. evaluated the acoustic properties of various natural-fiber-based com-
posites using the acoustic impedance tube. It was found that jute-fiber-based composites
showed maximum sound absorption performance for a wide range of frequencies, as
shown in Figure 5 .
Figure 5. The sound absorption coefficient of natural-fiber-based composites of thickness 1.8 cm
Maderuelo-Sanz et al. reported on the sound absorption performance of composites
produced from waste tires. The sound absorption coefficient of panels having 2.0 cm
thickness is shown in Figure 6 .
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Figure 6. Absorption coefficient of panels made from tire waste .
The real and imaginary part of acoustic impedance for panels are shown in Figure 7.
The real part is the resistance associated with energy losses, and the imaginary part is the
reactance, associated with phase changes [26,27].
Figure 7. (a) Real and (b) imaginary part of acoustic impedance for sound absorbers made from tire
Prabhu et al. investigated the sound absorption performance of sisal- and
tea-fiber-waste-based composites. As shown in Figure 8, tea-fiber-based composite sam-
ples with thickness 1.5 cm exhibited better sound absorption coefficients as compared to
other fibers .
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Figure 8. Sound absorption of composites with tea fibers .
Tiuc et al. investigated the acoustic properties of textile-waste-based rigid polyure-
thane closed-cell foam. Textile waste in the portion of 10–50% was used in the compo-
sites. As shown in Figure 9, better sound insulation was achieved as compared to pure
polyurethane material with a thickness of 2 cm .
Figure 9. The sound absorption coefficient of PU foam is based on different proportions of textile waste .
Elammaran Jayamani et al. investigated the sound absorption coefficient of natu-
ral-fiber-based epoxy composites. They used sugarcane bagasse, kenaf, and coconut fi-
bers in epoxy composites with a thickness of 1.8 cm. It was found that at the 500–2500 Hz
frequency range, sugarcane fiber composites provide better sound absorption, while co-
conut fiber composites also have good sound absorption at 2500–4000 and 5500–6000 Hz.
Due to higher density, sugarcane-fiber-based composite was found to be a better sound
absorber at relatively lower frequencies. It was also found that coconut-, kenaf-, and
sugarcane-fiber-based composites have sound absorption coefficients of 0.086, 0.086, and
Textiles 2021, 1, 1 63
0.085, respectively. Chen et al. also reported on the acoustic properties of ram-
ie-fiber-based composites with a thickness of 2.0 cm .
2.3.2. Effect of Fiber Size
A change in fiber size may be a change in length or diameter. Fiber size is considered
one of the major factors that affect acoustic properties. Lee et al. investigated the effect of
fiber diameter on the acoustic properties of composites. They used polyester with dif-
ferent fiber fineness, e.g., 1.25, 2, and 7 deniers. Non-woven samples with a thickness of
2.5 cm were developed from these fibers by using low melting polyester for binding
purposes. It was found that by reducing fiber diameter, the sound absorption coefficient
increases. It is because airflow resistance increases with smaller fiber diameter .
Koizumi et al. concluded that by decreasing fiber diameter, the sound absorption
coefficient increases, as shown in Figure 10. It was described that fiber denier ranging
from 1.5 deniers to 6 deniers results in better sound absorption than coarser denier. Fur-
ther, it was concluded that by using micro-denier fibers, a dramatic enhancement in
sound absorption coefficient can be achieved .
Figure 10. SAC comparison of different fiber diameters .
Ren et al. described that for achieving efficient SAC with the same volume density,
finer fibers are preferred as compared to coarser fibers. More fibers per unit area result in
a more tortuous path, which results in a higher sound absorption coefficient .
Bakri et al. investigated the SAC of banana-fiber-based epoxy composites. The re-
sults revealed that, by decreasing fiber diameter, flow resistivity increases, which causes
an increase in the SAC as shown in Figure 11 . Hasina Mamtaz et al. investigated the
acoustic behavior of various other natural-fiber-based composites with a thickness of 1.8
cm. They also found that fiber diameter is an important parameter for enhancing sound
absorption. By decreasing fiber diameter, SAC increases .
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Figure 11. Effect of fiber diameter on SAC of composites .
Luu et al. researched the acoustic properties of polydisperse fiber networks. They
used effective fiber diameter for modeling. During reconstruction, they considered two
types of polydisperse fiber matrix and bi-dispursed fiber matrix. Results reveal that fiber
diameter has a significant effect on the sound absorption coefficient. A significant in-
crease in the sound absorption coefficient has been observed by decreasing fiber diameter
Xiang et al. investigated the acoustic properties of kapok fibers. The results revealed
that long fibers have slightly higher SAC than short fibers. Further, the random orienta-
tion of fibers shows higher SAC than oriented fibers . V. Arumugam et al. conducted
a study on the effect of fiber orientation on acoustic properties of the glass epoxy lami-
nates. Different stacking sequence was used to investigate its mechanical properties. A
significant effect on acoustic behavior has been observed by changing the orientation,
which also changes overall porosity .
Chen et al. studied the morphology and properties of ramie-fiber-based PLLA
composites. They used fibers of different lengths. A fiber volume fraction of 30:70 ram-
ie/PLLA was used for composite manufacturing. The acoustic properties of the resultant
sample were measured through the standing wave tube method. Results revealed that
when sound hits the surface of the composite, the non-vertical angle fiber with incident
wave absorbs some part of the sound wave. Moreover, the composite with shorter ramie
fiber was found to be a better sound absorber .
2.3.3. Effect of Fiber Fraction
Jiang et al. studied and reported the sound absorption performance of seven-hole
hollow polyester fibers (SHPF)-based composite samples with a thickness of 1.5 cm .
The influence of increasing fiber fraction on the sound absorption coefficient is shown in
Figure 12. With increasing fraction of hollow fibers, the sound absorption performance is
observed to improve.
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Figure 12. Effect of hollow fiber fraction on the sound absorption performance of composites .
Sheng Jiang investigated the sound absorption coefficient of seven-hole polyes-
ter-based chlorinated polyethylene composite. It was found that the SAC of untreated
polyethylene was 0.2 at 100–2500 Hz frequency range. By increasing reinforcement con-
tent, remarkable improvement has been observed in SAC. At 20 wt% fiber content, SAC
is found to be 0.42 for sample thickness of 2 cm. As the thickness of the composites in-
creases, the SAC also increases. It was found that by increasing reinforcement content,
SAC is significantly improved, as shown in Figure 13 .
Figure 13. Effect of fiber content on CPE/SHPE composite .
Abdul Hakim Abdullah et al. investigated the sound absorption of natu-
ral-fiber-based composites. The study was mainly conducted to find out the SAC of ba-
nana fibers, sugarcane bagasse fibers, and their hybrid composites with thickness of 1 cm.
Polyester binders were used as a matrix. It was found that the sound absorption coeffi-
cient of sugarcane fiber is 0.6338 and of banana fiber is 0.6635, and by the combination, it
reaches up to 0.733 at 2325 Hz frequency. For 20 wt% fiber volume fraction, banana fiber
has 0.586, sugarcane has 0.71, and a combination of both fibers has 0.73 SAC at 2500 Hz
frequency. It was also established that by increasing fiber volume fraction, sound ab-
sorption increases .
Elammaran Jayammani et al. investigated the acoustic properties of epoxy-based
banana-fiber composites. It was found that by increasing frequency, the sound absorp-
tion coefficient increases. By increasing fiber content, sound absorption also increases. It
Textiles 2021, 1, 1 66
was also found that alkali-treated fiber composites have a higher sound absorption coef-
ficient as compared to untreated fiber composites. It is because materials with lower
molecular weight were removed, which reduces sound reflection and increases absorp-
An inspection of the effect of fiber loading on the sound absorption property of flax
fiber-based composites (FFRC) was conducted and presented in Figure 14 .
Figure 14. Effect of flax fiber content on SAC .
Reixach et al. also investigated the acoustic properties of natural-fiber-based com-
posites. The effect of fiber content on the transmission losses for sample thickness 1.1 cm
are shown in Figures 15 and 16. The effect of fiber treatment on the sound absorption
performance was also reported .
Figure 15. Transmission loss (TL) for the 20–50% polypropylene based composites .
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Figure 16. Transmission loss (TL) for the 30% polypropylene based composites .
2.3.4. Effect of Fiber Treatment
Elammaram Jayamani et al. investigated and found that alkali-treated and
heat-treated fiber-based composites have a relatively higher sound absorption coefficient
than untreated composites . They further investigated the effect of alkali treatment on
the sound absorption coefficient of other natural-fiber-based polymer composites. Sisal,
rice straws, betel nuts, luffa, and fruit bunch were used as reinforcement. Further, poly-
mers epoxy, unsaturated polyester (thermoset), and three types of thermoplastic poly-
mers zein, polypropylene, and polylactic acid were used as a matrix. Fibers were treated
with 5 wt% alkali solution. It was found that the sound absorption coefficient of natural
fibers is relatively good due to the presence of inter-fiber microvoids in the fiber struc-
ture. Alkali-treated fiber-based composites in all cases exhibited a higher sound absorp-
tion coefficient compared to untreated fiber-based composites. The alkali treatment
changes the composition of fibers, which causes the relative motion between polymers
and fibers [45–54]. The influence of alkali-treated betelnut fiber on SAC for 1 cm thick
samples is shown in Figure 17.
Figure 17. Influence of betelnut fiber content on sound absorption coefficients of composites .
S. Fatima and A.R. Mohanty reported a study on the effect of fiber treatment on the
acoustic properties and fire-retardant properties of jute fiber and its composites. Before
conducting testing, some physical properties of the fibers were measured, e.g., tortuosity,
porosity, and flow resistivity. For measuring the sound absorption coefficient, cylindrical
shape commercial-grade treated (TD4) and untreated (TD5) jute were used.
Two-microphone impedance tubes were used for measuring the sound absorption coef-
ficient. Results reveal that treatment removes the impurities and the surface becomes
rougher, which significantly enhances mechanical properties and acoustic absorption of
the fiber-based composites .
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2.3.5. Effect of Sample Thickness
The sample thickness is one of the most important characteristics that influence the
sound absorption performance. Change of thickness changes several other parameters,
including density and porosity. Coates et al. studied the effect of thickness on the acous-
tic behavior of porous materials. Effective absorption of sound is achieved at 1/10th of the
wavelength of incident sound waves in their measurements . Hirabayashi et al. de-
scribed that at resonance frequency, peak absorption can occur at one-quarter of the in-
cident sound wave. It is clear that there is a significant relationship between SAC and
thickness of the materials at high, medium, as well as low frequencies . The material
thickness should be a quarter of the wave length of the sound wave to be an effective
absorber. This is applicable to all measurements involving the impedance tube method
where hard backing is used.
Ibrahim et al. also described that increasing thickness causes an increase in SAC. At
relatively higher frequencies, sound absorption increases nonlinearly . The influence
of thickness on SAC is shown in Figures 18–20 from the reported literature .
Figure 18. SAC of 2.5 cm thick wool samples .
Figure 19. SAC of 1 cm thick wool samples .
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Figure 20. Effect of thickness on SAC .
Jiang et al. reported that by increasing the thickness of seven-hole polyester fiber
and chlorinated polyethylene composite, a significant improvement in sound absorption
coefficient was observed. They investigated the acoustic properties of a seven-hole hol-
low-polyester-fiber-based chlorinated polyethylene (CPE) composite. Polyester fiber
with 10Dtex fineness and 60 mm length was used in the study. During composite man-
ufacturing, a fiber volume fraction of 65:35 and a reinforcement/matrix was used with
three different thickness levels of 1, 2, and 3 mm. Test results conclude that by increasing
the thickness of the sample, the sound absorption coefficient will also be increased, as
shown in Figure 21 .
Figure 21. Effect of thickness on SAC .
The effect of thickness on the sound absorption coefficient is shown in Figure 22. A
higher sound absorption coefficient was obtained by increasing the thickness of compo-
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Figure 22. Influence of thickness of composites on sound absorption coefficients .
Hasina Mamtaz et al. investigated the sound absorption coefficient of fibro-granular
epoxy composites. Coconut/coir fiber was used along with rice husk grain as a granular
filler material. The results shown in Figure 23 reveal that by increasing thickness, the
SAC of composites also increases .
Figure 23. Effect of sample thickness on SAC .
They also validated the experimental measurement with the Johnson–Champoux–
Allard (JCA) model, as shown in Figure 24. The Johnson–Champoux–Allard (JCA) model
involves non-acoustical physical parameters, e.g., flow resistivity, tortuosity, porosity,
viscous characteristic length, and thermal characteristic length. This model is widely
used to describe the propagation of sound in porous media.
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Figure 24. The sound absorption coefficient of fibro-granular composite to validate the analytical
outcome through impedance tube measurement .
Taban et al. studied the sound absorption of palm-fruit-fiber-based composites. The
effects of fiber density and sample thickness are shown in Figure 25 . A higher
thickness and relatively lower density resulted in better sound absorption.
Figure 25. Effect of thickness and density onSAC of date-palm-fiber-based composites .
Shiney et al. investigated the acoustic properties of composite coir mats. Coir mats
with different weaving patterns like Boucle weaving, Panama weaving, and Herringbone
weaving were used along with the epoxy matrix. Acoustic properties were measured
through an impedance tube in the frequency range of 250–2000 Hz by following the
ISO-10534-2 standard. It was concluded that for all structures, an increase in thickness of
the sample results in an increase of sound absorption coefficient of the composite .
2.3.6. Effect of Gluing and Multiple Layering of Composites
Su et al. studied the effect of gluing multiple layers in a composite on the sound
absorption behavior . The sound propagation in multilayered composite is schemat-
ically shown in Figure 26.
Figure 26. Sound propagation model of multilayer materials .
The effect of the damping factor of the gluing material on sound absorption coeffi-
cient of the multilayer composite is shown in Figure 27.
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Figure 27. The effect of damping factor of gluing material on sound absorption coefficient .
The effect of damping factor of the gluing material on the sound transmission loss of
the multilayer composite is shown in Figure 28.
Figure 28. The effect of damping factor of gluing material on transmission loss .
2.3.7. Effect of Perforation
Yuvaraj et al. developed perforated acoustic panels from jute fiber based composites
as shown in Figure 29 . Substantial improvement in sound absorption performance
was observed in the case of perforated panels as compared to non-perforated material of
the same composition [64–68].
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Figure 29. Perforated jute fiber composite panel .
Figure 30 shows the influence of % depth penetration on the sound absorption coef-
ficient of the jute fiber based composites with a thickness of 1 cm.
Figure 30. Effect of depth of perforation on the SAC of the jute-fiber-based composites .
Sound transmission loss for different perforation depths is shown in Figure 31.
Figure 31. Effect of depth of perforation on sound transmission loss .
2.3.8. Effect of Adding Plasticizer and Flame Retardants
Researchers investigated the SAC of ramie-fiber-based polylactic acid composites.
They used plain weave ramie fabric and short fibers for the manufacturing of composites
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with thickness 1 cm. The results shown in Figure 32 reveal that by adding a plasticizer
and/or flame-retardant finish, the sound absorption is improved .
Figure 32. Effect of adding plasticizer on SAC of different composites . PLLA: Poly L-Lactic
acid, PBAT: poly(butylene adipate-co-terephthalate), ksFAB: ramie fabric treated by permanganate
acetone solution and silane coupling agent, ksFIB: ramie fabric treated by permanganate acetone
solution and silane coupling agent, F-ksFAB: ksFAB with flame retardant, F-PBAT–PLLA: PBAT–
PLLA with flame retardant
2.3.9. Effect of Frequency
Elammaran Jayamani et al. further investigated the sound absorption of a be-
tel-nut-fiber-based polymer matrix composite. Unsaturated polyester (thermoset) and
polypropylene (thermoplastic) were used as a matrix. It was found that by increasing
frequency, the sound absorption coefficient of the betel-nut-fiber-based unsaturated
polyester composite also increases. The sound absorption coefficient of sisal-fiber-based
polylactic acid biocomposites was also reported. It was found that by increasing fre-
quency, the sound absorption coefficient increases proportionally . Yang et al. con-
ducted a study on the acoustic properties of jute-, ramie-, and flax-fiber-based composites
. Samples were fabricated with 65% fiber volume fraction and overall thickness of 40
mm. An impedance tube was used to measure the sound absorption coefficient with a
frequency range of 63–10,000 Hz. Researchers indicated that frequency has a significant
effect on the sound absorption coefficient. An increase in sound absorption has been ob-
served by increasing frequency. Initially from 0–1000 Hz, rapid growth in sound absorp-
tion was recorded after the increase rate was reduced [70–75]. Several other researchers
have reported on the influence of frequency on the acoustic properties of composite ma-
terials based on fibers [76–80]. The influence of frequency on the sound absorption coef-
ficient of fiber-based lime-wool mortar is shown in Figure 33.
Textiles 2021, 1, 1 75
Figure 33. The acoustic absorption coefficient of lime-wool mortar at different frequencies .
2.3.10. Effect of Back Cavity Depth
Sound absorption mainly occurs in a specific frequency range, which shifts to the
lower region by increasing the depth of back cavity. Jiang et al. reported that absorption
values are maximal in the range 450–900 Hz, and there is a dependence between the
thickness and the reduced frequency. There is a decrease in sound absorption as the cav-
ity depth decreases. This type of behavior is a typical resonance effect . The sound
absorption coefficient for different depths of the back cavity is shown in Figure 34.
Figure 34. Effect of back cavity depth on the sound absorption performance of CPE/SHPF compo-
2.3.11. Effect of Temperature
Srivastava et al. investigated the effect of change in temperature on sound absorp-
tion. They found that the temperature change significantly affects the sound absorption
coefficient . Harris investigated the effect of temperature and humidity on sound
absorption in the air. He used a frequency range of 2000–12,500 Hz and six temperatures
ranging from 0.5° to 25.1° C at normal atmospheric pressure. The effect of temperature on
the noise attenuation coefficient with varying frequencies was measured. At 50% relative
humidity, the results revealed that at low frequency, the temperature does not show any
significant effect on the noise attenuation coefficient. However, at high frequency, the
highest attenuation coefficient was recorded at the lowest temperature. Sound absorption
continuously decreases with an increase in temperature .
Textiles 2021, 1, 1 76
Knudsen also investigated the effect of temperature and humidity on sound ab-
sorption in nitrogen, oxygen, and air. He used two reverberation chamber methods for
measuring sound absorption. Results revealed that by increasing temperature, sound
absorption decreases .
2.3.12. Effect of Porosity and Tortuosity
Researchers investigated the effect of porosity on the acoustic behavior of the mate-
rial. If the porosity of the material is higher, then the sound waves come frequently in
contact with the surface of the materials, which causes dissipation of acoustic energy. In
addition to that, the size and number of pores are also very important for the sound ab-
sorption behavior of any material. When sound waves interact with the porous surface of
the materials, the sound waves are damped . By increasing volume porosity, SAC
increases, as shown in Figure 35.
Figure 35. Effect of porosity on SAC .
Tortuosity is best described as the influence of internal structure on the acoustic
properties of the materials. It is the elongation of the way through pores. Sakagami et al.
investigated the acoustic properties of membrane-type sound absorbers. Results show
that less tortuous materials can absorb more sound as compared to more tortuous mate-
rials. Highly tortuous materials are more likely prone to large fluctuations in the sound
absorption coefficient. Researchers also stated that tortuosity does not have a significant
effect on noise transmission coefficient . Dupont et al. described that by increasing
porosity, sound absorption will also be increased .
2.3.13. Effect of Flow Resistivity
Flow resistivity is the ability to resist airflow from entering the core of any material.
It can also be defined as a measure of how much air can enter into a porous material. It is
one of the key factors that affect acoustic properties. The acoustic properties of materials
are affected by the flow resistance per unit thickness of the absorber material. In
non-woven materials, the interlocking of the fibers gives enough friction to resist the
motion of sound waves. When sound waves pass through the rough and tortuous path,
they cause friction, which results in a decrease in wave amplitude, and sound wave en-
ergy is converted into heat . According to Crocker, if the flow resistivity value of a
material is higher, then airflow resistance will be higher as well. Sometimes materials
with too-high airflow resistance result in more sound reflection than absorption . The
Textiles 2021, 1, 1 77
acoustic properties of materials mainly depend on intrinsic properties. The transmission
loss of the sound when it passes through a porous material mainly depends on the sound
wave frequency, thickness, and flow resistivity of the material. By increasing flow resis-
tivity, transmission loss will also be increased. Airflow resistivity has an inverse relation
to air permeability. Moreover, as the airflow resistivity of the material increases, then it is
difficult for sound waves to enter the material. Hence, sound absorption shows a signif-
icant decrease [85,86].
2.3.14. Effect of Density
Density is known to be one of the most important parameters that affect the acoustic
properties of materials. It was found that by increasing the density of the fibrous materi-
al, the sound absorption coefficient also increases, especially at higher and medium fre-
quencies. Koizumi et al. investigated the effect of density on the acoustic properties of
materials. They concluded that density has a direct relation to SAC at high and medium
frequencies . Ballagh et al. studied the effect of bulk density of wool fiber on sound
absorption of the material. It was found that materials having relatively lower densities
have higher sound absorption at low frequencies as compared to higher density materi-
als, which absorb high and middle-frequency sound waves. It is further described that
wool fibers perform very well at higher density with a frequency equal to or more than
500 Hz, because at higher frequencies, surface friction increases, which causes the dissi-
pation of more sound energy. By increasing the density of materials, fiber per unit area
will also be increased, which increases the sound absorption coefficient, as shown in
Figures 8 and 9 .
2.3.15. Effect of Compression
According to Castagnede et al. and Wang et al., the compression of fibrous or porous
material causes an increase in the sound absorption coefficient, as shown in Figure 36. It
is because by compression, the constituting fibers come closer to each other, thus in-
creasing density and decreasing the thickness. Further, an increase in compression causes
an increase in tortuosity as well as flow resistivity, which decreases the shape factor
Figure 36. Effect of compression rate on SAC .
Nor et al. studied the effect of compression on the acoustic properties of coconut coir
fibers. Experiments of coir fiber with different compression levels were conducted by an
impedance tube with 28 mm diameter, and standard ISO-10534-2 were used to validate
the analytical model. Samples were tested at different compression levels leading to dif-
ferent thicknesses, e.g., 50, 35, and 20 mm. It is obvious that compression significantly
affects the physical parameters like flow resistivity, porosity, and tortuosity of the sam-
Textiles 2021, 1, 1 78
ples. Results revealed that compression reduces the porosity of the sample, and it shifts
the absorption towards a higher frequency. Moreover, a higher compression rate causes
an increase in sound absorption .
Keshavarz et al. described that the compression of materials may cause an increase
or decrease in the sound absorption coefficient, which mainly depends on the method of
compression. The results indicated that increases in compression lead to decreases in the
sound absorption coefficient in the low and medium frequency region for samples with
thickness of 2 cm. For high frequencies, compression improves the absorption coefficient,
as shown in Figure 37 .
Figure 37. Influence of initial thickness and angle of sound incidence on SAC .
2.3.16. Effect of Location of Sound Absorbers
Everest investigated the effect of placement of the active absorbent material on
sound absorption. The sound absorber was placed at different positions such as ends,
sides, and the vertical, traverse, and longitudinal modes on the ceiling. The results re-
vealed that placement of sound absorbers along the edges and near corners of a rectan-
gular room shows effective absorption of the sound [91,92].
Lu et al. studied the effect of placement on the acoustic properties of the materials.
They concluded that if an air gap is added behind the sample, it will change the absorp-
tion behavior. Hence, the sound absorption of the materials is significantly increased by
the addition of an air gap between the sample and back surface .
S. Fatima et al. conducted studies on the effect of rigid backing and airgap backing of
treated and untreated jute fiber composites. Commercial grade jute fibers named TD5
were used in their study. An impedance tube was used with a frequency range of 0–4000
Hz. Air gaps of 25.4 and 50.8 mm were used between sample and rigid backing during
testing. Results revealed that a 50.8 mm air gap results in the highest sound absorption
coefficient, while rigid backing without an air gap leads to the lowest sound absorption
Textiles 2021, 1, 1 79
coefficient. Hence, by increasing the air gap between the sample and rigid backing, the
sound absorption coefficient can also be increased .
A detailed account of several fiber-based composite sound-absorbing materials as
reported by various researchers is given in Table 2.
Textiles 2021, 1, 1 80
Table 2. Summary of acoustical properties of fiber-based composites.
1 Banana Fiber Epoxy
Fiber loading 20 %
wt.%, Thickness 20
(1000 psi 24 °C) 0.11 (6000 Hz) 
2 Ramie Fiber Poly L Lactic
Thickness 3 mm,
Fiber Loading 30%
(Pressure 20 MPa,
Temperature 170 °C,
Time 4 min
0.12 (1600 Hz) 
Oil Seed Waste
Resin - -
0.8 (3200 Hz)
waste 0.9 (3200 Hz)
Wood Waste 0.9 (3200 Hz)
Steel Slag 0.65 (3200 Hz)
4 Rice Straw Urea
Thickness 10 mm,
fiber loading of 0–30
(500 psi 140 °C)
(8000 Hz) 
Epoxy Thickness 3 mm
0.6 (2000 Hz)
0.65 (2000 Hz)
Jute 0.65 (2000 Hz)
Fiber Loading 30% -
0.63 (4000 Hz)
0.68 (4000 Hz)
Composite 0.73 (4000 Hz)
8 Hemp Recycled latex Thickness 300 mm - 0.50 (2000 Hz) 
Thickness 40 mm 0.50 (3000 Hz)
Rice straw Polypropylene
Fiber Loading 10%
Temperature 190 °C,
Time 30 min
0.08 (2000 Hz)
yde 0.065 (2000 Hz)
10 Sisal Fibers Poly Lactic Acid
Fiber Loading 30 %,
Thickness 8 mm Hot Press Machine 0.085 (2000 Hz) 
Fiber Loading 20 %,
Thickness 10 mm
Machine, Pressure 7
MPa, Time 24 h,
Temperature 24 °C.
0.086 (6000 Hz)
Kenaf 0.085 (6000 Hz)
Sugarcane 0.083 (6000 Hz)
acid) 0.089 (1600 Hz)
Wheat straw Polypropylene
0.03 (1800 Hz)
Jute Zein 0.06 (5000 Hz)
Thickness 1 mm,
Fiber Loading 20%) -
0.42 (2500 Hz)
3 mm (Fiber Loading
20%) 0.695 (2500 Hz)
13 Wheat straw Polypropylene
Thickness of 3.2 mm,
fiber loading of 40–80
Hot compression 0.03–0.23
(3000 Hz) 
base Epoxy Fiber Loading 60%,
method, Pressure 2.5
0.369 (10 kHz)
0.293 (10 kHz)
Glass 0.324 (10 kHz)
Textiles 2021, 1, 1 81
0.32 (10 kHz)
Jute 0.419 (10 kHz)
Epoxy resin Fiber loading 50%,
Thickness 4 mm
Hot press machine
under pressure of 1
120 °C for 2 h for
0.96 (3200 Hz)
Balsa Wood 0.58 (3200 Hz)
Fiber Loading 25%,
Thickness 5 mm
Hot and cold
Sisal Poly-lactic Acid
Rice Straw Polypropylene
Oil palm Zein 0.095 (500–6000
17 Banana Fiber Epoxy Resins Fiber Loading 20%
Pressure 10 MPa
0.1 (500–6000 Hz)
18 Flax Epoxy Thickness of 3 mm Compression
(laminated) 0.11 (2000 Hz) 
In this review, the details of various factors affecting the acoustic behavior of natu-
ral-fiber-based materials and their composites were summarized. Natural fibers have
relatively good sound absorption capability due to their porous structure. Fiber-based
composites are widely used in buildings and constructions due to their good mechanical
and acoustic insulation properties. Researchers found certain factors that affect the SAC
of FRCs, i.e., fiber diameter, fiber type, fiber content, frequency, alkali treatment, sample
thickness, fiber length, fiber orientation, and addition of plasticizer and/or fire-retardant
finish [88–93]. Results reported by various researchers reveal that by increasing the con-
tent, frequency, sample thickness, and fiber inclination of fibers, the SAC of their com-
posites will also be increased. Decreasing fiber diameter causes an increase in the SAC of
composites. Alkali treatment of fibers causes enhancement of sound absorption. Further,
random orientations of fibers in composites can absorb sound more efficiently than
aligned fibers. The addition of a plasticizer and fire retardant finish to composites causes
a further increase in the SAC.
Author Contributions: All authors contributed to this review. Conceptualization, T.H., H.J. R.M.,
M.P. and M.M.; methodology, T.H, M.Q.K. and M.T.; software, H.J. R.M., M.P., and M.M.; formal
analysis, T.H., H.J. R.M., M.Q.K., M.P., M.T. and M.M.; resources, H.J. R.M., M.P., M.T. and M.M.;
writing—original draft preparation, T.H., H.J. R.M., M.Q.K., M.P., M.T. and M.M.; writing—review
and editing, T.H., H.J. R.M., M.Q.K., M.P., M.T. and M.M.; supervision, H.J. R.M., M.P. and M.M.;
project administration, H.J. R.M., M.P., M.T. and M.M.; funding acquisition, R.M., M.P., M.T. and
M.M. All authors have read and agreed to the published version of the manuscript.
Funding: The work was supported by the internal grant agency of the Faculty of Engineering,
Czech University of Life Sciences Prague (no. 2021:31140/1312/3108), project: Optimization of per-
formance and comfort properties of fire-resistant knitwear (No. NRPU/HEC/8980), project: “De-
velopment of comfortable compression socks for treatment of varicose veins or chronic venous
disease in legs” under HEC project Establishment of Technology Development Fund (TDF)
Textiles 2021, 1, 1 82
(No.TDF03-149), and by the Ministry of Education, Youth and Sports of the Czech Republic, the
European Union (European Structural and Investment Funds—Operational Program Research,
Development and Education) in the frames of the project “Modular platform for autonomous
chassis of specialized electric vehicles for freight and equipment transportation”, Reg. No.
Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.
Data Availability Statement: No data is associated with this work.
Conflicts of Interest: The authors declare no conflict of interest.
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