Effect of Different Factors on the Acoustic Absorption of Coir Fiber
Mohd Jailani Mohd Nor,
Mohammad Hosseini Fouladi
The aim of this study was to explore and analyze the effect of different factors on the absorption of coir fiber using the developed analytical techniques employing Johnson-Allard rigid frame model. Estimated results were verified by the measurements conducted in impedance tube on normal incidence sound absorption of coir fiber. It was found that the developed analytical method can provide a well consistent agreement with the experimental results. Factors that may have positive or negative effect are elaborated in this study. It describes how the physical elements of coir fiber absorber panel can change the absorption behavior. Results obtained show that layer thickness and fiber diameter have a significant effect on the absorption, whereas bulk density does not have any considerable effect. In addition, an example is presented in order to show the approaches of enhancing the absorption utilizing the advantage of modification in the physical elements. It exhibits that properly chosen fibers along with suitable amount of bulk density can increase the absorption for the same layer thickness. It indicates that these analyses can be powerfully exploited to improve the absorption of coir fiber and at the same time maintain a reasonable thickness which would be very efficient for limited space structure. Moreover, these results can serve as a guideline for the future implementation of acoustic absorber using naturally collected coir fiber.
Received: May 10, 2010;
Accepted: July 22, 2010;
Published: October 11, 2010
In practical application, most of the sound absorbing products used in the
building construction industry consist of synthetic materials. Because of the
dominance of these materials in the commercial market, the study of alternative
materials has been limited. However, several research works had been carried
out and findings have uncovered potential new materials for sound absorption
applications (Wassilieff, 1996; Ersoy
and Kucuk, 2009; Dias et al., 2007a, b;
Dias and Monaragala, 2006). Natural substances are in
the center of interest as they are recyclable and easily available source. Malaysia
has plenty of agricultural waste such as coconut fiber (Cocos nucifera),
rice fiber (Oryza sativa) and oil palm frond fiber (Elaeis Guinnesis)
which are abundant and usually burned or used as agricultural by-products (Zulkifli
et al., 2009a, b). These natural fibers,
such as coir fiber, are suitable as a substitute for synthetic fibers for acoustic
absorption purposes. Investigation of the sound absorption attribute of coir
fiber was initiated by Automotive Research Group laboratories, Universiti Kebangsaan
Malaysia (Nor et al., 2004; Zulkifli
et al., 2008, 2009a, b).
Thereafter, study was carried out to find out the analytical modeling techniques
to characterize the acoustic behavior of coir fiber and experimental observations
in impedance tube. Analytical modeling method of coir fiber and the analyses
of the absorption behavior can be found in authors previous works (Fouladi
et al., 2010a, b; Ayub
et al., 2010). Studies conducted on coir fiber showed that they have
high potential to be used as sound absorption panel (Ayub
et al., 2009). They can be very useful for various usages in many
structural and non-structural applications. However, the effects of different
physical factors that may have considerable contribution on the absorption of
coir fiber are not explored yet. This current study was aimed to elaborate the
effect of different factors such as thickness of the porous layer, bulk densityfiber
size of material on the acoustic absorption.
The study of the acoustical performance of natural substance coir fiber material is important to use it efficiently in applications such as transportation sector, buildings, automotive interior noise, wall lining, room interior surface, muffling system etc. Market demands increases for porous absorbers with the aspirations like lower frequency absorption ability, wide band frequency absorption capacity, thin structure for limited space absorber, specific acoustic absorption spectrum and low cost materials. To achieve these divergent targets and make an optimized absorber using a porous material, it is important to truly understand the relationship between the chemistry, structure, morphology and physical properties of an individual technology concerns to porous material. Therefore, modeling the coir fiber and exploring the effect of different physical and non physical elements on the absorption coefficient is vital and inevitable, since acoustic absorption characteristics have to be enhanced and optimized for commercial use. The main objective of this study is to analyze the effect of different factors on the absorption of coir fiber using the Johnson-Allard rigid frame model.
MATERIALS AND METHODS
Acoustic absorption properties of coir fiber was investigated for natural fresh samples and industrially made fibers mixed with binder in Acoustic Laboratory at Universiti Kebangsaan Malaysia (UKM). Fresh coir fiber samples were collected from fresh coconut husk available in local wet market and then made it dry in open sunny weather. After drying, it was compressed to make the sample using molds. Therefore, fresh coir fiber samples were not processed and sample contains almost same ingredients (including matrix granular part) as it was when collected from local market. Industrial coir fiber was prepared industrially using binder (latex) as a mixer with the fiber to keep it in shape. Samples were collected industrially as a large rectangular sheet and then cut into suitable circular shape for impedance tube.
Johnson-Allard rigid frame model (Allard, 1993) was implemented
to estimate the acoustic characteristics impedance and propagation constant
of coir fiberhence the absorption coefficient of the porous layer backed with
rigid wall. This model was accompanied by some compensation in the physical
parameters which was considered to make the model useful for industrially treated
coir fiber. It was assumed that binder became parts of fibers, covered their
surfaces and filled the porosity between them. Therefore, based on the analyses,
it was derived that the new diameter of fiber mixed with binder increased a
proportional amount based on the porosity of the material. As a result a new
parameter was considered for the diameter of fiber. A detailed analytical calculation
procedure along with the experimental validation can be found in authors
previous works (Fouladi et al., 2010a, b;
Ayub et al., 2010).
Experiments were conducted in impedance tube according to ISO 10534-2 (ISO
10534-2 1998) standard and on normal incidence sound absorption of coir
fiber. The measurement system included two impedance tubes with diameters 28
and 100 mm each contains two ¼@ microphones type GRAS-40BP, plane wave
source, dual channel Symphonie (01dB model) real time data acquisition unit
and 01dB software package. Calibrator type GRAS-42AB was used for microphone
sensitivity calibration at 114 dB and 1 KHz frequency.
Analyses on the absorption of coir fiber were conducted utilizing Johnson-Allard
model by varying different physical elements to explore and understand the effect.
Comparison between the analytical and experimental results showed that the developed
model can consistently provide a well estimation of the absorption coefficient
for coir fiber (Ayub et al., 2010). Performance
of the model was checked more specifically using the mean prediction error rate
(Kino and Ueno, 2007). It was observed that the mean
error rate shows discrepancies lower than 20% for the prediction of absorption
coefficient using Johnson-Allard rigid frame model (Allard,
1993). Results and verifications from the previous findings indicate that
developed model can be used efficiently for further analyses. Eventually, rigid
frame model is also implemented in this study to estimate the acoustic characteristics
of coir fiber in different conditions.
RESULTS AND DISCUSSION
Effect of fiber layer thickness: Figure 1 and 2 illustrate the acoustic absorption of fresh and industrial coir fiber for different thicknesses. Natural fiber has an average absorption of 0.8 for f >1360 Hz, f >940 Hz and f >578 Hz at thicknesses of 20, 30 and 45 mm, respectively. Unlike fresh coir fiber, industrial coir fiber has the average absorption varied within 0.65-0.8 for all sample thicknesses. For instance, 20 mm industrial fiber shows average absorption 0.8 for f >3190 Hz and absorption again decreases in high frequencies.
Industrial fiber with 35 mm thickness has average absorption 0.7 for f>1887
Hz despite a small decline in 3443-4584 Hz frequency band though it is more
than 0.65. Similarly, 50 mm sample shows 0.8 for f>1300 Hz and more than
0.7 for 1962-3984 Hz. It shows that increasing coir fiber layer thickness increases
the absorption and moves absorption peak towards low frequency for both cases.
||Experimental results of absorption coefficient for different
layer thickness of fresh coir fiber
||Experimental results of absorption coefficient for different
layer thickness of industrial coir fiber
Increasing the thickness of material enhanced the sound absorption in lower
frequencies having same average absorption coefficient. It indicates that the
absorption increases as impinged wave has to go long way through the material
and losses its energy. According to absorption phenomena inside a porous material,
long dissipative process of viscosity and thermal conductions in the fluid inside
the material due to increased thickness improve the absorption.
However, it is observed that fresh coir fiber has better absorption than industrial
fiber for the same thickness of the material. This is attributed to the fact
of lower moisture content and stiffness effect from binder, which is also addressed
in previous research works (Wassilieff, 1996). However,
flow resistivity of the fresh coir fiber might also be a reason. It is noted
that the flow resistivity of fresh coir fiber is greater comparatively than
that of industrial fiber for the same thickness of material. The main factors
influencing the flow resistivity of fibrous material are the fiber size and
bulk density of the material (Delany and Bazley, 1970).
During the preparation of fresh coir fiber, it was found that coir fiber contains
some matrix material with the fiber. Extra matrix granular part along with the
fiber increased the bulk density of the material, as a result flow resistivity
also increased. Nevertheless, fresh coir fiber without any treatment (or binder)
can not be used regularly as an absorber for long time period because of the
moisture and stiffness effect of the fiber (Wassilieff,
1996). It may decrease the thickness of the porous material later on and
change the absorption characteristics as mentioned earlier.
Despite the good acoustical absorption coefficient, coir fiber may not be used commercially in its natural form. It should be mixed with additives to keep it in shape and improve characteristics such as fire retardant, anti-fungus, etc. Here, binder was the only additive utilized by manufacturer to attach fibers together and adding stiffness. These samples had lower acoustic absorption; peaks were flattened and moved to higher frequencies. They exhibited weak absorption at low frequencies and tactics such as adding air gap or perforated plate are necessary to improve this shortcoming. However, these results demonstrate that more strategically designed layers and configurations of coir fiber could increase the noise reduction properties. Therefore, further analysis is conducted only for industrially treated fiber rather than fresh coir fiber.
Effect of bulk density: Absorption coefficient of 35 mm thick industrial coir fiber with varying bulk density is shown in Fig. 3 in order to reveal the effect on sound absorption of coir fiber. Six different mass of the coir fiber sample varying from 20 to 45 g with the duration of 5 g were considered for the circular sample of 100 mm diameter in order to change the bulk density of the material. Figure 3 shows that increasing bulk density of the porous material enhances the absorption of coir fiber and moves the peaks toward lower frequency. Enhancement of absorption occurs due to increased flow resistivity with increased bulk density.
However, the effect is not that much significant as it should be for usual
porous material characteristics due to the same layer thickness of coir fiber.
It can also be noticed that the profile of the graphs is slightly downward in
high frequency with the increased density and the peak is almost in the same
position though the additional bulk density increases the absorption. It means
that increasing bulk density does not change the position of absorption peak
considerably rather it increases the absorption coefficient in that peak position.
As long as there is no additional layer (coir fiber or air) with the existing
layer, no additional change occurs in the absorption peak. These results denote
that absorber prepared with larger bulk density has a small effect on the absorption
of coir fiber.
||Numerical simulations of absorption coefficient for 35 mm
industrial coir fiber with different bulk density
||Simulated absorption coefficient of 50 mm (mass = 34.13 g)
coir fiber for different fiber size varying from 250 to 100 μm (original
fiber diameter of industrial coir fiber = 252 μm)
However, increased bulk density can be a useful factor for enhancing the absorption
and it should be within a limit which will allow the sound wave to go through
the material. Otherwise, there will be a probability of sound wave to be reflected
by congested material surface rather than absorption due to compact material.
Effect of fiber size (Diameter): In the previous studies (Sun
et al., 1993; Koizumi et al., 2002;
Lee and Joo, 2003) on the various parameters that influence
the absorption properties of fibrous material, fiber size was shown to be an
important parameter that change the absorption significantly. In this section,
effect of coir fiber diameter on sound absorption is investigated and the results
are shown in Fig. 4. In the figure numerical simulation of
absorption coefficient is plotted for 50 mm thick industrial coir fiber with
different fiber diameter varying from 250 to 100 μm. It shows that absorption
coefficient changes dramatically with the variation of fiber size. Significant
enhancement in low frequency absorption is observed with the reduction of fiber
This behavior can be attributed to the fact of the changes of two basic material
properties; tortuosity and flow resistivity with the change of fiber size as
addressed from the previous works (Sun et al., 1993;
Lee and Joo, 2003). In general, flow resistivity of
porous material is inversely proportional to the fiber diameter for a given
porosity (Ingard, 1994), whereas tortuosity of the porous
material is an indicator of how much tortuous the transmission path of sound
wave within the absorbent (Cox et al., 2009). Thinner
fiber due to reduction of fiber diameter causes the requirement of more fiber
to reach an equal volume density for the same thickness. Addition of more thin
fibers results in a more tortuous path and higher airflow resistance of the
porous materials, which promotes the absorption coefficient and shifts the peak
towards low frequency as well. Moreover, thin fiber can move more easily than
thick fiber in sound waves which induces vibration in air. It results an increase
in airflow resistance by means of friction through the vibration of the air.
These results indicate that fiber size plays an important role for the improvement
of sound absorption of coir fiber. Since, coir fiber is a natural resource,
coir fiber absorber panel may not be prepared with just only by using thin fibers.
However, it can be achieved by making the absorber panel with mixing the fine
thin fibers with usual thick fibers from the treated coir fiber. It can also
be accomplished by mixing the thin fiber from other material (such as man-made
fabric, glass or mineral fiber) with the usual coir fiber.
Comparison of absorption performance by modifying the parameters: An
example is presented in Fig. 5 to demonstrate how the modification
in the fiber size, layer thickness and bulk density can improve the absorption
coefficient of coir fiber. As shown in Fig. 5, solid line
represents the absorption of 50 mm coir fiber layer in normal condition (fiber
diameter = 250 μm and mass of the material = 34.13 g); dotted line shows
the absorption plot for the same layer thickness of coir fiber with the only
modification in fiber size as 150 μm and same mass of the material (mass
of the material = 34.13 g); bold line corresponds to the absorption for 30 mm
coir fiber layer with the modification both in fiber size and mass, hence the
change of bulk density (fiber diameter = 150 μm and mass of the material
= 34.13 g) and dashed line concerns to the case of 30 mm iber layer in normal
condition (fiber diameter = 250 μm and mass of the material = 20.48 g).
It shows that decreasing the fiber diameter of 50 mm coir fiber by keeping the
same bulk density (mass of the material = 34.13 g) enhances the absorption significantly.
||Simulated results for the comparison of absorption performance
among different conditions of coir fiber with the modification in the layer
thickness, fiber size and bulk density
The advantage in the modification of fiber size can be utilized to reduce the
layer thickness as shown for the case of 30 mm fiber layer with the modification
in fiber size and mass of the material. It enables almost similar absorption
plot of 50 mm coir fiberat the same time it makes the absorber more suitable
for low space structure, whereas 30 mm fiber layer in normal condition shows
a very low profile absorption behavior especially at low frequency. Hence it
might be an efficient tool to reduce the thickness of acoustic isolators in
In this study, analysis have been conducted to explore the elements that may
affect the absorption behavior of coir fiber based on the analytical method
demonstrated in Authors previous works. In these analyses, Johnson-Allard model
were implemented to estimate the surface impedance of coir fiber in different
conditions. Variations of acoustic behavior of coir fiber with the change of
physical elements such as bulk density, fiber size layer thickness are illustrated.
It describes how the different factors can affect the absorption behavior of
coir fiber. Results also explore that proper design consideration of absorber
with the modification in the parameters can control and change the absorption
of coir fiber. An example has been shown to demonstrate the possible way of
improving absorption of coir fiber by utilizing the advantages of changing the
elements. However, since the coir fiber is collected naturally, its elements
can not be changed as it demands for the better absorption. In that case, absorbers
can be prepared using those coir fibers chosen with the proper fiber size and
thickness among all available fibers.
The authors thank to Ministry of Science, Technology and Innovation (MOSTI) and Ministry of Higher Education (MOHE), Malaysia for financially supporting this research. The laboratory facilities provided by the Department of Mechanical and Materials Engineering, Faculty of Engineering and Built environment, UKM, Malaysia are also gratefully acknowledged.
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