Technological Sciences
© Science China Press and Springer-Verlag Berlin Heidelberg 2012 tech.scichina.com www.springerlink.com
*Corresponding author (email: liyan@tongji edu.cn)
• RESEARCH PAPER • August 2012 Vol.55 No.8: 2278–2283 doi: 10.1007/s11431-012-4943-1
Sound absorption performance of natural fibers and their composites
YANG WeiDong & LI Yan
*School of Aerospace Engineering and Applied Mechanics, Tongji University, Shanghai 200092, China Received April 19, 2012; accepted May 25, 2012; published online June 30, 2012
This research aimed to study the sound absorption properties of natural fibers and their reinforced composites. Sound absorp- tion coefficients of three types of natural fibers, i.e., ramie, flax and jute fibers and their composites were measured by the two-microphone transfer function technique in the impedance tube. The results were compared with synthetic fibers and their composites. It was found that both natural fibers and their composites had superior capability of noise reduction. The mul- ti-scale and hollow lumen structures of natural fibers contributed to the high sound absorption performance. Moreover, the sound absorption properties of these natural fibers were also calculated by the Delany-Bazley and Garai-Pompoli models. They showed good agreement with the experimental data. It was concluded that multi-functional composite materials can be made by natural fibers so that both the mechanical and acoustical functions can be achieved.
natural fiber reinforced composites, sound absorption, impedance tube
Citation: Yang W D, Li Y. Sound absorption performance of natural fibers and their composites. Sci China Tech Sci, 2012, 55: 22782283, doi:
10.1007/s11431-012-4943-1
1 Introduction
Nowadays noise pollution has become the third pollution resource that has great adverse influences on the environ- ment, human health and economy. How to reduce the dam- ages of noise has become an important issue. Generally, the means of controlling noise include active control and pas- sive control [1]. The former is accomplished by reducing the production of noise at the locations of noise sources, but it can only control the noises of a narrow frequency range.
The passive control is normally achieved by utilizing high sound absorption materials, which can be used to absorb noises of a wide frequency range by effectively dissipating sound energy on the process of propagation of the sound wave.
Natural fibers such as ramie, jute, kenaf, etc. reinforced
composites have drawn a lot of attention from material sci- entists and engineers in recent years due to their good me- chanical properties, light weight, environmentally friendly and biodegradable [2, 3]. However, natural fibers, as fibrous porous materials, have also been interested in yielding the variety of new sound absorption structures in more recent years [4–7]. As a kind of fibers, natural fibers are supposed to have the same mechanism for acoustic absorption as oth- er conventional synthetic fibrous materials, like glass fiber and mineral wool. The mechanisms of sound absorption in porous fiber material are basically the viscous effects due to the internal friction between fiber wall and airflow, and thermal losses due to heat transfer among different fibers [8].
Previously, the sound absorption coefficient of a material was measured by the well-known standing-wave-ratio (SWR) method. However, the SWR method is relatively time consuming due to the requirements of a single fre-
quency operation, a travelling microphone inside the im- pedance tube, and a method of accurate location of the minimum sound pressure. In recent years, a method, called transfer function method [9], which calculates the transfer function between the sound pressures at two settled loca- tions, Mic.1 and Mic.2 (as shown in Figure 1), has been extensively used.
Delany and Bazley [10] studied the impedance and wave propagation properties of fibrous materials in 1970s. They normalized these parameters into dimensionless groups and presented simple power-law relations obtained by a large amount of experimental data of glass fiber and mineral wool.
The empirical model is still a good and fast approximation to the theoretical calculations because the model needs only one input parameter, the airflow resistivity. In addition, Garai and Pompoli [11] developed an empirical model based on a number of measurements upon polyester fibers.
Due to the differences of fiber diameters and the densities of the matrix materials, they corrected some parameters in order to apply the calculations of polyester fibers effectively.
It should be pointed out that the diameter and density of polyester are similar to natural fiber. Thus, using Garai and Pompoli model to predict the sound absorption parameter of natural fiber might give an accurate result.
In this study, the sound absorption coefficient of natural fibers, like flax, ramie and jute fiber, and their composites were studied from both experimental and theoretical. The mechanisms of the sound absorption were proposed. The results were also compared with those of synthetic fibers, which proved that natural fiber composites have superior acoustic absorption properties and are good candidates for sound absorption structures.
2 Materials and experiment
Three kinds of plain woven natural fibers which are ramie, flax and jute fibers were used to study the acoustic absorp- tion performance. All the natural fiber samples had the identical thickness of 40 mm for measurements. Three kinds of natural fibers reinforced epoxy composites were made by hot press. The fiber volume fractions were around 65% and the thickness was around 3 mm. The composite samples for
Figure 1 Schematic diagram of instrumentation for the transfer function method of measuring sound absorption coefficients of natural fiber rein- forced composites. Mic.1 and Mic.2 denote two microphones.
acoustic measurements were machined by water jet that guaranteed the sample to have superior dimension accuracy.
Plain woven glass and carbon fibers and their composites were also prepared in the same way as those of natural fi- bers for comparison.
The impedance tube for the transfer function method was employed to measure the sound absorption coefficients of fibers and composites. The facilities were made by BSWA Technology Co., Ltd, as shown in Figure 2. The tests were conducted according to ISO10534-2 standard. The meas- urements are composed of SW422, SW477 and SW499 impedance tubes with diameters of 100, 30 and 16 mm for measuring frequencies from 63–2000, 800–6300 and 2500–
10000 Hz, respectively. The sound absorption coefficients of fibers were measured at frequencies from 63 to 2000 Hz due to the edge constraint effects and the difficulties of achieving the dimensional accuracies of the fabrics for smaller samples [12]. The measurements were conducted at frequencies from 63 to 10000 Hz for composites at 25℃ and 60% relative humidity. The test samples of natural fiber and synthetic fiber reinforced composites are shown in Fig- ure 3.
3 Results and discussion
3.1 Sound absorption behavior of natural fibers Figure 4 compares the sound absorption coefficients of nat- ural fibers (ramie, flax and jute) with synthetic fibers (car- bon and glass fibers). It can be found that jute fiber has the
Figure 2 Impedance tube by transfer function technique.
Figure 3 Acoustic measurement samples of different fibers reinforced composites with diameters of 100, 30 and 16 mm, respectively.
Figure 4 Sound absorption coefficients of natural and synthetic fibers.
best sound absorption property especially at frequencies above 1000 Hz. The sound absorption coefficient is basi- cally higher than 0.9. Additionally, the sound absorption coefficients of flax and ramie fibers are about 0.8 and 0.6 at frequencies above 800 Hz, respectively. However, the val- ues for glass and carbon fibers are overall lower than those of natural fibers. In order to make an easy visual compari- son among the different fibers, the noise reduction coeffi- cients (NRCs), defined as the arithmetic mean value of sound absorption coefficients at 250, 500, 1000 and 2000 Hz, are summarized in Figure 5. It is found that NRCs of ramie, jute and flax are as high as 0.6, 0.65 and 0.65 while those of glass and carbon are 0.35 and 0.45, respectively.
The mechanism of fibrous materials to absorb sound en- ergy mainly involves three physical processes: Firstly, when the sound wave transmits into the fibers, the viscous effect between fiber frame and numerous air cavities will attenuate part of sound energy and convert it into heat; secondly, heat transfer will happen due to temperature distinction between different fibers and this process will further dissipate sound
Figure 5 Noise reduction coefficient (NRC) comparison of natural and synthetic fibers.
energy; finally, the vibration of air in the bulk materials will also lead to the vibration of fibers [13].
Figure 6 compares the cross-sections of natural fibers and synthetic fibers. Figure 6(a) shows the cross-section of a single sisal fiber (which can be a typical representative of the structures of natural fibers) and Figure 6(b) shows the cross-sections of several glass fibers. It is found that a sin- gle sisal fiber is made up of a bundle of hollow subfibers that have lumen inside. However, for glass fiber (also a typical representative for synthetic fibers), each fiber has the same regular and solid construction. Figure 7 further indicates the unique structural characteristics of natural fibers. Therefore, natural fibers are porous fiber materials which contain many connected open air cavities and those air cativities might be the major contributors of sound en- ergy absorption. The sound wave could propagate by vibra- tion through the air spaces and inside the lumen of natural fibers, in which the friction effect between cell wall and airflow would transform sound energy into heat. Thus the unique lumen structure endowed the superior sound absorption ability, compared to glass and carbon fibers.
Moreover, by further observing the microstructures of natural fibers, it can be realized that natural fiber possesses a multi-scale structure as shown in Figure 7. A single sisal fiber is made up of a bundle of hollow subfibers. The cellwall of a subfiber is made up of millions of nano fibrils [14]. The nano-sized fibrils would also lead to the extra vibration, which caused more sound energy dissispation.
3.2 Theoretical predication of sound absorption prop- erties of natural fibers
The Delany-Bazley empirical model needs the flow resisti-
Figure 6 Cross-section morphology of (a) sisal fiber and (b) glass fiber.
Figure 7 Multi-scale and hollow lumen structure of sisal fibers at differ- ent magnifications.
vity and then calculates the complex acoustic impedance, Zc
and propagation constant, kc both of which can be used to further determine the reflection coefficient.
0.754 0.732
0 0
0 1 0.0571 j0.087 ,
c
f f
Z c
(1)
0.7 0.595
0 0
1 0.0978 j0.189 ,
c
f f
k c
(2) where 0 andcare the density and speed of air media, respectively, is the flow resistivity, f is the frequency and 2 f is the angular frequency.
The Garai-Pompoli model was utilized to predict the flow resistivity, acoustic impedance and sound absorption coefficient of polyester fibers, the diameters and density of which are relatively close to those of natural fibers.
0.623 0.660
0 0
0 1 0.078 j0.074 ,
c
f f
Z c
(3)
0.53 0.571
0 0
1 0.121 j0.159 .
c
f f
k c
(4) The Delany-Bazley and the Garai-Pompoli models must
rely on the airflow resistivity values calculated with the Mechel models, which is expressed by
1.296 2 3
6.8 (1 ) , a
(5)
where is the viscosity of air (equal to 1.84×105 Pa s), ais the radius of the fibers and is the porosity which can be determined by fiber matrix density and bulk density.
Figure 8 shows that the measured sound absorption coef- ficient for the jute fiber in the impedance tube tended to follow the general trend of the theoretical calculations ob- tained by the predictions of the Delany-Bazley and the Garai-Pompoli models. The theoretical calculated parame- ters of natural fibers were shown in Table 1. The calculated results of the Delany-Bazley model were similar to the pre- dictions of Garai-Pompoli model, and the sound absorption coefficient curves were consistent with the predictions of the two models. The experimental data were a little larger than the calculated results of the two models. The experi- mental sound absorption coefficient of jute fiber at 1600 Hz was 0.92, while the values at the same frequency which were calculated were about 0.98 of those of the two theo- retical models.
3.3 Sound absorption behavior of natural fiber rein- forced composites
Figure 9 shows the sound absorption coefficients of these
Figure 8 Sound absorption coefficient curves of the jute fiber by experi- mental results compared with the theoretical predictions of Delany-Bazley model and Garai-Pompoli model.
Table 1 Theoretical calculated parameters of natural fibers Natural fibers Fiber diameter
(m) Fiber matrix density
(g/cm3) Fiber bulk density
(g/cm3) Calculated porosity
(%) Calculated flow resistivity (Pa s/m2)
Flax 15 1.5 0.341 77.29 83421
Ramie 25 1.5 0.443 70.44 123782
Jute 58 1.37 0.411 70.03 19790
Figure 9 Sound absorption coefficients of natural fibers (ramie, jute and flax), and synthetic fibers (glass and carbon fiber) reinforced composites.
composites at the frequencies from 63 to 10000 Hz. It is found that sound absorption properties of natural fiber rein- forced composites are overall better than those of the glass and carbon fiber reinforced composites. Jute fiber rein- forced composite has the best sound absorption property and the sound absorption coefficient can be as high as near- ly 0.9 at 10000 Hz. It indicates that natural fiber reinforced composites have superior noise reduction capability espe- cially at high frequency which would be very beneficial for the aeronautical applications due to the high sound fre- quency service environment. The density and porosity of the natural fiber reinforced composites and synthetic fiber rein- forced composites were measured by underwater weighing method and the results were shown in Table 2. It can be seen that compared to glass and carbon fiber reinforced composites, natural fibers (flax, ramie and jute) reinforced composites possessed lower density and much higher poros- ity. Therefore, the unique lumen structures of natural fibers resulted in the lower density and higher porosity of their reinforcing composites, which contributed to the more sound energy absorption, thus better acoustic absorption property. It should be pointed out that all the composites studied appeared the sound absorption trough at 8000 Hz.
This is because that when the thickness of material is equal to or even multiples of a quarter of incident wavelength, the sound pressure will be the maximum while the vibration rate of air particles is zero, which leads to the lowest sound energy loss caused by the viscous effect.
Figure 10 Sound absorption coefficient of epoxy resin.
Compared to natural fibers, the sound absorption proper- ty of their reinforcing composites apparently decreased.
Natural fibers possess excellent sound absorption property by themselves whereas the hollow lumen and air space would be significantly diminished. The resin would occupy some effective volume of airflow and the cavities between fibers and inside lumens were compressed by the pressure added during the process of composite manufacturing. Fig- ure 10 shows the sound absorption properties of epoxy resin and it can be seen that the sound absorption behavior of the resin system is very low. Additionally, the sound absorption property largely depends on the frequencies of sound wave.
The higher the frequency, the shorter the sound wave length and the longer the propagation path of sound wave in the composites. Therefore, more dissipation of sound energy happens in the composites. This explains why natural fiber composites have the best sound absorption performance at high frequencies.
4 Conclusion
The sound absorption property of natural fibers was superi- or to synthetic fibers such as glass and carbon fibers due to their unique hollow and multi scale structures. Natural fiber reinforced composites also possessed better acoustic ab- sorption behavior than synthetic fiber reinforced composite, especially at high frequencies, which might be very benefi-
cial for the aeronautical applications. The theoretical predi-
Table 2 Density and porosity of natural fiber and synthetic fiber reinforced composites
Reinforcing fibers Density (g/cm3) Porosity (%)
Flax 1.12 17.66 Ramie 1.09 16.78
Jute 1.04 18.26 Glass 2.07 4.08 Carbon 1.55 2.14
cation values obtained from Garai-Pompoli model and Delany-Bazley model showed good agreement with the experimental results, which implied that the sound absorp- tion coefficient of natural fibers and their composites can be predicted by these two models.
This work was supported by the National Basic Research Program of Chi- na (“973” Program) (Grant No. 2010CB631105).
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