CC BY
484Electronic Journal «Technical Acoustics» http://webcenter.ru/~eeaa/ejta/
2004, 3
Mohd Jailani Mohd Nor, Nordin Jamaludin, Fadzlita Mohd Tamiri*
Department of Mechanical and Materials Engineering, Faculty of Engineering,
Universiti Kebangsaan Malaysia.
E-mails: [email protected], [email protected], [email protected]
A preliminary study of sound absorption using multi-layer coconut coir fibers
Received 19.02.2004, published 27.03.2004
Acoustic treatment using absorbing materials are widely used to reduce reverberation properties of closed spaces and to increase the transmission loss properties of multi-layer sound absorption panels. Natural fibers such as coconut coir fiber have high potential to be used as acoustic materials. As the natural fibers are agriculture waste, manufacturing natural product is therefore an economic and interesting option. This paper discusses the sound absorption using multi-layer coconut fiber as absorbing material component. The effect of microperforated plate and airspaces layers towards the sound absorption of the multi-layer construction was also investigated in this paper. Acoustic absorption coefficient is the main parameter to be estimated in this research. Computer simulation using WinFLAG™ program was done to calculate the acoustic absorption coefficients. Simulation indicated that multi-layer coconut coir fibers and airspaces could increase the acoustic absorption coefficient. All simulation results obtained are based on diffused sound incidence situation.
INTRODUCTION
At the present time, green technology is widely used to manufacture materials from agricultural as a substitute to synthetic fibers and wood-based materials for acoustics absorption purposes. Malaysia has plenty of agricultural waste such as coconut (Cocos nucifera) fiber, rice (Oryza sativa) husk and oil palm (Elaeis guinnesis) frond fiber which are anticipated to increase in the future. Agricultural lignocellulosic fibers such as rice straw, wheat straw or oil palm frond can be easily crushed to chips or particles, which are similar to wood particle or fiber, and may be used as substitutes for wood-based raw materials [1]. Amongst the advantages of these fibers are: renewable, nonabrasive, cheaper, abundance and show less concern with health and safety during handling and processing.
Most practical sound absorbing products used in the building construction industry consist of glass- or mineral-fiber materials. Because of the dominance of these materials in the commercial market, the study of sound propagation in alternative materials has been limited. However, the growing concern about the potential health risks popularly seen as being
* Corresponding author Fadzlita Mohd Tamiri, address: Dept. of Mechanical & Materials Engineering, Faculty of Engineering, Universiti Kebangsaan Malaysia, 43600 Bangi, Selangor, Malaysia e-mails: [email protected], [email protected]
associated with fiber shedding from glass- or mineral-fiber materials provides an opportunity for wood-based sound absorbers to be developed for use in applications traditionally occupied by glass- or mineral-fiber products [2].
Several researchers have succeeded in developing particle composite boards using agricultural wastes [1, 3, 4]. Yang et al. produced rice straw-wood particle composite boards which properties are to absorb noise, preserve the temperature of indoor living spaces and to be able to partially or completely substitute for wood particleboard and insulation board in wooden construction. They reported that the sound absorption coefficient of rice straw-wood particle composite boards are higher than other wood-based materials in the 500-8000 Hz frequency range, which is caused by the low specific gravity of composite boards, which are more porous than other wood-based materials [3].
Multi-layer acoustic absorbers consisting perforated plates, airspaces or porous materials are commonly applied to absorb broadband noise. However, the acoustic absorption of these multi-layer acoustic absorbers is very dependent on their constructions. One of the objective of this work is thus to study the sound absorption of acoustic absorber using multi-layer natural fibers.
For the study of the multi-layer acoustic absorber in literature, Davern presented an experimental study for a three-layer assembly consisting perforated plate, airspace and porous material respectively. The results indicated that the porosity of the perforated plate and the density of the porous material would considerably change the acoustic impedance and absorption coefficient of the acoustic absorber. In addition, only the frequency bands near the resonance frequency achieved high acoustic absorption [5]. Lee & Chen provided a simple but accurate analytical acoustic transmission analysis for evaluating the acoustic absorption of multi-layer acoustic absorber which utilised several compartments including the layers of perforated plates, airspaces and porous materials together but only for normal sound incidence. From their demonstration, they reported that the acoustic absorption is generally more broadband and better for the assembly with three layers of perforated plate backed with airspaces than that with single layer of perforated plate backed with airspace [6]. Effect of glass fiber materials on the acoustic absorptions of single layer perforated plates were also discussed by Davern [5].
Microperforated panels are increasingly used for reverberation control. Microperforated panels are panels of arbitrary material with perforations of very small dimensions. The perforation diameter is typically less than a millimeter [7].
1. COCONUT COIR FIBER
Coconut coir is a natural organic resource which is the seed-hair fiber obtained from the outer shell (endocarp) or husk of the coconut. Coconut coir fiber then cleaned and compressed into bales and mattress sheet, mostly used as raw material for horticulture and agriculture application and for car seat filler or furniture application. Coconut coir fiber contains a high lignin ratio that makes fibers stiffer and tougher, high air porosity (95%), heat retardant, biodegradable and considered a renewable source. The coconut coir fiber samples in mattress sheet form (Fig. 1) were provided by supplier with 20 mm in thickness and density 74 kg/m3.
Figure 1. Raw coconut coir fiber
Sound absorption by porous material is based on the theory of energy transforming from sound energy to thermal energy. In general, common porous absorbers such as fibrous minerals wool and glass fiber, the acoustic absorption is due mainly to viscous losses as air moves within the pores. Porous materials can provide more absorption when they are located in positions where the particle velocity of the sound wave is large. Within porous materials, the sound velocity is lower than in air and the wavelength becomes shorter. Generally the absorption coefficient is low at low frequencies while at high frequencies the absorption coefficient is larger, particularly larger at those frequencies where the material thickness is equivalent to A/4. The thicker the material, the better the absorption at low frequencies. When improvement in absorption at low frequencies is required, an air space should be provided [8].
2. SIMULATIONS BY WinFLAG™ PROGRAM
In the simulation works, absorption coefficients of the panels were calculated by the computer program WinFLAG™, developed at NTNU [9]. This program also had been used for simulations to estimate the acoustic performance of constructions combining different material layers by other researchers [7, 9]. This program implements the transfer matrix method for a number of materials, including porous materials and perforated plates (slotted, with circular holes or microperforated etc.). WinFLAG™ does the job of calculating absorption coefficient, impedance and sound reduction index for such constructions. Calculations may be performed at single frequencies or as mean values in 1/3-octave bands, in both cases for a free field sound incidence as well as in diffuse field.
3. RESULTS AND DISCUSSION
3.1 Effect of airspace layer
Figure 2 shows the simulation values of sound absorption coefficient when the coconut coir fiber is mounted with 50 mm airspace in front of a rigid wall compared to the case where the coconut fiber is directly mounted on the wall without airspace. The airspace layer will enhance sound absorption coefficient in the low frequency range. This is because porous materials can provide more absorption when they are located in positions where the particle velocity of the sound wave is large which is maximum at a distance of A/4, % A and so on [8]. The absorption coefficient is slightly decreased at a distance of A/2, A and so on. In this case, when A/4 and % A is equal to 60 mm (distance from the center of the coconut coir fiber to the backwall surface), the maximum absorption coefficients should be around 1430 Hz and 4290 Hz which is similar to the simulation result and for A/2, around 2860 Hz. Theoretically, this could be the reason for the slight dip in the graph at frequency between 2000 Hz -3150 Hz as shown in Figure 2. For the case of coconut coir fiber without airspace, the fiber thickness should be equivalent to A/4 for maximum absorption. In this case, when A/4 is equal to 20 mm (coconut coir fiber thickness), the maximum absorption should be around 4290 Hz which is also similar to the simulation result.
0.8
c
<D
o
¡1=
<D
o
o
c
o
o
tn
_Q
ro
0.4
0.2
x'
.'X' X-
/
/
X
0 frf*
.x
.-x-
.X'
.X'
—— 1 layer (without airspaces)
- - ■X- - -1 layer (backed with airspaces)
50 80 125 200 315 500 „ ,,800 1250 2000 3150 5000
Frequency (Hz)
Figure 2. Comparison of the diffused incidence sound absorption coefficients of 20 mm thick coconut coir fiber sample of density 74 kg/m3 without airspaces and backed with airspaces
3.2 Effect of microperforated plate facing
For studying the effect of microperforated plate facing on the acoustic absorption, Figure 3 shows the simulation results of sound absorption coefficient when a coconut coir fiber is layered with 1 mm thickness of microperforated aluminum plate facing compared to the coconut coir fiber alone without microperforated plate facing. The microperforated plate facing will shift the absorption coefficient peak to low frequency range and the absorption coefficient will decrease in high frequency range. This is because of the resonant frequency of the microperforated plate. Theoretically, when a perforated panel separated from a rigid wall by an air space, a system of air cells should be visualized with imaginary partitioning for the air space behind each hole of the panel thus forming a series of Helmholtz resonators. Thus, the resonant frequency can be approximately calculated as follows [10]:
where, c is the sound speed, P is the hole opening ratio, L is the panel distance from wall, l is the panel thickness and 5 is the 0.8 hole diameter.
The microperforated plate principle is equal to the perforated plate described above except
is then superfluous to get a necessary resistance component for such a plate used in a resonance absorber [9]. This was agreed by Schultz which explained ‘self-flow-resistance’ of fine perforated metal screen. Schultz stated that if the holes are fine enough, they will act like the fine pores in a glass fiber absorptive blanket [11]. Meanwhile, Wassilieff expressed the microstructural of porous materials based on the Rayleigh model in attempt to predict acoustic properties by modeling the air space within the material as a collection of parallel cylindrical pores [2]. From the statements above, we know that the microperforated plate have same acoustic effect like porous materials which require other parameters to describe the materials such as airflow resistivity, porosity and tortuosity. An analytical expression to calculate the impedance of the holes in microperforated plate was put forward by Maa [12]. However, the simulation results of microperforated plate using the present program did not use the approximate formulas for the impedance given by Maa but the calculations are based on the full analytical solution which include Bessel functions with complex argument [9]. This does not imply, however, that the accuracy is very much better than when using the approximate formulas.
For the 1 mm thick of microperforated aluminum plate with 0.5 mm hole diameter and mounted 20 mm from the wall, the resonant frequency calculated is 1445 Hz which is a little bit higher compared to the simulation result (between 1000 Hz to 1250 Hz). This is because, in the simulation, the air space between the microperforated plate and the rigid wall was replaced with the coconut coir fiber. For studying the effect of porous materials behind the perforated plate, Lee & Chen found that porous material would distinctly promote the acoustic absorption and shift the acoustic resonance frequencies to lower frequency bands [6].
(1)
for the diameter of the holes should typically be less than 0.5 mm. A porous layer or a fabric
0.8
SE 0.6
<D
O
O
C
O
■& 0.4 o
0.2
— — 1 layer (without microperforated plate)
- - -X- - -1 layer (with microperforated plate
facing)
x **
• .
V
y\
/
/
¥--r*-r
X
.-X'.
X /
X.
X
50 80 125 200 315 500 800 1250 2000 3150 5000
Frequency (Hz)
0
Figure 3. Comparison of the diffused incidence sound absorption coefficients of 20 mm thick coconut coir fiber sample of density 74 kg/m3 without microperforated plate and with
microperforated aluminium plate facing
3.3 Multi-layer Coconut Coir Fibres
To study the effect of multi-layer coconut coir fibers backed with airspaces, the sound absorption coefficients for the assembly of one layer, two layers and three layers of coconut coir fiber backed with airspaces were simulated for two cases; without microperforated facing (Figure 4) and with microperforated facing (Figure 5). Figure 4 and 5 show the increase of sound absorption coefficients in the low- and middle-frequency region. However for Figure 5, the microperforated facing causes a rapid drop of sound absorption coefficient in the high-frequency region. From these result also known that the more layers of coconut coir fiber in the assembly, the better the sound absorption in low-frequency region. Theoretically, the absorption coefficient of porous material increases with thickness of the material, since particle velocity is maximum at a quarter wavelength from the substrate. The increase of coconut coir fiber layers also means that the fibers have more chance to contact with the sound wave. This causes more resistance by means of friction of viscosity through the vibration of the air [13]. Therefore, the inserted coconut coir fiber layers in the assembly of sound absorber contributed to increase the sound absorption coefficient through quite a wide range of frequency, because the reflected sound wave inside the sound absorber can be absorbed again and again through the multi-layer structure.
Frequency (Hz)
Figure 4. Diffused incidence sound absorption coefficients of 20 mm thick coconut coir fiber sample of density 74 kg/m3 backed with airspaces and without microperforated aluminium
plate facing
Frequency (Hz)
Figure 5. Diffused incidence sound absorption coefficients of 20 mm thick coconut coir fiber sample of density 74 kg/m3 backed with airspaces and with microperforated aluminium plate
facing
CONCLUSIONS
In this work, the diffuse sound absorption coefficients of multi-layer coconut coir fibers have been successfully simulated. From these simulations, the sound absorption coefficients for realistic multi-layer coconut coir fibers containing microperforated facing and airspaces layers can be calculated. Several acoustic features of sound absorbers are also discussed based on the theory. The effect of airspace layer will increase the sound absorption in the low frequency range. In the case of microperforated facing, the structure promotes the sound absorption coefficients in the low-frequency region, but it has the reverse effect in the high-frequency region. Therefore, many considerations are required for the purpose of sound control; on the other hand, the multi-layer coconut coir fiber contributed to increase the sound absorption coefficients. This provides a reliable guidance for the design of multi-layer sound absorbers. Future works will be carried out to compare sound absorption of plate structures with different size of holes and to compare sound absorption coefficient for coconut fiber and other types of natural fiber.
ACKNOWLEDGEMENTS
The authors would like to thank the Malaysian Ministry of Science, Technology and Environment for sponsoring this work under project IRPA 03-02-02-0016-SR0003/07-02 and for providing the scholarship of National Science Fellowship.
REFERENCES
1. Joseph Khedari, Sarocha Charoenvai, Jongjit Hirunlabh. New insulating particleboards from durian peel and coconut coir. Building and Environment, 2003, 38, 435-441.
2. Con Wassilieff. Sound Absorption of Wood-Based Materials. Applied Acoustics, 1996, vol. 48, N°4, 339-356.
3. Han-Seung Yang, Dae-Jun Kim, Hyun-Joong Kim. Rice straw-wood particle composite for sound absorbing wooden construction materials. Bioresource Technology, 2003, 86, 117-121.
4. Joseph Khedari, Noppanun Nankongnab, Jongjit Hirunlabh, Sombat Teekasap. New low-cost insulation particleboards from mixture of durian peel and coconut coir. Building and Environment, 2004, 39, 59-65.
5. W. A. Davern. Perforated Facings Backed with Porous Material as Sound Absorber - An Experimental Study. Applied Acoustics, 1977, 10, 85-112.
6. F.-C. Lee, W.-H. Chen. Acoustic Transmission Analysis of Multi-Layer Absorbers. Journal of Sound and Vibration, 2001, 4, 621-634.
7. Rolf Tore Randeberg. Perforated Panel Absorbers with Viscous Energy Dissipation Enhanced by Orifice Design. Doctoral Thesis, Department of Telecommunications, Norwegian University of Science and Technology, June 2000.
8. Z. Maekawa, P. Lord. Environmental and Architectural Acoustics. 1994.
9. T. E. Vigran. Manual for Program WinFLAG, Version 1. NTNU, 17 Feb, 2003.
10. Morse P. M. & Ingard K. U. Theoretical Acoustics. Princeton. 1968.
11. Theodore J. Schultz. Acoustical Uses for Perforated Metals: Principles and Applications. 1986.
12. Maa D. Y. Potential of microperforated panel absorber. Journal of Acoustical Society of America, 1998, 104, 5, 2861-2866.
13. Youneung Lee, Changwhan Joo. Sound Absorption Properties of Recycled Polyester Fibrous Assembly Absorbers. AUTEX Research Journal, 2003, vol. 3, N°2, 78-84.
