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Performance Improvement of Sound-Absorbing Materials Using Natural Bamboo Fibers and Their Application

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Performance Improvement of Sound-Absorbing Materials Using Natural Bamboo Fibers and Their Application Performance improvement of sound-absorbing materials using natural bamboo fibers and their application T. Koizumi, N. Tsujiuchi & K. Fujita Department of Mechanical Engineering, Doshisha University, Japan Abstract An acoustic material has been newly developed from the viewpoint of environmental protection. For this purpose, we used a natural resource, bamboo fiber, to manufacture sound-absorbing material. Until now, the sound-absorbing material has been developed using the crushed bamboo fibers. This material, however, is too heavy. To solve this problem, we have tried to develop a material using the explosion method. The normal incidence sound absorption coefficient of the bamboo fiber material was measured to be confirmed the basic properties. In addition, the characteristic impedance and the propagation constant of the material were measured. The contour lines of the sound absorption coefficient were calculated to find the relation of thickness and air space of the material with respect to density. In addition, we considered the influence of random incidence, and evaluated the material by adapting it to actual loudspeakers. The results show that the weight problem was solved, that the bamboo wool material works effectively in an actual product. Consequently, fundamental design criteria have been confirmed using the newly developed bamboo fiber material for sound absorption. Keywords: sound absorption coefficient, bamboo wool, fiber diameter, loudspeaker, acoustic property. 1 Introduction Acoustic materials play a number of roles that are important in acoustic engineering such as the control of room acoustics and traffic noise. Acoustic materials are divided into three types: porous material, boards, and resonance- type boards. Porous materials are the most effective for noises that occur over a High Performance Structures and Materials II, C.A. Brebbia & W.P. De Wilde (Editors) © 2004 WIT Press, www.witpress.com, ISBN 1-85312-717-5 462 High Performance Structures and Materials II broad frequency range. Acoustic materials have been developed for their use in sound absorption. On the other hand, acoustic material is claimed to have additional benefits, such as energy conservation, the advanced use and the re-use of resources from the viewpoint of earth protection [1]. Health concerns have been raised over glass fibers that become airborne and adhere to the body during building construction. Since glass wool is not the best material in terms of human health and the environment, we have adopted the safer, natural bamboo. Because it grows so quickly, we can harvest bamboo annually. Bamboo is effective as a resource because it is natural and easy to dispose of; there is no anxiety over ground pollution even if it is buried in the earth, and it does not emit poisonous fumes when burned. In this paper, an alternative sound-absorbing material is proposed that alleviates the load on the environment. 2 Sound absorption coefficient When the homogeneous layer has the thickness L , the characteristic impedance Zc , the propagation constant γ , and this layer has the acoustic impedance Z c in the back, the acoustic impedance of surface material Z c is expressed as: ZLZL2 coshγ + c sinhγ ZZ1 = c (1) ZLZL2 sinhγ + c coshγ while the sound absorption coefficient is expressed as [2]: 2 ()ZZ/− 1 α =1 − 1 air (2) ()ZZ1 /air + 1 Dual channel Personal Amplifier FFT analyzer computer Dx Lx LL0 Z1 Z2 Mic.1 Mic.2 Speaker Impedance tube Air space Porous material Movable piston Figure 1: Block diagram of the impedance tube and the sample. High Performance Structures and Materials II, C.A. Brebbia & W.P. De Wilde (Editors) © 2004 WIT Press, www.witpress.com, ISBN 1-85312-717-5 High Performance Structures and Materials II 463 3 Measurement method We used the measurement system shown in Figure 1 [3]. A random signal was generated from the loudspeaker, and the transfer function H between the two microphones was extracted using a two-channel FFT analyzer. The reflection coefficient and the sound absorption coefficient are expressed as follows, respectively, where k is the wave number for air. − jkDk − eH 2 ()+DLkj R = e kk (3) jkDk − He 2 α 1−= R (4) 4 Experimental results and discussions 4.1 Bamboo wool sound-absorbing material Until now, the sound-absorbing material has developed using the crushed bamboo fibers. In order to obtain a sound absorption effect similar to that of glass wool, however, the bamboo fiber material needs to be about four times as dense as glass wool [4, 5]. To solve this weight problem, we have attempted to develop a novel material using the explosion method. The resulting material is called “bamboo wool.” 4.1.1 Bamboo wool material The fiber was separated from the woody phase using the explosion method, and the very fine fibers, that is the bamboo wool material, were extracted using the home mixer. The thin composite fiber, polyolefin, was used as the binder material. The sheath part of this composite fiber consists of low-melting-point polyethylene, and its core is made of high-melting-point polypropylene. This difference between the two melting points allows a shaped fiber form to hold together. After thoroughly mixing the bamboo wool material and the binder material (10 % wt.), we molded the sound-absorbing material using a metal mold in a hot press machine. 1.0 1.0 0.9 0.9 0.8 0.8 0.7 0.7 Thickness Density 0.6 0.6 50 mm 32 kg/m3 Air space 0.5 0.5 Thickness 0 mm 0.4 50 mm 0.4 Density 0.3 Air space 0.3 20 kg/m3 0 mm 3 0.2 0.2 30 kg/m 25 mm 3 0.1 0.1 40 kg/m Sound absorption coefficient absorption Sound 3 Sound absorption coefficient 50 mm 50 kg/m 0.0 0.0 125 250 500 1k 2k 4k 125 250 500 1k 2k 4k Frequency [Hz] Frequency [Hz] (a) Air space depth. (b) Apparent density. Figure 2: Sound absorption coefficient. High Performance Structures and Materials II, C.A. Brebbia & W.P. De Wilde (Editors) © 2004 WIT Press, www.witpress.com, ISBN 1-85312-717-5 464 High Performance Structures and Materials II The sound absorption coefficient was measured by changing the air space depth and the apparent density of the material. Results show that the sound absorption coefficient increases in all frequency ranges and the first peak value of sound absorption coefficient moves from the high- to the low-frequency range as the air space depth of the sample increases or the density of the sample increases. These basic properties of porous sound-absorbing material are confirmed. These results are shown in Figure 2 (a) and (b), respectively. 4.1.2 Comparison with glass wool and bamboo fiber Table 1 gives the results of measurements of the fiber diameters of the bamboo wool, glass wool and the bamboo fiber using a digital microscope. Although the bamboo wool and glass wool have similar fiber diameters, we can confirm that the bamboo fiber has a fiber diameter over ten times larger. Figure 3 shows a comparison of the sound absorption characteristics of the bamboo wool, glass wool and the bamboo fiber. Also, although the densities of the bamboo wool and glass wool are the same, it is confirmed that the density of the bamboo fiber needs to be about four times as dense as them to have the similar effect. These results are explained by the sound absorption principle of porous material. For rigid-framed porous material, this absorption is mainly attributed to thermo- elastic damping and viscosity loss generated while the sound propagates through a large number of small air cavities in the material [6]. Of course, the number of the bamboo fibers per unit area can increase when the fiber diameter decreases. In addition, the energy loss increases when the surface friction increases. Consequently, the sound absorption coefficient increases. Table 1: Comparison of fiber diameters. Material Range of fiber size [μm] Bamboo wool 10.84 – 17.66 Glass wool 11.07 – 12.73 Bamboo fiber 125 – 210 1.0 0.9 0.8 0.7 0.6 Density 0.5 Bamboo wool 0.4 32 kg/m3 0.3 Glass wool 3 0.2 32 kg/m Bamboo fiber 0.1 3 Sound absorption coefficient 120 kg/m 0.0 125 250 500 1k 2k 4k Frequency [Hz] Figure 3: Comparison of bamboo wool, diameters High Performance Structures and Materials II, C.A. Brebbia & W.P. De Wilde (Editors) © 2004 WIT Press, www.witpress.com, ISBN 1-85312-717-5 High Performance Structures and Materials II 465 Taking the above results, into account, we can solve the weight problem posed by using the bamboo fiber material because we confirmed that the bamboo wool material has a similar sound absorption effect to glass wool. 4.1.3 Characteristic impedance and propagation constant The characteristic impedance and the propagation constant determine the state of the sound wave [7]. To clarify the acoustic effect in the bamboo wool material, the characteristic impedance and the propagation constant were measured for a material of densities ranging from 20 to 50 kg/m3. Results indicate that the characteristic impedance and the sound attenuation increase, and the sound velocity decreases, as the density rises. Figure 4: Optimization thickness and air space depth. 4.1.4 Optimization The sound absorption coefficient was calculated for the thickness and the air space depth of between 0 and 100 mm using the characteristic impedance and the propagation constant. The contour line was expressed as the sound absorption coefficient of averaged frequency between 500 and 4,000 Hz [8]. Although the sound absorption coefficient increases as the thickness increases, the air space is High Performance Structures and Materials II, C.A.
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