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Journal of Fiber Bioengineering and Informatics Regular Article Air Permeability and Acoustic Absorbing Behavior of Nonwovens Shu Yang, Wei-Dong Yu* College of Textiles & Center of Soft Materials, Donghua University, Shanghai, 20620, China Abstract: Several nonwovens were studied to explore the relationship between their structural characteristics, permeability and acoustic absorbing behavior. Fundamental structural parameters including thickness, gram square meter and porosity were considered. Results show that the permeability is not just linear to porosity, but also related to many other complex and difficult-to-measure parameters. Further in this the paper we compared absorption coefficient of nonwovens with and without air permeability back, the sound absorption principles of those are completely different. For nonwovens with rigid back, the absorption coefficient increases with increasing thickness. For samples tested with air gap, the increasing air permeability moves the absorption curve towards lower frequency, and enhances the initial slopes of the curves. And also the absorption coefficients over the whole frequency range were found to increase with air permeability. This finding also indicates that the capillary effect alone cannot sufficiently explain the acoustic absorbing behaviour of nonwovens. Accurate theories which can illustrate sound absorption property of nonwovens still need to be improved and more precise model is still to be developed to explain the acoustic absorbing behaviour of nonwovens. Keywords: Nonwovens, porosity, permeability, acoustic absorption, peak shift.1. Introduction and the sound absorption coefficient of material layers, provided the fiber diameter and density are known.Acoustical absorbing materials are often used in In this research, we studied several nonwovens toautomotive and building industries. At present, the explore the relationship between their structuralmost common materials being used are fibrous characteristics, permeability and acoustic absorbingmaterials, foam, glass, perlite and concrete. Fibrous behaviour. Fundamental structural parameters havematerials are considered to be the most ideal ones been considered including thickness, gram squarebecause of their low-cost, light-weight, no pollution meter and porosity. Furthermore, in this paper we haveand high-efficient absorbing ability. compared the sound absorption coefficient of Sound absorption behavior of nonwovens are nonwovens with and without air gap behind.studied by many researchers [1-3]. C. Zwikker and C.W. Kosten who provided the first monumental work on 2. Experimentthis subject [4], looked at the porous medium as amixture of two phases, air and solid material, which Six nonwoven samples are involved in this study.react differently with the sound wave. Yakir Shoshaniand Yakov Yakubov [5] used Zwikker and Kosten’s 2.1 Fundamental parameters measurementtheory to do numerical calculations of some intrinsiccharacteristics of nonwoven fiberwebs yielding the (1) Thicknesshighest sound absorption coefficients in the audiblefrequency range. For nonwoven fiber-based materials, The thickness of the nonwoven samples is measured byacoustical insulation is mostly related to the geometry YG141N digital fabric thickness gauge, whichof the fabric. The fiber denier, shape and length in complies with the standard ISO5084. The paper chosenonwoven fabrics are very important factors in sound press weight as 50cN and press time as 10s.absorption and insulation [6,7]. Mevlut Tascan andEdward A. Vaughn [8] studied the effects of total (2) Gram square metersurface area and fabric density on acoustical behaviorof needle punched nonwoven fabrics. N. Voronina [9] Using electronic balance, small round samples withinvestigated experimentally and derived a model which radius of 15mm are measured, further their gramcan be used to predict values of the acoustic impedance square meters are calculated.* Corresponding author’s email: wdyu@dhu.edu.cnJFBI Vol. 3 No.4 2011 doi:10.3993/jfbi03201103 203
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Air Permeability and Acoustic Absorbing Behavior of Nonwovens Shu Yang et al.(3) PorosityPorosity can be determined by m ε = 1− AL ρ , (1)where m is weight of nonwoven sample, A is samplecross-sectional area, L is sample thickness, and ρ isdensity of the fiber. Figure 1 SW260 double-microphones standing wave tube.2.2 Permeability measurement The measuring process must use a plain wave,The permeability of samples is measured by numerical whose wavelength is longer than the tube diameter. Fortype fabric air permeability instrument (YG461E), this reason, in this work we chose the narrowest tubewhich complies with the standard GB/T5453-1997. (30 mm in diameters) to obtain widest extent ofPressure is set as 200Pa, test area as 20cm2, and the working frequency. During the measurement, the twodiameter of nozzle is determined by permeability, microphones must be carefully matched.larger permeability needs bigger nozzle to match with. The transfer function technique is based on the fact that the sound reflection factor at normal incidence, r,2.3 Sound absorption measurement can be determined from the measured transfer function, H12, between two microphone positions in front of theThere are two types of methods to obtain acoustic material being tested. The complex acoustic transferabsorption coefficient: the reverberation room function, H12, is normally defined as[10]technique (ASTM C 423-84a) and the impedance tubetechnique (ASTM C 384-85). The latter one is adopted p2 e jk0 x2 + re − jk0 x2 = H12 = e jk0 x1 + re − jk0 x1 (2)here since it requires rather small sample, just 100 or p130mm in diameter. For normal incident sound waves,this method is faster and more accurate. There are also where p1 and p2 are the complex sound pressures at thetwo options available with the standing wave tube: two microphone positions; x1 and x2 are the distancesstanding wave ratio method and transfer-function between the two microphone positions from themethod. The only difference between them is that in reference plane (x = 0); and k0 is the wave numberthe latter one two microphones are fixed on the wall of defined as k0 = 2πf/c0, where f is the frequency and c0tube in place of one slipping microphone in the former the speed of sound.one. Compared with the standing wave ratio method, The transfer functions for the incident wave, HI, andthe transfer function method has a wider testing range. for the reflected wave, HR, can be calculated byThus in this study the transfer function method is used. The instrument adopts SW260 double-microphones H1 = e − jk0 ( x1 − x2 ) (3)standing wave tube, which is made in BSWA jk0 ( x1 − x2 )Technology Co., Ltd, complying with a standard HR = e (4)GB/T18696.2-2002 and ISO 10534-2:2001. It iscomposed of a signal generator, a loudspeaker, an Combining Eqs. (3) and (4), the normal incidenceimpedance tube, a portable dual-channel fast Fourier reflection factor, r, can be calculated usingtransform (FFT), a power amplifier and a precisionsound level meter as shown in Fig.1. The generator H12 − H I 2 jk0 x1 r= etransmits a broadband signal which is collected and H R − H12processed at the location of two microphones, where (5)the incident sound energy is separated from thereflected one, therefore the acoustic absorption Further the sound absorption coefficient, AC, can becoefficient and impedance at different frequencies can determined in terms of r by the following equationbe determined. 2 AC = r = (rr 2 + ri 2 ) 1− 1− (6) 204
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Air Permeability and Acoustic Absorbing Behavior of Nonwovens Shu Yang et al.3. Results 3000 25003.1 Fundamental parameters permeability mm/s 2000The results of fundamental parameters of nonwovensinvolved are listed in Table 1. These parameters 1500include raw material, thickness, gram square meter,porosity and mean pore diameter. 10003.2 Permeability properties 500The permeability properties of six nonwovens are 0 1 2 3 4 5 6shown in Fig. 2. Based on Kozeny Equation, the samplepermeability coefficient is [11-13] Figure 2 Permeability properties of samples. ε3 From Eq. (10) it can be seen that the permeability is K tfp = not just linear to porosity, but also relates to many η s 2κ , (7) other complex and difficult-to-measured parameters such as tortuosity, shape factor etc. where κ , the Kozeny constant, is 3.3 Sound absorption properties κ = k0 t f 2 , (8) The paper measures the sound absorption properties of and tf, the tortuosity, is the six samples without air gap, with 15mm air gap, and with 30mm air gap behind, respectively. The Le results are shown in Fig.3. It indicates that, the sound tf = L , (9) absorbing results of samples with and without air gap behind are completely different. Introducing air gapwhere ε is porosity of the sample, η is viscosity of the can enhance the absorbing effect of nonwovens greatly, especially at the frequency range of 1000-4000Hz.flow, s is channel wetted surface, k0 is shape factor, Fig.3 (a) shows the result of samples tested withand Le is effective channel length, or effective sample rigid back. It can be seen that the sound absorptionthickness. coefficient is improved by the increasing thickness. Mohammadi [14] has modified Kozeny Equation From Table 1 it can be seen that the order of thicknessusing Davies’ permeability coefficient equation kD [11], of samples is 5>6>3>2>4>1, whereas the thickness ofand derived the theoretical permeability qtp for fibrous samples 1-4 are nearly the same.structures with porosities ranging from 0.94 to 0.994 While Fig.3 (b) shows the sound absorbing result of d 2ε 3 ∆p samples tested with 15mm air gap, and Fig.3 (c) shows qtp = cm / s that of 30mm air gap behind. From these two figures, 16k Dη (1 − ε ) L 2 (10) we can see that the increasing air gap makes the absorption curve shift towards lower frequency, andwhere d is fiber diameter, ∆p is the pressure difference enhances the initial slopes of the curves.besides sample, and L is the thickness of the sample. Table 1 Properties of nonwovens involved No. Raw material Thickness Gram square meter Porosity (mm) (g/m2) 1 PET 0.523 42.46 0.941 2 PET/VS 70/30 0.779 84.50 0.923 3 PET 0.828 123.14 0.892 4 Superfine Fiber 0.673 141.12 0.850 5 Fiberglass 5.275 505.02 0.963 6 Basalt Fiber 3.490 581.03 0.937 205
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Air Permeability and Acoustic Absorbing Behavior of Nonwovens Shu Yang et al. z = 8η L ρ0 c0 ε a 2 , 1.0 (11) 0.9 0.8 where z is sound impedance, L is thickness, sound absorption coefficient η= 1.85 * 10-5 is viscosity of air, a is pore radius, ε is 0.7 sample 5 porosity, and ρoco= 415s characteristic impedance of 0.6 sample 6 air. 0.5 0.4 0.3 17.0781 sound impedance (calculated) 0.2 sample 3 sample 2 0.1 sample 4 sample 1 0.0 0 1000 2000 3000 4000 5000 6000 7000 f/Hz 4.1262 (a) 3.0574 sample6 1.0 0.9 sample4 0.3115 0.6107 0.7530 sample5 sound absorption coefficient 0.8 1 2 3 4 5 6 samples 0.7 0.6 sample3 0.9975 0.9992 0.9393 0.9576 0.5 sample2 0.4 sound absorption coefficient 0.7528 0.76513 0.3 0.61243 0.5828 0.2 sample1 0.5239 0.1 average sound absorption coefficient on whole frequency range (experimental) 0.3077 0.33498 maximum sound absorption coefficient 0.0 (experimental) 0 1000 2000 3000 4000 5000 6000 7000 0.20718 f/Hz (b) 1 2 3 4 5 6 samples 1.0 Figure 4 Comparison of calculated sound impedance 0.9 with experimental sound absorption coefficient. sound absorption coefficient 0.8 0.7 It can be seen that Eq. (11) does not consider the 0.6 sample3 issue of frequency f, however, the sound impedance is 0.5 sample5 changing with frequency. For the reason sound 0.4 sample6 absorption coefficient is proportional to sound 0.3 sample4 impedance, in this work we have compared the 0.2 sample2 calculated sound impedance with the two series of 0.1 sample1 experimental sound absorption coefficients of samples 0.0 with air gap behind. One is average coefficient on the 0 1000 2000 3000 f/Hz 4000 5000 6000 7000 whole testing frequency, the other is peak value of (c) coefficient. The results are plotted in Fig. 4. It can beFigure 3 Sound absorption coefficients of six understood from the figure that, the peak value ofnonwovens (a) without (b) with 15mm and (c) 30mm coefficient approximately has the same trend with thatair gap behind. of the calculated impedance.4. Discussion 5. ConclusionThe theory of capillary effect of porous materials[15] From modified Kozeny Equation and the permeabilityhas been used to explain the sound absorption experiment, it can be seen that air permeability is notproperty for a long time. It indicates that the sound just linear to porosity, but also relates to many otherabsorbing properties are related to thickness, pore size complex and difficult-to-be-measured parameters suchand porosity. The sound impedance can be calculated as tortuosity, shape factor etc.by 206
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Air Permeability and Acoustic Absorbing Behavior of Nonwovens Shu Yang et al. Furthermore, in this work we compared the [5] Shoshani Y, Yakubov Y. Numerical assessmentabsorption coefficient of nonwovens with and without of maximal absorption coefficients for nonwovenair gap behind, the sound absorption principles of fiberwebs. Appl Acoustics, 2000; 59(1): 77-87.these are completely different. For nonwovens with [6] Ballagh KO. Acoustical properties of wool. Applrigid back, the absorption coefficient increases with Acoustics 1996; 48(2): 101-120.the increasing thickness. For samples tested with air [7] Narang PP. Material Parameter Selection ingap, the increasing air permeability moves the Polyester Fiber Insulation for Sound-absorption curve towards lower frequency, and Transmission and Absorption. Appl Acousticsenhances the initial slopes of the curves. And also the 1995; 45(4): 335-358.absorption coefficients over the whole frequency [8] Tascan M, Vaughn EA. Effects of total surfacerange are found to increase with air. area and fabric density on the acoustical behavior Theory of capillary effect explains the absorption of needle punched nonwoven fabrics. Text Res Jcoefficient values very well. However, it ignores the 2008; 78(4): 289-296.issue of frequency. So the accurate theories which can [9] Voronina N. Improved empirical model of soundillustrate sound absorption property of nonwovens propagation through a fibrous material. Applstill need to be improved. Acoustics 1996; 48(2): 121-132. [10] Han FS, Seiffert G, Zhao YY, et al. AcousticAcknowledgements absorption behaviour of an open-celled aluminium foam. J. Phys. D: Appl. Phys 2003;We wish to acknowledge the 2008C01069 (Science 36(3): 294-302.and Technology Department of Zhejiang province) for [11] Carman PC. Flow of gases through porous media.the financial support. Butterworths Scientific Publications: 1956. [12] Piekaar HW, Clarenburg LA. Aerosol filters-theReferences: tortuosity factor in fibrous filters. Chem Eng Sci 1967; 22: 1817-1827.[1] Shoshani YZ. Effect of Nonwoven Backings on [13] Clarenburg LA, Piekaar HW. Aerosol Filters: I– the Noise Absorption Capacity of Tufted Carpets. Theory of the Pressure Drop Across Single Text Res J 1990; 60(8): 452. Component Glass Fibre Filters. Chem Eng Sci[2] Shoshani Y. Studies of Textile Assemblies Used 1968; 23(7): 765-771. for Acoustic Control. Tech. Textiles Int 1993; [14] Mohammadi M, Banks-Lee P, Ghadimi P. Air 2(3): 32-34. permeability of multilayer needle punched[3] Shoshani Y, Rosenhouse G. Noise Insulating nonwoven fabrics: Theoretical method. J Ind Blankets Made of Textiles. Appl Acoustics 1992; Textil 2002; 32(1): 45. 35: 129-138. [15] Allard JF, Daigle G. Propagation of sound in[4] Zwikker C, Kosten CW. Sound absorbing porous media: modeling sound absorbing materials. Elsevier: 1949. materials. J. Acoust. Soc. Am 1994; 95: 2785. 207
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