1. By: Virendra Kumar Saroj 2013TTE2764

2. Contents: Introduction Definition & measures Noise absorption by woven fabrics Sound absorbing curtains Noise absorption of tufted carpet Noise absorption behaviour of knitted spacer fabric Carbonized and activated nonwovens Pulp fibre-based sound absorbing materials (PSAMs) References 3. Introduction Sound is a sensation of acoustic waves (disturbance/pressure fluctuations setup in a medium). Unpleasant, unwanted, disturbing sound is generally treated as Noise and is a highly subjective feeling. Basically sound propagation is simply the molecular transfer of motional energy. Workplace and environmental noise pollution pose a significant threat to human comfort. A variety of ways are available to reduce noise and they can be basically grouped by passive and active mediums 4. Passive mediums reduce noise by disseminating energy and turning it into heat, while active mediums need the application of external energy in the noise reducing process. In practical applications often textiles are employed as sound absorbers, for example, textile interior parts of automobiles or carpets in rooms that are used to absorb sound energy. In these applications the absorbing material is mounted directly onto an acoustical hard surface. The textile can then be regarded as a porous medium and its sound absorption characteristic highly depends on its thickness. 5. Definition & measures Noise reduction means to reduce the intensity of sound either by converting the sound energy into heat energy (Passive medium)or by application of an external energy source (Active medium) which added exactly opposite sound or vibration waves. Noise reduction ability of a medium is given by NAC (Noise Absorbing Coefficient) or NRC (Noise Reduction coefficient) or in terms of Sound absorption efficiency. The NAC or NRC is a scalar representation of the amount of sound energy absorbed upon striking a particular surface. NRC of 0 indicates perfect reflection and NRC of 1 indicates perfect absorption. 6. ASTM C423 - 09a : Standard Test Method for Sound Absorption and Sound Absorption Coefficients by the Reverberation Room Method. In this method a band of random noise is used as a test signal and turned on long enough (about the time for 20 dB decay in the test band with the smallest decay rate) for the sound pressure level to reach a steady state. When the signal is turned off, the sound pressure level will decrease and the decay rate in each frequency band may be determined by measuring the slope of a straight line tted to the sound pressure level of the average decay curve. 7. The absorption of the room and its contents is calculated from Sabine formula : where: A = sound absorption, m2 V = volume of reverberation room, m3 , c = speed of sound (calculated according to 11.13), m/s d = decay rate, dB/s, 8. Noise absorption by woven fabrics For using woven fabrics as noise reduction application, it is of prime importance to know the noise absorption coefficients as a function of the intrinsic parameters characterizing the fabric. The transmission loss of a sound wave of a given frequency was predominantly dependent on the fabric cover factor and less on its weight and thickness. The intrinsic parameters of the woven fabrics have a very little effect on their noise absorption coefficients in the low frequency domain (f< 500 Hz) but a significant impact at the highest examined frequency (f= 4000 Hz). 9. Since the shorter the wavelength of the impinging sound wave, the higher the sensitivity to changes in the intrinsic structure of the fabric. The air gap behind the fabric has a very significant effect on the functional relations between NAC and the frequency. If defines the wavelength of the impinging sound wave, then it is clearly observed that when the depth of this gap, d, is approximately /4 or 3/4, there is maximum sound absorption, whereas when d = /2, it is a minimum, as expected. 10. Result shows that fabric density to be the major factor determining sound absorption, even more than fabric weight or thickness Sound absorption increased as fabric density increased to a certain point, showing the highest value at about 0.14 g/cm3. After this point, sound absorption decreased. This is assumed to be because high-frequency sound was absorbed more in dense fabric; in other words, in the fibre itself, whereas low-frequency sound was absorbed more in the air path or in the fabrics pores. Thus it is recommended that fabrics be manufactured with this density for high sound absorption. 11. Sound absorbing curtains A monolayer curtain of thin 100% porous cotton is found to be of low sound absorption efficiency (%). However, on doubling these curtains leaving an air space between the two layers, the absorption characteristics are appreciably improved in spite of showing minima at certain high frequencies. Results shows that a specific air space has to be determined to give optimum effectiveness. The replacement of one of these two thin porous cotton layers by a thin layer of non-porous cotton/polyester 35:65 is found to be of reduced effect. This arrangement, beside being sound absorbing, has the advantage of light insulation. 12. Noise absorption of tufted carpet Comparing the result between two fibre types, acrylic and wool, acrylic fibres generally imparts better sound absorption than wool. The difference (up to 10%) is more significant in the higher frequency range (f >1000Hz) and in the closed loop carpets with the thickest piles and highest densities. The average value of NRC (Noise Reduction Coefficient) does not any trend with pile height; it may increase or decrease or can have no apparent effect with increasing pile height. 13. For a given yarn count and pile height, increasing pile density results in an increase in NRC regardless of fibre type and construction. In all the samples when there is an air gap, NRC increases several hundred percent in low and medium frequency ranges (250 < f < 100). When the frequency of the sound wave increases, the vibrating air molecules move more rapidly and energy loss due to their friction with the piles is consequently increases. 14. Noise absorption behaviour of knitted spacer fabric The weft-knitted spacer fabric exhibits the typical sound absorption behaviour of porous absorber . The warp-knitted spacer fabric exhibits the typical sound absorption behaviour of micro perforated panel (MPP) absorber. The noise absorption coefficient (NAC) increases with increase of the frequency for both kinds of fabrics. The sound absorbability can be improved by laminating different layers of fabrics. 15. For the weft-knitted spacer fabric, the NACs significantly increase from one layer to four layers and thereafter more layers are no longer effective. However, for the warp knitted spacer fabric, the NACs can continuously be improved with increase of the fabric layers, but with a shift of the resonance region towards the lower frequency side. The combinations of weft-knitted and warp-knitted spacer fabrics can significantly improve their sound absorbability, but their arrangement sequence has an obvious effect. 16. At higher frequencies, the NACs of the warp-knitted spacer fabrics backed with weft knitted fabrics are much higher than those of the weft-knitted spacer fabrics backed with warp-knitted fabrics. However, at lower frequencies, the NACs of the warp- knitted spacer fabrics backed with weft knitted fabrics are much lower than those of the warp-knitted spacer fabrics backed with weft-knitted fabrics. The air-back cavity can be replaced by multilayered warp- knitted spacer fabrics to achieve high NACs at low and middle frequencies. 17. Yarn path notation weft knitted spacer fabric 18. Fig: NACs of single layer spacer fabrics without air back cavity, A=weft knitted B=warp knitted 19. Fig: NACs of weft knitted spacer fabrics laminated with different layers 20. Fig: NACs of warp knitted fabrics laminated with different layers 21. Carbonized and activated nonwovens Carbonization and activation is a thermochemical approach for producing active carbon products. Activated carbon materials feature exceptional adsorptive capacity and kinetics, because of their very high specific surface area up to 2500 m2/g and a high micropore volume up to 1.6 ml/g. Activated carbon materials are ideal for use as high- performance absorbents. Textile materials in the form of fibres, yarns, and fabrics can be converted into active carbon products by the process of pyrolysis and activation. 22. The use of fibres and fabrics as raw materials for making activated carbon products has tremendous advantages. First, activated carbon fiber has significantly different microporous structure that allows much more rapid dynamic adsorption and desorption with less material. Second, a wide range of fiber polymers can be used for producing activated carbon products, including celluloses, thermosets, and thermoplastics. Acoustic nonwoven structure consists of a base layer and a surface layer. 23. Based on six non-woven composites with two surface layers (ACF and glass fiber) and three base layers (cotton, ramie, and PP) it was evaluated that the non woven composites with ACF as a surface layer had significantly higher sound absorption coefficients than the glass-fiber- surfaced composites in both the low-frequency range (1001600 Hz) and high-frequency range (16006400 Hz). 24. Fig: Absorption coefficient of PP-based composites 25. Fig: Absorption coefficient of cotton-based composites 26. Fig: Absorption coefficient of Ramie-based composites 27. Fig: Absorption coefficient of ACF surfaced composites 28. Fig: Absorption coefficient of Glass fibre surfaced composites 29. Pulp fibre-based sound absorbing materials (PSAMs) It has been seen that decreases in important fibre characteristics (e.g., fibre length, width) after the beating process caused a significant increase in the sound- absorbing properties of the PSAMs. Scanning electron microscopy indicated that after the beating treatment, many new microfibrils detached from the cell walls and were reconstructed into new nanopores, which led to a decrease of the pore radius and an increase of the pore volume. These new nanopores served as new channels for sound wave movement and contributed to the increased consumption of sound energy. 30. Fig: Effects of beating degrees on the average sound-absorption coefficients and the thicknesses of the PSAMs. Note that pore structures consist of pore a (pores between fibres), pore b (pores within fibres), and pore c (pores between microfibrils). 31. Fig: Dependence of the raw pulp fiber-based PSAMs thickness on the basis weight, and the corresponding average sound-absorption coefficients. 32. Fig: Different sound-absorption coefficient curves for various samples thicknesses. 33. Fig: Comparison of the sound-adsorption coefficients of different sound-absorbing materials with different ratios of pulp fibres (P)/alumina silicate fibres (A)/glass fibres (G): bottom panel: P/A/G 100:0:0; middle panel: P/A/G 50:50:0; and top panel: P/A/G 50:0:50. 34. References 1. Sound Absorption Behavior of Knitted Spacer Fabrics ,Yanping Liu, Hong Hu1 Institute of Textiles and Clothing, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong,China TRJ 2. Noise Absorption by Woven Fabrics, Y. Shoshani, G. Rosenhouse ,Applied Acoustics 0003-682X/90/$03-50 O 1990 Elsevier Science Publishers Ltd, England 3. Sound Absorption Coefficients of Micro-fiber Fabrics by Reverberation Room Method, Youngjoo Na1,Jeff Lancaster and John Casali,Gilsoo Cho ,TRJ 4. Sound Absorbing Double Curtains from Local Textile Materials ,Y. I. Hanna & M. M. Kandil* National Institute for Standards, Shama al-Tahrir, Dokki, Cairo, Egypt 5. Effect of Pile Parameters on the Noise Absorption Capacity of Tufted Carpets, Y.Z. Shoshani and M.A. Wilding, Textile Research Journal 1991 61: 736 6. The effect of physical parameters on sound absorption properties of natural fiber mixed nonwoven composites,Merve Ku cuk1 and Yasemin Korkmaz2, TRJ 35. 7. Multi-fiber needle-punchednonwoven composites: Effects of heat treatment on sound absorption performance,Nazire Deniz Yilmaz1, Nancy B Powell2, Pamela Banks-Lee2 and Stephen Michielsen2, TRJ -2012 8. Carbonized and Activated Non-wovens as High Performance Acoustic Materials: Part I Noise Absorption ,Y. Chen1 and N. Jiang ,School of Human Ecology, Louisiana State University Agricultural Center, Baton Rouge, LA 70803, U.S.A. TRJ 9. Preparation and design of green sound-absorbing materials via pulp fibrous models,Detao Liu, Kunfeng Xia, Wenxiong Chen, Rendang Yang and,Bin Wang, Journal of Composite Materials 46(4) 399407) 2011