Experimental study of the Sound Absorption of Nylon Woven Fabric Fang Wang 1 Zenong Cai Xianhui Li Xiaoling Gai Tuo Xing Beijing Key Lab of Environmental Noise and Vibration, Institute of urban safety and environmental science, Beijing academy of science and technology No.55, Taoranting Road, Xicheng, Beijing, 100054, China

ABSTRACT Nylon is the world's earliest synthetic fiber, it has excellent performance and rich raw material re- source, so that it has been used until now. The purpose of this research is to investigate the acoustics of thin fabric. Acoustically, it can be seen as a thin layer composed of porous fibrous materials. Therefore, its sound absorption performance is the combination of at least two kinds of sound ab- sorption mechanisms. The main purpose of this paper is to study the relationship between sound absorption properties and structural parameters of thin fabric by means of experiment .The study concerning the transmission and attenuation of acoustic wave in thin fabrics is helpful for the design of thin fabric, and improving its applications in the field of noise control and indoor acoustics. 1. INTRODUCTION

Noise pollution, water pollution, air pollution and solid waste pollution are known as the four major pollutions today, according to a publication called burden of disease from environmental noise was prepared by experts in working groups convened by the WHO Regional Office for Europe, noise pollution has been regarded as the second killer of human public health after air pollution.

Noise has a great impact on people's hearing, cardiovascular, and mood [1] . Because of noise, work- ers are suffering from physical tiredness and slow response, work efficiency and work quality are significantly decreased, also the accident rate is higher [2] , therefore, it is necessary to take effective measures to improve the indoor sound environment. Noise absorption materials are categorized based on resonance absorption and porosity absorption.

Resonant sound-absorbing materials can be equivalent to multiple Helmholtz resonance connected in parallel, in which acoustic energy is consumed by internal resonance effects. [3-4] . The resonance sound absorption materials can be divided into microporous plate, thin plate and thin membrane sound absorption material [3-5] . This kind of structure has the advantage of low frequency sound ab- sorption, but the bandwidth of octave is narrow and the machining technology is high. For the porous absorber, there are a large number of small diameter and uniform distribution of the connected porous material interior [6] . Therefore, foams and fibers with internal pores and composites can be regarded as porous materials sound-absorbing materials. It has good sound absorption properties in the high frequency range, and the absorption coefficient in the low frequency range is very weak [6] . The

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production of porous absorption materials is low cost, and the application range is expanding gradu- ally.

Textile-based materials are usually classified as porous sound absorbing materials [7] . Depending on the thickness of the fibrous porous material, fiber materials are two forms: blocky and thin layer. The thickness of the block fiber materials is close to the acoustic wavelength like foam, while the thickness of the thin material is much smaller than the acoustic wavelength like woven or non-woven fabric. From the analysis of the sound absorption principle, it can be found that thin fabric is not only a porous sound absorbing material but also can be designed as a resonant sound absorbing material. In contrast to non-woven fabrics, the original structure in the woven fabric can realize the artificial design of noise control of the fabric.

Fabric can not only play a role in sound absorption and noise reduction, which also can decorate the indoor environment, beautify the visual sense. In this study, in order to optimize the sound ab- sorption coefficient and make the fabric smoother and softer, the light-transmitting woven fabric with twill weave structure made of polyamide fiber (commonly known as nylon) was selected, the sound absorption properties were studied by experiments. 2. METHODOLOGY

This is an experimental study. The theoretical basis of the experiment is introduced to obtain the sound absorption coefficient of the material. It is a fast and economical method to measure the sound absorption coefficient of materials by using double-microphone in impedance tube with vertical in- cidence. By solving the acoustic transfer function between the two microphones, the incident wave and the reflected wave can be separated effectively, and the normal acoustic impedance of the test sample can be calculated, the structure of the fabric with a back cavity is placed in the impedance tube as shown in Figure 1.

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Figure 1: Schematic diagram of sound propagation in an impedance tube The tested woven fabric material is located at the right end, the distance from the rigid back plate is D. The reflection coefficient r p is obtained by measuring the transfer function H i of the incident wave, the transfer function H r of the reflected wave and the transfer function H 12 of the total sound field between two microphone positions, then the normal sound absorption coefficient and normal acoustic impedance are calculated.

The sound pressures p 1 and p 2 at the positions of the two microphones in positions 1 and 2 are expressed by the incident sound pressure p i and the reflected sound pressure p r respectively, as in E quation1.

( ) ( ) ( ) ( ) 1 1 2 2 1 i r 2 i r e e , e e i t kx i t kx i t kx i t kx p p p p p p ω ω ω ω − + − + = + = + , (1)

The transfer function of the incident wave H i is expressed as:

( ) x ik x x ik p p H − − − = = = e e 1 2

i 2 i ， (2)

i1

2 1 x x x = − is the distance between two microphones. the transfer function Hr of the reflected wave is expressed as:

( ) x ik x x ik p p H e e 1 2

r 2 r = = = − , (3)

r 1

r i p p r p = According Equation 1 ,Equation 2, and , the transfer function of the total sound field is

e xpressed as:

ikx p ikx

−

e e

r

+ = = −

p p H +

2 2

, (4)

2 12 ikx p ikx

e e

r

1 1

1

On the basis of Equation 2, Equation 3, Equation 4, The sound pressure refle c tion coefficient is:

r 12 e ikx p H H r H H

− − = − , (5)

1 2 12 i

2 1 p r − = α Normal sound absorption coefficient is , normal acoustic impedance Z S is expressed

as :

0 0 S S S 1 1 c R R jX R Z ρ − + = + = (6)

2.1. The Three Basic Structures of Woven Fabrics

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Woven fabric is a set of warp yarns and a group of weft yarns perpendicular to each other on the loom according to the fabric organization of the fabric interwoven fabric, the common feature of the woven fabric is that the interweaving of warp and weft yarn is vertical and has detachability. The types of looms are divided into shuttle looms, shutterless looms, two-layer looms and jacquard looms. According to the warp and weft interweaving method, the fabric can be divided into three categories: plain, twill and satin, which is the basic structures of woven fabrics, see Figure 2. The new weaving

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process that constantly appeared was developed on the basis of the three original organizations, such as changing organization, complex changing organization, honeycomb organization, crepe organiza- tion, etc. Compared with plain woven fabric, twill fabric’s softness and gloss is better, meanwhile it is cheaper than stain woven fabric.

(b)Twill fabric

(c)Stain fabric Figure2: Three basic structures of woven fabrics diagram

(a)Plain fa b ric

The most outstanding advantage of Polyamide fiber (commonly known as nylon) is that its abra- sion resistance is superior to other fibers, and its elasticity is better, its elastic recovery rate is com- parable to wool, and its weight is light, the specific gravity is 1.14, among the commercialized syn- thetic fibers, it is second only to polypropylene (polypropylene, the specific gravity is less than 1) and lighter than polyester (the specific gravity is 1.38). Therefore, the fabrics made of polyamide fibers are very light, and feel fine, soft and smooth, good light transmittance. Because of a kind of fabric is needed which is soft texture, good gloss, high light transmittance and economy, then the nylon woven twill fabric was finally selected, the SEM microstructure of the fabric (see Figure 3(a) , (b)), the sample (see Figure 3(c)) and cross-sectional view of the sample fabric(see Figure 3(d))

(a)Sampl e 1

(b)Sample2

(c) 实物图

(d) 样品织物的剖面图 Figure 3: SEM microstructure of the fabric (a)(b), Sample(c), cross-sectional view of the sample fabric (d).

2.2. Testing in impedance tube

In this work, Two-microphone impedance tube (type 4206) of Bruel Kjaer is applied to measure the normal incident absorption coefficient of non-woven fabric materials according to the standard procedure detailed in ISO (10534-2) ， testing system schematic diagram see Figure4 。 The diameter of the impedance tube used in the experiment is 30 mm and 100mm. The frequency range of measures is from 50 to 6400 Hz. Fig. 4 is the test system of impedance tube

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Figure 4: Testing system schematic diagram 3. MEASUREMENT RESULT AND DISCUSSION

The aim of this study is to achieve better sound absorption by thin layer structure, the sound absorption coefficient curve of 25mm thick sponge and nylon fabric structure (0.008 mm thick fabric surface layer and 25mm back cavity) , see Figure 5b, the sound absorption effect of the thin layer fabric is much better than that of the ordinary sponge, which is a strong sound absorption sponge for the correction of the transfer function when the impedance tube is purchased, the fabrics in this study have good light transmittance and have little interference to the lighting system when they are used to optimize the indoor sound field.

Material acoustic characteristic test software Signal acquisition Power amplifier Microphone . Impedance tube microphon

Figure 5: Sound absorption coefficient curve of sponge and fabric See Figure6 is the sound absorption coefficient under a 100mm back cavity of nylon sheet fabric with different silk pitch, Figure3(a) and (b) can be seen even if it is the same fabric structure and the same fiber, the compactness of warp and weft threads has a great influence on their sound absorption performance. If the fabric with back cavity is only equivalent to the micro-perforated

plate, then the space between the silk diameters is equivalent to the pore size of the micro-perfo- rated structure, and the smaller the pore size, the better the sound absorption performance. How- ever, in Maa’s microperforated panel theory [8-10] , best sound absorption performance is the best combination of aperture, perforating rate and thickness, not determined by a single factor. That is to say, the thin layer fabric should also have the best configuration.

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Figure 6: Experiment sound absorption coefficient curve of two sample According to Maa’s microperforated panel theory, the sound absorption coefficient of sample2 is calculated see Figure7. It is found that the sound absorption coefficient obtained by directly substituting the material thickness and perforation rate into the micro-perforation plate prediction model has a large error with the measured value, so it is necessary to conduct in-depth research on sound absorption performance model of thin-layer fabric and establish a prediction model with simple but high accuracy for engineering application.

Figure 7: Experimental and numerical calculation of sample 2’s sound absorption coefficient value

k__—__|__|___{___] a oe Se

4. CONCLUSIONS

Through the experiment research, it is found that the fabric has the same structure, but the sound absorption coefficient is different with the different degree of compactness, but the higher the density, the better the sound absorption effect, there is an optimal solution for the wire diameter spacing. The prediction model for sound absorption properties of thin-layer fabrics is worthy of further study, which is also the work to be carried out in the following research.

5. ACKNOWLEDGEMENTS

This work is supported by BJAST Innovation Cultivation Programes (No. 11000022T000000468189) and BJAST Young scholar programs BYS202003. 6. REFERENCES

1. Kempen E. E., Kruize H., Boshuizen H. C. The association between noise exposure and blood pressure and ischemic heart disease: a meta-analysis. Environmental Health Perspectives, 2002, 110(3): 307-317 2. Estill C. F., Rice C. H. Morata T., et al. Noise and neurotoxic chemical exposure relationship to workplace traumatic injuries: A review. Journal of Safety Research, 2017, 60: 35-42. 3. Zhao X D , Yu Y J , Wu Y J . Improving low-frequency sound absorption of micro-perforated panel absorbers by using mechanical impedance plate combined with Helmholtz resonators[J]. Applied Acoustics, 2016. 4. Gai X L , Xing T , Li X H , et al. Sound absorption of microperforated panel mounted with helmholtz resonators[J]. Applied Acoustics, 2016, 114:260-265. 5. Zhao J , Li X , Wang Y , et al. Low frequency sound absorption of a membrane-type absorber with magnetic negative stiffness[J]. Shengxue Xuebao/Acta Acustica, 2017, 42(2):239-245. 6. Yang M, Sheng P. Sound absorption structures: from porous media to acoustic metamaterials[J]. 7. Annual Review of Materials Research, 2017, 47: 83-114 8. Maa DY. Theory and design of microperforated panel sound-absorbing constructions. Sci Sin 1975;18:55 – 71. 9. Maa DY. Microperforated-panel wideband absorbers. Noise Cont Eng 1987;29:77–84. 10. Maa DY. Potential of microperforated panel absorber. J Acoust Soc Am 1998;104:2861–4.

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