Functional Textile for Active Wear Clothing

*Ramratan Guru, Anupam Kumar and Rohit Kumar*

## **Abstract**

Moisture management property is an important aspect of any fabric meant for active wear fabric, which decides the comfort level of that fabric specially used as active wear garments. Regular physical activity is important to maintain consistency in human health. To achieve comfort and functional support during various activities such as walking, stretching, jogging etc., athletes and sports persons use active wear clothing. A fabric's moisture management performance is also influenced by its air and water vapour permeability. The moisture management finish (MMF) and Antimicrobial finish (AMF) have been used to increase moisture absorbency; improves wetting, wicking action and antimicrobial performance. In this study, influence of MMF and AMF finishes on the moisture management property of different knitted active wear fabrics had been carried out. For the study two different knit fabrics of 100% Polyester and 100% Nylon with three different GSM levels (100, 130 and 160) has been selected. Further two varieties of commercially available functional fabric finishes have been also taken for the study. The result shows that in case of finished fabric at certain concentration level, as the fabric GSM increases the value of Accumulative one-way transport index (OWTI) %, water vapour permeability but same time drying rate increases. The result shows that in case of finished fabric at certain concentration level, as the fabric GSM increases the value of accumulative one-way transport index (OWTI) %, water vapour permeability decreases but same time drying rate increases. The knitted fabrics of 100% Polyester and 100% Nylon composition follow the similar trend. Further with the increase of fabric finish concentration level, OWTI %, and water vapour permeability (WVP) factor decreases while the drying rate increases.

**Keywords:** Active wear knit fabrics, moisture management, dry rate performance, water vapour permeability

## **1. Introduction**

Moisture management properties of knit fabrics are important factors for deciding not only the comfort but also the performance of functional clothing like active wear, inner wear and sportswear. Comfort refers to the way clothing interacts with the body, with respect to dissipation of heat and moisture generated by the metabolic processes [1, 2]. During normal activity, human body loses heat by conduction, convection as well as radiation processes. Under normal condition, body cools itself by insensible perspiration where water vapour is lost from the body. When heat generation is excessive, the body breaks into a sweat or liquid moisture, also

known as sensible perspiration [3]. Those properties such as smoothness of the fabric surface, air permeability, heat transmittance, hydrophilicity, knit structure, and the presence of a bio-finish influence the comfort characteristics of the knitted fabric. Active sportswear is mostly made of polyester knitted fabrics. Polyester with a modified cross section like hexachannel in coolmax gives more comfort due to its rapid liquid transmission and drying [4].

Moisture management properties of fabrics are influenced by various constructional parameters of the fabric which give knit fabric a porous structure. Total porosity of a knit fabric comprises two types of porosity, viz. micro porosity caused by void spaces among the fibres in the yarns and the macro porosity, which is a consequence of void spaces among the yarns. The air permeability, UV transmission and screen printing depend on the macro porosity; absorption of liquids and capillary phenomenon depend on micro porosity; and thermal resistance and water vapour permeability of fabric depends on both micro- and macro porosity [5, 6]. Interaction of liquids with textile materials involve several physical phenomenon such as wetting of fibre surface, transport of liquid into assembly of fibres, adsorption on the surface or diffusion of liquid into the interiors of fibres, Evaporation of sweat during wear has the potential to cool the body besides restricting the additional weight of sweat being absorbed by the fabric [7].

Moisture is transported in textiles through capillary action or wicking. In textiles, the spaces between the fibres effectively form tubes, which act as capillaries, and transport the liquid away from the surface. The liquid moisture management performance of fabrics results from complex properties including their absorbent capacity, absorption rate, and evaporation [8, 9]. This observed that the water and moisture transmission process is controlled by the water vapour pressure gradient across the inner and outer faces of the fabric. The resistance to diffusion was governed by the fabric construction, i.e. the size and concentration of inter yarn pores and the fabric thickness. The efficiency of yarn wicking depends on the surface tension, i.e., wet ability of the fibre surfaces and on the size, volume and number of capillary spaces was determined by the choice of yarn and fabric construction [10]. The length of time for a fabric to dry depends mainly upon the amount of initial liquid water retained by the fabric per unit area for evaporation. Also, the drying process seems to be related to capillary penetration and porosity of the fabrics. The most significant influence of fibre properties was believed to be the manner in which fibre shape and surface reflect increased or decreased capillarity of the fabric, which in turn causes and enhanced or diminished water uptake on wetting and water retention on drying [11]. The noted finish on a fabric is the most important consideration when developing a dynamic fabric system, as the initial uptake of water depends on the presence of a hydrophilic finish on the fabric surface. This initial uptake is the rate-determining step of the wicking action and a hydrophilic surface finish enhances the moisture management capabilities of fabrics [12, 13].

Antimicrobial finish is manly important role play for the active or sportswear fabrics. The present time is more demand all textile products in better antimicrobial performances. Antimicrobial treatment apply on fabric surface are basically more reduces cross infection, microbial bacteria and skin infections like fungi and increases the performance of sports person infections [14].

Duration the sports activity is more generated sweat and temperature for this condition get for more growth bacteria. This bacteria and fungi cause loss for sports activity performance, ageing, staining, unpleasant odours and potential skin infections.

The basically is during the sports activity generated sweat and increases temperature. In this condition are increase bacteria. This bacteria and fungi cause loss for sports activity performance, ageing, staining, unpleasant odours and potential skin.

## **2. Fibre use for active or sportswear cloths**

Recently some year now, in active sportswear clothing are used for basically fashionable with more comfort performance. Active sportswears are one of the most lucrative segments within knits apparel. Performance of the clothing helps to remain cool, comfort and dry through the moisture management, thermal performance and other techniques. Polyester based knit has come up as a favourite for the performance of the apparel and also it can be engineered to wick transport moistures away from the body for the users' comforts.

The polyester is most common fibre used in active or sportswear cloths. Other fibres are used for active wear cloth like cotton, cotton-polyester, nylon-spandex, polyester- spandex, polypropylene and wool blend. Fibre crossection mainly used in active or sportswear cloth like irregular cross section and hollow structures fibre used [15]. Now it is more use blend with natural fibre in case for active wear cloths because improved thermo physiological performances. The basically fibre use sportswear clothing are mention in following (**Table 1**).


#### **Table 1.**

*Basically fibre used in active sportswear cloths.*

## **3. Design requirement for the active or sportswear cloths**

The textile materials are basically used in all sports as active or sportswear, games like for athletic clothing, football-cricket clothing, jackets, pants, shirts, shorts, socks, sweatshirts, swimwear and tennis clothing.

Plan prerequisites of dynamic and execution athletic apparel have delivered architects with abilities and information in illustrations, materials and style to imagine tastefully satisfying and ergonomically practical reaches which exploit the most recent advances in utilitarian and 'shrewd' materials [16]. Driving style fashioners have rushed to understand that the presentation has really become the feel in athletic apparel. It is the fabrics and innovation that set the precedent. Fuse of microfibres, breathable boundary fabrics, inventive stretch materials, shrewd materials, intelligent materials, for example, stage change materials and shape-memory polymers, and wearable innovation as a piece of the useful plan framework in active apparel, will get standard in the item improvement measure.

#### *Textiles for Functional Applications*

The development of new materials and designs for active or sportswears cloths has produced an exceptionally aggressive market for sports cloths design. The desires of customers for active wear sport and sportswear are concert, protection and comfort associated. The basically are all activewear cloths need for light weight, more durable, fast absorbing performance, heat- liquid regulating materials mainly used for functional design sportswear (**Tables 2**–**7**).

Knitted fabric is commonly used as base layer for functionally active wear due to greater elasticity and stretch ability compared to woven cloth, which is very imperative for freedom of body movement in sports. The tactile sensations by clothing


#### **Table 2.**

*Commercially use knitted structure for active sportswear.*


#### **Table 3.**

*Designing process for active sportswear cloths.*


#### **Table 4.**

*Basic requirement for active sportswear cloths.*


#### **Table 5.**

*Characteristics of yarn polyester and nylon.*

#### *Functional Textile for Active Wear Clothing DOI: http://dx.doi.org/10.5772/intechopen.96944*


#### **Table 6.**

*Yarn quality parameters.*


#### **Table 7.**

*Fabric geometrical characteristics.*

in direct contact with the wearer skin makes wearer more relaxed due to uneven surfaces provided by the knitted fabrics in comparison to smooth-surfaced woven cloth. In addition, the lesser number of contact points of fabric with skin results in reduced clinging sensation during sweat-wetted skin [17].

## **4. Basic mechanism in thermal and moisture transmission though active wear clothing**

The basic process implicated in heat and vapour transport is essential aspect which effects dynamic comfort of active wear garments. The basic phenomena heat can be transferred within active wear in the shape of conduction, convection, radiation and concealed heat transfer by vapour - liquid transport. Conduction, convection and radiation are overwhelmed by the temperature distinction between skin surface and climate and are thusly assembled as dry heat transfer. Then again, dormant heat transfer is accomplished by moisture transmission identified with water vapour pressure between the skin surface and the climate [18–20].

### **4.1 Essentials of heat transfer through garments system**

The active wear fabric layers can by heat transfer from conduction, convection, radiation and wind penetration mechanisms as shown in **Figure 1**.

### **4.2 Essentials of moisture transfer through garments system**

The basic phenomena moisture form garment may be transfer in liquid- vapour form. In vapour structure extraordinary framework like diffusion, sorption, absorption, convection and condensation are included while if there should arise an occurrence of liquid structure wetting and wicking are two components which are for the most part happen as shown in **Figure 2**.

#### **Figure 1.**

*The pathways for heat loss from the activities with human body.*

#### **Figure 2.**

*The pathways for moisture loss from the activities with human body.*

There are various finishes which are being applied nowadays on fabrics to improve its moisture management behaviour. So here in this research work various combinations of knit activewear fabrics (Polyester and Nylon) with varying moisture management finish and antimicrobial finish have been studied for its improvement in moisture management behaviours and antimicrobial activities for the activewear garments.

#### **5. Material for active wear cloths**

In this study work, polyester and nylon yarn count range has kept constant 120 denier, The mesh interlock knit activewear fabrics has prepared on circular knitting machine. Two different knitted fabrics of 100% Polyester and 100% Nylon were used for the study with three different GSM (100, 130 and 160). The fabrics used are scoured, bleached and ready for dyeing (RFD) fabrics (**Figures 3**–**5**).

*Functional Textile for Active Wear Clothing DOI: http://dx.doi.org/10.5772/intechopen.96944*

#### **Figure 3.**

**Figure 4.** *Schematic diagram of MMT apparatus testing.*

**Figure 5.**

*Schematic diagram of water vapour permeability tester equipment (left) and testing procedure (right).*

## **6. Moisture management (MMF) and antimicrobial finish (AMF)**

The fabrics of different type and different GSM are finished with (i) Evo soft MMF finish, (ii) Evo AMF finish. In Evo soft MMF finish Silicone micro emulsion is done which increases the hydrophilic and moisture management characteristics of the fabric. Similarly AMF finish, antimicrobial cloth is used especially for activewear and leisure activities to feel clean and safe or to control malodour.

Anti-microbial finished textile lowers down the psychological discomfort associated with foul odour arising out of microbial growth and by fungi causing skin infections which is an important aspect as human body sweats during various sports activities and the temperature of human body also increases, favouring microbial growth. They also create a powerful barrier against the spread of antibiotic resistant bacteria, which are responsible for medical infections in hospitals other activities.

## **7. Application of finishes**

Various finishes are applied on ready for dyeing fabrics, as per the following methodology. For treating the samples with MMF (i.e. to give moisture management finishing) solutions of 10 gpl and 20 gpl concentrations were prepared. For 10 gpl concentration, 10 gram of MMF was added to 1 gpl of acetic acid and 1 litter of water. Whereas, for 20 gpl concentration of finishing, 10 gram of MMF was added to 1 gpl of acetic acid and 1 litter of water. The same procedure was followed for preparing solution for other two finishes. Samples of dimension (25x 25) cm were prepared and treated with 100 ml of prepared solution by immersing it in


### **Table 8.**

*Vapour and liquid moisture management properties of standard samples without finish.*


**Table 9.**

*Influence of variation in (MMF) finish on moisture management properties of fabrics.*

*Functional Textile for Active Wear Clothing DOI: http://dx.doi.org/10.5772/intechopen.96944*


#### **Table 10.**

*Influence of variation in antimicrobial (AMF) finish on moisture management properties of fabrics.*

the solution contained in a beaker for 10 minutes. Then the sample was taken out & sand with between two transparent sheets & was passed through the padding mangle to squeeze out the solution. The squeezed samples were dried at 150 <sup>0</sup> c for 1 minute in oven dryer. The same procedure was repeated for 2 samples for each level. The whole experimental work was carried out for 100, 130, 160 GSM 100% polyester and 100% Nylon knit fabrics. The variation in fabric geometrical characteristics after applying various finishes and their concentration (level) of finish is tabulated in the (**Tables 8**–**10**). Same processes applying antimicrobial finishes.

## **8. Moisture management tester**

The knit fabric (Untreated and treated) samples were tested on SDL ATLAS M290 moisture management tester (MMT) according to AATCC test method 195–2009, 2011. The accumulative one-way transport index (OWTI) and the overall moisture management capacity (OMMC) measured by using the (Moisture management tester) MMT provide an insight about the liquid moisture transmission performance of fabrics. OWTC is the difference in the accumulative moisture content between the two surfaces of the fabric. OWTC reflects the one-way liquid transport capacity from the top (Inner next to the skin) to the bottom (Outer) surface of the fabric.

#### **8.1 Drying rate testing**

Dry rate testing was carried out using dry rate tester, which evaluates the weight of water evaporated in given time from the fabric. This device can be used independently to find a drying rate or in conjunction with the SDL Atlas Moisture Management Tester (MMT) in order to obtain a more complete understanding of the moisture management properties of a performance fabric. Sample size of 15 x 15 cm was used for the study, to which 2 ml water was added on its surface and allowed dry for required amount of time in the room conditions. The difference between initial and final weight gives the dry rate % of the fabric sample.

### **8.2 Water vapour permeability testing**

Water vapour permeability testing is carried out to determine the resistance of textiles and textile composites (Particularly action wear fabrics) to water vapour penetration using testing standard BS 3424. It was carried out in the water vapour permeability tester which consists of 8 containers with water reservoirs, a standard permeable fabric cover, sample holder ring and precision drive system. The water vapour permeability (WVP) of the fabric was calculated in g/m2 /day is using the Equation (1).

$$WWP = \frac{24 \text{ M}}{A\_{\text{r}}} \tag{1}$$

where, M- Loss of the assembly over the time period t (in g).

T- Time between successive weightings' of the assembly in hours.

A - Area of exposed test specimen (equal to the internal area of the test dish (in m2 ) in this case. A = 0.0054113 m<sup>2</sup> .

#### **8.3 Scanning electron microscope**

The surface of the coated fabrics was investigated using an SEM XL 30, Philips. According to SEM image confirm the impregnation of moisture management finish has used on the surface of the fabric. This can be also revealed from the SEM images of the moisture management finish shown as below **Figures 6** and **7**. I have used coating on the polyester fibre with a particle size ranging 10 nm. The similar trend has also found for the nylon fibre.

This can be also perceived from **Figures 8** and **9** in SEM images at the uniform coating of the antimicrobial finishes on the polyester fabrics surface with a particle size ranging 10 nm. The similar trend has also found for the nylon.

**Figure 6.**

*SEM images of untreated and treated polyester fabric with moisture management finishes.*

**Figure 7.**

*SEM images of untreated and treated polyester fabric with moisture management finishes.*

**Figure 8.**

*SEM images of untreated and treated polyester with antimicrobial finish.*

**Figure 9.**

*SEM images of untreated and treated polyester fabric with antimicrobial finish.*

## **9. Influence of moisture management finish (MMF) on fabric moisture management properties**

In case of 100% polyester fabric it can be observed from the **Figure 10**. That as the fabric GSM increases from 100 to 160 grams, the value of accumulative one-way transport index (OWTI) % decreases. It is due to the increase in the thickness of the fabric with the increase in GSM as shown on the **Table 9**. The increased thickness

**Figure 10.** *Effect of MMF finish concentration and GSM on OWTI% in polyester fabric.*

offers more restriction to the flow of moisture across the plane of fabric (reduced conductivity), which reduces the OWTI %. Also it was observed that the increased finishing concentration decreases the OWTI% of polyester fabric. It is due to the increased decreased pore size after finishing. HDS finish provides a surface finish on the fibre surface to increase its moisture management property. Since the finish is applied on the surface of the fibre, the fibre diameter increases and pore size decreases after finishing. The decreased pore size also decreases the air permeability of the fabric as shown in the **Table 9**. It can be seen that other fabric 100% Nylon also follows the similar trend but the rate of reduction in pore size and OWTI% was different for different fabrics. Basically the HDS softness to penetrate deeply into fibres with amorphous structure to create and increase core hydrophilicity and softness to the fabrics.

## **10. Influence of moisture management finish (MMF) on drying rate of different fabrics**

In case of 100% polyester fabric it can be observed from the **Figure 11**. That as the fabric GSM increases from 100 to 160 grams, the value of drying rate increases. This is because of the increase in the thickness of the fabric with the increase in GSM. Increase in the thickness causes the water to spread in wider volume which causes the fabric to dry easily. Further with the increase of finish concentration level, drying rate increases. It is due to the blocking of pores of the fabric and so water remains on surface of the fabric not inside the pores and facilitating easy drying. It can be seen that other fabric 100% Nylon also follows the similar trend but the rate of increment is different due to its different physical properties than polyester.

**Figure 11.** *Effect of MMF finishes concentration and GSM on rate of drying in polyester fabric.*

## **11. Influence of moisture management finish (MMF) on water vapour permeability of different fabrics**

In case of 100% polyester fabric it can be observed from the **Figure 12**. That as the fabric GSM increases from 100 to 160 grams, the value of water vapour permeability (WVP, gm/m2 /day) decreases. It may be due to the increase in the thickness of the fabric

**Figure 12.** *Effect of MMF finish concentration, GSM on water vapour permeability in polyester fabric.*

with the increase in GSM. Further with the increase of finish concentration level, WVP decreases. It is due to the increase in the fabric thickness after finishing, blinding of the fabric structural pores and reduction in fabric porosity with the increase of finish level. This may also be attributed that the reason of blocking of natural capillary action of the fibre/fabrics softener (HDS) [13]. It can be seen that other fabric 100% Nylon also follows the similar trend but the rate of reduction is different.

## **12. Influence antimicrobial (AMF) finish on fabric moisture management properties**

In case of 100% polyester fabric it can be observed from the **Figure 13**. That as the fabric GSM increases from 100 to 160 grams, the value of accumulative one-way

**Figure 13.** *Effect of AMF finish concentration and GSM on OWTI%% in polyester fabric.*

transport index (OWTI) %decreases. It is due to the increase in the thickness of the fabric with the increase in GSM as shown on the **Table 10**. The increased thickness offers more restriction to the flow of moisture across the plane of fabric (reduced conductivity), which reduces the OWTI %. Also it was observed that the increased finishing concentration decreases the OWTI% of polyester fabric. It is due to the increased decreased pore size after finishing. PEH finish provides a surface finish on the fibre surface to increase its moisture management property. Since the finish is applied on the surface of the fibre, the fibre diameter increases and pore size decreases after finishing. The decreased pore size also decreases the air permeability of the fabric as shown in the **Table 10**. It can be seen that other fabric 100% Nylon also follows the similar trend but the rate of reduction in pore size and OWTI% was different for different fabrics.

## **13. Influence of antimicrobial (AMF) finish on drying rate of different fabrics**

In case of 100% polyester fabric it can be observed from the **Figure 14**. That as the fabric GSM increases from 100 to 160 grams, the value of drying rate increases. This is because of the increase in the thickness of the fabric with the increase in GSM. Increase in the thickness causes the water to spread in wider volume which causes the fabric to dry easily. Further with the increase of finish concentration level, drying rate increases. It is due to the blocking of pores of the fabric and so water remains on surface of the fabric not inside the pores and facilitating easy drying. It can be seen that other fabric 100% Nylon also follows the similar trend but the rate of increment is different due to its different physical properties than polyester.

**Figure 14.** *Effect of AMF finish concentration and GSM on rate of drying in polyester fabric.*

## **14. Influence of antimicrobial (AMF) finish on water vapour permeability of different fabrics**

In case of 100% polyester fabric it can be observed from the **Figure 15**. That as the fabric GSM increases from 100 to 160 grams, the value of water vapour permeability (g/m2 /day) decreases. It may be due to the increase in the thickness of the fabric with the increase in GSM. Further with the increase of finish concentration

**Figure 15.** *Effect of AMF finish concentration, GSM on water vapour permeability in polyester fabric.*

level, WVP decreases. It is due to the increase in the fabric thickness after finishing, blocking of pores of the fabric and reduction in fabric porosity with the increase of finish level [14]. It can be seen that other fabric 100% Nylon also follows the similar trend but the rate of reduction is different.

## **15. Conclusions**

In this research an attempt has made to study the influence of MMF and AMF finishes on the moisture management behaviour, dry rate performance, water vapour permeability properties on different knit activewear fabrics. Therefore from the various combinations of fabrics, GSM, finishes and finish concentration level the following conclusions are drawn:


#### *Textiles for Functional Applications*


## **Author details**

Ramratan Guru\*, Anupam Kumar and Rohit Kumar Department of Textile Engineering, Giani Zail Singh Campus College of Engineering and Technology, Maharaja Ranjit Singh Punjab Technical University, Punjab, India

\*Address all correspondence to: ramratan333@gmail.com

© 2021 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

*Functional Textile for Active Wear Clothing DOI: http://dx.doi.org/10.5772/intechopen.96944*

## **References**

[1] Uttam D. Active sportswear fabrics. International Journal of IT, Engineering and Applied Sciences Research, 2(1), 34-40, 2013.

[2] Ramratan and Choudhary A. K. Thermo-physiological study of active knitted sportswear: A critical review. Asian Textile Journal, 27(7), 53- 64, 2018.

[3] Kothari, V. K. Fibers and Fabrics for Active Sportswear. Asian Textile Journal, 12 (3), 55-61, 2003.

[4] Manshahia M and Das A. High active sportswear a critical review. Indian Journal of Fiber and Textile Research, 39(2), 441-449, 2014a.

[5] Zupin Z, Hladnik A and Dimitrovski K. Prediction of one layer woven fabrics air permeability using porosity parameters. Textile Research Journal, 82(1), 117-128, 2011

[6] Moretz Herbert L and Brier Daniel L. Moisture-management garment and support pouch garment. 08/047,841. Accessed 30 Oct2012.

[7] Kissa, E. Wetting and wicking. Textile Research Journal, 66(1), 660-668, 1996

[8] Yoo S and Barker R. L. Moisture management properties of heat-Resistant work wear fabrics effects of hydrophilic finishes and hygroscopic fiber blends. Textile Research Joumal, 7491), 995-1000, 2004

[9] Manshahia, M and DAS, A. Comfort characteristics of knitted active sportswear heat and mass transfer. Research Journal of Textile and Apparel, 17(3), 50-60, 2013.

[10] Lomax, G.R. The designs of water proof, water vapour permeable fabrics. Journal of Coated Fabrics, 1(1), 15-18, 1985

[11] Cimilli, S, Nergis, B.U. Candan, C. and Ozdemir M. A Comparative study of some comfort related properties of socks of different fiber types. Textile Research Journal, 80(10), 948-957, 2010.

[12] A.C. Anand, D.A. Holmes and T. Rowe, In search of dynamic fabric for active wear. Textile Asia, 8(1), 77-80, 1996

[13] Ramratan and Choudhary A. K. Moisture management functionalization of sportswear fabrics with finishing on hydrophilic silicone softener. Asian Dyer Journal, 16 (3), 34-38, 2019 a.

[14] Ramratan and Choudhary A. K. Antimicrobial functionalization of activewear fabrics with quaternary ammonium compounds. Asian textile Journal 28(6), 71-75, 2019 b.

[15] Chakraborty J N, Deora D. Functional and interactive sportswear. Asian Textile Journal, 22(9), 69-74, 2013

[16] Oner, E., Okur, A. Thermo physiological comfort properties of selected knitted fabrics and design of Tshirts. Journal of The Textile Institute, 106, (12) 1403-1414, 2015

[17] Das A, Alagirusamy R. Sciences of clothing comfort. Woodhead Publishing India, New Delhi, 2010

[18] Bhatia D and Malhotra U. Thermo physiological wear comfort of clothing. Journal of Textile Science & Engineering, 6(2), 1-8, 2016.

[19] Manshahia, M and DAS, A. Moisture management of high active sportswear. Fibres & Polymers, 15(6), 1221-1229, 2014b.

[20] HU, Y. L, Li, Y. and Yeung, K.W. Liquid moisture transfer, Cambridge, Woodhead Publishing Limited, 2006.

## **Chapter 12** Textiles for Noise Control

*Mallika Datta, Srijan Das and Devarun Nath*

## **Abstract**

This chapter includes the mechanism of sound absorption and the classes of sound absorbing material to control the noise. The basic phenomena related to the reduction of sound by allowing it to soak in and dissipate also were introduced first, which, can be realised by viscous effects, heat conduction effects, and internal molecular energy interchanges. Porous absorbers are materials where sound propagates through an interconnected pore network resulting in sound energy dissipation. They are only effective at the mid-to-high frequency range, which is most sensitive to the human ear. The applications of different textile fibres and their various forms were identified later in the chapter. Finally, specific discussions are given to sound parameters, noise absorption coefficient, and its measurement technique. The chapter also deals with various factors influencing sound absorption.

**Keywords:** noise absorption coefficient, noise control, nonwoven, porous absorber sound absorption

## **1. Introduction**

"The International Committee of Standardization of Acoustical Terms" has defined noise as 'sound not desired by the recipient' i.e. 'unwanted sound'. Generally, the machines that have been developed for industrial purposes, for high speed transportation, or improved livelihood of human beings are accompanied by noise [1, 2]. A noise system is comprised of three component [1, 3].

Noise Source: The component which perturbs the air;

Noise Path: It is the medium that promotes the propagation of the acoustical energy from one point to other

Noise Receiver: The component which has the potential to adjudge the quantity or level of noise at a point of interest.

The unwanted noise can be reduced by [4, 5]: 1) Source Treatment 2) Transmission Path Treatment 3) Receiver Treatment. In general, there are four basic principles involved in noise control: isolation, absorption, vibration isolation, and vibration damping [6–8]. The noise control options are thus said to be: 1) Absorbers 2) Barriers 3) Composites 4) Enclosures 5) Lag Treatment.

## **2. Mechanism of sound absorption**

Sound absorbers are soft, porous, open-celled materials such as Baffles or Quilted fibrous system blankets that reduce the reflection of sound waves by allowing them to soak in and dissipate also. The dissipation mechanism of sound absorption results in the conversion of acoustic energy to heat energy. Attenuation or dissipation of acoustic energy as a sound wave moves through a medium attributed to three basic mechanisms [4, 6]: Viscous effects, Heat conduction effects, Internal molecular energy interchanges. Dissipation of acoustic energy due to fluid friction while moving through the medium is a thermodynamically irreversible propagation of sound responsible for viscous effects. In Heat conduction effects, heat transfer between high and low temperature regions in the wave results in non-adiabatic propagation of the sound. The sum of these two mechanisms, viscous and heat conduction, is called the classical attenuation, α<sup>n</sup> is given by

$$a\_n(classical) = \frac{2\pi^r f^2}{\rho c\_p^{-3}} \mu \left(\frac{4}{3} + \frac{\rho - 1}{P\_r}\right) \tag{1}$$

where,

μ is fluid viscosity, φ specific heat ratio (1.667 for monatomic gases and 1.400 for diatomic gases), Pr is Prandtl number and equals to μcp,/kt where kt = thermal conductivity cp = specific heat at constant pressure ρ is the density of the fluid and f is frequency [4].

Attenuation of sound energy in fluid results from the finite time required converting translational kinetic energy into internal energies. This is associated with the rotation and vibration of the molecules. The attenuation coefficient can be written in terms of the sum of the individual contributions as follows [4]:

$$a = a\_n(classical) + \sum a\_{\vec{\eta}} \tag{2}$$

where, the *αij* are the contributions of the various vibration energy relaxation effects.

#### **3. Sound absorbing material**

The sound absorption performance of the material is influenced by the amount of acoustic energy absorbed and reflected by the same. The concept of a perfect absorber can be understood by an open window that transfers all the incidence energy to the other side of the window, thereby results in 100% absorption (α= 1.0). An open window of 1 m<sup>2</sup> area gives 1 Sabine of absorption [5]. The maximum absorption of sound is found when the natural impedance of the material is equal to the characteristic impedance of the air (medium). Thus the sound absorbing capacity of a material is the function of its natural impedance (Zc). The sound absorption phenomenon of an acoustic material varies with the frequency and angle of incidence of the sound waves impinge upon the material.

There are mainly four types of sound absorption materials [2] available to achieve sound absorption namely porous absorber, Helmholtz resonator, membrane absorber and perforated panel absorber [7, 8] as discussed below which are

#### **3.1 Helmholtz resonator**

Helmholtz resonator can be used to estimate sound absorption at a lower frequency range [4, 8]. The quality factor (Q) indicates the quality of the resonator. The bandwidth in Hz is estimated from resonance frequency in the case of the Helmholtz resonator following the equation as described below [5].

*Textiles for Noise Control DOI: http://dx.doi.org/10.5772/intechopen.99274*

$$\mathbf{Q} = \frac{f\_{\rm res}}{\Delta \mathbf{f}} \tag{3}$$

where Q is the quality factor, fres is resonance frequency in Hz and Δf is bandwidth in Hz

A Helmholtz resonator is a cavity filled with air with a small opening or neck. The resonator is considered to be undamped when no porous fibres are found in the cavity. Equation (4) is used to determine the undamped resonance frequency of a Helmholtz resonator [9].

Under the condition of no porous fibres inside the cavity,

$$f\_{res} = \frac{1}{2\pi\sqrt{BC}}\tag{4}$$

where,

B is inertance and C is an acoustical capacitance

$$\mathbf{B} = \frac{\rho\_0 (L + \mathbf{1.7R})}{\pi R^2} \tag{5}$$

where,

*ρ*<sup>0</sup> = Density of the air kg/m<sup>3</sup> (L+1.7R) = Effective length of the neck, m *πR*<sup>2</sup> = Area of the opening, m<sup>2</sup>

R = Radius of the hole in m

$$\mathcal{C} = \frac{V}{\rho\_0 c\_0^2} \tag{6}$$

where,

*C* = Acoustical capacitance

*V* = Volume of the chamber, m3 .

*c*<sup>0</sup> = Speed of sound in air, m/sec.

*ρ*<sup>0</sup> = Density of air, kg/m<sup>3</sup>

Alternatively, the approximate resonance frequency, fres (for nonporous fibrous material) could be calculated using the following equation [9]

$$f\_{res} = \frac{\text{CD}}{2\pi\sqrt{vV}}\tag{7}$$

where

*c*<sup>0</sup> = Speed of sound, m/s

*D* = Area of the neck, m<sup>2</sup>

*v* = D (L + 1.7R) Effective volume of the neck, m<sup>3</sup>

*V* = Volume of the chamber, m3

The sound absorption performance of the Helmholtz resonator at its resonance frequency can be then estimated using the following equation

$$a\_{\rm B} = 0.159 \left( \frac{c\_0}{f\_{res}} \right)^2 \tag{8}$$

where,

α<sup>B</sup> = Sound absorption, m<sup>2</sup> (Sabine)

c0 = Speed of sound in air, m/sec

#### **3.2 Membrane absorber**

A Membrane absorber [5] or diaphragmatic absorber is used to absorb low frequencies sound energy. The membrane absorber offers resistance to rapid flexing and the surrounding enclosed air also shows resistance to compression during vibration at the low frequencies of sound and converts that to heat energy. A membrane type absorber is made of plywood or rubber stretched and attached to a rigid support/panel placed at some distance with respect to a solid wall. The stiffness of the panel and the method of fixing of membrane on the panel influence performance of the absorber as the panel itself tends to vibrate.

#### **3.3 Perforated panel absorber**

A perforated panel absorber [5] is the rigid thin perforated sheet with a circular opening/aperture. An air cavity is found behind the perforated panel absorber (PPA) as it is mounted at a distance from the wall. The performance of PPA is improved to tackle broader frequency range when the cavity is filled with porous fibre. The resonance frequency of the perforated panel can be estimated using the following equation [9].

$$f\_{res} = \frac{c}{2\pi} \left\{ \frac{P}{d(L+1.7R)} \right\} \tag{9}$$

where

*c* = Speed of sound in air, m/sec

*P* = Perforation ratio (hole area/plate area)

*d* = Distance of the perforated panel from the wall, m

*L* = Perforated panel thickness, m

(*L* + 1.7*R*) = Effective length of the neck

*R* = Radius of the hole, m.

An air gap between the porous fibre and the wall increases the thickness of the perforated panel and also the depth of the air gap lowers its resonance frequency. Therefore, by varying the depth of the air space and the thickness of the perforated panel, the broader frequency range of sound absorption performance could be achieved. A perforation ratio of more than 20% with a small aperture does not affect the sound absorption of porous fibre. However, a smaller perforation ratio reduces the higher frequency sound absorption performance of porous fibre [7].

The thickness of the PPA increases with cavity depth or the air gap between rigid wall and absorber which in turn lowers its resonance frequency. Thereby, the change in cavity depth/depth of the air space and the thickness of the perforated panel make PPA more suitable for the broader frequency range of sound absorption performance. The small aperture/perforation with more than 20% perforation ratio has no effect on sound absorption by PPA. However, a smaller perforation ratio reduces the higher frequency sound absorption performance of porous fibre [7].

#### **3.4 Porous absorber**

In porous absorbers [10, 11], sound propagates through an interconnected pore [12] network resulting in sound energy dissipation. They are only effective at the mid-to-high frequency range, which is most sensitive to the human ear [12]. The porous absorbers are found their applications in noise control for industries, automobiles, building acoustics, and sound recording studios. The magnitude of undesirable sound/noise reflected from hard interior surfaces and reverberant noise level

#### *Textiles for Noise Control DOI: http://dx.doi.org/10.5772/intechopen.99274*

is reduced in presence of such absorbers [8]. The various classes of porous absorbers are i) cellular ii) granular, and iii) fibrous materials. The cellular absorbers are made of foam of polyurethane and sometimes metal like aluminum [8]. Sound absorbing foam with an open structure allows the propagation of air from one to the other face through the interconnected pores [12]. The polymer based foam is associated with fire hazard and the generation of combustible toxic gases while the metallic foam offers higher mechanical strength [13]. Some common examples of granular absorbers are panels made of wood chips, pervious road, and, porous concrete. The granular absorber can be used in the form of a consolidated structure made with the suitable binder(s) or in the loose form [13]. The used tires, rubber particles, and waste foam have been identified in making granular absorbers [12, 14]. Fibrous materials as reported compose of glass, mineral, or organic fibres in the form of nonwoven fabrics, boards, or preformed elements. The granular materials can achieve a broadband absorption limited to around 0.8, while in the case of fibrous absorbent, the value can rise to unity [12]. Fibrous and granular absorbers are often produced by bonding the fibres or granules with a binder. They are generally covered with a thin perforated sheet such as highly perforated panels of metal, wood or gypsum giving better aesthetic value protecting from damages and prevent the particles from polluting the air which may harm the eco-system [8].

## **4. Textile materials as porous absorber**

## **4.1 Textile fibre**

The textile fibre based porous absorber may be made of felt, glass wool, rock wool, polymer foams, waste cloth fillers [15–21]. The structure of the absorber consists of cavities that promote internal reflection of sound of waves, trap the sound energy and dampen the oscillation of the air particles by friction with absorber material [22]. They are, however, effective for the medium to high frequency range only [23]. The type of fibrous porous absorber is based on the raw material viz., metal, synthetic polymer, and natural. The fibrous forms of metal and its alloy viz., stainless steel, nickel, aluminum, etc., are also identified as suitable noise absorbers in harsh environments [19, 23, 24].

Presently, synthetic fibres have largely been used owing to some advantageous attributes viz., large specific surface area, good mechanical strength, and good permeability [23, 25, 26]. The various structural forms of such fibre as acoustic absorber include the felt, woven cloth, and fibre reinforced composites [23]. The various synthetic fibre options in noise control are glass wool, rock wool, basalt, carbon fibre [15, 17, 19, 20, 24, 27–30]. The crude based fibres like polyester [26, 31–38], polypropylene [39–41], nylon [42, 43] were also used for noise control study. Polyester microfibre felts showed improved noise absorption in the middle frequency, ranged from 1200 Hz to 4000 Hz [44]. The study on polyester and nylon microfibre fabric concluded that sound absorption increased with fabric density up to 0.14 g/cm [17, 45]. Synthetic fibres and their blend at different ratios towards the optimised acoustic absorption coefficient have been identified by several researchers [37, 38, 40, 46–50].

The use of metallic fibre in noise controls is limited due to their poor flexibility, heavyweight, poor formability [23], while the synthetic polymer fibres are nonbiodegradable and cause serious health hazards during manufacturing, installation, and disposal [10]. The limitation of metallic and synthetic based fibrous porous absorbers wrenches the researcher in the exploration of suitable alternatives from a renewable resource.

Different works had been reported on exploration of using various natural fibres [51–53] viz., wool [54], cotton [50, 55], kapok [56], kenaf [57], hemp [58–60], ramie [61], flax [57, 61–63], banana [64], broom [65], coir [66], jute [51, 61, 67–70], tea leaf [71], combination of Luffa fibres with cotton layer [72] and agro residues viz., straw, oil palm [57] for producing sound absorbers. Most of the fibres showed encouraging sound absorption property.

The textile materials in various forms viz., woven [73–76], carpet [77, 78], nonwoven [70, 76, 79–88], knitted [89], and composite [31, 48, 59, 68, 90–93] were used for control of noise.

The major disadvantage associated with fibrous absorbers is in tackling long wavelengths of low frequency sound energy. Different approaches were attempted by various researchers to improve the noise control performance of porous absorbers. Some works are available in the literature on the improvement of absorption performance of noise of low frequency range by adjusting the nonacoustical parameters viz., thickness, areal density, and bulk density of porous absorber [94].

#### **4.2 Forms of textiles in noise control**

#### *4.2.1 Nonwoven fabric*

The use of nonwoven or felt structures developed from different fibres and their blend or in composite form is gaining interest to use as a noise control solution [3, 13, 26, 31, 81, 84, 87, 95]. Effect of fibre fineness, fibre cross section, structure parameters of nonwoven viz., total surface area, the density of fibre packing, thickness and physical parameters including fibre mixing ratio (blend ratio), bulk density [51, 70, 96, 97] were investigated. The results revealed that total surface and fabric density determined sound absorption positively, and fibres with profiled cross-section shapes show a higher noise reduction coefficient [26]. The acoustic absorption coefficient increased with the increase of thickness of nonwoven/felt [98, 99]. Pore diameter [100], porosity, and the air gap behind the mounted nonwoven samples influenced the sound absorption at low frequency [101]. Absorption behaviour changes with different fibre content in their blend [46, 47, 98, 99] and there was an optimum bulk density for noise reduction [98]. The use of fibre with a larger lumen in the mix of natural with synthetic fibres enhanced the noise control performance [98, 99]. The increase in hollow fibre percentage in nonwoven fabric was directly related to its thickness and sound absorption efficiency [85]. The use of nonwoven made from natural fibres was suitable for application in vibration control for the automobile industry due to their excellent strength and renewable properties [102] and offered excellent absorption in the mid-to-high frequency ranges [86]. The work revealed [40] that the orientation of fibres in nonwoven did not affect acoustic absorption whatever might be the pile orientation of 0°/90° and 45°/45° [40]. The smaller size of aerogel embedded in a nonwoven fibre matrix has positive effects on acoustic absorption [103]. Introduction of nano fibre in making nonwoven improve both sound absorption and sound transmission loss [104]. Acoustical nonwovens with activated carbon fibre on the surface exhibited improved acoustic properties [79–81].

#### *4.2.2 Woven fabric*

The use of woven structure in noise control was identified by various researchers [105–107]. The study on the effect of weave type, weft yarn linear density, thickness created by the layering of test fabrics, yarn spinning system, and depth of air

#### *Textiles for Noise Control DOI: http://dx.doi.org/10.5772/intechopen.99274*

space at the back of samples revealed that the sound absorption coefficient of woven fabrics is influenced by both density and porosity of fabrics. The weight and cover of woven cloth as an upshot thread density and thread count [107, 108] influenced transmission loss of sound transmitted through the structure. Higher thickness of woven fabric associated with improved noise reduction coefficient [106]. The plain weave structure offered higher sound absorption in comparison to other weave designs. Higher absorption woven structure was found with finer and low twisted weft yarns [109]. The woven structure made of fabrics rotor-spun yarns exhibited the highest absorption in comparison to structure made with ringspun or compact yarns.

Pile carpet as noise control material was investigated [77] and it was found that pile thickness and weight of carpet have a minor influence on transmission loss of sound moving through the carpet. Effect of pile parameters namely fibre type, pile height, and carpet construction, pile density, air gap behind the mounted tufted carpets was studied which identified suitability of various factors in controlling sound at audible frequency range [78]. The air gap behind the carpet enhances the noise absorption capacity at low to medium frequencies.

### *4.2.3 Knitted fabric*

NAC for plain knitted structures with the same thickness but different pore changes with the size of pore diameter. The smaller diameter pore under the influence of smaller stitch sizes with lower porosity offers a higher degree of sound absorption sizes [110]. For the same value of pore radius and porosity, the NAC changes with thickness in the case of knitted structure [111]. Introduction of spacer fabric inside the knitted structure improved the noise control performance of the knitted structure.

## **5. Factors influencing sound absorption**

Studies on various parameters that influence the sound absorption properties of fibrous materials have been published widely in the literature [7, 10, 12, 94, 112, 113]. The factors in detail can be described as follows:

#### **5.1 Textile factor**

#### *5.1.1 Fibre Size*

The sound insulation behaviour of wool fibre based material sound absorption coefficient increase with a decreasing fibre diameter [54]. It is found that thin fibres can move more easily than thick fibres on sound waves. Moreover, with fine denier, more fibres are required for the same volume density which generates a more tortuous path and higher airflow resistance [94]. Studies of Tancan [82] revealed that the fine fibre increases sound absorption coefficient values due to an increase in airflow resistance through increased viscosity resulting from the vibration of the air.

#### *5.1.2 Thickness*

The thickness of the porous absorber directly influences the low frequency sound absorbing performance [94, 114]. The material with an apparent thickness (includes cavity depth from the rigid back) equal to the one-quarter wavelength at a resonant frequency gives peak absorption [94], however, the threshold thickness

for effective absorption is one-tenth of wavelength. A study also showed that sound absorption increases with the increase in thickness of the material only in the case of low frequencies. Thickness becomes insignificant at higher frequency.

#### *5.1.3 Density*

The sound absorption performance of a material is a function of the bulk density of the material [10, 54]. It is to be kept in mind that the density of the acoustic material affects its cost. The sound absorption value of the absorber at the middle and higher frequency (> 500 Hz) increases with the density of the sample [115]. When the number of fibres increases per unit area, the apparent density (considering the entrapped air) is high. Conversion of sound energy to heat increases as the surface friction at the viscous boundary layer increases [12] and so the sound absorption coefficient, especially for nonwoven fibrous materials [23, 53, 94]. Open and light porous structure absorbs the sound of low frequencies (<500 Hz), while denser structure suitable for frequencies above 2000 Hz.

### *5.1.4 Porosity*

The sound absorption mechanism of the porous absorber can be explained in the light of, pore parameters viz, number, size, and shape [100]. Dissipation of sound energy is owing to frictional resistance offered by the pores that allowing the propagation of sound through it. The porosity is generally thus defined as the ratio of the volume of the voids in the material to its total volume [12, 116]. Equation (9) defines porosity (∅) [53, 117].

$$\text{Porosity} = (\mathcal{Q}) = \frac{\mathbf{V\_a}}{\mathbf{V\_m}} \tag{10}$$

where:

Va = Volume of the air in the voids Vm = Total volume of the sample of the acoustical material being tested

#### *5.1.5 Tortuosity*

Tortuosity is a measure of the crookedness of the passageway through the pores, compared to the thickness of the sample. Tortuosity enumerates the influence of the internal structure of a material on its acoustical properties. Con Wassilieff [118] describes it as a measure of the deviation of the pores from the normal, or meander about the material. The location of the quarter-wavelength peaks of sound energy is influenced by tortuosity, while the height and width of the peaks are persuaded by porosity and flow resistivity. The degree of crookedness/tortuosity determines the behaviour of absorbing porous materials at the high frequency level.

#### *5.1.6 Compression*

The porous fibrous textile structures are compressible in nature and experience compression on the application of load and thickness decreases. The factors like density, porosity, tortuosity, airflow resistivity, porosity, and density also vary with changes in thickness. The studies [119, 120] found that in the event of reduction of the thickness of a homogeneous layer of porous fibrous porosity and characteristic lengths (shape factor) [12, 94] decrease while the density and tortuosity or crookedness in the structure increase. The effect of compression of fibrous structure is

found to be more profound in the case of automotive acoustics. The weight of the passenger causes cyclic compression and expansion of the seat padding that results in squeezing down the porous materials (fibrous or cellular) which in turn results in the variation of the above mentioned physical parameters [3].

## *5.1.7 Airflow resistance*

One of the most important parameters that influence the sound absorbing characteristics of fibrous material is the specific flow resistance per unit thickness of the material [12, 32, 94, 116, 121]. The characteristic impedance and propagation constant, which describes the acoustical properties of porous materials, are governed to a great extent by the flow resistance of the material [94].

The presence of fibrous peg (due to interlocking of fibre at the time of needling) as frictional elements in the case of a needled nonwoven, provide resistance to acoustic wave motion. As the sound wave enters a fibrous nonwoven structure, its amplitude is decreased by friction while moving through the tortuous path, and sound energy is converted into heat [94]. The friction resistance of the material to the flow of air is called 'airflow resistivity' and is expressed as:

$$
\sigma = \frac{\Delta P}{\Delta T} \times \frac{1}{u} \text{Pa.s/m}^2 \tag{11}
$$

where,

σ = airflow resistivity Pa.s/m<sup>2</sup>

*u* = Air velocity through sample m/sec

*Δp* = Sound pressure differential across the thickness of the sample measured in direction of particle velocity, N/m2

*ΔT* = Incremental thickness [3, 94]

Based upon the airflow test following ASTM D737 [122], flow resistivity *σ* of the sample is obtained from the following equation:

$$
\sigma = \frac{P}{ct} \tag{12}
$$

where,

*P* = Static pressure differential between both faces of the sample, dyne/cm<sup>2</sup> (10�<sup>1</sup> Pa)

*c* = Air velocity, cm/s

*t* = Thickness of sample, cm

The airflow resistance per unit thickness of a porous material is proportional to the coefficient of viscosity of the fluid (air) and inversely proportional to the square of the pore size of the material. For a fibrous material with a given porosity, the flow resistance per unit thickness is inversely proportional to the square of the fibre diameter

#### *5.1.8 Surface impedance*

For a given layer of thickness, the acoustic resistivity of an absorber is directly related to its ability to dissipate sound energy. Moreover, the surface impedance [123] of the layer increases with the degree of air resistance offered by the structure that results in a more reflection of sound from the surface layer. Thereby, sound absorption by the fibrous structure reduces. The mechanism of sound absorption is

frequency dependent, thus at a lower frequency as the thickness of the layer increases resistivity decreases.

### **5.2 Other factor**

#### *5.2.1 Placement/position of sound absorptive materials*

The placement/position of sound absorptive materials is known to affect the sound absorption of the material. It has been reported by Alton Everest [5], that if several types of absorbers are used, material applied to the lower portions of high walls can be as much as twice as effective as the same material placed elsewhere [124]. The porous structure behaves like a frequency dependent membrane of a certain mass under the influence of an air cavity behind a material. The presence of air inside the cavity has an analogy to a mechanical spring. The absorption property of the porous fibrous absorber enhances significantly with the air filled cavity between the absorber and the rigid back wall [125].

#### *5.2.2 Temperature*

The study [126] revealed that the sound absorption characteristics of mineral wool remained unaffected by the change in temperature in the range of 10–50 °C. Least square method was used to develop a theoretical relation between the noise reduction coefficient and the thermal conductivity at different temperature conditions.

#### *5.2.3 Process parameters*

Process parameters during absorbent material formation have an important impact on sound absorption due to their effects on the characteristics of the absorbent material. It reported that 'air laid' web based nonwovens offered higher sound absorption compared to carded ones irrespective of the fibre content. This might be due to higher flow resistivity of air laid nonwovens because of relatively random placement, and thus, higher tortuosity, the higher number of pores with smaller sizes, higher number of fibre to fibre contact points, and gradient in porosity due to gravity. Among web bonding methods [3], did not find a significant difference between needled and needled plus thermally bonded nonwovens. Thermal bonded and needle punched nonwoven (punching density of 28 cm<sup>2</sup> ) from polypropylene and needle punched polyamide nonwoven offered maximum absorption of sound at a material density of 100 kg/m<sup>3</sup> over a frequency range of 63 to 8000 Hz [88]. The fibres of diameter 10 to 40 μm were used to manufacture the nonwovens with thickness varying from 3 to 20 mm. The study revealed that the needle punched nonwovens had more dependency on the frequency of sound energy as well as on the diameter of fibres compared to thermally bonded webs.

#### **6. Testing and characterisation of sound absorbing material**

#### **6.1 Absorption coefficient**

When a beam of sound wave (*Ei*) strikes against a barrier, gets divided into three parts: i) *Er* is reflected part ii) *Ea* is the part absorbed in the barrier iii) *Et* is transmitted part to the other side of the barrier as shown in **Figure 1**. These phenomena can be written as

#### **Figure 1.** *Typical behaviour of sound wave striking a wall.*

$$E\_i = E\_r + E\_a + E\_t \tag{13}$$

Then the sound absorption coefficient, α is defined as follows [7, 93].

$$\infty = \frac{E\_i - E\_r}{E\_i} = \frac{E\_a + E\_t}{E\_i} \tag{14}$$

Equation (13) shows that all portion of the sound energy which is not reflected is considered to be absorbed.

When an infinitely large boundary plane presents between two media, the path of movement of the sound wave traveling from Medium 1 to 2 is normal to the boundary plane, as shown in **Figure 2**. The relation of reflected and transmitted sound can be expressed as:

$$P\_i + P\_r = P\_t \tag{15}$$

$$V\_i + V\_r = V\_t \tag{16}$$

where, *Pi* is incidence sound pressure, *Pr* is reflected sound pressure, *V* = *P*/*Z* and *Z* is the inertance or impedance

$$Z\_1 = \rho\_1 c\_1 \text{ and } Z\_2 = \rho\_2 c\_2$$

The sound pressure reflection coefficient *rp* is

$$r\_p = \frac{P\_r}{P\_i} = \frac{Z\_2 - Z\_1}{Z\_2 + Z\_1} \tag{17}$$

The relationship between the absorption coefficient and sound pressure reflection coefficient between two media is given by:

$$\mathbf{a\_{r}} = \frac{\mathbf{E\_{r}}}{\mathbf{E\_{i}}} = \left( \left| \frac{\mathbf{P\_{r}}}{\mathbf{P\_{i}}} \right| \right)^{2} = \frac{\left(\mathbf{Z\_{2}} - \mathbf{Z\_{1}}\right)^{2}}{\left(\mathbf{Z\_{2}} + \mathbf{Z\_{1}}\right)^{1}} \tag{18}$$

$$\alpha = \mathbf{1} - \left| \mathbf{r\_p} \right|^2 = \mathbf{1} - \frac{\mathbf{E\_r}}{\mathbf{E\_i}} = \mathbf{1} - \left( \left| \frac{\mathbf{P\_r}}{\mathbf{P\_i}} \right| \right)^2 = \mathbf{1} - \frac{\left( \mathbf{Z\_2} - \mathbf{Z\_1} \right)^2}{\left( \mathbf{Z\_2} + \mathbf{Z\_1} \right)^1} \tag{19}$$

#### **6.2 Principles of sound absorption measurement**

Two standards and one relative measurement technique are used to measure the noise control property of absorbing material and describe in detail as follows.

#### *6.2.1 Impedance tube method*

This technique (Normal Incidence Technique) [4, 7, 127] is based on normal incident sound energy and requires a tube (**Figure 3**) where its diameter is smaller than the wavelength. The absorption coefficient can be measured in impedance tube as per ASTM E 1050-12 and ISO 10534-2: 1998 standards [128, 129]. The circular sample of φ 30 mm and φ 100 mm are tested against the rigid back wall using two tubes. The larger diameter tube (φ 100 mm) is used to measure the absorption coefficients in the frequency range, 63–1600 Hz, and a smaller diameter tube (φ 30 mm) for measuring in the frequency ranges from 1000 Hz–6300 Hz. The samples are tested five times to minimize the influence of variation of thickness and areal density.

#### *6.2.2 Reverberant field method*

Measurement of sound absorption is concerned with the performance of a material exposed to a randomly incident sound wave, which technically occurs when the material is in a diffusive field (Random Incidence Technique) [7, 130]. EN ISO 354 (2003) testing standard method [131] is employed to measure the

**Figure 3.** *Schematic diagram of impedance tube.*

reverberation time (RT) to determine the absorption coefficient. This technique requires a reverberation chamber with a volume of 200 m<sup>3</sup> . The diffused acoustic field is created inside the room to test the sample with an area between 10 m<sup>2</sup> and 12 m2 . The absorption coefficient α<sup>r</sup> is calculated using the following formula from RT.

$$a\_r = \frac{KV}{s} \left[ \frac{1}{T\_m} - \frac{1}{T\_0} \right] + \overline{a} \tag{20}$$

Where,

*V* = Volume of reverberation chamber, m<sup>3</sup>

*S* = Surface area of the material tested, m<sup>2</sup>

*Tm* = time with tested material, sec.

*T*<sup>0</sup> = time empty, sec

*α* = Average sound absorption coefficient of reverberation chamber

*K* = constant=0.16

#### *6.2.3 Clemson-Boston differential sound insulation method*

Clemson-Boston Differential Sound Insulation (CBDSI) Tester [95] is used to measure sound insulation of the material as shown in **Figure 4**.

CBDSI is comprised of a computer to process sound signal, an amplifier to amplify the sound signal, sound source, sound chamber, sample holder to mount the test sample, and a detector to detect the sound. The sound source generates white noise [7] in the frequency range of 73 Hz to 20,000 Hz for testing the sound insulation property of the material under investigation. The signal amplifier amplifies the signals generated by the computer which are then converted into sound waves via the sound source. The material under investigation mounted on the sample holder interferes with sound energy as it is in the path of sound. The sound moves through the sample and strikes the detector, which changes the received sound signals to electric signals that are then analyzed by the signal-processing computer. The test is also conducted with no sample in the sample holder to measure the insulation property of the material in reference to the background.

**Figure 4.** *Schematic diagram of Clemson-Boston differential sound insulation tester.*

CBDSI tester provides a direct comparative analysis of various types of samples as they are tested under the same conditions. Sound insulation property in terms of 'Transfer Function Magnitude' in dB unit is evaluated by this instrument.

## **7. Commercial players and market overview**

Currently, a wide range of synthetic fibres is taken for noise reduction applications. Various structural forms of synthetic fibre use in noise control are woven, nonwoven, knitted, fibre felts, and fibre reinforced composites. The different crosssections of synthetic fibre such as hollow [35, 85], and triangular, trilobal are beneficial to improve acoustic absorption properties. It has been reported that hollow fibre has higher sound absorption and of lighter weight. Superior mechanical responses [23] of synthetic fibre and their various cross sectional in combination with natural fibre [22, 52, 53, 72] make them suitable for optimized applications in noise control. The use of recycled synthetic fibre [38] as a noise control solution addresses the concern related to the disposal of hard generated after the use of virgin nonbiodegradable synthetic fibre.

The forecasted growth size of the global noise control system market will be nearly 200% during 2017-2027 [132]. The noise control materials generally used in residential, industrial, and commercial applications in the form as follows

Acoustic panels Acoustic tiles Sound curtains Acoustic surface Sound insulating flooring Sound barrier walls Baffles Sound blanket Sound doors

#### **8. Conclusion**

In this chapter, the mechanism of sound absorption has been discussed first followed by a detailed description of the type of sound absorbers such as Helmholtz resonator, membrane absorber, perforated panel absorber, porous absorbers with emphasis on textile materials as sound absorbers. Then the effect of fibre size, thickness, density, porosity, tortuosity, compression, etc. on sound absorption has been discussed followed by a brief description of the principles of sound absorption measurement.

#### **Conflict of interest**

The authors declare no conflict of interest.

*Textiles for Noise Control DOI: http://dx.doi.org/10.5772/intechopen.99274*

## **Author details**

Mallika Datta\*, Srijan Das and Devarun Nath Government College of Engineering and Textile Technology, Serampore, Serampore, India

\*Address all correspondence to: dattamallika8@gmail.com

© 2021 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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## **Chapter 13**
