IOP PUBLISHING MEASUREMENT SCIENCE AND TECHNOLOGY

Meas. Sci. Technol. 18 (2007) 2657–2666 doi:10.1088/0957-0233/18/8/042

Analysis of sound absorption of tuckspacer fabrics to reduce automotive noiseTilak Dias, Ravindra Monaragala, Peter Needhamand Edward Lay

William Lee Innovation Centre, Textiles and Paper, School of Materials,The University of Manchester, Manchester, UK

E-mail: tilak.dias@manchester.ac.uk

Received 12 March 2007, in final form 28 April 2007Published 11 July 2007Online at stacks.iop.org/MST/18/2657

AbstractTextiles are widely used in the automotive industry to provide both comfortto the passengers and an aesthetic appearance to the automotive interior.They can also be used to reduce automotive interior noise, which can makeautomotive travel safer and more comfortable. Knitted fabrics are usedwidely in automotive upholstery; however, the sound absorbency of a singlelayer of a knitted fabric is inadequate for the reduction of automotive interiornoise. This paper investigates the sound absorbency of a novel knitted spacerfabric, which can be used in automotive upholstery and has the potential forgreater sound absorbency than a conventional plain knitted fabric and itsderivatives. The spacer fabric is modelled as a porous sound absorber and itssound absorbency is studied with regard to its structural parameters.

Keywords: interior noise reduction, automobile industry, spacer knittedfabric, noise absorption, construction industry

(Some figures in this article are in colour only in the electronic version)

1. Introduction

The OEM (original equipment manufacturers) strive toimprove the comfort of the automotive interior as it reflects onthe brand’s prestige [1]. Automotive interior noise contributesto driver fatigue, a major cause of road accidents, and thusreducing it is a valuable contribution to road safety [2]. Theproblem can be overcome by using two approaches, whichare known as active and passive [3]. The passive techniquehas several methods for reducing noise, of which the soundabsorption technique is most suitable [4]. Generally poroussound absorbers comprising polyurethane and foam are usedas sound absorbing materials [5]. However, they tend to beenvironmentally unsound. Textiles can be manufactured fromrecycled material and therefore are of ecological and industrialinterest. Textiles are used widely in the automotive interior, asthey are light in weight, and inexpensive [6]. Knitted fabricsmay be superior to other fabric structures mainly because oftheir superior drapability. Moreover, they can be knitted withany design to suit the OEM requirements with ease. However,a single layer of a knitted fabric is a poor sound absorber. Toimprove its sound absorbency, several layers of the fabric have

to be laid over each other to make the overall thickness severalcentimetres [7].

This paper studies the sound absorbency of a novel spacerfabric, which has a thickness in the region of 1 cm and hasreasonable sound absorption properties. It is light in weight,flexible and can be knitted with any design to suit the OEMand automotive brand requirements. These fabrics can be usedin automotive upholstery, parcel shelf, headliner and doorpanels, to provide both comfort and noise reduction to theautomotive interior. The fabric is superior over other textilestructures used in the automobile in terms of flexibility andease of manufacture. However, its drapability is reduced incomparison to a simple knitted fabric.

The tuck spacer fabric consists of top and bottom plainknitted layers. These two layers are interconnected with amesh of yarn.

This structure thus represents a tight mesh or bundleof yarn sandwiched between two plain knitted layers. Theinterconnecting mesh of yarn is oriented at an angle betweenthe top and bottom layers (figures 1 and 2).

The pictorial view of a tuck spacer fabric is shown infigure 2. As can be seen in this figure, the tuck spacer fabric

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Figure 1. Structural representation of the cross section of a tuckspacer fabric.

can be fabricated as a single thick fabric as compared to thelaminated layers of plain knitted fabrics and their derivatives.

2. Analytical prediction of the sound absorption oftuck spacer fabrics

The tuck spacer fabrics may be analysed as porous soundabsorbers. This is because the gap between the top and bottomlayers of the fabric is filled with yarns oriented at an angle andrepresents a porous structure (figures 1 and 2). The poresof the tuck spacer fabric are considered to be composed ofidentical slits oriented at an angle from the surface as shown infigure 3.

When a sound wave impinges normal to the tuck spacer,when it is placed on a solid metal enclosure (figure 3),the acoustic impedance (ZB) of the air inside a slit in thefabric near its surface at point B in figure 3 can be given byequation (1) [8]:

ZB = −jZc cot(β · l) (1)

where Zc is the characteristic impedance of air, of theimpinging sound wave on the fabric, and β is the wavenumber of the sound wave. These quantities are given byequations (2) and (3) [8]:

Zc =√

K · ρ (2)

β = ω√

ρ/K (3)

where ω is the angular frequency of the sound wave, ρ is thedynamic density and K is the dynamic bulk modulus of the airinside the pores. Zwikker and Kosten [9] have modelled theairflow inside a cylindrical pore in a rigid frame as a laminarflow and derived relationships for ρ and K, treating the thermal

Figure 2. Pictorial view of a tuck spacer fabric.

Figure 3. Sound absorption of the tuck spacer fabric to a soundwave impinging normal to its surface.

exchange effects between the air and the walls of the cylinder,and by considering the viscous effects of the laminar flow ofair in the cylinder as two separate issues. Biot and Allard[8, 10] have modified the dynamic density function derived byZwikker and Kosten for a rigid frame which is presented asfollows:

ρ = ρ0ks − jσφ · F (ϑ)

ω(4)

where ρ0 is the density of air, φ is the porosity of the fabric,ks is the tortuosity and σ is the airflow resistivity of the fabricwhich is an intrinsic property of the fabric to a laminar airflowdefined in [8]. The quantity F(ϑ) for a slit is [8]

F(ϑ) = −√

j · ϑ · tanh(ϑ√

j)

3 · [1 − { tanh(ϑ

√j)

ϑ√

j

}] . (5)

The dimensionless parameter ψ for a circular cylinder is givenas [8]

ψ = S

√8ωρ0ks

σφ(6)

where S is the frequency-independent shape factor of the pore.This value is related to the dimensionless parameter of a slit(ϑ) as [8]

ϑ = 34ψ. (7)

The dynamic bulk modulus derived by Zwikker et al [9] hasbeen considered as the dynamic bulk modulus of the slits which

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Figure 4. Yarn path notation of a tuck spacer fabric.

is given as

K = κP0

1 + 2Bψ

√−j· (κ − 1) · J1(Bψ

√−j)J0(Bψ

√−j)

(8)

where κ is the thermal conductivity of air, B is the square rootof the Prandtl number [8] and P0 is the atmospheric pressure.

The acoustic impedance at the surface of the tuck spacerfabric at point A (ZA) is a function of the acoustic impedanceat point B (ZB) and the porosity () of the fabric. This is givenby equation (8) [8, 9]. Furthermore, the noise absorptioncoefficient (NAC) of the fabric can be given by equation (9)from the acoustic impedances at points A and B of the fabric[8]:

ZA = ZB

φ(9)

NAC = 1 −∣∣∣∣ZA − Zc

ZA + Zc

∣∣∣∣2

. (10)

Thus, to analytically determine the NAC of the fabric withthe use of equations (1)–(10), the following parameters of thefabric are required.

(a) Fabric thickness (l).(b) Fabric porosity (φ).(c) The airflow resistivity of the fabric (σ ).(d) The tortuosity (ks) and the pore shape factor (S).

The constants, thermal conductivity of air (κ), Prandtlnumber (B), density of air (ρ0) and the atmosphericpressure (P0) at 18 ◦C temperature, which are required forequations (4), (6) and (8), can be represented as follows.

(a) κ: 1.4(b) B2: 0.71(c) P0: 1.0132 × 105 Pa(d) ρ0: 1.213 kg m−3.

Table 1. Details of the yarns used for the fabrics.

Tuck spacerfabric Yarn used for the top and bottom layers

TS1 200 dtex Tencel spun yarnTS2 200 dtex Tencel spun yarnTS3 167 dtex textured polyester multifilament yarnTS4 167 dtex textured polyester multifilament yarnTS5 972 dtex double covered elastomeric yarnTS6 167 dtex textured polyester multifilament yarn

3. Fabric samples

3.1. Fabric sample construction

The tuck spacer fabrics were knitted from a seven-gauge ShimaSeiki flat bed knitting machine. The fabrics were steamed onboth sides by placing them on a steam table. The front andback layers of the fabrics have been plain knitted with the yarnsindicated in table 1. These two layers are then interconnectedthrough a series of tucks by five ends of 167 dtex texturedpolyester multifilament yarn (figure 4).

The number of rows of the interconnecting yarn, tuckedbetween the front and back beds in a plain knitted course ofa fabric, affects its density and porosity. More rows of tucksbetween a plain knitted course result in increased density.

3.2. Fabric sample details

The details of the yarns used for the fabrics are given intable 1. The structural details of the fabrics are given intable 2.

It can be observed from table 2 that there is a correlationbetween the fabric porosity and its airflow resistivity. This isshown in figure 5.

Table 2 indicates that more rows of interconnecting yarnbetween the front and back beds of a plain knitted course ofthe fabric result in a higher density. This would yield a lesserporosity. Moreover, it can be seen from figure 5 that when theporosity increases there is a reduction in the airflow resistivity

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Table 2. Structural details of the tuck spacer fabrics.

Number of courses TotalCourses per cm Wales per cm of the interconnecting fabric Airflow

Tuck spacer of the top and of the top and yarn between alternating Density Porosity thickness resistivityfabric bottom layers (C) bottom layers (W) layers of the fabric (kg m−3) (%) (mm) (σ ) (N m−4 s)

TS1 5 5 12 115.064 91.6 9.35 27 096.1TS2 6 5 8 92.61 93.2 10.10 17 172.9TS3 7 4 12 122.557 91.1 10.05 33 653.8TS4 8 5 8 86.0355 93.8 12.10 15 490.3TS5 12 7 12 216.0575 84.35 15.10 76 401.5TS6 8 4 6 89.537 93.5 9.45 18 211.5

Figure 5. Relationship between the porosity and airflow resistanceof the tuck spacer fabrics.

of the fabrics. Thus, with the increase of fabric density, thereis an increase of airflow resistivity.

4. Measurement of structural parameters of tuckspacer fabrics

The structural parameters, namely the thickness, porosity andthe airflow resistivity of the fabric samples, were measuredas described in sections 4.1, 4.2 and 4.3, respectively. Theparameters tortuosity and shape factor of the pores in the fabricwere approximately determined as described in sections 4.4and 4.5, respectively.

4.1. Measurement of the total fabric thickness

The thicknesses of the fabrics as indicated in table 2 weremeasured with the use of a Zwick Roell tensile tester operatedin compression mode. A 1000 N load cell was used for thispurpose.

4.2. Measurement of porosity and density

Porosity can be defined as the ratio of the void space or volumeof pores within the boundaries of a solid material, to the totalvolume [8]. The porosity or void fraction of a solid material

is usually expressed as a percentage, and can be given byequation (11) [4, 11]:

φ = 100

[1 − M

tρm

](11)

where M is the mass per unit area of the fabric, t is the thicknessof the fabric and ρm is the density of the fibres in the fabricwhich is taken as 1.38 g cm−3 [12] for the polyester yarn usedin the fabrics (table 1).

To measure the porosity of a particular fabric, threesamples were prepared, each cut as a circle of 19 cm diameterfrom the fabric under test. The weights of these samples weremeasured in grams and the average was taken. This value isthen divided by the area of a sample (283.53 cm2), to obtainthe mass per unit area (M) for the fabric in g cm−2.

This value and the measured thickness of the fabric fromtable 2 are used in equation (11) to obtain the porosity of thefabric. The results are given in table 2.

The density of the fabrics can be determined by

ρf = M

t. (12)

The densities of the fabrics are given in table 2. The density ofconventional porous or fibrous damping material is in the rangeof 20–200 kg m−3 [13]. In terms of commercially availablesound absorption and insulation materials, polyurethane foamhas a density of 25–30 kg m−3 and felt sheets have a density of250 kg m−3 [14]. Glass wool mats, which are used for soundabsorption and insulation, have densities from 19 to 80 kg m−3

[15]. Thus, the density of the tuck spacer fabrics is generallyhigher than that of polyurethane foam being similar to that offibrous acoustic materials.

4.3. Measurement of airflow resistivity

The airflow resistivity of the fabrics was measured usingan orifice airflow tube system (shown in figure 6) based onthe direct flow method described in the BS EN 29053:1993standard. The orifice airflow tube system was designed basedon the BS EN ISO 5167-2:2003 standard.

As shown in figure 6, a laminar flow of air is generatedinside the tube by operation of the vacuum pump. The suctionrate of the pump can be adjusted by the Variac transformer,which controls the ac voltage supply to the equipment.

The tube internal diameter is 35 mm and it is 2 m long.The orifice diameter is 12 mm. The fabric is clamped on tothe sample holder, which is part of the duct connected to thetube. The inlet diameter of the duct at the sample holder endis 150 mm. Thus, the airflow occurs from the atmosphere into

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Analysis of sound absorption of tuck spacer fabrics to reduce automotive noise

Figure 6. Airflow resistivity measurement system.

the system via a circular area having a diameter of 150 mm inthe fabric under test.

The orifice plate generates a low airflow velocity insidethe tube and through the fabric under test. This low airflowranges from 1.1 mm s−1 to 200 mm s−1 which is required bythe BS EN 29053:1993 standard to maintain a turbulent-freelaminar flow of air inside the apparatus.

The pressure transducers convert the air pressure insidethe tube at the respective test points to corresponding voltagelevels. These values are then the input to the PC via thedata acquisition (DAQ) hardware. The application program inthe PC uses these data to calculate the pressure difference oneither side of the fabric and the airflow velocity inside the tubeaccording to the BS EN ISO 5167-2:2003 standard. Thesedata are used with the fabric thickness and its cross-sectionalarea to calculate the airflow resistivity of the fabric under testaccording to the procedure given by the BS EN 29053:1993standard.

To measure the airflow resistivity of the fabrics, threesamples from each fabric having a circular shape of 190 mmdiameter were prepared. Each sample was subjected to anairflow variation from 1.1 mm s−1 to 200 mm s−1 inside thetube and the corresponding airflow resistivity was obtainedfrom the software. The region where the change in airflowresistivity is more or less constant was then selected andthe average was obtained as the airflow resistivity for theparticular sample. This region indicates a laminar flow of airfor the particular fabric sample as indicated by the standard.The procedure was repeated for all the three samples and theaverage was taken as the airflow resistivity of the respectivefabric.

4.4. Determination of tortuosity

The pores in the fabric are assumed as slits whose walls arebounded by the mesh of yarns. The analytical model considersthese pores to be inclined at an angle (θ ) normal to the surfaceof the fabric as shown in figure 7.

Figure 7. Orientation of a pore in the fabric.

The tortuosity (ks) of the fabric is related to θ as follows[8, 9]:

ks = 1

cos2(θ). (13)

The angle of inclination of a pore can be assumed to be thesame as the angle of inclination of the interconnecting yarnbetween the front and back surfaces of the fabric. This anglecan be determined using the spacing between tuck loops onalternate wales on the front and back face of the fabric (d) andits thickness (t) as shown in figure 8.

From figure 8 the triangle OAB gives the angle ofinclination as

θ = tan−1

(d

t

). (14)

The spacing d can be determined from the number of wales percm (W) of the plain knitted face of a particular fabric and thenumber of needle positions between the two alternate wales(p) as

d = (p + 1) +10

(W − 1). (15)

Thus, with the use of equations (12)–(14) and the W data ofthe fabrics from table 2, the tortuosity of the fabrics can bedetermined. The calculated values are given in table 3.

These values predict the tortuosity approximately, sincethose sound waves that propagate within the fibres of aninterconnecting yarn have not been considered.

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Figure 8. Orientation of an interconnecting yarn.

Figure 9. Yarn path notation showing the location of consecutive tucks on the front face of the fabrics.

Table 3. Tortuosity data of the fabrics.

Fabric p Tortuosity

TS1 5 3.574TS2 7 4.921TS3 5 4.96TS4 7 3.732TS5 5 1.439TS6 5 5.479

4.5. Pore shape factor

The tuck loops of the interconnecting yarn at the frontface of the structures of the fabrics can be grouped intotwo configurations as shown in the yarn path notations infigure 9.

As seen from figure 9, the interconnecting yarn is tucklooped into every stitch in a course, and in configuration B

into alternate stitches of a course. The result on the plainknitted fabric is simulated in figure 10. Furthermore, the tuckloops at the corners of a rectangle form a pore within the fabric(figure 10).

The pore cross section is more or less a shape between arectangle and a circle. Further, the interconnecting yarns bulgein the steaming process of the fabric; this causes the pore crosssection to be non-uniform throughout its length. This valuehas to be adjusted in order to improve the correspondencebetween the experimental results and the model discussed insection 2 [10].

Considering the dimensions of the rectangles in figure 10,the pore area (Apore) can be approximated as

Apore = 10p · Cs

W − 1(16)

where p is 1 for configuration A, and 2 for configuration B.The calculated pore areas for the fabrics are given in table 4.

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Analysis of sound absorption of tuck spacer fabrics to reduce automotive noise

Figure 10. Pores formed by the interconnecting yarns in configurations A and B.

Figure 11. Relationship between the pore shape factor and porearea.

Considering the pore area, the pore shape factor (S) wasadjusted from a series of values between 0.1 and 0.7 forthe analytical prediction given in section 2 to best fit theexperimental data. The optimum pore shape factor valuesfor the fabrics are given in table 4.

Table 4 shows the configurations in which the fabrics aregrouped and the related pore shape factor.

The relationship between the pore area and the pore shapefactor for the fabrics is shown in figure 11.

Figure 11 indicates an inverse second-order relationshipbetween the pore area and the pore shape factor, which can begiven as

S = 0.0092A2pore − 0.1685Apore + 0.8318. (17)

This relationship is an approximation and may have to berefined with different tuck spacer fabrics. It can also beobserved upon comparison of tables 2 and 3 that the reduction

Table 4. Pore shape factor of the fabrics.

Pore area Designated poreFabric Configuration (Apore) (mm2) shape factor (S)

TS1 A 6.25 0.17TS2 B 10 0.07TS3 A 5.556 0.15TS4 B 7.143 0.1TS5 A 1.515 0.6TS6 B 9.524 0.06

in pore area results in an increased density which subsequentlyincreases its airflow resistivity and reduces its porosity.

5. Sound absorbency of a tuck spacer fabric basedon its structural properties

The sound absorbencies of the tuck spacer fabrics weremeasured with a two-microphone impedance tube accordingto the ISO 10534-2 standard [4]. The effect of thickness andairflow resistivity parameters of the tuck spacer fabrics ontheir sound absorbencies are analysed in this section. Thetheoretical data are obtained using the procedure indicatedin section 2 using the porosity (φ), airflow resistivity (σ ),thickness (l) values for the fabric under consideration fromtable 2. The tortuosity and pore shape factor for analyticalprediction were considered from tables 3 and 4, respectively.

For the sake of clarity, the sound absorbencies ofthe fabrics were grouped into two, based on their airflowresistivity. Group A consists of TS1, TS3 and TS5 consideringtheir higher airflow resistivity. Group B consists of TS2, TS4and TS6 based on their lower airflow resistivity. The resultsare shown in figures 12 and 13.

There is a fair agreement between the theoretical and theexperimental data except for the fabric TS5. The differencemay be due to the following.

(a) The theoretical analysis considered the pores formed bythe interconnecting yarn to be cylindrical slits. However,

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Figure 12. Sound absorbency of group A.

Figure 13. Sound absorbency of group B.

in practice, as the yarn bulges due to the steaming processof the fabrication, the pores are not true cylindrical slits.

(b) The tortuosity data for the fabrics are an approximationas it is based on the inclination angle of the cylindricalpores formed by the interconnecting yarn. However, thereare interstices between fibres of the interconnecting yarnwhere sound propagates and is damped.

(c) The analytical model considers propagation of the soundthrough the cylindrical pores only. However, there maybe lateral propagation between the pores.

There is a considerable difference between the analytical andexperimental data for the fabric TS5. The top and bottomlayers of this fabric are composed of plain knitted structures

from elastomeric yarn (table 1); thus, nearly all interstices,which allow the incident sound to enter into the pores formedby the interconnecting yarns, are virtually closed as shown infigure 14.

Thus, most of the incident sound energy beyond 1100 Hzmay be reflected away from the top layer and does not penetrateinto the fabric. This may be the cause for the experimentaldata deviating from the expected pattern. However, the fabricTS5 has better sound absorbency in the lower frequency regionfrom 200 to 1000 Hz than the other tuck spacer fabrics and ifengineered suitably (with several layers) may well be suitablefor dampening the low frequency tones of automotive interiornoise.

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Figure 14. Electron microscope image of the front face of the TS5tuck spacer fabric.

5.1. Effect of the thickness of a tuck spacer fabric on itssound absorbency

To analyse the effect of thickness on sound absorbency, thesound absorbencies of the fabrics TS2, TS4 and TS2, TS6were compared. These two classes of fabric have more or lessthe same airflow resistivity (table 2) but their thicknesses aredifferent within a class.

Comparing the data in figures 12 and 13 shows that thesound absorbency of the fabric increases with the increase ofthickness while the airflow resistivity remains more or less thesame.

5.2. Effect of the tuck spacer fabric’s airflow resistivity on itssound absorbency

To analyse the effect of the airflow resistivity of the tuck spacerfabric on its sound absorbency, the tuck spacer fabric classesTS1, TS6 and TS2, TS3 were chosen. These fabric classeshave nearly the same thickness (table 2); however, their airflowresistivity is different within a class.

Figures 12 and 13 indicate that the sound absorbencies ofthe fabrics increase with the increase of airflow resistivity,whereas the thicknesses remain more or less the same.Moreover, as shown in figure 6, an increase in airflowresistivity gives rise to a reduction in porosity; thus, the soundabsorbency of the fabric increases with the reduction in itsporosity.

5.3. Simultaneous effect of the thickness and airflowresistivity of tuck spacer fabrics on sound absorbency

To analyse the simultaneous effect of the airflow resistivityand thickness of tuck spacer fabrics on their sound absorbency,fabrics TS1 and TS3 were chosen. The thickness and airflowresistivity of these fabrics increase simultaneously in the orderof TS1 and TS3, respectively (table 2).

Figure 12 indicates that for TS1 and TS3 fabrics the soundabsorbency increases with the simultaneous increase of bothairflow resistivity and thickness. Moreover, as the porosityreduces with the increase of airflow resistivity, as indicated in

figure 5, the sound absorbency of tuck spacer fabrics increaseswith a simultaneous increase in thickness and reduction inporosity.

5.4. Independent effects of thickness and airflow resistivity onthe sound absorbency of tuck spacer fabrics

To analyse this effect, the tuck spacer fabrics TS3 and TS4were chosen. The thickness and airflow resistivity parametersare inverted with respect to each other (table 2). Figures 12and 13 show that the sound absorbency of the fabric increaseswith the increase of airflow resistivity, in spite of the reductionof thickness. Therefore, the effect of airflow resistivity hasa greater effect on the sound absorbency of the tuck spacerfabric than its thickness. Moreover, as shown in figure 5,the airflow resistivity is more or less inversely proportionalto the porosity of the fabric; the effect of porosity thus hasa greater effect on its sound absorbency. Further, this wouldresult in the sound absorbency of these fabrics increasing withdensity.

5.5. Application of the fabrics

From the sound absorbency data given in figures 12 and 13,it can be observed that the fabric TS3 has the optimumsound absorbency. However, this fabric has the highestdensity apart from TS5. For a single layer of this fabric,its sound absorbency is greater than 0.5 beyond 2000 Hz.Consequently, this fabric would be suitable for dampeningthe higher frequencies of automotive interior noise. Noise atthis range (wind noise) occurs when travelling at high speeds(specially when the door seals are poor) [15]. Moreover,this fabric can be used in city buses as its interior noise hassignificant intensity at the frequency range of interest [16].

The fabric TS5 is suitable for reducing automotive interiornoise below 2000 Hz. Several layers of this fabric maysignificantly dampen frequencies below 1000 Hz.

Tuck spacer fabrics can be knitted with a greater densityresulting in better sound absorbency than TS3. This wouldimprove the sound absorbency from 1000 to 2000 Hz. Severallayers of the TS3 fabric can be layered together to improvethe sound absorbency in this range. Moreover, a combinationof the fabric TS3 with thick spacer fabrics [17] may reducenoise intensity below 1000 Hz. Future work will be directed atthis. However, to effectively reduce any engine-related noisein the range below 100 Hz, a fabric speaker-based active noisecontrol system should be investigated [18].

The advantage of using these fabrics is that they can beseamlessly integrated with automotive upholstery by meansof the available current advanced knitting technology in anydesign to suit the OEM brand. Further, they do not deterioratewith moisture and alter their sound absorbency, as may be thecase with conventional porous sound absorbers.

6. Conclusion

The sound absorbency of these fabrics increases with bothairflow resistivity and thickness.

The porosity is more or less inversely proportional tothe airflow resistivity of the fabrics; therefore, the sound

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absorbency increases with the decrease in porosity andincreased density. However, the effect of density is morepredominant in terms of sound absorbency than thickness.The fabrics can be made denser by having more rows of theinterconnecting yarn between a plain knitted course of thefront and back beds.

The fabric is a suitable alternative to utilizing severallayers of plain knitted fabrics for achieving better soundabsorbency, and may be used conveniently in automobileupholstery to reduce interior noise.

The sound absorbency of the tuck spacer fabric is effectiveonly from 2000 Hz onwards when its noise absorptioncoefficient (NAC) is greater than 50%. Therefore, future workon these fabrics will be directed at the improvement of itssound absorbency in the region less than 2000 Hz.

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