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CHAPTER 2

LITERATURE REVIEW

2.1 INTRODUCTION

The terms ‘sound’ and ‘acoustics’ are similar, but there is a

difference in their functionality representation. Acoustic is defined as the

scientific study of sound which includes the effect of reflection, refraction,

absorption, diffraction and interference. Sound wave can be considered as a

phenomenon. A sound wave is a longitudinal wave where particles of the

medium are temporarily displaced in a direction parallel to energy travelling

and then return to their original position. The vibration in a medium produces

alternative waves of relatively dense and sparse particles which are termed as

compression and rarefaction respectively.

The resultant variation to normal ambient pressure is received by the

ear and perceived as sound.

A simple wave for sound is shown in Figure 2.1. This wave can be

described in terms of Amplitude, Frequency, Wavelength, Period and

Intensity.

Figure 2.1 Simple waves for sound

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Amplitude refers to the difference between maximum and minimum

pressure. Frequency of a wave is measured as the number of complete back

and forth vibration of a particle of the medium per unit of time. A common

unit for frequency (f) is the Hertz (Hz).The wave length ( ) of a wave is the

distance which a disturbance travels through the medium in one complete

cycle of the wave. As the wave repeats the pattern for every wave cycle, the

length of one repeat is called as wave length and the time required for the

completion of one cycle of wave motion is called period. The average rate at

which the sound energy is transmitted through unit area is known as the

intensity of sound wave.

WeiyingTao et al (1997) mentioned that relation between

frequency and wavelength can be represented by the following equation:

Wavelength = [ / ][ ]

(1)

Like any wave, the speed of sound refers to how fast the disturbance

is transferred from particle to particle. Under normal condition of pressure

and humidity at sea level, sound wave travels at approximately 344 m/s

through air.(Paul N Chermisinoff et al 1982). Frequency refers to the number

of vibrations, which an individual particle makes per unit of time, while speed

refers to the distance which the disturbance travels per unit of time.

The unwanted or painful sound is called as noise. The high

production machine in all the industrial sectors and high speed vehicles

produces enormous noise.

The three elements of noise systems are noise source, noise path and

noise receiver;

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The Noise Source-medium of emission.

The Noise Path- passage of acoustical propagation.

The Noise Receiver-hearing elements

The above three elements are essential factors to be considered for

the noise control.

Table 2.1 Acoustical properties (absorption) of some conventional and

sustainable materials

S.No Materials Thickness

(mm) Density(kg/m2)

Absorption coefficient( )N.R.C250Hz 500Hz 1000Hz 2000Hz

1 Glass wool 50 50.0 0.45 0.65 0.75 0.80 0.663

2 Rock wool 50 80.0 0.29 0.52 0.83 0.91 0.638

3 Polystyrene 50 28.0 0.22 0.42 0.78 0.65 0.518

4 Polyurethane 50 30.0 0.30 0.68 0.89 0.79 0.665

5 Polyethylene 50 32.0 0.25 1.00 0.40 0.70 0.588

6 Polyester 45 20.0 0.56 0.85 0.98 0.95 0.835

7 Hemp fibers 40 40.0 0.59 0.60 0.56 0.52 0.568

8 Kenaf fibers 50 50.0 0.48 0.74 0.91 0.86 0.748

9 Mineralized wood fibers

50 470.0 0.25 0.65 0.60 0.55 0.513

10 Flax 35 43.0 0.66 0.84 0.79 0.53 0.705

11 Coconut fibers 35 70.0 0.28 0.40 0.64 0.74 0.515

12 Reed grating 50 130.0 0.46 0.86 0.71 - 0.676

13 Sheep wool 60 25.0 0.24 0.38 0.62 0.84 0.520

14 Cellulose 50 28.0 0.60 0.90 0.75 0.53 0.695

15 Rubber grains 5 1400.0 0.20 0.82 0.50 0.56 0.520

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The reduction of first two elements will control the noise and

minimize the sensitivity to high noise level by the third component which

reduces the noise level. Another important parameter to develop noise control

system is the cost factor. Treatment of the noise path is the simplest and

therefore the most common approach to noise problem.

Natural fibers are generally good sound absorbers. The extremely

wide varieties of natural fibers allow finding a suitable material for almost

every sound absorbing need.

Table 2.1 reports the coefficients of absorption as well as the Noise

Reduction Coefficient (NRC), for some conventional and sustainable

materials. (Asdrubali, F. 2006). The NRC rating is an average of absorption

coefficient ( ) of the materials at four frequencies (250, 500, 1000 and 2000

Hz).

This chapter focuses on various types of acoustic absorptive

materials used by different research scholars and their findings. The

mechanism of acoustic absorption in fibrous materials, applications of

acoustic absorptive materials, various factors which influence the acoustic

absorption phenomena, acoustic measurement and performance analysis of

acoustic absorption are also dealt in this chapter.

2.2 MECHANISM OF ACOUSTIC ABSORPTION IN FIBROUS

MATERIALS

Porges, G (1977) detailed that generally acoustic absorbents rely

for their action upon the frictional losses which occur when the alternating

pressure of the incident sound wave causes a to and fro movement of the air

contained in the pores of the materials and so the acoustical behavior of a

porous absorbent can be determined almost completely by;

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The porosity, represented by the percentage volume of air

contained by the material.

Resistance to air flow through the material, which depends

upon the diameter of the pores.

The thickness of the material.

The greater these three factors, the greater the noise absorption

coefficient (NAC) of the material would be. Sadao aso et al (1964) in his

investigation discussed the influence of several factors of fiber assembly in

sound absorption, It was concluded that the Type of fiber, Fiber fineness,

Fiber Orientation, Porosity of the material, Thickness of the material, Sound

speed, and Propagation constant influences the sound absorption. (Jing Li

2011)

The additional factors influence the sound absorption of various

textile materials are Fiber size, Fiber surface area, Compression, Surface

treatments (coating or finishes) and position or placement of sound absorptive

materials. ( Davern 1977). The absorption of sound results from the

dissipation of acoustic energy to heat. Many authors have explained this

mechanism of sound dissipation in the past. Constable et al (1977) describe

the sound dissipation as: when sound enters porous materials, owing to sound

pressure, air molecules oscillate in the interstices of the porous material with

the frequency of the exciting sound wave. This oscillation results in frictional

losses. A change in the flow direction of sound waves, together with

expansion and contraction phenomenon of flow through irregular pores,

results in loss of momentum. Owing to exciting of sound, air molecules in the

pores undergo periodic compression and relaxation. This results in change of

temperature. Because of long time, large surface to volume ratios and high

heat conductivity of the fibers, heat exchange takes place isothermally at low

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frequencies. At the same time it takes place adiabatically. In the high

frequency region compression between these isothermal and adiabatic

compression, the heat exchange results in loss of sound energy. This loss is

high in fibrous materials if the sound propagates parallel to the plane of fibers.

So altogether the reason for the acoustic energy loss when sound passes

through sound absorbing materials are due to Frictional losses, Momentum

losses and Temperature fluctuations.

2.3 ACOUSTIC ABSORPTIVE TEXTILE MATERIALS

Materials that reduce the acoustic energy of a sound wave as the

wave passes through it by the phenomenon of absorption are called acoustic

absorptive materials. They are commonly used to soften the acoustic

environment of a closed volume by reducing the amplitude of the reflected

waves. Absorptive materials are generally resistive in nature; either fibrous or

porous materials are special cases reactive resonators as discussed by

Asdrubali (2006). Classic examples of resistive materials are nonwovens,

fibrous glass, mineral wools, felts and foams. Resonators include hollow core

masonry blocks, sintered metal and so on. Most of these materials provide

some degree of absorption at nearly all frequencies and performance at low

frequencies typically increases with increasing material thickness. The

detailed accounts of these acoustic absorptive materials were discussed by

Bies et al (2003), Mulholland and Attenborough et al (1981) and Faulkner et

al (1976).

2.4 FUNCTIONS OF SOUND ABSORBING MATERIALS

For porous and fibrous materials, acoustic performance is defined

by a set of experimentally determined constants, namely absorption

coefficient, reflection coefficient, acoustic impedance, propagation constant,

normal reduction coefficient and transmission loss. There are different

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methods available to determine these acoustical parameters but all of these

methods mainly involve exposing materials to known sound fields and

measuring the effect of their presence on the sound field.

The performance of sound absorbing materials in particularly is

evaluated by the sound absorption coefficient ( ). Alpha ( ) is defined as the

measure of the acoustical energy absorbed by the material up on incident and

usually expressed as a decimal value varying from 0 to 1.0.

If 55% of the incident sound energy is absorbed, the absorption

coefficient of that material is said to be that absorbs all incident sound waves

will have a SAC of 0.55. The maximum material absorption coefficient is 1.

The sound absorption coefficient ( ) depends on the angle at which

the sound wave impinges upon the material and the sound frequency values

are usually provided in the standard frequencies of 125, 250, 500, 1000 and

2000 Hertz. The other important acoustic parameters that need to be

considered while studying the acoustical absorptive properties are as follows;

Sound reflection Coefficient: Ratio of the amount of total reflected

sound intensity to the total incident sound intensity.

Aoustic Impedence: Ratio of sound pressure acting on the surface

of the specimen to the associated particle velocity normal to the

surface.

Harris et al (1998) give four factors that affect the sound absorption

coefficient. They are

- Nature of the material itself

- Frequency of the sound

- The angle at which the sound wave strikes the

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surface of the material

- Air gap

More basically, all sound absorptive materials can be characterized

by two basic parameters namely Characteristic Impedance and Complex

Propagation Constant.

Characteristic impedance is the measure of wave resistance of air. It

is the ratio of sound pressure to particle velocity. Attenuation and phase

constant which are included in the propagation constant are the measure

of how much sound energy is reduced and the speed of propagation of

sound respectively. Even other parameters were tried by researchers

in order to include various effects like material internal structure, viscous

and thermal loss, which are not discussed here.

2.5 APPLICATIONS OF SOUND ABSORPTIVE MATERIALS

Acoustical material plays a number of roles that are important

in acoustic engineering such as the control of room acoustics, industrial

noise control, sound studio acoustics and automotive acoustics. Mulholland

et al (1981) and Attenborough et al (1981) describe that Sound absorptive

materials are generally used to counteract the undesirable effects of sound

reflection by hard, rigid and interior surfaces and thus help to reduce the

reverberant noise levels. They were used as interior lining materials for

auditoriums, halls, apartments, automotives, aircrafts, ducts and enclosures

for noise equipments and insulations for machineries.

Sound absorptive materials may also be used to control the

response of artistic thereby affecting the performance spaces to steady

and transient sound sources, character of the aural environment, the

intelligibility of unreinforced speech and the quality of unreinforced musical

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sound. Combining absorptive materials with barriers produces composite

products that can be used to lag pipe or provide absorptive curtain

assemblies. All noise control problem starts with the spectra of the emitting

source. Therefore, sound absorbing materials are chosen in terms of material

type, dimension and based on the frequency of sound to be controlled.

2.6 INFLUENCING FACTORS OF ACOUSTIC ABSORPTION

Various influencing factors on acoustic absorption property of

textile materials are discussed below:

2.6.1 Influence of type of fibers

Acoustic absorption constitutes one of the major requirements of

human comfort. Sound insulation requirements in manufacturing

environments, heavy equipment and automobiles generating higher sound

pressure drive the need to develop more efficient and economical ways of

producing sound absorbing materials. An industrial application of sound

absorption generally includes the use of fibers like cellulose, hemp, Kenaf,

wood, Flax, coconut, rubber grains, sheep wool, polyethylene, polyester,

polystyrene, polyurethane, glass wool, rock wool, foam, mineral fibers and

their composites.

Wang et al (2001) observed that Sound Absorption Coefficient

(SAC) of rock wool found to be similar to glass fiber. Yang et al (2001) in

their research work developed a porous laminated composite material by

molding of premix, preheating and lamination exhibited a very high acoustic

absorption coefficient property in the frequency range of 500 to 2000 Hz.

Murugesan et al (2006) stated that two stage compression molding of recycled

polyolefin based packaging wastes along with plastic coated aluminum foils,

expanded polystyrene and coir pith offers sound absorption properties

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comparable to glass wool. Kosuge (2005) in his findings of sound absorption

with combination of nonwoven fabric and para - aramid paper showed higher

performance than that of glass wool.

Jamaluddin et al (2003) found that coir fiber compressed into bales

and mattress sheet demonstrated good absorption coefficient. When compared

to a single layer, multilayer coir fibers with airspace layers increase the

absorption coefficient at lower frequencies ( Leo 1971). Sintered Al fiber with

a relative density of 0.6 and 10mm thickness showed a sound absorption

coefficient of 0.7 for the frequency range of 800- 2000Hz. Similarly metal

foam yields good SAC between 2000 – 4000Hz, stated by Hur and Park

(2005).

Hong et al (2007) observed that the recycled rubber particles with

perforated, polymer material results comparable SAC. The sound absorption

of the composite material is dominated by recycled rubber when the rubber

particle size is small, whereas the property is influenced by polymer porous

material when the rubber particle size is larger. A composite structure with

the combination of perforated panel, rubber particle, porous material,

polyurethane foam and glass wool were found to demonstrate significant

sound attenuation. Usually waste rubber particle demonstrates lower SAC at

higher frequencies. This can be altered by combining with polypropylene and

polystyrene particles resulting in higher SAC stated by Hong Zhou et al

(2007).

Yang et al (2003) developed composites boards of random cut rice

straws and wood particles that showed higher SAC than particle board, fiber

board and plywood for the frequency range of 500-8000Hz.

Koizumi et al (2002) stated that bamboo material formed into a fiberboard

yields superior SAC property when compared to plywood with similar

density.

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Shoshani et al (1991) stated that one of the oldest applications of

jute or shoddy mat was noise damping. Zulkifi et al (2008) observed that

agricultural waste like coir fiber, rice husk, oil palm frond fiber can be used

for acoustic absorbing material that are renewable, nonabrasive, cheaper,

abundant and shown less health and safety concern during handling. Zulkifi

et al (2009) developed particle board from agriculture waste and investigated

for its SAC, resulted in good performance. Wambua et al (2003) observed

that agricultural ligno cellulosic fibers such as rice straw, wheat straw or oil

palm frond can be easily crushed to chips particles and may be used as sound

absorbing material.

Wang et al (2001) stated that Polymers act as effective sound

insulators owing to their viscoelastic properties. Loss factor characterizes

damping and the wave equation of plane stress wave in a linear viscoelastic

solid demonstrates the quantitative relationship between acoustic absorptive

coefficient of polymers and their loss factors, sample thickness and measured

acoustic frequencies.

2.6.2 Influence of Fiber Size

Youngjoo et al (2007) in his research examined the possibility of

using micro fiber fabrics as sound absorbent materials. The results of sound

absorption coefficients of micro fiber fabrics were superior to conventional

fabric with the same thickness or weight and the micro fiber fabric density

was found to have more effect than fabric thickness or weight on sound

absorption.

Rashit et al (1995) observed that the hollow fiber fabric show

higher sound absorption because of increased air flow channel by the

complicated structure , increased surface area , higher total surface area and

greater possibility of sound to interact with fibers. Jute fiber having polygonal

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cell structure with a central hole or lumen, comprising about 10% of the cell

area of cross section performs similar to hollow fiber in sound absorption.

The nonwoven produced from Polypropylene along with short staple wool

with higher dimensional stability performs good absorption. The headliner in

automotives required more dimensional stability.

Koizumi et al (2002) reported that the increase in sound absorption

coefficient with decrease in fiber diameter. That is , thin fibers can move more

easily than thick fibers on sound waves. Moreover, with fine denier fibers

more fibers are required to reach equal more fibers for same volume density

which result in a more tortuous path and higher air flow resistance. A study

by Young Eung Lee et al (2004) concluded that the fine fiber content

increases SAC values. The increase was due to an increase in air flow

resistance by means of viscosity through the vibration of the air.

A study of Koizumi et al (2002) also showed that fine denier fibers

ranging from 1.5 to 6 filament denier (dpf) perform better acoustic absorber

than coarse denier fibers. Moreover it has been reported by Koizumi et al

(2002) that micro denier fibers (less than 1 dpf) provide a dramatic increase

in acoustical performance. Youn Eung Lee et al (2003) in their research work

concluded that the absorption coefficient is higher for nonwoven having more

fine fibers.

2.6.3 Influence of Fiber Surface Area

Mevlut Tascan et al (2008) reported that the surface area of the

fabric is directly related to the denier and cross sectional shape of the fibers in

the fabric. Smaller deniers yield more fibers per unit weight of the material,

higher total fiber surface area and greater possibilities for a sound wave to

interact with the fibers in the structure. Fabric density also affects the

geometry and the volume of the voids in the fabric structure.

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Kyoichi et al (1999) indicated that there is a direct correlation

between sound absorption and fiber surface area. Their study explained the

fact that friction between fibers and air increases with fiber surface area

resulting in a higher sound absorption. Moreover it has been said that, in the

frequency range from 1125 Hz - 5000 Hz, the fibers with serrated cross

sections (e.g., Kenaf) absorb more sound compared to ones with round cross

sectional area. Bo-Young Hur et al (1989) explained that the sound absorption

in pororus material is due to the viscosity of air pressure in the pores or the

friction of pores wall.

Therefore, sound absorption increases with specific surface area of

fiber with increase of relative density and friction pore wall. Man made fibers

are available in various cross sectional shapes, for instance, hallow, trilobal,

pentalobal and other novel shape fibers. These cross sectional shapes can add

acoustical value by providing more surface area contact.

2.6.4 Influence of airflow Resistance

One of the important factor that influence the sound absorbing char-

acteristic of a nonwoven material is the specific flow resistance per unit

thickness of the material. The characteristic impedance and propagation

constant which describe the acoustical properties of porous materials

are governed to a great extent by flow resistance of the material .

Fibers interlocking in nonwovens are the frictional elements that provide

resistance to acoustic wave motion. In general, when sound enters these

materials, its amplitude is decreased by friction as the waves try to move

through this friction passages, the tortuous converted into heat. Thus,

acoustic energy’s quantity which can be expressed by resistance of the

material to airflow is called airflow resistance and is defined in equation as:

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R = mks Rayls/m (2)

Where; Ri = Specific flow resistance, mks Rayls/m

u = Particle velocity through sample, m/sec

p = Sound pressure differential across the thickness of the

sample measured in direction of particle velocity ,

newton/m2

T = Incremental thickness, m

The unit, that is generally used for the flow resistance is Rayls

(N.S/m x10).

According to Delany et al (1970) flow resistance is proportional

to the material bulk density and fiber size. Fiber packing density decreases

the air permeability with a resultant increase in pressure drop and hence flow

resistance.

Based upon the air flow test ASTM D-1564, the flow resistance Rf

of the sample is obtained from the following equation

Rf = (3)

Where;

P = static pressure differential between both faces of the sample,

dyn/cm2 (10-1 Pa)

V = Air velocity, cm/s

l = Thickness of sample in cm

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Andrea zent et al (2007) stated that the best material properties are a

function of the application such as material thickness and boundary

conditions. Thinner materials require significantly more flow resistivity than

thicker materials. The specific air flow resistance of around 1000 mks rayls

(Pa s/m) can yield good absorption regardless of the thickness of the material.

The flow resistivity of a material may be increased to improve absorption at

lower frequencies at the cost of lower absorption at higher frequencies. One

common method of increasing flow resistivity is the addition of a flow

resistant scrim layer, which increases the specific air flow resistance without

adding too much weight or thickness. It is also possible to increase the flow

resistivity by increasing the surface density of the material (adding density

without changing the thickness); however, this method adds weight to the

material.

2.6.5 Influence of Porosity of the materials

Number, size, types of pores are the important factors that one

should consider while studying sound absorption mechanism in porous

materials. To allow sound dissipation by friction, the sound wave has to enter

the porous material. This means, there should be enough pore on the surface

of the material for the sound to pass through and get dampened. The porosity

of a porous material is defined as the ratio of the voids in the material to its

total volume.

The following equation gives the definition for porosity (H).

Porosity (H) = (4)

Where; Va = Volume of the air in the voids

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Vm = Total volume of the sample of the acoustical material being

tested

Shoshani et al (1992) stated that, in designing a nonwoven web to

have a high sound absorption coefficient, porosity should increase along

the propagation of the sound wave. Shoshani et al et al (2000) reported

that textile material should be designed such that the porosity should be

maximum in the middle of the material.

Atalla et al (1996) compared an approximate general method of

predicting the surface impedance at low frequencies for non homogeneous

thin porous layers based on non-propagative representation of the acoustic

field in the layer to a finite element based method for different three

dimensional porous patch works. They found comparable results and

concluded that propagative phenomena in sound absorption for non-

homogeneous thin porous layers are not important.

Acoustic of media with double porosity studied by Auriault et al

(1994) using the periodic structures homogenization method applied to multi

scales materials. They showed that the macroscopic behavior highly depends

on the inter scale ratio of the materials.

In case of rigid porous materials, Boutin et al (1998) found that

when comparing the pores and micro pores the micro pores satisfy the

diffusion of sound waves. Boutin et al (1999) stated that macroscopic

behaviors of the porous materials were highly depending on the permeability

of the materials.

2.6.6 Relation between Air Flow Resistance and Sound Absorption

The investigations done by Sadao aso et al (1964) formulated results

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regarding air flow resistance and absorption of cotton fabrics, The influence

of the flow resistance of fabrics on their absorption characteristics has been

investigated by measuring the flow resistance and the absorption

characteristics. To deal with the subject from the point of view of the

design and density of fabrics, thirteen different kinds of cotton fabrics

were woven as samples. The results obtained were as follows:

(1) The relation between flow resistance R of fabrics and

flow speed V can be given as follows:

R = Ai+BiV (5)

where Ai and Bi are constants fixed by the design and density of a

fabric. In a range of small densities, the value of Bi is nearly zero, while

Ai and Bi increase together in value as the density of a fabric increases.

(2) There are two types of absorbing mechanisms: the viscosity

resistance type and the resonance type depending on the kinds

of fabrics. A fabric is of the viscosity resistance type if its

flow resistance depends only on air viscosity in a small

range of flow speeds, namely,

R = Ai (6)

(3) A fabric is of the viscosity resistance type if it has an air

space behind it, provided the relation among frequency fo,

which shows the maximum absorption coefficient, depth ‘d’

of the air space, and R can be given as follows :

fo = (c/4 - aR) d-I (7)

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where c is the speed of a sound wave and a is a constant fixed by the

design of the fabric. This empirical formula means that a fabric has the

maximum absorption coefficient when it is placed at a shorter distance

than the place where the particle velocity is a maximum.

(4) The relation between maximum absorption coefficient ‘ ’

and of ‘R’ fabrics woven with the same design is:

= a'+a" R (8)

Where a’ and a" are constants fixed by the design of the fabrics.

Teli et al (2007) in his research on efficiency of nonwoven material

for sound insulation elucidated that the efficacy of a material as a sound

(noise) barrier depends on frequency of the sound wave to which material is

exposed to, GSM, air permeability, thickness and orientation of the fibers. It

is also reported by him that the extent of sound reduction increases with

decrease in air permeability while with the increase in air permeability; the

extent of sound reduction by the material is decreased.

2.6.7 Influence of thickness

Jing li et al (2007) reported that the thickness of the nonwoven

materials are the most influencing factor on their sound absorbing capacity. In

his findings, he said that if the thickness of the nonwoven is less than 3.5mm

little sound absorption is achieved, if the thickness is more 9.03 mm best

sound absorption is achieved.

The various studies on sound absorption in porous materials have

stated that low frequency sound absorption has direct relationship with

thickness. The rule of thumb that has been followed is the effective sound

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absorption of a porous absorber is achieved when the material thickness

is about one tenth of the wavelength of the incident sound.

Peak absorption occurs at a resonant frequency of one quarter wave

length of the incident sound (ignoring compliance effect). A study by

Ibrahim et al (1978) showed the increase of sound absorption only at low

frequencies, as the material gets thicker. However, at higher frequencies,

thickness has insignificant effect on sound absorption. When there is air space

inside and behind the material, the maximum value of the sound

absorption coefficient moves from the high to the low frequency range .

Shoshani et al (2000) while referring the acoustical absorption, the

thickness of textile materials are important criteria. A numerical method of

calculating acoustic performance of nonwovens has been proposed by

Shoshani et al (1992) in a study and concluded that the noise absorption

coefficient of a fiber web is shown as a function of thickness and porosity.

2.6.8 Influence of Density

Density of a material is often considered to be the important factor

that governs the sound absorption behavior of the material. At the same time,

cost of an acoustical material is directly related to its density. A study by

Koizumi et al (2002) showed the increase of sound absorption value in

the middle and higher frequency as the density of the sample were increased.

The number of fibers increases per unit area when the apparent density

is large. Energy loss increases as the surface friction increases, thus, the

sound absorption coefficient increases. Moreover, a presentation by him

showed the following effect of density on Sound absorption behavior

of nonwoven fibrous materials. Less dense and more open structure absorbs

sound of low frequencies (500 Hz). Denser structure performs better for

frequencies above than 2000 Hz.

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2.6.9 Influence of fiber compactness

Bernard Castagnede et al (2000) stated that, compression of fibrous

mats decreases the sound absorption properties. He explained that, under

compression the various fibers in the mat are brought nearer to each other

without any deformation (without any change in the fiber size). This

compression results in decrease of thickness. He also observed the other

physical variation that occurs during compression. Compression resulted

in an increase in tortuosity and airflow resistivity and decrease of porosity

(Shape factor). Bernard Castagnede et al (2000) and Everest F (2001) despite

these physical parameter variations in the compressed material, he stated

that the reason for the decrease in sound absorption value is mainly

due to decrease in sample thickness (Ballagh 1996). The influence of

compression on sound absorption can play an important role in the field of

automotive acoustics. The seat padding in the vehicle is subjected to

compression / expansion cycle due to passenger’s weight. This results in

squeezing down the porous materials (fibrous or cellular) which in turn results

in variation on physical properties.

2.6.10 Surface finishing of acoustical materials

As acoustical materials are used inside buildings and these material

save to satisfy some requirements such as good light reflecting behavior and

good appearance. Often when used inside buildings, acoustical materials

are coated with paints or some finishes. These surface coatings affect the

absorption behavior. Thin layer of paint coating should

be applied over the material surface. This can be done with the help obtaining

a desirable surface finish to cover the surface of the fabric with

perforated paneling of the Helmholtz resonator type. Several authors

have studied the effect of such cover screen on sound absorption.

The study by Ingard et al (1998) showed the increase of sound absorption

30

at low frequencies at the expense of higher frequencies. Sometimes,

fibrous materials are covered with film in order to improve the sound

absorption properties at low frequencies by the phenomenon of surface vibrati

on of film .

Parik et al (2006) observed that plasma treatment has both chemical

and mechanical effects on fibers, surface etching and ionic charging. Etching

occurs when the ions with high kinetic energy hit the surface, removing the

weak part or contaminated region of the fibers. Consequently, it changes the

surface morphology or increases the surface area of fibers. After plasma

treatment, polyester changes in surface morphology, weight loss, higher

thickness and higher fullness and air permeability are increased as a result of

increasing porous space between fibers. As the plasma treatment increases the

surface area and change in surface morphology the acoustic absorption

increases.

Hargeth et al (2001) stated that the SAC of jute decreases when

exposed to plasma treatment. Three seconds of exposure do not give any

change, but six seconds decrease the SAC of 7.7 to 10.5 %. The decrease is

seemed to occur by fiber damage, as the jute has been etched and split by the

treatment, which results in 3.3 to 7.9 % loss of fabric weight.

Kwon et al (2002) and Jung et al (2006) observed that etching and

fiber surface damage due to extended time of treatment which reduces the

SAC of the fabric.

2.6.11 Positioning of Sound Absorptive Materials

It is a known fact that sound absorption of a material depends

on the position and placement of that material. It has been reported by

Porges, G. (1977) that if several types of absorbers are used, it is desirable to

31

place some of each type on ends, sides and byceilings so that all three axial

modes (longitudinal, transverse and vertical) will come under their influence.

In rectangular rooms it has been demonstrated that absorbing material placed

near corners and along edges of absorbents that are room surfaces is

most effective. In speech studios, some effective at higher audio frequencies

should be applied at head height on the walls. In fact, material applied to the

lower portions of high walls can be as much as twice as effective as the same

material placed elsewhere. Moreover, it is recommended that untreated

surfaces should never face each other.

2.6.12 Surface Impedance

The higher the acoustic resistivity of a material, the higher is its

dissipation, for a given layer of thickness. At the same time, the surface

impedance of the layer also increases with resistivity resulting in a greater

amount of reflections on the surface layer, giving a lower absorptive

capability stated by Yunseon Ryu (2002). Moreover the whole process is

frequency dependent, so that for lower frequency bands the necessary layer

thickness increases as resistivity decreases (Takahashi et al 2005).

2.6.13 Additional factors for acoustic absorption

The surface of rooms, offices, schools, hospitals, restaurants,

industrial plants or any enclosed area in which the occupants are exposed

to noise must satisfy varying degree of structural and architectural

requirements. Some of the properties apart from high sound absorptivity that

a sound absorbing material should posses are appearance, decorative effect,

light reflectivity, maintainability and durability (Yunseon Ryu 2002).

32

2.7 PREVIOUS WORK ON ACOUSTIC ABSORPTION

Factors such as fiber, fabric and chemical treatments are discussed

below:

2.7.1 Previous work on acoustic absorption in fibers

Chen et al (2007) observed the SAC of six nonwoven with two

surface layers (activated carbon fiber (ACF) and glass fiber (GF)) and three

base layers (coconut, ramie, and polypropylene). The impedance tube

instrument was used to measure the normal incident SAC fabric. The

comparison of the sound absorption was carried out by statistical method of

variance analysis. The results show that the nonwoven with ACF as a surface

layer had significantly higher SAC than the GF surfaced in both low

frequency range (100- 1600 Hz) and high frequency range (1600 – 6400 Hz).

In particular, the ACF nonwoven exhibited an exceptional ability to absorb

low frequency noises (with absorption coefficient always above 0.5 at a

frequency of 500Hz). Mean while, the ACF surface layer seemed to dominate

this high sound absorption no matter what type of fiber was used for the base

layer nonwoven. The analysis also revealed that , In comparison with the

glass fiber and polypropylene nonwoven, ACF and cotton was 4.6 times

lighter to weight and 14% higher in low frequency absorption and 7% higher

in high frequency absorption.

Youneung lee et al (2003) observed that the effect of the fiber

content on the SAC usually depends on the content of the fine fiber. The

nonwoven which has more fine fiber have more chance to contact to sound

waves. This causes more resistance by means of friction of viscosity through

the vibration of the air. The nonwoven absorber which has an un-oriented web

in the middle layer has a higher SAC than nonwovens which have totally

oriented web structure, but the difference is very marginal.

33

The sound absorption of an industrial waste, developed during the

processing of tea leaves has been investigated by Sezgin Ersoy et al (2008).

Three different layers of tea -leaf fiber waste materials with and without

backing provided by a single layer of woven textile cloth were tested for their

sound absorption properties. The experimental data indicate that a one cm

thick tea leaf fiber waste material with backing, provides SAC, which is

almost equivalent to that provided by six layers of woven textile cloth.

Twenty millimeter thick layer of rigidly backed tea leaf fibers and nonwoven

fiber materials exhibit almost equivalent SAC in the frequency of 500 –

3200Hz.

Parikh et al (2006) stated that Natural fiber composites having

excellent appearance, environmental benefit and are lighter than fiber glass.

The acoustic properties of potential floor coverings used either alone or in

combination with cotton nonwoven under pad were determined. Using

various weight ratios of natural to synthetic fibers, air laid needle punched

and carded needle punched moldable composites were produced from kenaf,

jute, waste cotton and flax with recycled polyester and off quality

polypropylene. Control fabrics were made from PET and PP. ASTM E 1050

was used to determine acoustical properties of the composites. Each of the

natural fiber based nonwoven floor coverings contributed to noise reduction

because of their absorptive properties as compared to control fabrics. Soft

cotton under pad greatly enhanced the sound absorption properties of the

nonwoven floor coverings.

Parikh et al (2006) observed the SAC of four aesthetically pleasing

(to vision and touch) velour nonwoven fabrics and of the stacked velour fabric

and high loft pads that make trunk lining systems were determined. The trunk

lining systems have excellent sound absorption capabilities and are used as

sound proofing materials in European automobiles. Velour nonwovens are

34

attractive because of the silky, soft hand of the short, thick pile giving a rich

textile feel that is compliant, pliable and inexpensive.

Parikh et al (2006) stated that eliminating unwanted noise in

passenger compartments of vehicles is important to automobile

manufacturers. The ability to reduce noise inside the vehicle enhances the

perceived value of the vehicle to the consumer ,and offers a competitive

advantage to the manufacturer. Several methods are presently employed to

reduce noise and its sources, one of which uses sound absorbing materials

attached to various component s such as floor coverings, package trays, door

panels, head liners and trunk liners. Natural fibers are noise absorbing

materials that are renewable and biodegradable, making them an effective

choice for the automobile industry. Floor coverings using natural fibers

(kenaf, jute, waste cotton and flax) in blends with polypropylene (PP) and

polyester (PET) were developed as carded needled punched nonwovens. The

acoustical absorption coefficient of these floor coverings and of floor

coverings in combination with an under pad (either a rebounded polyurethane

foam or a soft cotton nonwoven) were evaluated by ASTM E-1050in the

frequency range of 100 to 3200 Hz. Noise was significantly reduced with a

floor coverings using either of the under pads. The natural fiber nonwoven

floor covering contributed the SAC of 0.54 – 0.84 at 3200Hz. The most

absorption occurred with polyurethane as 1 at 3200 Hz.

Rozli Zulkifli et al (2009) observed the acoustic properties of two

natural organic fibers; coir and oil palm fibers. During the processing stage,

coir fiber sheet has been treated with latex and the oil palm fiber sheet has

been treated with PVA. Both are compressed under pressure using high

precision hydraulic machine for 30 minutes to form the fiber sheets. The

density of the coir fiber sheet is determined to be 74 kg/m3 while the density

of the oil palm fiber is 130 kg/m3.The SAC values of coir fiber gives an

35

average value of 0.50.It shows a good SAC for higher frequencies but less for

lower frequencies.

The oil palm fiber gives an average SAC of 0.64.The oil palm fiber

shows a good absorption coefficient for higher frequency region compared to

lower frequency.

Mohammad et al (2010) investigated the SAC of coir fiber from

natural source and industrial prepared fibers mixed with binders. Two

analytical approaches were implemented for analysis, namely; Allard

analytical model based on wave transmission and Delany – Bazley technique

that is derived from empirical equations. Experiments were also conducted in

impedance tube to support the analysis. The Allard technique had the

advantage that not only showed overall pattern but also predicted resonances

very well. But formulation was complicated and compensations would be

considered for industrial fibers. The Delanny – Bazley method was a good

approximation for overall broad band trend of acoustical behavior. Moreover

it was easy to use without need to modify any part of formulae for stiffened

industrial type which generally had lower peaks. Natural fiber had an average

absorption of 0.8 for f > 1360Hz, f > 940 and f >578 at thicknesses of 20

mm,30 mm and 45 mm. Modeling the industrial fiber is vital and inevitable,

since natural coir fiber has to be enhanced for commercial use. This includes

characteristics such as stiffness, fire retardant, anti fungus and flammability.

Here, binder was the only additive utilized by manufacturer to attach fibers

together and adding stiffness. These samples had lower acoustic absorption,

peaks were flattened and move to higher frequencies. They exhibited weak

absorption at low frequencies and tactics such as adding air gap or perforated

plate are necessary to improve this shortcoming.

Thilagavathi et al (2010) observed that natural fibers are noise-

absorbing materials, renewable and biodegradable nonwovens have been

36

developed using natural fibers such as banana, bamboo and jute fibers for the

automotive interiors to reduce noise, which currently contain traditional

materials such as glass and other manufactured fibers and foams that are

difficult to recycle. Three types of nonwovens were developed using needle

punching technique by blending bamboo, banana and jute fibers with

polypropylene stable fibers in the ratio of 50:50. SAC was tested by

impedance tube method (ASTM E 1050).Comparison of physical properties

such as areal density, thickness, stiffness, tensile strength, elongation,

structural properties and comfort properties such as air permeability and

thermal conductivity were performed for all samples. It was observed that the

bamboo / polypropylene nonwoven with its compact structure showed higher

values of tensile strength and stiffness and lower values of elongation, thermal

conductivity and air permeability and good SAC than others and it is suitable

for automotive interiors. At 800 Hz, the SAC of bamboo / polypropylene and

jute / polypropylene is equivalent to the target level, but it is lower by 22% in

banana/polypropylene. But at higher frequencies (1600 Hz), there is a

reduction from the target level in all the nonwovens, which could be

improved by increasing the thickness of the nonwovens.

Shah Huda et al (2009) stated that Three to four billion pounds of

chicken feather are wasted in the United States annually. These feathers pose

an environment challenge. In order to find a commercial application of these

otherwise wasted feathers, composites have been prepared from feathers.

Flexural, impact resistance and sound dampening properties of composites

form chicken feather fiber (FF) and high density polyethylene / polypropylene

(HDPE/PP) fiber have been investigated and compared with pulverized

chicken quill- HDPE/PP and jute - HDPE/PP composites. Sound dampening

by FF composites was 125% higher than jute and similar to quill although

mechanical properties were inferior to later two. In ground form, FF and jute

composite properties were similar except for 34% higher modulus of jute;

37

under the same formulation and processing condition, ground FF composites

had nearly 50% lower mechanical properties compared with ground quill

composites. It was found that voids and density of composites have effect on

mechanical and sound dampening properties; however, no direct relationship

was found between mechanical properties and sound dampening.

Mohammad Hosseini et al (2009) stated that coconut is one of the

important harvests in Malaysia .Industrial prepared coir fiber is obtained from

coconut husk combined with latex and other additives to enhance its

structural characteristics .unfortunately such inevitable process diminishes

the acoustical features of materials. Therefore perforated plate (PP) was added

to the multilayer structure to further enhance the sound absorption in this area.

Analysis were accomplished through three PP modeling approaches (Allard,

Beranek and Ver, Atalla and Sgard) and Allard transfer function(TF) method.

Experiments were conducted in impedance tube to support the analytical

results. Outcome showed that Allarf TF method was generally closer to

measurement values and implemented for additional analysis. Two possible

conditions of putting PP in front of fiber layer or between fibers – air gap

layers were investigated. Both arrangements were suitable to enhance the

sound absorption. Although, when PP was backed by coir fiber and air gap,

porosity of the plate had great influence in adjusting the amount of low

frequency absorption. Result derived that PP might improve the low

frequency absorption of coir fiber but at the same time the medium frequency

absorption was reduced. This effect was noticed previously in coir fiber air

gap structure while the air gap thickness increased. The advantage of using PP

was that it assisted in greatly reducing the air gap thickness under the same

acoustical performance. Hence it is an efficient tool to reduce the thickness of

acoustic isolators in practical purpose.

38

Yakir Shoshani et al (2003) have worked with the Zwikker and

Kosten model (Zwikke Kosten) sound absorbing materials, Oxford : Elsevier

Pub Co.,1949) for sound propagation through porous flexible media is used

for numerical calculations of some intrinsic characteristics of nonwoven fiber

webs yielding the highest SAC in the audible frequency range. These results

can serve as guide line for the optimal design of acoustic elements made of

textile materials.

Min – Der Lin et al (2009) stated that developing efficient sound

absorption materials is a relevant topic for large scale structures such as

gymnasiums shopping malls, air ports and stations. This study employs

artificial neural network (ANN) algorithm to estimate the SAC of different

perforated wooden panels with various setting combinations including

perforation percentage, backing material and thickness. The training data sets

are built by carrying out a series of experimental measurements in the

reverberation room to evaluate the sound absorption characteristics of

perforated wooden panels. A multiple linear regression (MLR) model is also

developed for making comparisons with ANN. The analytical results indicate

that the ANN. The analytical results indicate that the ANN exhibits

satisfactory reliability of a correlation between estimation and truly measured

absorption coefficients of approximately 0.85.However, MLR cannot be

applied to nonlinear cases.ANN is useful and reliable tool for estimation

sound absorption coefficients estimation.

Al – Nawafleh et al (2005) observed that the operation conditions

of the industrial equipment, acoustic parameters essentially depend on type of

the sound field (free, diffuse or mixed).Besides the effect of the noise control

of the machine itself, the field type also important, which in turn depends on

the degree and quality of the acoustic preparation of the workshop. The

criterion is an estimation of applicability broad band absorbents, which

39

enables to solve the problem related to the ecology of noise control at various

manufacturing processes.

Vijayanand et al (2003) made an attempt to identify the acoustical

characteristics of textile materials using precision woven mono filament

fabrics as model textiles. The experiments try to eliminate the effect of

entrapped air pockets in the fabric on an ultra sound wave field. The results of

the experiment reveal that the power consumption of the ultra sound horn

remains practically constant after introducing the textile at different positions

in the standing wave field. Measurements of transmitted acoustic pressure

amplitude through the textile reveal that fabrics form an almost transparent

boundary for acoustic waves. A simple model involving the structural and

hydrodynamic characteristics of the textile is proposed to determine their

impedance. The overall conclusion of the study is that the absence of

entrapped air, textiles does not have any individual impact on the ultra sound.

2.7.2 Previous work on acoustic absorption in Porosity of the

materials

Sadao aso et al (1964) Explained about the influence of several

factors relating to the make – up of a fiber assembly have on sound

absorption characteristics was investigated by measuring the normal incident

absorption coefficients of fiber assemblies from 250 to 200 c/s at intervals of

1/3 octaves ,the results obtained are:

There are two other types of absorption characteristics besides

the well known viscosity resistance type (I). one is a fibrous

resonance type (II) of which the absorption characteristics

show resonance absorption at low frequency but which, in a

high frequency range, belongs to type (I).the other is an

intermediate type (III),which is between (I) and (II).

40

The absorption characteristics of a fiber assembly belongs to

type (I) if the fibers are arranged parallel to the direction of the

propagation of the sound wave. The air in the fiber assembly

plays a part in absorbing action. If the fibers are arranged so as

to divide the air space in the assembly into small sections, the

value of its absorption coefficient is high.

It is experimentally established that the absorbing mechanism

of a fiber assembly comes mainly from the frictional action

between the surface of fibers and air in the assembly. Fiber

assemblies are equal to one another in their absorption

characteristics if the fibers are the same in total surface area,

even if they differ in length or fineness.

To increase the absorption coefficient of a fiber assembly in a

low frequency range, it is better to increase its thickness than

to reduce its porosity. The thickness of a fiber assembly has an

effective value which increases the absorption coefficient to a

maximum for a certain frequency and a certain porosity

degree.

The relation between frequency ( f ) and effective porosity

(Pe):when porosity increases the absorption coefficient at that

frequency increases to a maximum value, is shown as

follows:

f = K (100 - Pe) -1.3 (9)

Where K is a constant which is decided by the kind of fiber material,

its fineness, the fiber orientation and thickness of the fiber assembly. If K is

obtained experimentally at a certain frequency, the value of Pe for every

frequency is calculated by the above equation. There is the most effective

41

porosity (Pme) giving the greatest value among the maximum absorption

coefficient in Pe of all frequencies. The larger the total surface area is the

greater the Pme is and the lower the frequency is.

Andrea zent et al (2007) observed that the sound absorption

performance of the porous materials used in automobiles are not so much a

function of type of material(cotton shoddy, PET or glass fiber) as it is a

function of how well the material construction can be executed to achieve

desirable properties for sound absorption Chao-Nan Wang (2001). For open-

faced materials or materials with porous scrim, the flow resistivity is very

important Narang (1995).

2.7.3 Previous work on acoustic absorption in chemical treatments

Youngjoo et al (2010) investigated the SAC of polyester and

cellulose – polypropylene nonwovens of vehicle headliner components

available in the commercial market and the influence of plasma treatment of

these nonwovens on SAC. The hallow fiber polyester or jute fiber display the

higher sound absorption than regular fiber polyester nonwoven or kenaf

nonwoven even with similar web structure. This is due to their high surface

area and the finer and more fibers in the web. Smaller and more pores in the

web with high porosity prove the higher possibility for the sound wave of

high frequency to interact with the fibers. Higher the viscoelastic property,

web has the higher sound absorption.

The plasma treatment alters the sound absorption and the Visco

elastic property depending on the fiber type. In the case of regular polyester

fiber fabric, due to a little change in pore size, weight loss and visco-elastic

property, its sound absorption property and displays almost no change , while

42

as far hollow polyester fiber fabric , all of the sound absorption, visco-elastic

property and pore size increase after plasma treatment. Thus, in the case that

the changes of pore size and weight loss are small, if visco-elastic property

increases by treatment, SAC increases as for hollow polyester fabric.

The cellulosic fibers are easily attacked by plasma, thus proper

exposure time and intensity is needed to increase sound absorption with less

weight loss in the case of jute fabric. Jute fabric is weaker than kenaf fabric

to the treatment and jute fabric receives more damages in fiber itself in

addition to the separation of lateral bondage between the neighboring cells,

where they form the cementing material of the middle lamina providing

strong lateral adhesion between the ultimate.( Allard (1989). As for natural

fiber webs, such as jute or kenaf fabrics with the higher weight loss than

polyester webs from plasma treatment, the sound absorption usually

decreased. But if the treated fabric overcomes its weight loss with the

increased number of smaller pore size, higher surface area by bundle split and

the unchanged viscoelastic property, its sound absorption could increase, as

for kenaf fabric. Therefore, even the untreated fabrics of hollow polyester

fabric or jute fabric are good acoustical materials in automotive industry, the

plasma – treated kenaf is found to be a potential candidate in terms of

economy scale.

2.8 MEASUREMENT OF SOUND ABSORPTION COEFFICIENT

Komkin et al (2003) explained the measuring techniques

available to quantify the acoustical behavior of porous materials.

In general the following properties can be measured in regarding with

acoustic behavior:

43

Sound absorption coefficient ( ),

Reflection coefficient (Rc),

Surface impedance (Z).

2.8.1 Acoustic Measurements

Measurement techniques used to characterize the sound absorptive

properties of a material are :

Reverberant Field Method

Impedance Tube Method

Steady State Method

2.8.2 Reverberant Field Method

This method which is used for measuring 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 diffusive

field . However creation of a diffusive sound field requires a large and costly

reverberation room. A completely diffuse sound field can be achieved only

rarely. Moreover, an accurate value of complex impedance cannot be derived

from the absorption coefficient alone. Since sound is allowed to strike the

material from all directions, the absorption coefficient determined is called

random incidence sound absorption coefficient, RAC. This method is clearly

explained in ASTM C 423 - 72.

44

Figure 2.2 Reverberant method

2.8.3 Impedance Tube Method

This method uses plane sound waves that strike the material

straight and so the sound absorption coefficient is called normal incidence

sound absorption coefficient. This impedance tube method needs circular samples,

either 35 or 100 mm in diameter (according to the type of impedance tube method,

sound waves are confined within the impedance tube). And thus the size of the

sample required for test needs only be large enough to fill the cross section of

the tube. Thus this method avoids the need to fabricate large test sample with

lateral dimensions several times the acoustical wavelength. The impedance tube

method employs two techniques to determine NAC, namely:

1. Movable microphone which is one-third-octave frequencies

technique (ASTM C 384) is based on the standing wave

ratio principle and uses an audio frequency spectrometer

45

to measure the absorption coefficients at various

centre frequencies of the one-third-octave bands.

2. Two fixed microphone impedance tube or transfer function

method (ASTM E 1050), which is relatively recent development. In

this technique, a broad band random signal is used as a

sound source. The normal incidence absorption coefficients

and the impedance ratios of the test materials can be

measured much faster and easier compared with the

first technique .

Figure 2.3 Impedance tube set up (50Hz – 6.4 kHz) Type 4206.

(courtesy of Bruel &Kjaer)

46

Figure 2.4 Impedance tube kit set up (50Hz – 6.4 kHz) Type 4206.

(courtesy of Bruel &Kjaer)

2.8.4 Two Microphone Impedance Tube Technique (Transfer

Function Method)

The transfer function method (ASTM E1050) covers the use of an

impedance tube, with two microphone locations and a digital frequency

analysis system for the determination of normal incident sound absorption

coefficient and normal specific acoustic impedance ratios of materials. This

test method is similar to ASTM C 384 in that it also uses an impedance tube

with a sound source connected to one end and the test sample mounted at the

other end.

Rather than probing the sound field to determine sound maxima and

minima pressure level as in standing wave tube method, in the two microphon

e method the ratio between the sound pressure amplitudes at two-fixed

microphone positions is measured. Quantities are determined as a function

47

of frequency with a resolution determined by the sampling rate of a digital fre

quency analysis system. The usable frequency range depends on the diameter

of the tube and the spacing between the microphone positions. An extended fr

equency range may be obtained by using tubes with various diameters

and microphones spacing. By this method acoustical parameters like

absorption coefficient, reflection coefficient and surface admittance for a

small samples exposed to plane waves can be determined . The reflection

coefficient (Rc) of the sample can be obtained from the equation .

Rc = e ( ) (10)

Where; Hl = Frequency Response Function (FRF) of the impedance tube

Hi = FRF associated with the incident wave components

Hr = FRF associated with the reflected wave components

k = Wave number

l = Distance between the microphone and the sample

s = Spacing between the microphone

By using equation, Noise absorption coefficient, NAC ( )

can be determined.

= | | (11)

Also surface impedance (Z) can be calculated using the equation:

= (12)

48

Where;

= Air density (kg/m2)

c = Sound velocity in air (ms-1)

Outline of the theory behind the calculation of sound absorption coe

fficient by using transfer function method is given by Frank Fahy and many

others.

2.8.5 Steady State Method

This method is mostly used when the other will not work. This

particular method is described in ASTM E336-71. To measure the

transmission coefficient of the materials a third microphone or even

a second pair of microphone can be placed behind the test sample in a

second impedance tube. All the developed samples in this work were

tested by using the two-microphone impedance tube method (ASTM E

1050).

2.9 MEASUREMENT OF SOUND RESISTANCE

A sound of particular decibel is created by the sound source and the

receipt decibels have been measured by the decibel meter with and without

sample. The sound insulation by the fabric samples can be calculated by the

following derivation derived by Teli et al (2007), Surajit Senguptha et al

(2010) and Constable et al (1977).

SR % =dBwos dBws

dBwos (13)

49

Where;

SR - sound reduction

dBwos - sound level without sample and

dBws -the sound level with sample.

2.9.1 Fabrication of testing apparatus for measuring the sound

insulation

A novel testing apparatus has been fabricated to measure the sound

insulation property of the textile materials.

Figure 2.5 sound resistance tester

It consists of a box 100 cm X 100 cm made out of wood with

removable top lid. In the left hand side of the box a sound source which will

produce definite decibel of sound is fixed and in the right hand side the

decibel meter is fixed coaxially opposite to sound generator to measure the

sound intensity. In between the sound source and the receiver the fabric

sample can be fixed in different positions