CHAPTER 2 LITERATURE REVIEW

Transcription

1 10 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

2 11 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. noise receiver; The three elements of noise systems are noise source, noise path and

3 12 The Noise Source-medium of emission. The Noise Path- passage of acoustical propagation. The Noise Receiver-hearing elements the noise control. The above three elements are essential factors to be considered for Table 2.1 Acoustical properties (absorption) of some conventional and sustainable materials S.No Materials Thickness Density (mm) (kg/m 2 ) Absorption coefficient( ) 250Hz 500Hz 1000Hz 2000Hz N.R.C 1 Glass wool Rock wool Polystyrene Polyurethane Polyethylene Polyester Hemp fibers Kenaf fibers Mineralized wood fibers Flax Coconut fibers Reed grating Sheep wool Cellulose Rubber grains

4 13 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;

5 14 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

6 15 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

7 16 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 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

8 17 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

9 18 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: 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

10 19 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 Hz. Similarly metal foam yields good SAC between Hz, 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 Hz. Koizumi et al (2002) stated that bamboo material formed into a fiberboard yields superior SAC property when compared to plywood with similar density.

11 20 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 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

12 21 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 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.

13 22 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 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 Influence of airflow Resistance One of the important factor that influence the sound absorbing characteristic 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:

14 23 R = mks Rayls/m (2) Where; R i = 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/m 2 T = Incremental thickness, m (N.S/m x10). The unit, that is generally used for the flow resistance is Rayls 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 Where; Rf = (3) P = static pressure differential between both faces of the sample, dyn/cm 2 (10-1 Pa) V = Air velocity, cm/s l = Thickness of sample in cm

15 24 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 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; V a = Volume of the air in the voids

16 25 V m = 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 nonhomogeneous 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 Relation between Air Flow Resistance and Sound Absorption The investigations done by Sadao aso et al (1964) formulated results

17 26 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 = A i +B i V (5) where A i and B i are constants fixed by the design and density of a fabric. In a range of small densities, the value of B i is nearly zero, while A i and B i 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 = A i (6) (3) A fabric is of the viscosity resistance type if it has an air space behind it, provided the relation among frequency f o, which shows the maximum absorption coefficient, depth d of the air space, and R can be given as follows : f o = (c/4 - ar) d -I (7)

18 27 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 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

19 28 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 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.

20 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 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

21 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 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

22 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 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) 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).

23 PREVIOUS WORK ON ACOUSTIC ABSORPTION below: Factors such as fiber, fabric and chemical treatments are discussed 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 ( Hz) and high frequency range ( 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.

24 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 Hz. 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

25 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 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/m 3 while the density of the oil palm fiber is 130 kg/m 3.The SAC values of coir fiber gives an

26 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 noiseabsorbing materials, renewable and biodegradable nonwovens have been

27 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;

28 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.

29 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

30 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 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).

31 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 (P e ):when porosity increases the absorption coefficient at that frequency increases to a maximum value, is shown as follows: f = K (100 - P e ) -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 P e for every frequency is calculated by the above equation. There is the most effective

32 41 porosity (P me ) giving the greatest value among the maximum absorption coefficient in P e of all frequencies. The larger the total surface area is the greater the P me 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 openfaced materials or materials with porous scrim, the flow resistivity is very important Narang (1995) 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

33 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:

34 43 Sound absorption coefficient ( ), Reflection coefficient (Rc), Surface impedance (Z) Acoustic Measurements Measurement techniques used to characterize the sound absorptive properties of a material are : Reverberant Field Method Impedance Tube Method Steady State Method 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

35 44 Figure 2.2 Reverberant method 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

36 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 (courtesy of Bruel &Kjaer)

37 46 Figure 2.4 Impedance tube kit set up (50Hz 6.4 khz) Type (courtesy of Bruel &Kjaer) 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