Abstract:
A three-dimensional (3D) nonwoven structure is provided having fibers oriented in different directions and superficial shapes in predefined areas. The structure comprises at least three layers, a top layer, a bottom nonwoven layer and a randomly oriented fiber layer, connected with fibers in predefined areas. The layers are connected using a special technique based on the needle board configuration of the needle-punching machine, and may be needled with different needle positions. A special needle loom feeding system to feed the different layers and to produce the product is also disclosed.

Description:
FIELD OF THE INVENTION  
       [0001]    The present invention relates to three-dimensional (3D) nonwoven structures with superficial shapes in predefined areas using at least three layers, a conventional needlepunching machine with an altered feeding system ( 10 ) and a special technique based on the needle board configuration of the needle-punching machine, which may be needled with different needle positions. 
       BACKGROUND ART  
       [0002]    Textile structures play an important role in the performance of the global textile system. Three-dimensional structures (3D) are organized and integrated set of fibres with multiaxial orientation. Due to the different orientations of the fibres, this type of structures presents a high performance behaviour being suitable for a wide range of applications, such as acoustic insulation. 
         [0003]    Nonwoven products with various functions and applications are increasingly used in technical end-uses due to their different functions and advantages. Nonwoven products present, in this way, an important position in the technical textile market. The most important advantages of nonwoven structures (herein after called ‘nonwovens’) are their low weight, flexibility and versatility, combined with the direct production from fibres that make them faster and cheaper to produce. 
         [0004]    The combination of the advantages of conventional nonwovens with the superior properties of three-dimensional fibrous structures, leads to three-dimensional nonwoven products with a wide number of applications. 
         [0005]    Nonwoven fabrics with three-dimensional structure are required for many technical applications, such as geotextiles, insulation, composite reinforcements, and civil engineering, amongst others. This type of nonwovens can be used alone, or in combination with other materials. Currently they are produced from flat webs. In this case, in addition to the high cost of the conversion processes, irregularity is inevitably introduced into final product due to the joints. There have been several attempts to produce 3D nonwovens structures directly. 
         [0006]    Acoustic insulation can be an important application of this type of fibrous structure. Many acoustic articles have been developed to offset the unpleasant attributes of noise pollution. Known sound absorbing materials frequently come in the form of panels or laminates. These products are mainly used in motor-vehicles, and other devices including, airplanes, trains, commercial and residential structures. 
         [0007]    Many acoustic problems cannot be solved satisfactorily merely by using primary soundproofing measures which are applied to a sound source, and additional secondary measures are required. Secondary measures are those that, as a rule, intervene in the transmission path of the acoustic energy. Either the energy is reflected, that is to say deflected, or the energy is converted into a different energy form, mostly heat. In the first case, insulation is used, and in the latter case, the sound is attenuated. In conventional sound attenuation, the prior art methods convert the acoustic energy in the medium frequency to high-frequency range into heat through the use of porous sound absorbers, wherein the extent of conversion depends on the frequency range of the sound. The fact that porous absorbers are generally tried and tested only in the medium to high-frequency range is based on their physical attenuation properties. In order to attenuate an acoustic wave with the highest possible absorption, the thickness of the attenuating material must be at least one quarter of the wavelength, to be attenuated, since the displacement range of a sound wave is the greatest over that range. Therefore, low frequencies determine the required bigger thickness of insulation material due to their longer wavelength. This effect can also be achieved by means of thinner material thickness in combination with air gap. The insulation material is in this case arranged at a distance corresponding to λ/4. However, degree of absorption of airborne sound a describing the attenuation capacity is in such a configuration marked by dips in the higher-frequency range. 
         [0008]    Conventional sound insulating materials include materials such as cork, foams, compressed fibres, fibreglass bats, felts and nonwoven webs of fibres. Considering nonwoven web of fibres, meltblown fibres have been widely used in sound insulation materials. In addition, laminates of meltblown nonwoven webs have been used as acoustical insulation. 
         [0009]    The selection of a particular sound insulation material is governed by several factors, including cost, thickness, weight and the ability to attenuate sound and the use of the insulation material. Sound insulation attenuates sound by either absorbing sound waves striking the insulation material or reflecting such sound waves outwardly and away from a receiving area. Sound attenuation is measured by the ability of a material to absorb incident sound waves (sound absorption), and/or by the ability of the material to reflect incident sound waves (reflection). Ideally, a sound attenuation material has a high sound absorption coefficient and/or a high transmission loss value. 
         [0010]    U.S. Pat. No. 5,841,081 published in Nov. 24, 1998, presents a method for attenuating sound waves that pass from a source area to a receiving area, that comprises a moulded three-dimensional nonwoven web of organic microfibres, and 15 percent in weight or greater of heat activated staple fibres. The nonwoven web has a thickness of at least 0.5 cm or greater, and has a density of less than 250 kg/m 3 . The heat activated staple fibres are bonded to each other, and to the microfibres at various contact points. In this way, the resulting article could be moulded into a variety of shapes. By adding an appropriate amount of high denier bulking fibres, superior attenuation is achieved, while also obtaining good flexural strength. 
         [0011]    U.S. Pat. No. 6,376,396 B1 published in Apr. 23, 2002 relates to a soundproofing material made of nonwoven materials containing thermoplastic fibre for the acoustic frequency range of 100 to 5000 Hz. This material is specially used in combination with porous insulation materials, to increase the acoustic absorption at low frequencies. This nonwoven material containing thermoplastic fibres is permanently compacted to a specific flow resistance of RS=800-1400 Ns/m 3  in two stages by a mechanical compaction process and a subsequent pressure/heat treatment. 
         [0012]    U.S. Pat. No. 6,893,711 B2 published in May 17, 2005 relates to an acoustical insulation material for sound attenuation, containing a nonwoven web of thermoplastic fibres having an average fibre diameter of less than about 7 microns, wherein the acoustical insulation material has a thickness less than about 3 mm, and a density of greater than about 50 Kg/m 3  and is effective as a sound insulation material. 
         [0013]    U.S. Pat. No. 6,220,388 B1 published in Apr. 24, 2001 relates to an acoustical insulation panel to absorb and attenuate sound energy. The acoustical insulation panel comprises an inner core including a plurality of cells formed therein, and an outer membrane disposed on at least one side of the inner core, to cooperatively form a plurality of sound attenuating chambers. Each outer membrane comprises an inner substrate of nonwoven meltblown microfibre acoustical absorbing fabric extending tuft, and an outer facing to protect the inner substrate of nonwoven meltblown microfibre acoustical absorbing fabric. The inner core comprises a honeycomb structure formed by a plurality of interconnecting cell sidewalls to form a plurality of cells. 
         [0014]    U.S. Pat. No. 6,723,416 B1 published in Apr. 20, 2004 relates to a three-dimensionally structured composite fibrous web and a method for manufacturing a try-dimensionally structured fibrous web. The three-dimensionally structure is produced with continuous-filament layers, which alternate perpendicular to the surface plane, and dense short-fibres layers, that are permanently thermally bonded in a continuous or spot-like manner to the filament layers. The wide-mesh continuous filaments layers representing a scrim, lattice or netting, have folds or wave shaped elevations on the short fibre layers. In the manufacturing process, all the layers of the laminate are subjected together to a shrinkage process at a certain temperature, which lies between the softening and melting points of the scrim material. To obtain the three-dimensionally composite, at least two nonwoven fabric layers are bonded in each case, to one scrim layer. The nonwoven fabric layers are made up of fibres that are bonded to each other mechanically and/or thermally and that, in the surface direction, pass a fold-like pattern in the form of geometric, repeating elevations or undulations. The three-dimensionally composite is composed of shrunk scrim and both nonwoven fabric layers. They are bonded to the shrunk scrim but not to each other, such that, on both sides of the scrim, elevations and depressions are formed on the nonwovens fabrics. Between and beneath the elevations are located hollow spaces which are permeable to fluid media and which absorb particles and dust. 
         [0015]    U.S. Pat. No. 7,060,344 B2 published in Jun. 13, 2006 relates to three-dimensional molded structures preferably comprising a nonwoven substrate formed of small diameter fibres and/or filaments. Nonwovens with a random fibre orientation distribution and a high degree of crimp with partially oriented fibres are preferred for use in forming the deep molded structure. The process to produce the three-dimensional structure is based on heating, deformation and cooling. During the heating process, the fibres approach their onset of melting and are only partially melted. The structure relies on the thermoplastic components in the structure for molding capability but the structure may be composed of both thermoplastic and non-thermoplastic components. The drawing characteristics of fibres are important, and also the process for molding the structure. The structures are formed by a combination of heat and pressure, such as those commonly used in solid phase pressure forming, vacuum bladder match plate molding, stamping, pressing or calendaring. 
         [0016]    These cited patents are based in the use of thermoplastic materials (fibres), in order to enable the shapeability by heat application due to fibres melting, and to increase the acoustic performance especially at low frequencies. The three-dimensional shaped nonwoven presented in this invention does not present any thermoplastic materials, and the shapes are developed automatically in the nonwovens production step, using a special technique. The acoustic performance at low frequencies is very good due to its 3D fibrous structure. Moreover, the three-dimensional shaped nonwoven, materials are produced in a single step, and do not need additional processes like heating and cooling. 
         [0017]    The combination of the advantages of conventional nonwovens, with the superior properties of three-dimensional fibrous structures, leads to the production of a fibrous material required for many technical applications, such as geotextiles, insulation, composite reinforcements, and civil engineering, amongst others. This type of nonwovens can be used alone or in combination with other materials, such as composites. Currently they are produced from flat webs. In this case, in addition to the high cost of the conversion processes, irregularity is inevitably introduced into the final product due to the joints. There have been several attempts to produce 3D nonwovens structures directly in the non-woven process. 
         [0018]    WO/2005/007962 published in Jan. 27, 2005 presents a three-dimensional (3D) nonwoven spacer fabric, comprising at least two separate but interconnected layers, each of the layers being provided with discrete interconnections so as to provide discrete voids between the two layers of fabric. These are very thin products with absorbent proprieties. Thickness varies between 1 to 9 mm, and weight in the range 20-1.000 g/m 2 . The method of manufacturing this product includes two different steps: in the first step is formed a nonwoven fabric from fibre or filament webs in both e sides of a spacer device, in the second step, causing fibres in at least one web to be transferred between the gaps in the spacer device towards the adjacent web to form an integrated structure. High-pressure water jets are used to interconnect groups of fibres in the layers between the spacer elements. 
         [0019]    U.S. Pat. No. 5,475,904 published in Dec. 19, 1995 presents a method and device for producing composite laps and composites thereby obtained, where two laps are needled together between longitudinal guides tubes, which maintain spacing between the laps and downstream of the needling position, the tubes release an interleaving material between the laps and between the rows of bridges formed by needling. The interleaving material is consisting of resin, powder, fibres, tubes, wires, threads and/or electrical conductors. The composite product obtained presents some mechanical strength, while retaining the ability to be formed. In order to achieve this, a pasty mass that is soaked with water is placed between two laps, and the two laps are needled through the pasty mass. The thickness of the product obtained is uncertain. 
         [0020]    Patent WO 2007/125248 published in Nov. 8, 2007 present a process characterized by the use of a folding element ( 5 ) to apply, to an incoming web containing a proportion of thermoplastic fibres and/or filaments or fixing material, an initial folding of the incoming web in the form of peaks and troughs, brought about by blades extending radially on ends of the folding element, this initial folding being applied either by maintaining the folding element itself at a set temperature, or by heating the incoming web. The invention is more particularly applicable to the textile industry, especially the manufacture of nonwovens. 
         [0021]    The present invention has novelty and implicate an inventive step regarding all the others described above in the prior art. The system to produce three-dimensional nonwoven structures of this invention is not using any spacer plate between the layers, and any means for advancing and guiding the two basic layers, and in this way is possible to use one conventional needlepunched nonwoven. Moreover, in this invention, it is not used high-pressure water jets. in combination with a rigid spacer device to produce the final product. 
         [0022]    Different mechanisms and devices have been introduced in the last decades to improve the performance of needlepunched nonwoven machines, with the objective of creating some patterns and/or configurations of the fibres in the final product. Examples of such mechanisms are disclosed in the following documents: U.S. Pat. No. 0,162,203 A1 published in Nov. 7, 2002, U.S. Pat. No. 6,584,659 B2 published in Jul. 1, 2003, U.S. Pat. 7,107,658 B2 published Sep. 19, 2006. U.S. Pat. No. 6,622,359 B2 published in Sep. 23, 2003 and U.S. Pat. No. 6,785,940 B1 published in Sep. 7, 2004 present an apparatus for needling a nonwoven material with a support which receives at least one needleboard and is drivable in a reciprocating manner, both in the needle penetration direction as well as in the direction of the passage of the nonwoven material. 
         [0023]    By other hand, UK 2376473A published in Dec. 18, 2002 describe a needling machine with a movable base plate and a movable stripping plate to provide advantageous structural conditions, to allow the needle density of the needleboard to be increased. The needling machine includes a needleboard that is driven in a reciprocation manner in both the needle penetration direction and the direction of the passage of a nonwoven material, and comprises a movably mounted base plate and a movably mounted stripping plate which together define a guide means for the nonwoven material. The base plate and stripping plate are movable in the direction of passage of the nonwoven material and can be driven synchronously with respect to the needleboard which is mounted above the stripping late. The base plate and stripping plate may be driven by separate eccentric drives. 
         [0024]    U.S. Pat. No. 0,103,975 A 1 published in Jun. 3, 2004 describes a ball-covering needlefelt produced by needling a fibre batt in a range of angles including a plurality of angles which are non-perpendicular to the plane of the bat. The range of angles is preferably achieved by the batt being curved during needling, the batt conveniently bkkeing curved in its direction of travel through the needling machine. The needleboard of the needling machine is preferably correspondingly curved. 
         [0025]    U.S. Pat. No. 0,011,060 A1 published in Jan. 20, 2005 presents an apparatus for needling a nonwoven material with at least one needleboard, which is drivable in reciprocating manner in the needle-penetration direction and with a stitch base which is opposite of the needleboard and is made of a continuous revolving brush belt forming a conveyor for the nonwoven material. The needleboard comprises rows of needles, which have an even distance of the rows and extend in at least one section, in the revolving direction of the brush belt. The brush belt and the needleboard can be moved back and forth in a reciprocating manner, by means of a traversing drive during the conveyance of the nonwoven material transversally to the revolving direction of the brush belt, to an extent corresponding to the distance of the needle rows relative to each other, or an multiple thereof. In order to achieve a marked pile formation during the needling of the nonwoven material, the nonwoven material is conveyed on a stitch base made of a continuously revolving brush belt through the needling device, so that the needles penetrating the nonwoven material needle on the pile fibres in a loop-forming manner into the brush belt. The pile formation occurs only in sections so the nonwoven material can be provided with a surface pattern formed by the pile fibres. It is necessary to ensure an even wearing of the brush belt, because the surface structures of the brush belt are imaged on the surface of the nonwoven during the needling of the nonwoven material. 
         [0026]    Patent DE 1660783 published in Jan. 15, 1976, presents a device to needle fibre felts or similar things due to the use of needle boards (connected to a needle beam) which are performing an up and down stroke. This needle beam is divided into segments, which are placed next to each other. These segments are guided in there up and down stroke, so that they cannot move out of their position. These needle boards are moved with one main driving shaft and some according clutches. The different needle boards can provide different stitch densities. 
         [0027]    DE 518907 published in Feb. 5, 1931, U.S. Pat. No. 2,148,511 published in Jul. 30, 1937 and U.S. 1,742,133 published in Dec. 31, 1929 are related to a needling machine where the needling operation is performed by reciprocating banks of needles which push the fibres into the base fabric through the down stroke of the needles. The needle beam is divided into more than one segment, which can be moved separately from each other. A main driving shaft drives the segmented needle beams. The segmented boards will also do a movement in the direction of the material flow. This movement in the machine direction is so long as the needle is in action in the felt. The machine may be conveniently built in various widths simply by employing a greater or less number of needle bank units and operating means thereof. The units will be of a size corresponding to the smallest type of machine that it may be desired to build say 60″. 
         [0028]    Patent DE 1785256 published in Sep. 2, 1968 presents a needling machine with a needle beam divided in various segments. A second staple fibre source feeds the production process, creating with the varied movement of the various segments of needles, patterns in the final structure. 
         [0029]    Patent DE 10036821 A1 published in Mar. 22, 2001 presents a needle board nonwoven webs has at least two spaced independent needleboards in succession with needles that work with another stitching devices that are in the base plate, to create some loops to give a wide range of patterning with the needle bonding action. The speed variation and the work frequency of the needleboards help to the creation of different patterns in the final nonwoven. 
         [0030]    All these patent documents refer to some improvements in needlepunched machines to create some special nonwoven materials, with different fibre orientations, and special configurations and patterns in the final nonwovens products. This new technology however is different in the process to produce the three-dimensional shaped nonwoven since is not based in segmented needles, but on a special configuration of the needles in a single needleboard. The different configurations or position of the needles in the board is able to create some superficial shapes in predefined areas. In other hand, this special technique can be applied in a conventional needlepunched machine, presenting low cost and easy and quickly production. 
       SUMMARY OF THE INVENTION 
       [0031]    The present invention relates to a three-dimensional nonwoven structure, composed by a top and a bottom nonwoven layers, and a third layer based on randomly oriented fibres. These layers are connected by fibres in predefined areas, using special, needle punching technique, leading to the formation of shaped voids located according to the requirements. The voids thus created enable the creation of superficial shapes in predefined areas on the top and bottom layers of the non-woven structure. These three-dimensional nonwoven structures are produced in a single step, in several configurations, and may be applied where the final configuration of the product leads to a better performance, mainly for acoustic insulation purposes. 
         [0032]    To obtain this three-dimensional shaped needlepunched nonwoven structure a special technique is used, based on the needle board configuration of the needle-punching machine, which may be needled with different needle positions and, if required, with different needle lengths. These three-dimensional nonwoven structures are produced in a single step, using an intermittent process, in several configurations. In the production of this type of structures it is possible to use all types of fibres. 
         [0033]    The final product developed using this special technique presents important characteristics when comparing to other products used currently in acoustic insulation such as: low cost, easy and quick production, versatility according to thickness and dimensions required and good performance for different kind of specific applications. 
     
    
     
       DESCRIPTION OF THE DRAWINGS  
         [0034]      FIG. 1  presents the cross-section of the three-dimensional (3D) needlepunched nonwoven structure, with regular superficial shapes in predefined areas. ( 1 )—Superficial shapes; ( 2 )—Top layer; ( 3 )—Bottom layer; ( 4 )—Randomly oriented fibres; ( 5 )—connections. 
           [0035]      FIG. 2  presents the top view of three-dimensional (3D) needlepunched nonwoven structure, with regular superficial shapes in predefined areas. ( 1 )—Superficial shapes; ( 4 )—Randomly oriented fibres; ( 5 )—connections. 
           [0036]      FIG. 3  presents the adjustable components of the needle-punching machine. These components can be variable to produced products with specific properties. ( 6 )—Needleboard; ( 7 )—Stripper plate; ( 8 )—Bedplate; ( 9 )—Felting needles; (D)—Distance between stripper and bedplate. 
           [0037]      FIG. 4  presents some needle board configurations that can be uses to produce three-dimensional nonwoven structures with superficial shapes in predefined areas. 
           [0038]      FIG. 5  presents the feeding and take out system developed to produce three-dimensional nonwoven structures with superficial shapes in predefined areas. 
           [0039]      FIG. 6  presents the comparison between the acoustic insulation performance of the three-dimensional non-woven structure with superficial shapes from the invention and of a plain nonwoven structure (from the prior art). 
           [0040]      FIG. 7  presents the acoustic performance of several products existing in the market in comparison with the three dimensional non-woven structure with superficial shapes from the invention. 
           [0041]      FIG. 8  presents a table with a comparison between materials used in the three-dimensional non-woven structure with superficial shapes from the invention and of a plain nonwoven structure (from the prior art). 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0042]    The system to produce the three-dimensional nonwoven fabrics is composed by feeding and mechanical devices, in combination with special technique based on the needle board ( FIG. 3 , component  7 ) configuration of the needle-punching machine, which may be needled with different needle positions. 
         [0043]    This three-dimensional (3D) needle punched nonwoven structure comprise at least two separate, but interconnected, layers ( 2  and  3 ), filled with randomly oriented fibres ( 4 ) fed directly from the cross-lapper, and has an irregular shaped surface ( 1 ). These three-dimensional nonwoven structures are produced in a single step, using an intermittent process, in several configurations. In the production of this type of structures it is possible to use all types of fibres. 
         [0044]    To obtain the 3D needle punched nonwoven proposed, two nonwoven layers are fed from a special feeding system ( FIG. 5 ) into the needle-punching machine, where the needle-board with barbed needles, transfer fibres from one layer to another, to form links or bridges between the separate layers in defined areas of the structure, creating a three-dimensional structure with fibres in different orientations. In the other areas, where the connection between the two layers is inexistent, the randomly oriented fibres inserted create an irregular superficial shape. A large number of three-dimensional nonwoven shaped structures may be achieved. 
         [0045]    Within needle board configuration technique, different needle positions are used in the same board to provide the ability to produce three-dimensional nonwoven structures. This technique provides a fibre connection in the machine direction, just in the required areas, according to the configuration chosen for the needles in the board. This configuration is based on the needles position, once some parts of the needleboard are not needled. 
         [0046]    The position of the needles in the board can be chosen, taking into account some selected configurations for the needle board, depending on the 3D connection required. The non-needled areas in the needle board lead to unconnected areas between the different nonwoven layers, or to the creation of some voids in the final structure, where it is possible to insert randomly oriented fibres in order to create irregular superficial shapes. Moreover, needles with different lengths can be used in some specific positions to create a greater entanglement of the fibres in the structure. 
         [0047]    The specific needles used in the process are chosen according to the material requirements, used for the production of the three-dimensional shaped nonwoven structure. Different types of needles can be used without any restriction. 
         [0048]    The distance or gap between the stripper and the bedplates (D), can be varied to control the thickness of the final product, taking into account the quantity of material that is feed. At the same time, the bedplate corresponds to the base where are allocated the materials control the penetration depth. Therefore is one important parameter to control the rigidity of the structure and the configuration of the superficial shape. Furthermore, if different needle lengths are used, the variation of the bedplate permits to control the penetration depth of the needles with different length. 
         [0049]    To obtain continuous superficial shapes in the needlepunched nonwoven structure, an intermittent process is used where the feeding is stopped when the structure is needled, and the needle board is stopped when the feeding is running. The process of needling is controlled according to a determined time that is variable between 4 and 60 seconds according to the stitch required for a specific application. The stitch depends on the strokes/min and on the needle density of the board used. 
         [0050]    To produce this three-dimensional needlepunched nonwoven structure by applying this special technique, a conventional needlepunched machine can be used. The needleboard needs to be configurated manually according to the superficial shapes required, and two devices need to be added to feed the two nonwovens layers (top and bottom). These two devices can be easily add and removed from the conventional needlepunched machine. 
         [0051]    Different materials can be used to produce the tree-dimensional shaped nonwoven structure. The variation in the type of fibre used can occur in the bottom and top layers, and in the randomly oriented fibres used in the middle, according to the requirements. The top and the bottom layer are pre-needled and present low thickness. The randomly oriented fibres are integrated in the conventional line of nonwoven production, being preferably carded (web formation) before the cross-lapper, in order to orient and put parallel the fibres. 
         [0052]    For acoustic insulation, the three-dimensional (3D) needle punched nonwoven with superficial shapes within this invention, performs better than conventional plain nonwoven (without shapes), and with the same structural and composition characteristics, for the whole range of Frequencies (Hz), showing higher Absorption Coefficient ( FIG. 6 ), for nonwoven structures produced according to the specifications presented in Table 1. The peak obtained for the 3D nonwoven is obtained at about 800 Hz with an Absorption Coefficient of 0.83. The plain nonwoven reaches its peak at 4000 Hz corresponding to a 0.61 of Absorption Coefficient. 
         [0053]    The performance difference reaches more than 100% for 1000 Hz, about 55% for 1500 and 2000 Hz, about 30% at 2500 Hz and more than 20% from 3000 to 5000 Hz. 
         [0054]    When comparing to other products in the market, the three-dimensional (3D) needle punched nonwoven structure with superficial shapes, developed within the present invention, enhances substantially the acoustic performance of the nonwoven panels when comparing to those referred in the literature of the prior art ( FIG. 7 ). The testing results performed show that these innovative 3D shaped nonwoven structures present better airborne sound insulation, when comparing with conventional products used with higher thickness. 
         [0055]    Considering the results obtained and taking into consideration the characteristics of the nonwovens, it is possible to conclude that the arrangement of the fibres in the three-dimensional (3D) needle punched nonwoven structure with irregular superficial shapes is influencing positively the acoustic performance, leading to higher acoustic insulation performance.