Abstract:
The invention concerns a multilayered perforated sound absorber ( 1 ) with a microperforated plastics film ( 4 ) and a thermally deformable absorber ( 3 ) which is a foam layer, a fabric or a fibrous non-woven layer. The invention further concerns a method for producing a corresponding sound absorber.

Description:
1) FIELD OF THE INVENTION 
     The invention relates to a multilayered perforated sound absorber comprising a microperforated plastic sheet and a thermodeformable absorber, which is a foam layer, a fibrous woven fabric, or a fibrous non-woven layer. Further, the invention includes a process for producing a corresponding sound absorber. 
     2) BACKGROUND 
     Absorbers that are covered on one side thereof with a microperforated sheet towards a sound source are known. The absorption can be influenced by the perforation geometry, the distance between perforations and the open surface area of the sheet. In the automobile field, such structures are known specifically with a perforated metal foil and an absorber for the engine compartment. 
     DE 10 2004 050 649 A1 describes a sound-absorbing heat shield. A sound-absorbing heat shield with a sheet metal carrier plate which holds an acoustically and thermally effective insulation layer is disclosed. The heat shield comprises a protective sheet having microperforations and provided on the side of the insulation layer facing a sound source when in use. 
     A heat- and sound-insulating lining for the engine compartment of motor vehicles, which is described in DE 101 43 167 A1, consists of a heat reflector microperforated on the engine side, in contact with a polyurethane foam layer which is impregnated on the engine side with thermoset material having long-term temperature resistance of 200° C., especially of 150° C., for three weeks, in contact with a cover layer facing away from the engine side. 
     The production of microperforated sheets, especially metal foils, has long been known. For example, reference may be made to U.S. Pat. No. 7,838,125 B2, in which a specific embodiment of a microperforated metal foil is described. It is described that the metal foil is deformed by needle punching in such a way that a crater elevated with respect to the mean sheet layer level whose lower edge frays when the needle punctures through is formed against the needle puncture direction. 
     DE 10 2004 053 751 A1 describes a lining part for a vehicle, especially an underbody panel. It possesses a porous middle layer and at least one cover layer on each side, the porous middle layer having such a structure that it has acoustic transparency or acoustically absorbing efficiency. The acoustically absorbing porous middle layer may be covered on one or both sides thereof with one or more acoustically transparent or absorbing cover layers. 
     Structures with an absorber and a microperforated polymer sheet are known for the interior area of motor vehicles. EP 1 101 218 B1 describes a microperforated polymer sheet for use in a sound absorber, which is formed with a polymer sheet of a defined thickness and several microperforations, wherein the microperforations each have a narrowest diameter that is smaller than the sheet thickness, and a broadest diameter that is greater than 125% of the narrowest diameter. 
     SUMMARY OF THE INVENTION 
     A multilayered perforated sound absorber ( 1 ) comprising a thermodeformable absorber ( 3 ) and a microperforated sheet ( 4 ), bonded together surface to surface, characterized in that
         said microperforated sheet ( 4 ) contains of a two-layered material including a first, higher melting plastic layer provided with a defined perforation geometry, and a second plastic layer that is lower melting as compared with said first plastic layer, wherein said lower melting plastic layer comprises an intermediate layer ( 2 ), adjacent to the absorber ( 3 ), consisting of absorber components and said lower melting plastic, and   the ratio of the flow resistances of the cover layer of said higher melting plastic layer and said intermediate layer ( 2 ) to said absorber ( 3 ) is from 1:3 to 1:30, and the overall flow resistance R of the sound absorber ( 1 ) is within a range of from R=800 Nsm −3  to R=8000 Nsm −3 .       

     Thus, a microperforated plastic sheet with a defined perforation geometry depending on sheet thickness is described. In particular, perforation channels of a column-shaped or funnel-shaped type with variable diameters are described here. 
     For the absorber structures, the perforation geometry, the number of perforations and the mutual distance thereof are considered preconditions of good acoustic properties, which are influenced by the mentioned quantities. 
     In all these structures, the alteration of the two-layered sheet during the processing process remains unconsidered. In accordance with the needle punching of the sheet, relaxation processes leading to a reduction of perforation diameters occur in cold-needled higher melting sheets. On the other hand, perforation increase occurs during sheet deformation. The lower melting sheet layer relaxes and, together with the absorber components, leads to an additional reduction of the flow resistance of the cover layer. 
     All effects together change the flow resistance of the cover layer, and it is only this value that is important to the practical effect. 
     Thus, there is the object to produce a component (sound absorber) comprising a thermodeformable absorber and a microperforated sheet in such a way that it has a high absorption and a partially increased insulation in a broad frequency range, and especially at low frequencies. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention is illustrated and described herein with reference to the various drawings, in which: 
         FIG. 1  illustrates the funnel-shaped design of the needle channel in accordance with the present invention; and 
         FIG. 2  illustrates the structure of the sound absorber in accordance with the present invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The above object is achieved by a multilayered perforated sound absorber  1  comprising a thermodeformable absorber  3  and a microperforated sheet  4 , bonded together surface to surface, characterized in that 
     said microperforated sheet  4  contains of a two-layered material including a first, higher melting plastic layer provided with a defined perforation geometry, and a second plastic layer that is lower melting as compared with said first plastic layer, wherein said lower melting plastic layer comprises an intermediate layer  2 , adjacent to the absorber  3 , consisting of absorber components and said lower melting plastic, and the ratio of the flow resistances of the cover layer of said higher melting plastic layer and said intermediate layer  2  to said absorber  3  is from 1:3 to 1:30, and the overall flow resistance R of the sound absorber  1  is within a range of from R=800 Nsm −3  to R=8000 Nsm −3 . 
     Thus, the present invention relates, in particular, to a three-layered sound absorber  1  consisting of a microperforated sheet (cover layer)  4  and a thermodeformed absorber  3  having such a structure that a coupling between the two absorbers is achieved by the formation of an intermediate layer  2 . 
     If the specific flow resistance of sheet  4  is too high, the sound cannot penetrate into the rearward absorber  3 , and is reflected in the wide area. If the flow resistance of sheet  4  is too low (open), it is only the absorber  3  that has an essential effect, and all influences of the perforated sheet absorber  4  are negligibly small. 
     The frequency characteristics of both kinds of absorbers are basically different. While the porous sound absorbers  3  have a broad-range absorption effect that increases with frequency and thus can be employed mainly for the absorption of medium and high frequencies, the microperforated sheets  4  preferably absorb in a narrow frequency range, the resonance region, which is at medium or low frequencies. The scattering of the flow resistance over the sheet  4  results in an adjustable broadening of the absorption. 
     The microperforated sheet  4  preferably consists of or contains a plastic material having a melting temperature (softening temperature) above the processing temperature, such as polyamide, polyethylene terephthalate (PET), polyetherimide, polysulfone, polyethersulfone, polyetheretherketone, and a second plastic material, such as polyethylene (PE) or polypropylene (PP), having a melting temperature (softening temperature) below the processing temperature (backing temperature). This lower melting plastic material serves as an adhesive, bonds to particles of the absorber  3 , forms an intermediate layer  2 , and changes the perforation pattern and thus the acoustic and rheological properties of sheet  4 . 
     The softening temperatures of the higher and lower melting plastic materials preferably differ by at least 30K, especially at least 100 K. 
     In particular, the thermodeformable absorber  3  consists of a fibrous woven fabric, a fibrous non-woven layer, or a foam. The latter can have a one-layer or multilayer design, and may consist of combinations of such materials. 
     For example, the sound absorption layer  3  in contact with the above mentioned microperforated sheet  4  may comprise a foam layer, more preferably according to the present invention, one containing an open-cell polyurethane (PUR), melamine resin or lightweight foam. 
     In the same way, the fibrous woven fabric or fibrous non-woven layer as components of the sound absorber  1  according to the invention may also contain synthetic fibers, such as polyester fibers, especially PET, polyamide (PA) fibers, especially nylon 6 and/or nylon 66; polyolefin fibers, especially PP and/or PE; acrylic fibers, and mixed fibers thereof, including bicomponent (BiCo) fibers and multicomponent fibers, and/or mixtures of natural fibers, especially raw cotton, hemp, coconut, kenaf, jute and/or sisal fibers, with the above mentioned synthetic fibers. 
     Coupling of all sound absorption effects is achieved by forming an intermediate layer  2  between the high melting plastic layer (cover layer) having a defined perforation profile  4  and a low melting plastic layer with bonded absorber components, wherein the ratio of the flow resistances of sheet  4  and intermediate layer  2  on the one hand and the absorber on the other is from 1:3 to 1:30, and the difference in flow resistance within the component is at least 20%. 
     Accordingly, the second plastic layer, which is lower melting as compared to the first plastic layer, is facing the absorber  3 . 
     Another embodiment of the present invention includes a process for producing a sound absorber by back-molding a non-perforated PA/PE sheet onto a preformed fibrous non-woven absorber  3  consisting of binding and breaking fibers (BiCo, cotton, wool, PET), placed into a backing mold. 
     In the backing process, an intermediate layer  2  consisting of absorber components and PE is formed from the low-melting PE and the adjacent absorber layer  3 . 
     The perforation of the PA layer and of the ready-formed intermediate layer  2  is effected in a cooling/calibrating tool in which the mold piece facing the PA layer is provided with appropriate needles. Thus, the perforation is effected in the direction of the fibrous non-woven absorber  3 . Thus, the needle punching (piercing) channel goes inward into the material composite, to the absorptive layer  3 . The funnel which is thus provided gives the additional effect of a Helmholtz resonator. 
     The acoustic effect of a component  1  to be prepared can be varied by means of the volume of the piercing channel, the channel diameter, and the channel length, in accordance with the desired circumstances (number and arrangement of the perforations in sheet  4 ). 
     The funnel-shaped design of the needle channel is shown in  FIG. 1 . It can be achieved, in particular, with a conical needle punching tool whose cone length is preferably a multiple of the thickness of plastic sheet  4 . 
     The frayed material funnel formed by the needling, which extends into the intermediate layer  2  of the low melting plastic layer with bonded absorber components, to the absorber  3  after the backing, has an advantageous effect on the absorption behavior. 
     The shaping of the absorber  3  into the component is effected, for example, with supplying heat, which may lead to considerable changes of perforation geometry and thus to the acoustic properties when the sheet  4  is integrally inserted by extension and relaxation. Accordingly, the absorber  3  is preferably thermally preformed (into its final configuration), before the backing process is initiated. The microperforated sheet ( 4 ) and the preformed thermodeformable absorber ( 3 ) are deformed in a hot mold, especially using steam. Subsequently, needling is performed. 
     Example 
     The following Example, see  FIG. 2 , shows a structure of a sound absorber  1  according to the invention in an exemplary way. 
     A 65 μm thick PA/PE sheet  4  with a 30 g/m 2  PE layer, perforated with 227 E/cm 2  and a perforation diameter of 0.07+/−0.05 mm, was placed onto a preformed fibrous non-woven absorber  3  consisting of binding and breaking fibers (BiCo, cotton, wool, PET), inserted in the backing mold. The backing mold temperature was 135° C., and the backing time was 55 s. 
     In the backing process, an intermediate layer  2  consisting of absorber components and PE is formed from the low-melting PE and the adjacent absorber layer. 
     The afterwards determined overall flow resistance (of sound absorber  1 ) was from R=2400 Nsm −3  to R=3200 Nsm −3 .