Abstract:
Acoustical insulation for attenuating sound entering the passenger compartment of motor vehicles. The acoustical insulator comprises dual nonwoven layers in which one nonwoven layer is an airflow control layer and the second nonwoven layer is a support layer positioned between the cap layer and the sheet metal of the firewall separating the passenger and engine compartments. The areal density of the cap layer is less than the areal density of the support layer and, preferably, comprises polyethylene terephthalate (PET) fibers. The cap layer has a specific airflow resistance ranging from about 200 mks Rayls to about 1200 mks Rayls.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to motor vehicles and, in particular, to acoustical insulation for use on interior and exterior surfaces of the passenger compartment of motor vehicles.  
       BACKGROUND OF THE INVENTION  
       [0002]     Sound attenuating materials are provided for acoustically insulating motor vehicles to reduce the level of noise inside the vehicle passenger compartment. External noises, such as road noise, wind noise, engine noise, vibrations, etc. may be attenuated through the use of various acoustical materials applied to the internal and external surfaces of the passenger compartment. Noises emanating from sources within the passenger compartment may be attenuated through the use of various acoustical materials applied to the internal surfaces of the passenger compartment.  
         [0003]     The attenuation of airborne noise from external sources transmitted through the body structure and components to the passenger compartment is commonly referred to as sound transmission loss. The attenuation of internal airborne noise incident on the interior surfaces of the vehicle is commonly referred to as sound absorption. The specific-airflow resistance, of a material is defined as the air pressure difference across the thickness of the material divided by the linear velocity of the airflow and defines the resistance to air movement through a material. The specific airflow resistance may be expressed in units of mks Rayls (Pa·sec·m −1 ). The airflow resistance of fibrous materials depends among other parameters upon the areal density of the fibrous material, fiber orientation, fiber blend, and fiber diameter. The sound transmission loss and sound absorption of a single layer of material are determined by the weight and airflow resistance.  
         [0004]     There are two types of sound attenuating material used in vehicles. The first type of sound attenuating material or acoustical insulation comprises a molded barrier mat located on the interior surface of the firewall separating the passenger compartment from the engine compartment. This barrier mat system comprises a layer of fiber or foam which rests against the firewall and is attached to a thermoplastic barrier which can be made from ethyl vinyl acetate (EVA), polyvinyl chloride (PVC), or a thermoplastic polyolefin (TPO). These molded barrier mats are designed to have high transmission loss. Increasing the areal mass of the barrier mat and the thickness of the fiber-foam layer increases the transmission loss. Generally, these dual layer sound insulating materials are designed to provide high sound transmission loss at the expense of the sound absorption, which is low because the barrier mat is impermeable.  
         [0005]     The other type of sound attenuating material or acoustical insulation comprises a molded mat located on the interior surface of the firewall separating the passenger compartment from the engine compartment. Resinated cotton and phenolic impregnated polyester fibers are two common types of sound absorption substrates. However, these materials rely on phenolic resin as a strengthening and binder agent. Phenolic resins are undesirable due to the presence of formaldehyde and odors, as well as the need to utilize a high-tonnage press to manufacture a shaped product. Generally, such single layer sound insulating materials are not optimized for sound absorption and transmission loss. To optimize these properties, three or more layers of fibrous material are combined into a laminate in which the individual layers contribute to sound absorption and acoustical transmission loss. However, these are complex structures that require multiple process steps to successfully form into a shaped component.  
         [0006]     It would be desirable to provide an improved sound attenuating material for vehicle passenger compartments in which sound absorption dominates as a sound attenuation mechanism rather than sound transmission loss.  
       SUMMARY OF THE INVENTION  
       [0007]     In an embodiment of the present invention, an acoustical insulator comprises a first nonwoven layer having a first areal density and a second nonwoven layer coupled with the first nonwoven layer to define a laminate. The laminate is adapted to be applied to a surface of a motor vehicle with the first layer positioned between said second nonwoven layer and the surface when applied to an interior surface of a motor vehicle. The second nonwoven layer has a second areal density less than the first areal density and specific airflow resistance between about 200 mks Rayls and about 1200 mks Rayls.  
         [0008]     The invention therefore provides a sound attenuating material that includes a layer of nonwoven material and an underlying layer supporting the single cap layer. This simple two-layer structure provides acoustical attenuation effective to significantly reduce the audibility of common externally-originated noises, such as road noise and engine noise. In comparison with conventional barrier type acoustical insulators, the layered acoustical insulator of the present invention is realized with substantial cost savings and a lightweight construction. The acoustical insulator is biased toward providing sound absorption rather than sound transmission loss as the mechanism for acoustical attenuation because of the construction of the cap layer, which represents a benefit in comparison with conventional acoustical insulators used to sonically insulate the passenger cabin in motor vehicles.  
         [0009]     These and other benefits and advantages of the invention shall become more apparent from the accompanying drawings and description thereof. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]     The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and, together with a general description of the invention given above, and the detailed description given below, serve to explain the principles of the invention.  
         [0011]      FIG. 1  is a perspective view of a portion of a passenger compartment partially covered by an acoustical insulator in accordance with the present invention; and  
         [0012]      FIG. 2  is a detailed cross-sectional view of the acoustical insulator of  FIG. 1  showing the individual layers of a laminated structure. 
     
    
     DETAILED DESCRIPTION  
       [0013]     With reference to  FIG. 1 , a portion of a passenger cabin  10  of a motor vehicle  11  is shown with the instrument panel (not shown) removed to reveal the underlying firewall  12  separating the passenger compartment from the engine compartment. The firewall  12  is almost completely covered by an acoustical insulator  14 . The acoustical insulator  14  may be attached to the firewall  12  with mechanical fasteners or by other attachment methods, such as adhesive, familiar to those skilled in the art. The acoustical insulator  14  functions by absorbing the sound that is transmitted though the firewall  12  and registered holes  16  and cutouts  17  and then reflected from the surface of the instrument panel onto the surface of the acoustical insulator  14 .  
         [0014]     There are various openings or cutouts  17  defined in the sheet metal  12  and registered holes  16  in the insulator  14  for the steering column, brake booster, pedals, cables, hoses, etc., which are commonly referred to by a person of ordinary skill in the art as pass-thru&#39;s. Despite variations in the size and number of these registered holes  16  and cutouts  17  among vehicle types, the registered holes  16  and cutouts  17  generally degrade the transmission loss of the acoustical insulator  14  by defining regions through which noise from the engine can pass unimpeded by the acoustical insulator  14 . The area occupied by the holes  16  may be as much as 5% to 20% of the total surface area of the acoustical insulator  14 , contingent upon the vehicle type. Moreover, portions of the acoustical insulator  14  immediately surrounding the holes  17  are typically thinner than other portions of the acoustical insulator  14  more distant from the holes  17 .  
         [0015]     In addition to the interior surface of firewall  12 , the acoustical insulator  14  may find other applications for acoustically insulating the passenger cabin  10  of motor vehicle  11 . For example, the acoustical insulator  14  may be used as a sound insulator for the wheel houses located behind the vehicle rear quarter panels in a sports utility vehicle, minivan, etc.  
         [0016]     With reference to  FIG. 2  in which like reference numerals refer to like features in  FIG. 1 , the acoustical insulator  14  has a laminated structure that includes a nonwoven support layer  20  and a nonwoven cap layer  22  that provides stiffness and structural integrity. The nonwoven cap layer  22  is supported structurally by the support layer  20  and has a lower areal density (i.e., mass per unit area) than the support layer  20 . The cap layer  22  may be formed from a lofted layer in which the constituent fibers are bound together to supply structural integrity to the porous structure and then calendered to thickness less than 1.5 mm to provide a consolidated nonwoven layer. The support layer  20  and the cap layer  22  have an at least partially contacting face-to-face relationship and are bonded together as understood by persons skilled in the art during the manufacturing process to form a shaped construction suitable for use inside the passenger compartment.  
         [0017]     The support layer  20  is placed into contact with sheet metal  24  of the firewall  12  ( FIG. 1 ) and the cap layer  22  is separated from the sheet metal  24  by the support layer  20 . The support layer  20  provides the structural integrity to the acoustical insulator  14  required for handling, installation, and function and may be manufactured from various natural and synthetic fibers or a porous foam material, such as a polyurethane (PUR). The support layer  20  makes a major contribution to the sound attenuation. The cap layer  22  contributes increases the airflow resistance of the acoustical insulator  14  and significantly improves the sound absorption in a frequency range from 250 Hz to 10 kHz. Typically, the support layer  20  has an areal density ranging from about 80 grams·ft −2  to about 150 grams·ft −2  and the cap layer  22  has an areal density from about 10 grams·ft −2  to about 25 grams·ft −2 .  
         [0018]     In one specific embodiment of the present invention that provides particularly advantageous sound insulation properties, the cap layer  22  is a composite synthetic matrix that includes a mixture of high melt matrix or staple fibers each formed from a homopolymer or copolymer of polyester, which is generally termed polyester herein unless otherwise indicated, and preferably polyethylene terephthalate (PET), and low melt binding fibers each formed from polyester. The cap layer  22  is formed from a layer that is initially about 30 mm to about 10 mm thick and constituted by a mixture of stable and binding fibers. This initial layer is heated to a temperature effective to soften the binding fibers and compressed to less than 1.5 mm, which binds the collection of stable and binding fibers together upon cooling to form cap layer  22 . In alternative embodiments, the binding fibers may be replaced with a thermoplastic powder binder that binds the stable fibers upon heating and compression. The support layer  20  is an underpad consisting of cotton fibers blended with polyester fibers and may include recycled materials. Cap layer mats suitable for use in this embodiment of the present invention are commercially available from, for example, Owens Corning (Toledo, Ohio).  
         [0019]     The cotton and polyester fibers in support layer  20  are preferably cross-lapped to impart structural integrity and strength during the molding process. Cross-lapped fiber mats suitable for use as support layer  20  are commercially available from, for example, Hobbs Fibers (Waco, Tex.).  
         [0020]     To make the acoustical insulator  14 , continuous lengths of layers  20  and  22  are unrolled from individual rolls, paired in a face-to-face arrangement, and cut into blanks. The blanks can be heated using convection, infrared, microwaves, radio frequency, conduction through heated plates, and other conventional methods familiar to persons of ordinary skill in the art. The layers  20 ,  22  are preferably heated for about 40 seconds to about 90 seconds at about 300° F. to 360° F. to consolidate the layers  20 ,  22  and, thereafter, are transferred to a mold of a form tool. When the mold is closed, the layers  20 ,  22  are preferably compressed for approximately 40 seconds to approximately 50 seconds to form the acoustical insulator  14 , which has a three dimensional molded shape that is retained, due to the cooling, after ejection. The mold may be optionally chilled to reduce the cycle time. The formed acoustical insulator  14  is then ejected from the mold, trimmed, and shipped to an assembly line. Alternatively, cold blanks of layers  20  and  22  can be loaded directly into a heated tool without any pre-heating. When the mold of the heated tool is closed, the layers  20 ,  22  are heated to about 360° F. to 450° F. and compressed for approximately 25 seconds to 60 seconds to consolidate the layers  20 ,  22  to form the acoustical insulator  14  with the three dimensional molded shape that is retained, after cooling.  
         [0021]     In the final product, the acoustical insulator  14  preferably has a total thickness of in the range of about 4 millimeters (mm) to about 37 mm, with the cap layer  22  contributing less than 1.5 mm of the total thickness and the support layer  20  accounting for about 2.5 millimeters to about 35.5 millimeters of the total thickness. In this configuration, the cap layer  22  has a specific airflow resistance between about 200 and about 1200 mks Rayls (Pa·sec·m −1 ), and preferably between about 400 and 700 mks Rayls. In addition, the support layer  20  has a specific airflow resistance less than about 10,000 mks Rayls, and preferably between about 500 and 3500 mks Rayls. The acoustical insulator  14  may be placed on the sheet metal  24  so that the support layer  20  is coextensive with the sheet metal  24  and the cap layer  22  is spaced from the sheet metal  24  by the support layer  20 .  
         [0022]     While the present invention has been illustrated by the description of one or more embodiments thereof, and while the embodiments have been described in considerable detail, they are not intended to restrict or in any way limit the scope of the appended claims to such detail. Additional advantages and modifications will readily appear to those skilled in the art. The invention in its broader aspects is therefore not limited to the specific details, representative apparatus and methods and illustrative examples shown and described. Accordingly, departures may be made from such details without departing from the scope or spirit of Applicants&#39; general inventive concept.