You are an expert at summarizing long articles. Proceed to summarize the following text:

You are an expert at summarizing long articles. Proceed to summarize the following text: 
BACKGROUND OF THE INVENTION 
       [0001]    The invention relates to building products and, in particular, to acoustical ceiling tile. 
       PRIOR ART 
       [0002]    Suspended ceilings customarily comprise a suspended metal grid and panels or tiles closing the spaces between the grid elements. Normally, the panels are constructed with selected materials and/or surface treatments to absorb sound. The ability of a panel to absorb sound is conventionally reported as its Noise Reduction Coefficient or NRC. NRC can range between 0 (no absorption) and 1 (full absorption) with a rating of 0.5, meaning it absorbs 50% of the sound energy striking it, being required to qualify a panel as “acoustical”. In the industry, panels rated at 0.7 are considered to have good acoustical performance. A need exists for acoustical tiles that achieve excellent NRC values and especially have the ability to absorb sound at target frequencies, have a high post consumer recycle content, resist sagging over time, are relatively light in weight, and are relatively inexpensive to produce. 
       SUMMARY OF THE INVENTION 
       [0003]    The invention provides a ceiling panel with high level acoustical absorption properties using a core made of ordinary corrugated fiberboard, sometimes called cardboard. The core construction consists of numerous narrow strips of corrugated fiberboard laminated together. The corrugated board is cut perpendicular to the corrugations or flutes so that the flute openings lie in front and back planes of the panel core corresponding to the geometry of the finished panel. The front of the panel is covered with a suitable sheet of acoustically transparent material with proper air flow resistance and the back of the panel is optionally closed with another sheet, preferably with acoustical isolating properties. 
         [0004]    In addition to high acoustical performance, the panel of the invention has the potential to be economically produced, light in weight, and have a high post-consumer recycle content. Corrugated fiberboard is typically produced on high speed machines with relatively low energy consumption and with high recycled paper content. Because the inventive panel is largely air space, it is relatively light in weight. 
         [0005]    The disclosed vertical orientation of a flat liner board component of the corrugated fiberboard in the finished panel makes the panel sag resistant and capable of spanning large grid modules. The inventive panel can be produced directly from reclaimed corrugated fiberboard since there is no criticality in the uniformity of the flute size, flute alignment, and/or number of walls of the corrugated fiberboard used in a particular panel. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]      FIG. 1  is a perspective view of an acoustical panel made in accordance with the present invention; 
           [0007]      FIG. 2  is a fragmentary schematic showing of one manner of assembling a core of the inventive panel; and 
           [0008]      FIG. 3  is a perspective view of a three-dimensional block from which the inventive panels are cut in an alternative manner of producing a core of the inventive panel. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0009]      FIG. 1  illustrates an example of an acoustical ceiling panel  10  of the invention; the panel is a nominal 2 foot by 2 foot unit and can have a nominal thickness of 1 inch. Dimensions discussed herein will be understood to include industry metric equivalents. The panel  10  includes a corrugated fiberboard core  11 , a face sheet  12 , and a backing sheet  13 . The core  11  is made by assembling numerous corrugated fiberboard layers  15  side-to-side such that the combined total thickness of the layers is equal to the length of an edge of the panel  10 . 
         [0010]    As shown in  FIG. 2 , each layer  15  can comprise a corrugated medium  16  and a single flat liner board  17 , the combination of these elements sometimes being referred to as a single-sided or single face corrugated board. The paper compositions and fabrication of corrugated fiberboard is well known to the relevant industry. The corrugated medium  16  is a paper, typically, in the United States having a weight of 0.026 lbs./square foot. The paper is heated, moistened and formed into a fluted pattern on geared wheels. Typically, the fluted or corrugated medium  16  is joined to the flat liner board  17  with a starch-based adhesive to form the single face board comprising the layer  15 . As is typical, the liner board stock can have the same weight as the paper of the medium  16 . The flutes or corrugations of the medium  16  are essentially entirely curvilinear in cross-section and resemble a sine wave. The size of the flutes, designated  19 , is ordinarily stated by the number of flutes in a foot length of the corrugated fiberboard. ASTM Standard D4727 sets out the following flute sizes, applicable to single face, as well as singlewall, doublewall and triplewall corrugated fiberboard (referred to below). 
         [0000]    
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
                   
               
               
                   
                   
                   
                 Flute 
                 Flute 
               
               
                   
                 Flutes/ft 
                 Flutes/m 
                 Height (in.) 
                 Height (mm) 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                 A-Flute 
                 30 to 39 
                  98 to 128 
                 0.1575 to 0.2210 
                 4.00 to 5.61 
               
               
                 B-Flute 
                 45 to 53 
                 147 to 174 
                 0.0787 to 0.1102 
                 2.00 to 2.80 
               
               
                 C-Flute 
                 35 to 45 
                 115 to 148 
                 0.1300 to 0.1575 
                 3.30 to 4.00 
               
               
                 E-Flute 
                 70 to 98 
                 229 to 321 
                 0.0445 to 0.0550 
                 1.13 to 1.40 
               
               
                   
               
             
          
         
       
     
         [0011]    Tests have indicated good acoustical properties, with an NRC in the order of 0.70, can be obtained with all of these standard flute sizes. Moreover, the panel construction, such as the panel thickness, can be selected to absorb sound at targeted frequencies. 
         [0012]    By way of example, the thickness of the corrugated fiberboard core can be, as mentioned above, nominally 1 inch.  FIG. 2  schematically illustrates one method of manufacturing the core  11 . Single face stock or board  15 , i.e. having only one flat liner board  17  and one corrugated medium  16 , is slit into 1 inch wide strips. The length of the strips can be equal to one of the nominal planar dimensions of the finished panel  10 . The strips are stacked on each other with their longitudinal slit edges in registration. Glue or adhesive is applied to a side of a strip at the interface between adjacent strips. The stack height is built up until it reaches the nominal planar dimension of the finished panel perpendicular to that represented by the length of the laminated strips. 
         [0013]      FIG. 3  illustrates another method of forming the core  11 . Flat rectangular sheets  21  of corrugated fiberboard having at least one planar dimension equal to a nominal planar dimension of the finished panel  10  are stacked to a height equal to the other nominal planar dimension of the panel. The sheets are permanently attached to one another with glue or adhesive at their interfaces. The result is a block  22 , which in the illustration of  FIG. 3  is a cube. The block  22  is sliced with a saw along a plane denoted by lines X-X and Y-Y spaced nominally 1 inch from a side of the block to form a core. Successive cores  11  are formed by more cuts, each spaced a distance of 1 inch from the preceding cut. 
         [0014]    The flutes  19  of the core  11  extend perpendicularly to its major planar faces. The face sheet  12  is an acoustically transparent medium or film, optionally painted with proper air flow resistance that can serve as the appearance side visible to an observer in a room in which the panel  10  is installed. The face sheet  12  is adhered to the core  11  with a suitable adhesive. The face sheet  12  can be coated with a paint of a type used on the face of conventional ceiling tiles to improve its appearance and/or light reflectance and to obtain overall air flow resistance in a proper range. An example of a suitable face sheet  12  is a non-woven fabric such as fiberglass scrim with a caliper of 0.02 inch, basis weight of 125 g/m 2 , and specific air flow resistance of 45.6 Pa.s/m coated with a paint. The choice of face sheet  12  with proper air flow resistance was found to be important to the overall acoustical performance of the inventive panel; if the air flow resistance is too low or too high, the acoustical performance is impaired. 
         [0015]    The side of the core  11  opposite the facing sheet  12  is preferably covered with the backing sheet  13  which can be a kraft paper laminated with a metal foil as used in some commercially available ceiling tile products. Other non-foiled paper can be used for the backing sheet  13 . The backing sheet  13  can be used to obtain a good CAC (Ceiling Attenuation Class) value. A suitable adhesive is used to attach the backing sheet  13  to the core  11 . 
         [0016]    The single face board  15  illustrated most clearly in  FIG. 2  is the most efficient corrugated fiberboard style from a material usage standpoint. As shown in  FIG. 2 , the flat liner board  17  of one board  15  can serve as a liner board of an adjacent single face board when it is adhesively attached thereto. From an acoustical standpoint, singlewall, doublewall and triplewall corrugated fiberboard work satisfactorily and can be used in place of the illustrated single face board  15 . It is contemplated that where there is a reliable source of used quality corrugated fiberboard stock is available, the core  11  can be made by reclaiming this used material and converting it directly into a core. Since the standard flute ranges are comparable in acoustical performance in a core construction, it is possible to produce a core with mixed flute sizes and without layer to layer flute registration. This flute size and registration free compatibility can make use of reclaimed corrugated fiberboard stock in the manufacture of the inventive panel  10  more practical. 
         [0017]    It should be evident that this disclosure is by way of example and that various changes may be made by adding, modifying or eliminating details without departing from the fair scope of the teaching contained in this disclosure. The invention is therefore not limited to particular details of this disclosure except to the extent that the following claims are necessarily so limited.

Summary:
An acoustical ceiling panel comprising a flat core and an acoustically transparent face sheet adhesively attached to one of two oppositely facing major sides of the core, the core comprising a multitude of layers of corrugated fiberboard laminated together, the corrugated fiberboard layers each having a corrugated medium adhesively attached to a flat liner board, the corrugated medium forming regularly spaced flutes of curvilinear cross-section, the flutes of the layers of fiberboard being arranged in parallel directions extending perpendicularly to the major faces of the core.