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BACKGROUND OF THE INVENTION 
     The invention relates to building materials and systems and, in particular, to an acoustical panel for constructing monolithic ceilings and interior walls. 
     PRIOR ART 
     Sound absorption in buildings is commonly achieved with ceiling tiles carried on a suspended grid. Generally, the sound absorbing capacity of the tiles is achieved by material selection and/or characteristics of the room facing surface. Ceiling tile installations have the advantage of affording ready access to the space above the ceiling, but the divisions between the tiles, even when the grid is concealed, remain visible. Architects and interior designers have long sought a monolithic, texture free look in an acoustical ceiling particularly when there is no expected need for access to the space above the ceiling. Ordinary gypsum panel drywall ceiling construction does not achieve a sufficiently high noise reduction coefficient (NRC) that would qualify as acoustical. Perforated gypsum panels may achieve an acceptable NRC level but they are not monolithic in appearance. 
     SUMMARY OF THE INVENTION 
     The invention resides in the discovery that ordinary gypsum panels, such as drywall sheets, can be modified to construct an acoustical ceiling or wall with a monolithic plain face and surprising acoustical properties. Such panels can achieve an NRC of 0.70 or more. 
     In accordance with the invention, the gypsum core is made with a multitude of perforations or holes distributed throughout its planar area. The perforations or holes are restricted, preferably with a painted non-woven porous scrim fabric or veil at the front face and, optionally, a non-woven porous acoustical fabric at the back side. 
     The gypsum panel can be made, for example, by perforating standard sheets of drywall and thereafter covering the perforated sides of the sheet with additional laminated sheets or layers. These perforating and laminating steps can be performed by the original manufacturer of the drywall sheets or by a separate entity independent of the original drywall manufacturer. 
     Variations in the construction of the gypsum panel are contemplated. Common among these variations is a panel with a perforated gypsum core and with a face covered by a structure that is porous while appearing essentially imperforate to the unaided eye. 
     The disclosed gypsum-based panels can be installed in the same manner or a like manner as ordinary drywall. For ceiling applications, the acoustical panels of the invention can be screwed to a conventional drywall suspension system of grid tees or “hat channels” carried on black iron channels typically used in commercial applications or they can be attached to wood framing more often used in residential construction. Acoustical walls can be built by attaching the inventive acoustical panels to vertical studs, serving as spaced support elements. It will be seen that the inventive panels can be readily taped and painted like ordinary drywall, using the same or similar materials, equipment, tools and skills, to produce a smooth monolithic ceiling or wall. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a fragmentary, schematic, isometric view of a monolithic acoustical ceiling; 
         FIG. 2  is a fragmentary, cross-sectional view, on an enlarged scale, of the monolithic ceiling; and 
         FIG. 3  is a fragmentary, enlarged, cross-sectional view of a modified form of an acoustical panel of the invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to  FIG. 1 , there is shown a schematic partial view of an acoustical monolithic ceiling installation  10 . Portions of layers of the ceiling  10  are peeled away to reveal constructional details. The ceiling  10  is a suspended system including a drywall grid  11 , known in the art, comprising main tees  12  spaced on 4 ft. centers and intersecting cross tees  13  spaced on 16 in. or 2 ft. centers. Dimensions used herein are typically nominal dimensions and are intended to include industry recognized metric equivalents. The main tees  12 , to which the cross tees  13  are interlocked, are suspended by wires  14  attached to a superstructure (not shown). A perimeter of the grid  11  is conventionally formed by channel molding  15  secured to respective walls  16 . 
     Acoustical panels  20  are attached to the lower sides of the grid tees  12 ,  13  with self-drilling screws  21 . The illustrated acoustical panels are 4 ft. by 8 ft. in their planar dimensions, but can be longer, shorter and/or of different width as desired or practical. The size of the panel  20  and spacing of the grid tees  12  and  13 , allows the edges of the panel to underlie and be directly attached to a grid tee, assuring that these edges are well supported. 
     Referring to  FIG. 2 , the acoustical panel  20  of the invention is characterized with a perforated gypsum core  24 . One method of providing the core  24  is to modify a standard commercially available sheet of drywall by perforating it through a front paper face  23 , the gypsum core  24 , and a rear paper side or face  25 . Perforations  28  can be formed by drilling, punching, or with other known hole-making techniques. The perforations  28  are preferably uniformly spaced; by way of an example, the perforations can be round holes of 8 mm diameter on 16 mm centers. This arrangement produces a total area of the perforations substantially equal to 20% of the full planar area of a panel  20 . Other hole sizes, shapes, patterns and densities can be used. For example, tests have shown that a hole density of 9% of the total area can achieve good results. Marginal areas, as well as intermediate areas corresponding to centers of support grid, joists, or studs, of a sheet can be left unperforated to maintain strength at fastening points. 
     Sheets  29 ,  30  are laminated to both full sides of the perforated drywall sheet thereby at least partially closing both ends of the perforations  28 . At a rear side of the drywall, the backer sheet or web  30  is preferably an acoustically absorbent non-woven fabric known in the acoustical ceiling panel art. By way of example, the backer fabric can be that marketed under the trademark SOUNDTEX® by Freudenberg Vliesstoffe KG. It has a nominal thickness of 0.2 to 0.3 mm and a nominal weight of 63 g/m 2 . Specifically, the main components of this non-woven fabric example are cellulose and E-glass with a synthetic resin binder such as polyacrylate, poly(ethylene-CO-vinylacetate). Alternatively, for example, the backer sheet  30  can be a porous paper layer. The sheet  30  can be provided with a suitable adhesive for binding it to the rear paper side  25  of the modified drywall sheet  22 . 
     At a front side of the drywall sheet  22 , a sheet or web in the form of a non-woven fabric scrim layer  29  is attached with a suitable adhesive. The facing layer or sheet  29  is porous; a suitable material for this application is that used commercially as a cover or face for conventional acoustical ceiling panels. An example of this type of veil material is that marketed by Owens Corning Veil Netherlands B.V. under the product code A125 EX-CH02. This scrim fabric comprises hydrated alumina fiberglass filament, polyvinyl alcohol, and acrylate copolymer. The unpainted scrim  29  has a nominal weight of 125 g/m 2  and an air porosity, at 100 Pa, of 1900 l/m 2  sec. To avoid blocking the face scrim  29 , the adhesive can be initially applied to the panel or sheet  22 . The facing sheet  29  should be sufficiently robust to withstand field finishing operations described below. It should also be compatible with drywall joint compound or similar material and commercially available paints, typically water-based paints such as that described below. 
     The panel  20  with other identical panels is hung on the grid  11  in the same manner as ordinary drywall is installed. Similarly, as shown in  FIG. 1 , joints  33  are taped in the same way as regular drywall is taped. Drywall joint compound or similar material  34  is used to adhere a tape or similar material  35  to adjacent margins of two abutting panels  20  by applying it directly to the sheets  29  and over the tape  35  to conceal the tape. Typically, the long edges of the panels  20  are tapered to receive the joint tape  35  below the plane of the major part of the panel faces. The joint compound  34  can be conventional drywall joint compound and the tape  35  can be conventional drywall paper or mesh tape. The screws  21  securing the panels  20  to the spaced support elements  12 ,  13  forming the grid  11  are countersunk, as is conventional in drywall construction, and are concealed with joint compound  34  applied with a taping knife or trowel in the same manner as if applied to ordinary drywall. The panels  20  can be adhesively attached to vertical stud supports when constructing a wall. When dry, the joint compound  34  can be sanded or wet sponged to blend it into the plane of the surface of the face sheet  29 . 
     After the joint compound  34  has been sanded or sponged smooth, the front sheets  29  and remaining joint compound are painted with a commercially available acoustical paint  31  used for painting acoustical tile. An example of a suitable water-based paint, sometimes referred to as a non-blocking paint, is available from ProCoat Products, Inc. of Holbrook, Me. USA, sold under the trademark ProCoustic. To improve the uniformity of the finished appearance of the ceiling, the taped joints can be covered with strips of the veil fabric  29 , wide enough to cover the joint compound, prior to painting. The paint application should leave as much porosity through the layer  29  as is desired but leave the appearance of an essentially imperforate surface to the unaided eye so that the perforations  28  are not seen. Alternatively, where high NRC is not necessary, satisfactory results can be obtained by using a conventional primer and a coat of interior latex paint  31  to complete the installation of the ceiling  10 . When the term monolithic is used herein, it is to denote that essentially the entire visible surface of a ceiling or wall appears to be a seamless expanse without joints. 
     A ½ or ⅝ in. drywall-based panel  20 , having the described perforation arrangement and front and rear sheets  29 ,  30  and customary space behind the panel can exhibit NRC values up to and above 0.70, a rating equal to the performance of better-grade acoustical ceiling tile. 
     Presently, the preferred characteristics of the gypsum-based core  24  are:
         Thicknesses: 0.5-0.625 in.   Open area: 9.6-27.7%   Hole diameters: 6-12 mm.   Hole spacing: 15-25 mm.       

     Following are airflow characteristics of the backer layer  30  of the non-woven SOUNDTEX® material described above and the face layer  29  of the non-woven scrim material described above before and after painting with a proprietary acoustical coating and the acoustical ProCoustic coating. 
     
       
         
               
               
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
               
             
           
               
                   
               
               
                   
                   
                   
                   
                   
                   
                   
                 Airflow 
                   
                 Airflow 
                   
               
               
                   
                   
                   
                   
                   
                   
                   
                 Resistance R 
                 Specific 
                 Resistivity 
                   
               
               
                   
                   
                   
                   
                   
                   
                   
                 mks 
                 Airflow 
                 r o   
                 Airflow 
               
               
                   
                   
                   
                 P 
                   
                   
                   
                 acoustic 
                 Resistance r 
                 mks 
                 Resistivity 
               
               
                   
                 in. 
                 U 
                 in. 
                 v 
                 U 
                 P 
                 ohms, 
                 mks rayls, 
                 rayls/m, 
                 r o   
               
               
                   
                 thick 
                 l/min. 
                 H 2 O 
                 mm/s 
                 m 3 /s 
                 Pascal 
                 (Pa · s/m 3 ) 
                 (Pa · s/m) 
                 (Pa · s/m 2 ) 
                 MPa · s/m 2 ) 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 Backer 
                 0.009 
                 2.00 
                 0.0156 
                 16.4 
                 3.33E − 05 
                 3.9 
                 116,574 
                 236 
                 1.09E + 06 
                 1.09 
               
               
                 Unpainted 
                 0.019 
                 2.00 
                 0.0027 
                 16.4 
                 3.33E − 05 
                 0.7 
                 20,176 
                 41 
                 8.47E + 04 
                 0.08 
               
               
                 Scrim 
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 Painted 
                 0.020 
                 2.00 
                 0.0143 
                 16.4 
                 3.33E − 05 
                 3.6 
                 106,859 
                 217 
                 4.26E + 05 
                 0.43 
               
               
                 Scrim w/ 
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 Proprietary 
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 Coating 
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 Painted 
                 0.020 
                 2.00 
                 0.0144 
                 16.4 
                 3.33E − 05 
                 3.6 
                 107,606 
                 218 
                 4.29E + 05 
                 0.43 
               
               
                 Scrim w/ 
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 ProCoustic 
               
               
                   
               
             
          
         
       
     
     The tables printed below show NRC values for the inventive board and boards of other constructions for comparison purposes. As in the preceding table, unless otherwise noted, the backer is the SOUNDTEX® material and the face is the scrim identified above. 
     
       
         
               
             
               
               
               
               
               
             
           
               
                   
               
               
                 TEST I: 
               
               
                 *Perforated Panel = ⅝ in. FC30 (drywall) with ⅜″ diameter 
               
               
                 perforations, 16 mm o.c. spacing - 27.7% open area 
               
             
          
           
               
                   
                   
                 NRC 
                   
                   
               
               
                   
                 Panel Configuration 
                 Mounting 
                 4FA 
                 NRC 
               
               
                   
                   
               
               
                   
                 A Perforated panel only 
                 E400 
                 0.1967 
                 0.20 
               
               
                   
                 B Panel + backer 
                 E400 
                 0.6572 
                 0.65 
               
               
                   
                 BB Panel + backer used as 
                 E400 
                 0.6215 
                 0.60 
               
               
                   
                 unpainted face 
               
               
                   
                 H Panel + backer + unpainted 
                 E400 
                 0.7442 
                 0.75 
               
               
                   
                 scrim face 
               
               
                   
                 I Panel + backer + painted scrim 
                 E400 
                 0.7314 
                 0.75 
               
               
                   
                 face 
               
               
                   
                 E Panel + backer + paper face 
                 E400 
                 0.1978 
                 0.20 
               
               
                   
                 F Panel + backer + painted 
                 E400 
                 0.2963 
                 0.30 
               
               
                   
                 paper face 
               
               
                   
                 G Panel + painted scrim face 
                 E400 
                 0.5772 
                 0.60 
               
               
                   
                 K Panel + painted scrim face + 
                 E400 
                 0.6376 
                 0.65 
               
               
                   
                 unpainted scrim backer 
               
               
                   
                 C Panel + unpainted scrim face 
                 E400 
                 0.4028 
                 0.40 
               
               
                   
                   
               
             
          
         
       
     
     
       
         
               
             
               
               
               
               
             
           
               
                   
               
               
                 TEST II: 
               
               
                 *Perforated Panel = ½ in. Ultralight (drywall) with 6 mm 
               
               
                 diameter perforations, 15 mm o.c. spacing,      borders-hole 
               
               
                 pattern = 12.6% open area, overall panel = 9.6% open area 
               
             
          
           
               
                   
                 NRC 
                   
                   
               
               
                 Panel Configuration 
                 Mounting 
                 4FA 
                 NRC 
               
               
                   
               
               
                 Perforated panel only 
                 E400 
                 0.1937 
                 0.20 
               
               
                 Panel + backer + unpainted scrim face 
                 E400 
                 0.5947 
                 0.60 
               
               
                 Panel + backer + painted scrim face 
                 E400 
                 0.4825 
                 0.50 
               
               
                   
               
             
          
         
       
     
     
       
         
               
             
               
               
               
               
             
           
               
                   
               
               
                 TEST III: 
               
               
                 Panel A (small holes) = ½ in. Knauf 8/18R with 8 mm. diameter 
               
               
                 round perforations, 18 mm o.c. spacing &amp; no borders-15.5% open area 
               
               
                 Panel B (large holes) = ½ in. Knauf 12/25R with 12 mm. 
               
               
                 diameter round perforations, 25 mm o.c. spacing &amp; no borders- 
               
               
                 18.1% open area 
               
             
          
           
               
                 Panel Configuration 
                 NRC Mounting 
                 4FA 
                 NRC 
               
               
                   
               
               
                 Panel A only (with backer) 
                 E400 
                 0.6480 
                 0.65 
               
               
                 Panel B only (with backer) 
                 E400 
                 0.7191 
                 0.70 
               
               
                 Panel A + backer + unpainted scrim 
                 E400 
                 0.6245 
                 0.65 
               
               
                 face 
               
               
                 Panel B + backer + unpainted scrim 
                 E400 
                 0.6810 
                 0.70 
               
               
                 face 
               
               
                 Panel A + backer + painted scrim 
                 E400 
                 0.5782 
                 0.60 
               
               
                 face 
               
               
                 Panel B + backer + painted scrim 
                 E400 
                 0.5652 
                 0.55 
               
               
                 face 
               
               
                 Panel A + backer + painted scrim 
                 E400 
                 0.6192 
                 0.60 
               
               
                 face over 1 in. fiberglass panel 
               
               
                 Panel B + backer + painted scrim 
                 E400 
                 0.6031 
                 0.60 
               
               
                 face over 1 in. fiberglass panel 
               
               
                   
               
             
          
         
       
     
     Panel E of Test I had a heavy manila paper face with a basis weight of 263.50 gm/m 2 , a caliper of 17.22 mils, a density of 0.60 c/m 3  and a porosity of 58.97 seconds. This test sample illustrates that a face, although porous, but with too high an air flow resistivity is unsuitable for use with the invention. Panel BB of Test I indicates that a face with a higher air flow resistivity (see above table) than a painted scrim face can achieve a satisfactory NRC. 
     The acoustical panel of the invention can be manufactured in additional ways and with different constructions, but maintaining the perforations effectively restricted on at least the face (room) side of a completed panel. For example, where high NRC values are not needed, the rear layer  30  may be omitted. Porous paper may be substituted for either of the non-woven layers  29 ,  30 . 
     It has been further discovered that NRC can be measurably increased by orienting the perforations obliquely to the plane of the panel. Such a construction is illustrated in  FIG. 3 . The perforations  28  can, for example, be oriented at 20 degrees off a line perpendicular to the plane of the panel. The reason or reasons for this improved acoustical performance is not presently completely understood, but could be the result of a greater perforation volume and/or internal reflection of sound waves due to the oblique angle, and/or a greater effective open area at the face. 
     The foregoing disclosures involve modification of a conventional drywall sheet to convert it to the acoustical panel of the invention. However, the inventive acoustical panel can be originally manufactured with perforations in the gypsum core while it is being originally formed or immediately after it is formed and prior to attachment of one or both cover sheets or layers to its front face and rear side. The perforations, for example, can be cast into the gypsum body. The cross-section of the perforation in the various disclosed embodiments can be accircular when not drilled. 
     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 panel for forming a monolithic ceiling or wall, the panel extending across a rectangular area, and having a core made primarily of gypsum, the core being essentially coextensive with the panel area such that it has two opposed sides, each of an area substantially equal to the area of the panel, the core having a multitude of perforations extending generally between its sides, the perforations being distributed substantially uniformly across the full area of the core and being open at both sides of the core, the face side of the core being covered by a porous layer, the perforations being optionally restricted at a rear side of the core, the porous layer at the face side of the core being suitable for adherence of drywall joint compound and a water-based non-blocking paint.