Abstract:
An improved radio frequency wave attenuating wall (ceiling or floor) or door material comprises a laminated structure having as an integral part thereof one or more layers of a viscoelastic material which also functions as a glue and one or more electrically conducting layers. An electrically conducting material such as tape or a formed metal channel provides an electrical connection between the electrically conducting material and an exposed outer surface of the laminated structure. In one embodiment the electrically conducting material is paint. In one embodiment, standard wallboard, typically gypsum, comprises the external surfaces of the laminated structure and one or more conductive layers are constructed between the gypsum exterior. In one embodiment, the conducting layer material is selected to provide physical security in addition to radio frequency wave attenuation. The construction is such that acoustical attenuation is also achieved.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is related to U.S. patent application Ser. No. 10/658,814 filed on Sep. 8, 2003, by Kevin J. Surace and Marc U. Porat, entitled “Acoustical Sound Proofing Material and Methods for Manufacturing Same”, which is incorporated by reference herein in its entirety. 
     FIELD OF THE INVENTION 
     This invention relates to materials for improving the security of rooms and buildings and in particular to novel laminated construction materials which reduce radio frequency (“RF”) signal transmission compared to the RF signal transmission through normal building materials thereby to prevent undesired RF signal transmission from or into the protected room or building. 
     BACKGROUND OF THE INVENTION 
     The security of information, such as that transmitted by radio frequency waves, is emerging as a privacy, economic, security, and anti-terrorism issue. In spite of security technologies such as data encryption, information may be surreptitiously obtained by sensing the emissions of keyboards, wireless mouse pointing devices, computer monitors, security systems and such. Unintended listeners may receive the emissions of telephone systems outside the user&#39;s premises. Employees may make calls with cell phones and inadvertently transmit background sounds or conversations. Some restaurants, theaters, churches and other public places want to prevent cell phones or pagers from being used. In some cases the information represented by the signals may not be important, but the signals themselves interfere with nearby sensitive equipment. Thus it is important in many circumstances to contain emissions within, and/or to prevent emissions from penetrating, a certain room or building. 
     Prior solutions have typically used electrically conducting materials, such as wire mesh or sheet metal, to enclose the volume of interest. The conducting materials are mechanically and electrically connected together and then grounded. Installation is done by building an enclosure a layer at a time. For example, one method in use today is to weld steel plates together on six sides of a room, with an opening for a door, typically steel. This method is labor intensive, requires a highly trained labor force, may require heavy duty material handling equipment, and the resulting structure slowly loses its RF shielding ability over time as small cracks develop in the welds due to building motion and/or settling. 
     In areas requiring high security it is also desirable to be able to detect attempts to penetrate the protected area to, for example, install a secret listening or viewing device. 
     Accordingly, what is needed is a new material and a new method of construction to reduce the transmission of RF signals into or out of an enclosure which allows easier construction, does not deteriorate over time, and is tamper resistant. The ability to simultaneously reduce acoustic energy (i.e. sound) transmission from or into the enclosure is also desirable. 
     SUMMARY OF THE INVENTION 
     In accordance with this invention, a new laminated structure and associated manufacturing process is provided which significantly improves the ability of a wall, ceiling, floor or door to reduce the transmission of RF waves from or into a room. As an added feature, the materials of this invention resist tampering. 
     The materials of the laminated structure of this invention are similar to those described in the aforementioned U.S. patent application Ser. No. 10/658,814, but with the center material (FIG. 1 and FIG. 2 of the application Ser. No. 10/658,814) specified to be electrically conductive. As described in application Ser. No. 10/658,814, the outermost materials of the laminated structure of this invention (sometimes hereinafter referred to as a “panel” or “stack”) may be gypsum, wood, or other suitable materials. Relative to the laminated structure described in application Ser. No. 10/658,814, one embodiment of the present invention further comprises a conductive tape which is in electrical contact with the electrically conductive center material and which extends beyond at least one end of the laminated structure by an amount which will allow the tape to be wrapped around at least one end or edge of the structure and attached to an exterior surface of the structure. When the resulting structure, which in one embodiment is typically provided in 4×8 foot sheets, is attached to electrically conductive metal studs by electrically conductive screws (typically metal) which penetrate the conductive center material, the panel is thereby electrically connected to the metal studs. The metal studs are electrically connected to ground potential. 
     In accordance with an embodiment of the invention, a strip of electrically conductive tape is used to cover the seam where two adjacent panels come together. The electrically conductive tape covering the seam is in electrical contact with the electrically conductive tape extending from the abutting edge of each panel (this last tape is further connected with the electrically conductive center material). As a result electrical connection is provided between all of the panels so connected, as well as to the metal studs. The electrically conductive materials of the panels so connected are at essentially the same potential and when grounded or held at a fixed potential the net effect is to mitigate any impinging RF waves. 
     Walls and ceilings are typically constructed using panels having gypsum outer layers; floors and doors are typically constructed with panels having wood for the outer layers. Of course, other appropriate materials can be used for the outer layers if desired. 
     Another embodiment of the invention provides for room construction using wood instead of metal studs or floor joists. The panels are installed on the wooden studs in a similar fashion to their installation on metal studs as described above, but with one or more shorting bars located on the studs at one or more predetermined locations such that the metal screws attaching the panels to the studs will penetrate the one or more shorting bars, which in turn are grounded. In one embodiment the screws are wired together across the back side of the panels. 
     In another embodiment the panels are installed in at least two courses (i.e. two or more layers) wherein the panels of the second (outer) course are staggered so as to cover up the seams between the pairs of abutting panels of a first inner course. The metal screws attaching the second course panels also penetrate the conductive layer of the panels of the first course, thereby electrically connecting all panels. By covering the seams between panels of the first course, the second course of panels prevents RF signals from leaking through the seams. 
     While in one embodiment tape is used to cover all seams in the multi-layer structure of panels, alternative embodiments of this invention using two or more layers of panels omit the additional strip of conductive tape between panels of the first course, or omit the additional strip of conductive tape between panels of the second course, or omit the additional strip of conductive tape between the panels of the first and second courses. 
     As an added benefit of this invention, the panels of this invention not only attenuate RF signals but also attenuate acoustic signals. 
     If desired, three or more courses of panels can be used as part of a wall or ceiling or floor and the additional strips of conductive tape which cover the seams between abutting panels of each course can be omitted selectively from one or more of the courses. 
     An advantage of this invention is that the panels are all electrically connected together and to ground and seams between adjacent panels in each layer of panels are sealed either by electrically conductive tape or by an additional course of panels placed over the seams or by both tape and the additional course of panels. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       This invention will be described in more detail in conjunction with the following detailed description taken together with the drawings. 
         FIG. 1  shows a laminated structure fabricated in accordance with this invention for reducing the transmission of RF through the structure. 
         FIG. 2  shows another embodiment of a laminated panel fabricated in accordance with this invention and a method for attachment of the panel to a metal stud. 
         FIG. 3  shows a method for attaching adjoining panels to a metal stud. 
         FIG. 4  shows a method of attaching panels to a conductive, grounded shorting bar and non-conducting studs. 
         FIG. 5  shows a method of attaching multiple courses of panels to metal studs. 
         FIGS. 6A through 6H  are graphs showing the attenuation ability of the laminated panels of this invention as a function of the frequency of the signals being attenuated. 
         FIG. 7  shows an embodiment of this invention for electrically contacting a cut edge on a panel. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Definition of Terms: 
     
       
         
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 DCID 6/9 
                 A directive from the Director of the 
               
               
                   
                   
                 Central Intelligence (DCI) titled: 
               
               
                   
                   
                 Physical Security Standards for 
               
               
                   
                   
                 Sensitive Compartmented Information 
               
               
                   
                   
                 Facilities 
               
               
                   
                 PSA 
                 Pressure sensitive adhesive. 
               
               
                   
                 RF 
                 Radio frequency waves. 
               
               
                   
                 SCIF 
                 Sensitive Compartmented Information 
               
               
                   
                   
                 Facility: An accredited area, room, 
               
               
                   
                   
                 group of rooms, buildings, or 
               
               
                   
                   
                 installation where Sensitive 
               
               
                   
                   
                 Compartmented Information (“SCI”) may 
               
               
                   
                   
                 be stored, used, discussed, and/or 
               
               
                   
                   
                 processed. SCIF performance 
               
               
                   
                   
                 requirements and design details are 
               
               
                   
                   
                 given in DCID 6/9. 
               
               
                   
                 WIFI 
                 “Wireless fidelity”: popular term for 
               
               
                   
                   
                 a high-frequency wireless local area 
               
               
                   
                   
                 network (WLAN). 
               
               
                   
                 PANEL 
                 A laminated structure constructed in 
               
               
                   
                   
                 accordance with the present invention. 
               
               
                   
                   
                 The panel may be further attached to a 
               
               
                   
                   
                 structure defining a room or a wall. 
               
               
                   
                   
               
             
          
         
       
     
     The structure of  FIG. 1  is an example of the laminated structure of one embodiment of the present invention. The layers in the structure will be described from the top to the bottom with the structure oriented horizontally as shown. It should be understood, however, that the laminated structure of this invention will be oriented vertically when placed on vertical walls and doors, as well as horizontally or even at an angle when placed on ceilings and floors. Therefore, the reference to top and bottom layers is to be understood to refer only to these layers as oriented in  FIG. 1  and not to the actual use of this structure wherein this structure may be placed vertically, horizontally, or at an angle between vertical and horizontal. A detailed discussion of the method of construction and the materials of the laminated structure of  FIG. 1  and various alternative embodiments is given in the aforementioned U.S. patent application Ser. No. 10/658,814 and is not repeated here. 
     In  FIG. 1 , a portion of two adjacent panels  100 - 1  and  100 - 2  is shown. Note that the spaces shown between the layers and the edges of the panels are for illustrative purposes only. References to the “center material” or “center layer”, as in layer  110  of  FIG. 1 , are understood to mean the “electrically conductive layer” of the laminated layers of a panel, whether or not this layer is physically in the center. For example, in some embodiments the conductive layer is last in the stack of layers. As described in U.S. patent application Ser. No. 10/658,814, top layer  102  is glued to a center layer  110  using a thin viscoelastic adhesive  106 . The material of layer  110  is selected for its electrical conduction properties. In one embodiment, the conductive material of center layer  110  is a sheet of metal, such as silicon steel plus copper. In other embodiments center layer  110  can be a copper alloy or aluminum, or a steel sheet with an adjacent copper wire mesh, mu metal or the like. The thickness of the conductive layer  110  may be as thin as foil or up to about a half-inch thick, selected for a certain purpose depending upon the degree of rigidity, acoustic damping, RF signal transmission reduction or physical security desired. In one embodiment the conductive center layer  110  is conductive paint or conductive adhesive. 
     A conductive tape  122 , such as 3M-1345 obtained from 3M Manufacturing, is attached to the center layer. The tape that is preferred for use with this invention to seal the cracks between abutting panels is 3M-1345 tape which is described by 3M as “embossed tin-plated copper foil shielding tape”. As described in the 3M data sheet, this tape “consists of an embossed 1-ounce deadsoft tin-plated copper foil backing and an aggressive pressure-sensitive acrylic adhesive. The edges of the embossed pattern pressed into the foil cut through the adhesive layer to establish reliable metal-to-metal contact between the backing and the application substrate.” This tape is available in standard and custom widths and lengths. Widths vary from ¼ inch to 23 inches. This tape is intended for use in “applications requiring excellent electrical conductivity from the application substrate through the adhesive to the foil backing. Common uses include grounding and EMI shielding in equipment, components, shielded rooms . . . [and similar structures]. The tin plating on the copper coil backing facilitates soldering and improves resistance to oxidation and discoloration.” The 3M data sheet reports that “typical shielding effectiveness (far field) is in the range of 75 dB to 95 dB (30 MHz to 1 GHz).” 
     In accordance with this invention, the tape applied to the edges of the panels is long enough to extend completely along (i.e. to “span”) each edge of a panel (such as the edge which extends perpendicular to the plane of the paper in  FIG. 1 ) and is sufficiently wide such that a portion ( 122 - 2  and  122 - 3 ) of the tape  122  extends out from the edge far enough to be pulled around to the top surface of layer  102  as shown in  FIG. 1 . The conductive tape  122  may have on one surface thereof a PSA for convenience of construction. The PSA provides for the electrically conductive material of the tape to make electrical contact with a portion of a surface of electrically conductive layer  110  by rolling or otherwise applying pressure to the tape. The conductive tape is applied to and extended along at least the major (longer) edges of a panel. In some embodiments the conductive tape is applied to all edges. In another embodiment conductive tape  122  is replaced by a metal channel or strip. The channel or strip is sized to extend along an edge of a panel in electrically conductive contact with the center conductor  110  with a portion of the channel strip extending out from the edge and being bent to contact the front surface, as described above and in  FIG. 1  in conjunction with tape. The formed metal channel or strip is fastened to the layers of the structure using rivets, screws, PSA, or other electrically conductive attachment means. 
     If desired, a slight indentation can be provided in the portion of the surface of electrically conductive layer  110  to which the tape  122  or metal channel/strip is attached to make the outer surface of the tape  122  or metal channel/strip flush with the adjacent surface of layer  110 . 
     A thin layer of viscoelastic glue  114  is applied to the lower surface of center layer  110  so as to attach the center layer  110  to the bottom laminated layer  118 . In one embodiment layers  114  and  118  are omitted so that layer  110  is visible and is one external side of the panel  100 . 
     Upon installation, for example attaching 4×8 foot panels side by side to a wall, another layer  126  of conductive tape is affixed over the seam between the panels as shown in  FIG. 1 . Tape  126  electrically connects the tapes  122  of adjoining panels. Of course, in an actual structure, the tapes  122  of abutting panels will ideally be in physical contact. 
     The dimensions of a room utilizing panels constructed according to the invention may not be the same as the dimensions of one or more panels. Panels may be cut to any arbitrary size. However, such cutting exposes an edge of the laminated structure without conductive tape  122 . Referring to  FIG. 2 , conductive tape  122  is applied to extend over and along the edge exposed by cutting the panel so as to cover completely this edge. The edges  122 - 3  and  122 - 4  of the tape  122  are bent ninety degrees (90°) back over the panel  100  so as to electrically connect these edges to the edge-adjacent portions of the front and back surfaces of the panel  100 . An electrically conductive metal screw  204  is inserted through the top portion  122 - 3  of tape  122 , conductive layer  110 , and metal stud  206 , as shown in  FIG. 2 . A portion  122 - 4  of tape  122  may extend far enough across the bottom of layer  118  ( 110  in some embodiments) for screw  204  to penetrate this portion of tape  122 . The center portion  122 - 2  of tape  122  electrically contacts the edge of center layer  110  exposed by cutting. 
     In one embodiment, stud  206  may be of a nonconductive material and screws  204  may be grounded by other means, such as an electrically conductive strip of material  402  ( FIG. 4 ) extending along the face of the nonconductive stud  206  but in electrical contact with screw  204  and electrically grounded or held at a fixed potential. 
       FIG. 3  illustrates one installation of two panels  100 - 1  and  100 - 2  shown mounted with adjacent edges of the two panels  100 - 1  and  100 - 2  abutting and attached to a single metal stud  304 . A metal screw  204 - 1  attaches the laminated panel  100 - 1  to stud  304 . Screw  204 - 1  penetrates center layer  110 - 1 , thus completing an electrical connection between the center layer  110 - 1  and the grounded stud  304 . The space shown between the edges of panels  100 - 1  and  100 - 2  is for ease of explanation and, of course, does not exist in the actual structure. 
     Panel  100 - 2  is similarly attached to stud  304  by means by metal screw  204 - 2 . Metal screw  204 - 2  again penetrates through electrically conductive tape  126  and through panel center layer  110 - 2  of panel  100 - 2  extending into metal stud  304 . Thus the center layer  110 - 1  of panel  100 - 1  is electrically connected via electrically conductive metal screw  204 - 1 , electrically conductive metal stud  304  and electrically conductive metal screw  204 - 2  to center layer  110 - 2  of panel  100 - 2  and the two panels will be at the same electrical potential. In addition, electrically conductive tape  126  is placed over the two edges  103 - 1  and  103 - 2  of panels  100 - 1  and  100 - 2 . While screws  204 - 1  and  204 - 2  are shown as having their heads external to tape  126 , in an alternative embodiment these screws will have their heads covered by tape  126  and in some embodiments the heads will be countersunk into the panels  100 - 1  and  100 - 2  so that the tops of the heads are flush with the surfaces of these panels. Tape  126  will then lie flat over these countersunk heads. Electrically conductive tapes  122 - 1  and  122 - 2  of the panels  100 - 1  and  100 - 2  will be in electrically conducting contact with each other when the panels  100 - 1  and  100 - 2  are mounted on stud  304  such that edges  103 - 1  and  103 - 2  are in physical contact with each other (i.e. directly abut). The structure of  FIG. 3  thus ensures that panels  100 - 1  and  100 - 2  are electrically grounded and at the same electrical potential thereby to effectively reduce if not eliminate RF transmissions through these panels from one side to the other. 
     Referring to  FIG. 4 , in one embodiment wood studs  404  shown in side view are substituted for metal studs. A grounded conductive shorting bar  402  is arranged behind panels  100 - 1  and  100 - 2  and at least one metal screw  204  per panel  100  connects the center layer  110  (not shown) to shorting bar  402 . In another embodiment at least one metal screw  204  per panel  100  is wired to ground. 
     In an alternative embodiment to that shown in  FIG. 4 , the grounded conductive shorting bar  402  can be replaced by a vertical electrically conductive shorting bar (not shown) placed along each wooden stud such that screws  204 - 1   a  through  204 - 1   g  through panel  100 - 1  go through the electrically conductive shorting bar running parallel to and attached to the stud  404 - 2 . In this case, a separate electrical connection connecting each of the individual electric shorting bars can be provided although in one embodiment such electrical connection is inherently provided by center layer  110  of each panel  100 . 
     Furthermore, as shown in  FIG. 4 , a plurality of metal screws  204 - 1   a  through  204 - 1   g  are used to attach panel  100 - 1  to center stud  404 - 2 . Likewise, a similar plurality of electrically conductive metal screws  204 - 2   a  through  204 - 2   g  are used to attach panel  100 - 2  to the same stud  404 - 2 . As shown in  FIG. 4 , panels  100 - 1  and  100 - 2  are attached to stud  404 - 2  such that the directly adjacent edges of these panels covered respectively by tapes  202 - 1  and  202 - 2  mounted along the edges of the panels as described above, directly abut and therefore are in electrically conductive contact with each other. To ensure, however, that these tapes  202 - 1  and  202 - 2  are at the same potential, the electrically conductive screws  204 - 1   a  through  204 - 1   g  and  204 - 2   a  through  204 - 2   g  connect the tapes  202 - 1  and  202 - 2  to the center layers  110 - 1  and  110 - 2  within panels  100 - 1  and  100 - 2  respectively via grounding bars. Grounding bars  402  mounted horizontally across the studs  404 - 1 ,  404 - 2  and  404 - 3  as shown in  FIG. 4  are perpendicular to each of the studs  404 - 1  through  404 - 3  and ensure that the panels such as  100 - 1  and  100 - 2  are at essentially the same electrical potential at all points within the panels. 
     Although not shown in  FIG. 4 , electrically conductive tape  126  can be placed over the seam between panels  100 - 1  and  100 - 2  shown in  FIG. 4  to extend along the edge portions of tapes  202 - 1  and  202 - 2  which are visible in  FIG. 4  so as to cover these edge portions and the electrically conductive screws  204 - 1   a  through  204 - 1   g  and  204 - 2   a  through  204 - 2   g.    
     Referring to  FIG. 5 , an alternative construction is shown. To further attenuate any RF which might escape through the seams between two panels assembled, for example, in accordance with  FIG. 3 , two courses  502  and  503  of panels are utilized. A first course  502  of panels is secured to a stud assembly (per  FIG. 1  or  FIG. 3 ) using screws  204 . Tape  126  (shown over a seam between screws  204 - 3  and  204 - 4 ) is applied over each seam and also over the heads of countersunk screws in the middles of the second, outer, course of panels as shown in  FIG. 5 . In one embodiment tape  126  is omitted from the first course  502  of panels. The second, outer course  503  of panels is secured over the first course  502 . The screws  204  of the second course  503  are at least long enough to penetrate the center layer  110  of the panels of the first, inner, course  502 , thereby providing an electrical connection between the two courses. The first course  502 , having previously been physically and electrically connected to the studs, provides a grounding connection for the second course  503 . In one embodiment, the screws  204  of the second course are long enough to also make a direct electrical connection with the studs  205 - 1 ,  205 - 2  and  205 - 2  and to assist in holding both courses on the studs. Conductive tape  126  is applied over the seams of adjacent panels in the second course  503 . In one embodiment tape  126  is omitted. 
     Alternatively, tape  126  can be applied over all seams of both the first course  502  and the second course  503  if desired. The tape  126  when so applied will be pressed by the compressive forces generated by screws  126  being extended into the underlying studs such that the resulting structure is sufficiently rigid to provide structural integrity and to attenuate if not eliminate all RF transmissions through the wall. 
     In some embodiments other construction is used instead of studs. For example, the invention may be practiced by placing panels which have wood or other appropriate material for their outer layers upon foam, the foam being over a concrete floor. A foam thickness of ¼ to ½ inch is recommended. The panels are electrically connected to each other using in one embodiment electrically conductive strips of material placed on or in the foam as well as to the walls, all of which are held at a fixed potential, typically ground, thus providing the RF shielding effect. The foam improves the attenuation of sound. Other structures for electrically grounding the floor panels will be obvious to those skilled in the electrical arts. 
     The center material  110  of each panel may be selected not only for electrical properties but for physical strength as well. For example, a center material made of a certain thickness of sheet steel provides resistance to a blast, bullets, or other projectiles. Such a material also resists secretive drilling of a hole for the insertion of a sensing device, or at least makes such a penetration obvious upon inspection. 
     The laminated structure described provides a panel which may be handled by two people. Depending upon the material selected for center layer  110 , the panel may be cut with a conventional circular saw using blades intended for cutting wood. An RF attenuating room constructed using panels produced by practicing the invention is easier to construct and enjoys a lower total cost than equivalent solutions available today. 
     The laminated structure of the invention is consistent with some embodiments of the invention disclosed in U.S. patent application Ser. No. 10/658,814. Accordingly, panels constructed according to the present invention will attenuate sound as described in application Ser. No. 10/658,814. 
     Table 1 shows the estimated RF and acoustic attenuation provided by enclosures constructed using panels constructed in accordance with the present invention. 
     
       
         
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                   
                   
                 Acoustic 
                 Acoustic 
               
               
                   
                   
                 Standard 
                 standard 
               
               
                 Assembly 
                   
                 Achieved using 
                 achieved using 
               
               
                 method of room 
                 RF attenuation 
                 single studs 
                 double studs 
               
               
                   
               
             
             
               
                 Single layer 
                 up to 95 dB 
                 STC 54 
                 STC 66 
               
               
                 of panels for 
               
               
                 walls, ceiling 
               
               
                 and floor 
               
               
                 Double 
                 up to 120 dB 
                 STC 60 
                 STC 74 
               
               
                 overlapping 
               
               
                 panels for 
               
               
                 walls, ceiling 
               
               
                 and floor 
               
               
                   
               
             
          
         
       
     
     Panels constructed in accordance with the present invention, using a minimum 0.011 inch thick steel plate for center material  110 , are believed to be compliant with DCID6/9. 
     While the invention has been described in conjunction with complete panels as manufactured at the plant, in practice, panels will have to be cut on site to fit the particular sizes intended to be covered. When this is done, tape will not exist on the edge on the panel which is exposed by the cut. While  FIG. 2 , described above, shows one solution to this problem, another solution is shown in  FIG. 7 . As shown in  FIG. 7 , a conductive cord  127  fabricated of a mixture of butyl and nickel-coated carbon slivers or nickel filings inserted into the butyl up to about 80% by weight, is placed on the exposed metal edge of the internal electrically-conductive metal layer  110  in the panel. Once the butyl cord containing adequate conductive filings has been placed over the edge, a conductive tape  122  can then be placed over both the butyl cord and the remainder of the edge exposed by the cut. Each conductive tape  122  has one or two edge portions such as portions  122 - 1  and  122 - 3  which extend beyond the panel edge and thus can be folded over onto the adjacent portions of the surfaces of the panel. When two panels so cut are then abutted against each other as shown in  FIG. 7  (the space between the panels and the conductive tapes  122  on the edges of the panels is exaggerated for illustrative purposes) then a third conductive tape  126  can be placed over the seam between the panels  100 - 1  and  100 - 2  to electrically contact the bent edges  122 - 3  and  122 - 4  of the conductive tape on the edges thereby to ensure that the entire structure is at a fixed potential such as electrical ground. Such a structure then is highly effective in preventing the transmission of RF signals. 
     The conductive cord has been found by experiment to require approximately 80% by weight of the conductive metal filings such as conductive nickel filings or nickel-coated carbon slivers in order to be electrically conductive and thereby ensure that the internal electrically conductive metal layer  110  is at approximately the same potential (preferably ground) as the electrically conductive tape  122 . Other electrically-conductive metals can also be used, if desired, in cord  127 . As a result, the invention is capable of being used with cut panels as well as with prefabricated panels having the conductive tape already in contact with the internal electrically conductive layer  110 . As described above, a portion of the edge-connected conductive tape will extend beyond the panel for ease of electrical contact. 
     The effectiveness of this invention in reducing the transmission of RF signals through walls is shown in  FIGS. 6A through 6H .  FIGS. 6A through 6H  show the attenuation as a function of frequency of the RF signal varying from 19 MHz to 10 GHz for different structures identified at the top of each figure. Thus,  FIG. 6A  shows the attenuation for a laminated panel with no seams and horizontal antenna polarity to range from 80 dB at approximately 20 MHz to 100 dB just below 200 MHz and then dropping to between 60 to 70 dB at 1 GHz. Beyond 1 GHz the attenuation is shown to be relatively flat with negative and positive spikes as a function of frequency. 
       FIG. 6B  shows the attenuation for a laminated panel with no seams and vertical antenna polarity. The attenuation varies from approximately 76 dB at 20 MHz to as high as over 100 dB in the range of 100 MHz to about 180 MHz dropping in a spike back to 80 dB at 200 MHz and then remaining between 90 and 110 dB until approximately 800 MHz thereafter dropping to approximately 70 dB between 1 GHz and 10 GHz. 
       FIG. 6C  shows the RF signal attenuation for a laminated panel with a seam along the manufactured panel edges and horizontal antenna polarity. This structure shows attenuation varying from a little better than 80 dB at 20 MHz down to about 40 dB in the range of about 4½ GHz to 7 GHz. 
       FIG. 6D  shows the RF signal attenuation for a laminated panel with a seam along the manufactured panel edges with a vertical antenna polarity which has the attenuation varying from a little over 70 dB at 20 MHz to approximately 80 to 90 dB in the range of 100-200 MHz and then dropping to approximately between 50 dB and 60 dB in the 1 GHz to 10 GHz range. 
       FIG. 6E  shows the RF signal attenuation for a laminated panel with the seam along field modified panel edges and horizontal antenna polarity. The attenuation varies from between 50 dB and 60 dB for a frequency of 20 MHz to as high as 90 dB for a frequency of 200 MHz and then drops to a value of approximately 35 dB attenuation for 2 GHz climbing to approximately 70 dB attenuation for between 9 GHz and 10 GHz. 
       FIG. 6F  shows the RF signal attenuation associated with a laminated panel with a seam along field modified panel edges and vertical antenna polarity. The attenuation varies from 70 dB at 20 MHz to as high as 100 dB at approximately 200 MHz and then drops with certain spikes to as low as approximately 43 dB between 3 and 4 GHz. 
       FIG. 6G  shows the RF signal attenuation associated with a two-layer system of laminated panels, one continuous and one with a seam along field modified panel edges and with horizontal antenna polarity. The attenuation is shown to be much better using this structure than in some of the previous structures, varying from approximately 89 dB attenuation at 20 MHz to as high as 120 dB at 300 MHz and dropping to between 50 dB and 60 dB in the range of 2 GHz to 3 GHz and then rising again to approximately 82 dB or 83 dB at 10 GHz. 
       FIG. 6H  shows the RF signal attenuation associated with a two-layer system of laminated panels structured in accordance with this invention, one a continuous panel covering a seam in an underlying layer of panels created by abutting two field modified panel edges (such as shown in  FIGS. 2 and 7 ) using vertical antenna polarity. The attenuation varies from about 77 dB at 20 MHz up to a peak of between 105 to 120+ dB at approximately 150 mHz to 170 MHz with some gradual drop to within the range of 52 dB to 70 dB for signals with a frequency of 1 GHz to 10 GHz. 
     As can be seen from  FIGS. 6A to 6H , the structure of this invention significantly attenuates RF signals in frequency ranges commonly used with many types of communication systems. 
     The present disclosure is to be taken as illustrative rather than as limiting the scope, nature, or spirit of the subject matter claimed below. Numerous modifications and variations will become apparent to those skilled in the art after studying the disclosure, including use of equivalent functional and/or structural substitutes for elements described herein, and/or use of equivalent functional steps for steps described herein. Such variations are to be considered within the scope of what is contemplated here. For example, while the described structures are shown as rectangular in shape, structures with other shapes, such as circular, hexagonal or other polygonal shapes can also be used, if required or appropriate. This invention is not limited to any particular shape. 
     Thus the invention is not limited to the embodiments described above. 
     The following claims are not to be taken as limiting Applicant&#39;s right to claim disclosed, but not yet literally claimed subject matter by way of one or more further applications including those filed pursuant to 35 U.S.C. §120 and/or 35 U.S.C. §251.