Abstract:
An acoustical panel construction useful as a suspended ceiling tile having a rectangular shape bounded by edges and establishing a face area comprising at least one corrugated layer or layers of a total thickness, the layer or layers having a multitude of parallel flutes extending across an expanse of the rectangular shape substantially from one edge of the panel to an opposite edge, the flutes being formed by walls of the layer or layers and being of known volume, a series of apertures each of known area through the wall or walls of the flutes communicating with the atmosphere at the face, the aperture area, flute cavity volume associated with an aperture, and the total thickness of the corrugated layers associated with an aperture being arranged to produce a maximum absorption frequency between 200 and 2,000 Hz.

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to acoustical panels particularly suited for use in suspended ceilings. 
     PRIOR ART 
     Acoustical panels typically used as ceiling tiles or on walls, serve to absorb unwanted noise as well as to enclose a space and/or serve an architectural function. 
     Most conventional ceiling panels are made from a water-felting process or a water-based cast process. Usually a panel has a homogeneous porous core capable of absorbing sound. Lower cost products of these types are susceptible to sagging over time as a result of moisture absorption and have limited noise absorption capabilities measured as noise reduction coefficient (NRC). Higher grade products are typically more expensive to produce and can be relatively heavy. For the most part, water felted and water cast products exhibit relatively low sound absorption efficiency below 800 Hz. and are especially ineffective below 400 Hz. 
     SUMMARY OF THE INVENTION 
     The invention provides an acoustical panel formed of an apertured corrugated layer or layers with highly desirable sound absorbing properties. The panel is arranged to absorb those sound frequencies audible to the human ear and can be readily tuned to absorb sound in the lower frequencies of normal human hearing range. The invention is applicable to corrugated panels made of, for example, cardboard or plastic, either of which can be of a high recycled content. 
     The invention is based on the realization that corrugated panels perforated in a particular manner behave as pseudo Helmholtz resonating cavities able to produce relatively high NRC values and capable of being tuned to absorb a maximum of sound energy at a relatively low targeted frequency or frequencies. 
     More specifically, the invention relies on the discovery that the individual flutes of a corrugated panel can be treated like Helmholtz resonating cavities. By adjusting the relative size of the flutes, apertures, and aperture spacings, the frequency of maximum absorption can be determined. This frequency can be selected to target a specific noise or frequency band. Studies have shown corrugated panels can achieve ENRC (estimated noise reduction coefficient) as high as 0.8 with an absorption coefficient of 0.98 at a maximum absorption frequency below 600 Hz., for example. Moreover, these studies have shown a high correlation between classic Helmholtz cavity parameters and the analogous parameters discovered in the apertured corrugated acoustical panels of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an isometric view of a first embodiment of an acoustical panel constructed in accordance with the invention; 
         FIG. 1A  is a fragmentary view of the panel of  FIG. 1  on an enlarged scale; 
         FIG. 2  is a fragmentary isometric view of a second embodiment of the invention; 
         FIG. 3  is a fragmentary isometric view of a third embodiment of the invention; 
         FIG. 4  is a fragmentary isometric view of a fourth embodiment of the invention; 
         FIGS. 5 and 6  are graphs of the acoustical absorption properties of examples of panels constructed in accordance with the invention; 
         FIGS. 7 and 8  are graphs showing the linear correlation between calculated and observed absorption frequency of panels with apertures formed, respectively, by round holes or slits; and 
         FIG. 9  is a schematic illustration of a suspended ceiling system employing the acoustical panels of the invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the various embodiments disclosed below, the invention is applied to ceiling panels for use, ordinarily, with suspended ceiling grid. In the industry, such panels have nominal face dimension of 2′×2′ or 2′×4′ or metric equivalents. 
       FIG. 1  shows an acoustical tile or panel  10  formed of three layers of extruded corrugated plastic sheet. In this construction, each layer  11  has a pair of main walls  12  between which are webs  13  parallel with one another and perpendicular to the main walls  12 . Adjacent pairs of webs  13  and areas of the main walls  12  form flutes or elongated cavities  14  that extend from one edge  16  of a panel to an opposite edge  16 . The main walls  12  of abutting layers  11  are suitably bonded together with an adhesive, by welding or other technique. The layers  11  can be extruded polyethylene copolymer; a suitable source for the layers  11  is Coroplast™, Dallas, Tex. USA. Apertures  17  are drilled, punched, or otherwise formed in a face  18  of the panel  10  formed by an outer wall  12  of one of the layers  11  and all of the other walls  12  of the layers  11  except the layer at a rear face  19  of the panel opposite the face  18 . Thus, in the illustrated arrangement of  FIGS. 1 and 1A , each hole on the face  18  overlies a series of coaxial holes or apertures in the inner walls  12  of the sandwiched layers  11 . The panel of  FIG. 1  and other panels described below and illustrated in the drawings are inverted from a normal installed orientation when they are used in a suspended ceiling where the apertured face  18  will be facing downwardly towards the interior of a room. In practicing the invention, at least one and ordinarily more than one set of coaxial apertures is formed in each flute  14 . 
     It has been discovered that an apertured or perforated corrugated panel such as shown in  FIG. 1 , and on a larger scale  FIG. 1A , forms a series of pseudo Helmholtz resonating cavities. The classical Helmholtz formula for a cavity with a necked opening is: 
               j   H     =       ν     2   ⁢   π       ⁢       A       V   o     ⁢   L                 
where:
 
f H  is the resonance frequency;
 
ν is the speed of sound;
 
A is the cross-sectional area of the neck;
 
V o  is the volume of the cavity; and
 
L is the length of the neck.
 
     For the embodiment of  FIG. 1  and other embodiments including those discussed below, extensive research has demonstrated that certain dimensional parameters of the corrugation flutes and apertures are analogous to the dimensional parameters of the classic Helmholtz formula. These analogous parameters are: 
     area of aperture A o  correlates to A, the neck area; 
     internal volume V f  of a flute between adjacent apertures or holes (essentially a measure of two half flute volumes on each side of an aperture) correlates to V o ; 
     the distance T from the apertured face to the opposed blind wall, taken as the thickness of the panel, correlates to L. 
     A maximum absorption frequency of a panel can be determined in accordance with the invention using these correlated parameters in the classic Helmholtz equation. 
     Sound frequency audible to the human ear and that is of concern, for example, in the NRC rating ranges between 200 Hz and 2,000 Hz. While traditional water-felted or cast ceiling tiles absorb sound at the higher ranges of these frequencies, they are of very limited effectiveness at or below 400 or 500 Hz. Moreover, it is difficult to economically produce a traditional tile with an NRC value greater than 0.7. It has been found that apertured corrugated panels such as disclosed in  FIG. 1 , can be readily tuned for maximum absorption at selected frequencies between 200 and 2,000 Hz. Such panels can be especially useful, as compared to conventional tile construction, in targeting noise at 800 Hz. or less. By way of example, ENRC test samples using an impedance tube according to ASTM 384 on three-layer 10 mm Coroplast™ had the following results. 
     3-Layer Data—10 mm Coroplast™ 
     
       
         
               
               
               
               
               
               
               
             
           
               
                   
               
               
                 Number 
                 Hole 
                 Segment 
                 Number 
                 Max Abs 
                 Absorption 
                   
               
               
                 of Layers 
                 Diameter 
                 Length 
                 of Holes 
                 Freq (Hz) 
                 Coefficient 
                 ENRC 
               
               
                   
               
             
             
               
                 3 
                 0.075 
                 2.00 
                 16 
                 436 
                 0.694 
                 0.643 
               
               
                 3 
                 0.101 
                 2.00 
                 16 
                 526 
                 0.870 
                 0.716 
               
               
                 3 
                 0.128 
                 2.00 
                 16 
                 596 
                 0.982 
                 0.809 
               
               
                 3 
                 0.157 
                 2.00 
                 16 
                 676 
                 0.999 
                 0.588 
               
               
                 3 
                 0.199 
                 2.00 
                 16 
                 792 
                 0.982 
                 0.546 
               
               
                   
               
             
          
         
       
     
     The foregoing table shows the effect of aperture size on the maximum absorption frequency. The smaller the aperture or perforation, the lower the absorption frequency. 
     Maximum absorption frequency is affected by the spacing between apertures measured in the lengthwise direction of the flutes. The greater the spacing the greater the resonant cavity volume, and consistent with the analogy to Helmholtz&#39;s equation, the lower the frequency. 
     It can be demonstrated that as the panel is made thicker and therefore the effective parameter T analogous to the Helmholtz neck opening length L is increased, the maximum absorption frequency will decrease. 
       FIG. 2  represents a panel  20  as a second embodiment of the invention utilizing conventional cardboard that includes a corrugated paper sheet. Similar to the panel  10 , the panel  20  comprises several corrugated layers  21  with each layer comprising a flat paper sheet  22  and a curvilinear corrugated paper sheet  23  bonded to the flat sheet at contact lines  24  between flutes  26 . Apertures  27  are drilled, punched or otherwise formed through the corrugated and flat sheets of the layers  21  except the sheet  22  at a panel face  28  opposite a face  29  from which the apertures are formed. The apertures  27  through the several sheets  22 ,  23  are of the same size and are coaxial along an axis perpendicular to the faces  28 ,  29 . 
     The analogous parameters of the panel corresponding to the Helmholtz cavity resonant frequency equation are essentially the same as those given above in connection with the Coroplast™ 10. These analogous parameters are: 
     A o =the area of an aperture; 
     Vf=the volume of a flute taken as the cross-sectional area of a flute times the distance between apertures; 
     T=taken as the total thickness of the panel. 
     It is possible to form apertures through the various layers  21 , except for the last sheet, centered between the flutes  26  so as to utilize the spaces between the flutes as additional resonant cavities. 
     A third embodiment of an acoustic panel  30 , represented in  FIG. 3 , is similar to that of  FIG. 1  in that it comprises three extruded double wall corrugated layers  31 . All of the main walls, designated  32  and web walls designated  33 , except for the main wall on a rear face  34  of the panel  30  are cut with vertical slots or slits  36 , extending perpendicularly to the lengthwise direction of flutes  37  of the corrugated layers  31 . The slots  36  create individual apertures  38  for each of the flutes  37 . The analogous parameters of the panel  30  shown in  FIG. 3  are as follows: 
     A o =Aperture area is the slot width times the flute width, i.e. the distance between adjacent flutes; 
     Vf=the volume of a flute between slots  36 ; or half the flute volume on each side of a slot; 
     T=the thickness of the panel  30 . 
     Note that the flute volume relationship holds true for each of the disclosed embodiments. It is contemplated that the flutes could be blocked midway between the apertures extending along a flute such as by crushing or collapsing the walls locally and the same acoustic results would be obtained. 
       FIG. 4  illustrates an acoustical panel similar to the panel of  FIG. 3 . The panel  40  is constructed of corrugated cardboard like the panel of the embodiment of  FIG. 2 . Three corrugated cardboard, single wall layers are shown. The corrugations form flutes  42 . Flat walls  43  and corrugated sheets  44 , except for a flat wall on a rear face  45  of the panel  40  are cut through with vertical slots  46  perpendicular to the lengthwise direction of the flutes  42 . Where the slots  46  cross the flutes  42 , apertures  47  are formed. 
     The analogous parameters of the panel  40  are as follows: 
     A o =the width of the slot  46  times the distance between flutes; 
     Vf=the volume of a flute  42  between adjacent slots  46 ; 
     T=the thickness of a panel  40 . 
     Spaces  48  intervening the flutes  42 , being of substantially the same volume as the flutes, will absorb sound at substantially the same maximum absorption frequency as that of the flutes. 
     The panels illustrated in  FIGS. 1-4  are exemplary of applications of the invention. In these embodiments, three corrugated layers have been shown, but it will be understood that as few as one and as many of four layers have been found to be practical. 
       FIGS. 5 and 6  are graphs of the sound absorption characteristics of apertured corrugated acoustic panels constructed in accordance with the invention. It will be seen that the frequency of sound at maximum absorption is about 600 Hz in  FIG. 5  and about 900 Hz in  FIG. 6 . By adjusting the parameters of a panel, the maximum absorption frequency can be reduced or increased as desired. 
     As indicated, the flute cavities can be treated as pseudo Helmholtz resonating cavities that produce maximum sound absorption at the resonant frequency. Extensive studies have shown a high linear correlation between a calculated resonant frequency of maximum absorption using the analogous parameters discussed above. Examples of the correlation between calculated and observed frequency are shown in  FIGS. 7 and 8 . 
     If certain parameters are initially determined such as panel thickness, flute cross-sectional area, and distance along the flutes between apertures, two or more samples can be made with a different aperture size. A resonant or maximum absorption frequency can be calculated and be determined by empirical results for the samples. If an ideal actual resonance frequency is not obtained, with these samples, simple extrapolation of these data points can be used to modify the values of the analogous parameters to quickly reach a proper value of a selected variable or variables to obtain a desired maximum absorption frequency. By selecting the proper values of the analogous parameters, essentially any sound frequency between, say 200 and 2,000 Hz. can be established as a maximum absorption frequency. The invention, when practiced as described, is especially useful to produce a panel with a maximum absorption frequency at a value between 200 and 800 Hz. Sound absorption in this audible range is not readily obtained by traditional wet felted or cast ceiling tile. 
       FIG. 9  schematically illustrates a suspended ceiling of generally conventional construction, including metal runners or tees  49  forming a rectangular grid and acoustic panels  51  of the corrugated construction described above. Different panels  51  tuned to absorb different frequencies of, for example, 250, 500, 1,000 and 2,000 Hz. to thereby obtain a broad sound absorption range. Alternatively, a single panel can have a plurality of distinct areas that each provides different maximum absorption frequency. In either of the latter examples, a ceiling system can be designed to absorb sound through a broad human audible range. The apertured faces of the panels can be covered with an acoustically transparent scrim or veil to visually conceal the apertures. The hollow nature of the various disclosed panel embodiments permits them to exhibit the characteristics of a sandwich panel including a high stiffness in proportion to mass. Relatively high sag resistance is achievable, for example, by treating the paper forming the corrugations with humidity-resistant material. 
     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.