Abstract:
A wall-mountable acoustic device for absorption of sound in an indoor area is constructed of self-supporting interbonded rockwool mat material. The primary component made of the mat material is shaped in semi-cylindrical configuration. End panels of rockwool mat of similar composition are attached to the opposite ends of the semi-cylindrical configuration to enclose the configuration except for an open rear extremity of rectangular perimeter. When attached to a flat wall, a totally enclosed semi-cylindrical chamber is defined. In a preferred embodiment, further improvement of sound absorption properties is achieved by way of a diaphragm disposed within the semi-cylindrical configuration.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   This invention relates to devices employed for modifying the acoustic characteristics of large indoor areas bounded by vertical wall structure, and more particularly concerns a device which, when mounted upon at least one wall of a room achieves controlled selective diffusion and absorption of sound within said room. 
   2. Description of the Prior Art 
   It is often sought to diminish the noise level in indoor rooms, auditoriums, gymnasiums, restaurants, hallways, cafeterias, manufacturing plants and other indoor areas. In theaters where music is performed, the quality of the music heard by the audience is enhanced when the acoustic characteristics of the theater minimizes echoes, reverberations and ambient noise. 
   Various types of sound-absorbing rigid panel products have been employed as ceiling tiles, and various rigid and soft wall coverings have been disclosed for sound absorption. In most cases the sound-absorbing panels constitute a uniform array in their wall or ceiling installations. It has been found however, that panels intended to alter the characteristics of sound in an indoor enclosure are of greatest effectiveness when the nature and placement of the panels is custom-designed to accommodate the characteristics of the area being serviced and the type of sound encountered. 
   In situations where a customized sound-interactive system is being installed, it is often necessary to employ considerable trial and testing to optimize the system in terms of the types of panels employed, and their placement and interrelationships. An array of acoustic wall panels may, for example be comprised of an interactive assembly of different panels whose individual specific functions are to reflect, diffuse or absorb sound. With suitable trial and testing, the most suitable combination and arrangement may be found for the various panels. 
   Flat rectangular sound absorbing panels suitable for wall mounting in an abutting assemblage are disclosed in U.S. Pat. Nos. 5,644,872; 6,158,176 and elsewhere. Sound absorbing wall panels having trapezoidal or wedge shapes are disclosed in U.S. Pat. Nos. 5,141,073 and 6,209,680. Panels having a plurality of projections for the purpose of minimizing reflection of sound are disclosed in U.S. Pat. No. 3,498,405. Pyramidal panels for enhancing reflection of sound in an audience area are disclosed in U.S. Pat. No. 4,356,880. 
   U.S. Pat. No. 4,548,292, which concerns a floor-standing acoustic device of cylindrical shape adapted to be located in a corner of a room, discusses the difficulties in absorbing low frequency sounds, namely sounds having a frequency below 125 Hz. U.S. Pat. No. 4,319,661 discloses cylindrical acoustic devices equipped with Helmholtz resonators for absorption of low frequency sound. The Helmholtz resonators are generally defined to be comprised of a hollow chamber bounded in part by a perforated rigid panel. Although effective, Helmholtz resonators are usually heavy because of the nature of the rigid panel, which is generally of metal construction. 
   Although the aforementioned acoustic devices provide specialized advantages in selected installations, further improvement is needed, especially where the devices can provide versatility of performance in accommodating the specific requirements of different indoor areas. 
   It is accordingly an object of the present invention to provide a wall-mountable acoustic device for desirably modifying the subjectively perceived quality of sound in an indoor area. 
   It is another object of this invention to provide an acoustic device as in the foregoing object which is highly efficient in absorbing low frequency noise. 
   It is a further object of the present invention to provide an acoustic device of the foregoing object which is easily mountable upon a substantially flat wall surface. 
   It is a still further object of this invention to provide an assemblage of a plurality of the aforesaid acoustic devices uniformly mounted upon a vertical wall surface. 
   An additional object of the present invention is to provide an acoustic device of the aforesaid nature of light weight, fireproof construction amenable to low cost manufacture. 
   It is yet another object of this invention to provide an acoustic device of the aforesaid nature having an anesthetically pleasing appearance. 
   These objects and other objects and advantages of the invention will be apparent from the following description. 
   SUMMARY OF THE INVENTION 
   The above and other beneficial objects and advantages are accomplished in accordance with the present invention by a wall-mountable acoustic device for diffusion and absorption of sound in an indoor area, comprising:
     a) a self-supporting mat of compacted and interbonded rockwool fibers, said mat linearly elongated between opposite end extremities and bounded by 1) a convex exterior surface of circular cylindric shape extending 180° in circular curvature, 2) a concave interior surface substantially concentric with said exterior surface and extending 180° in circular curvature, 3) two diametrically opposed straight flat rear surfaces in parallel and coplanar juxtaposition, having identical widths which represent the thickness of the mat as measured orthogonally between said interior and exterior surfaces, and 4) opposed flat end surfaces having a semicircular perimeter,   b) a thin facing material tautly embracing said exterior surface and extending across said rear surfaces and onto said interior surface, and   c) an end panel of flat contour disposed upon each end surface and extending between said exterior surface and rear surfaces, whereby,   d) when said device is mounted upon a wall by way of abutment with said rear surfaces, said mat, end panels and wall define a fully enclosed internal chamber of semi-cylindrical configuration.   

   
     BRIEF DESCRIPTION OF THE DRAWING 
     For a fuller understanding of the nature and objects of the invention, reference should be had to the following detailed description taken in connection with the accompanying drawing forming a part of this specification and in which similar numerals of reference indicate corresponding parts in all the figures of the drawing: 
       FIG. 1  is a front, top and side perspective view of a first embodiment of the acoustic device of the present invention. 
       FIG. 2  is an enlarged top view thereof, with portions broken away to reveal interior details. 
       FIG. 3  is an enlarged lateral sectional view taken in the direction of the arrows upon line  3 — 3  of  FIG. 1 . 
       FIG. 4  is a vertical sectional view taken in the direction of the arrows upon line  4 — 4  of  FIG. 2 . 
       FIG. 5  is a front view of an assemblage of said acoustic devices mounted upon a wall as a uniformly spaced array. 
       FIG. 6  is a rear view of the acoustic device of  FIG. 1 . 
       FIG. 7  is a lateral sectional view of a second embodiment of the acoustic device of this invention. 
       FIG. 8  is a vertical sectional view of the embodiment of  FIG. 7 . 
       FIG. 9  is a perspective view of an assemblage of said acoustic devices arranged for testing purposes. 
       FIG. 10  is a plan view of a testing chamber room which accommodates the assemblage of  FIG. 9 . 
       FIG. 11  is a graphical presentation of data obtained by testing the assemblage of  FIG. 9 . 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Referring now to  FIGS. 1–4 , an embodiment of the acoustic device  10  of this invention is shown comprised of mat  11  assembled with end panels  14 , said assembly being covered by a facing material in the form of fabric  12 . 
   Mat  11  is self-supporting, constructed of compacted and interbonded rockwool fibers, and is linearly elongated between opposite end extremities  15 . Said mat is bounded by convex exterior surface  16  of circular cylindric shape extending 180° in circular curvature, concave interior surface  17  substantially concentric with said exterior surface and also extending 180° in circular curvature, two diametrically opposed straight flat rear surfaces  18  in parallel and coplanar juxtaposition, and opposed flat end surfaces  19  having a semicircular contour  13 . Said rear and end surfaces have an identical width  20  which represents the thickness of the mat, namely the orthogonally measured distance of separation between said interior and exterior surfaces. 
   The thickness of the mat may range between 1 and 3 inches, and the length of the mat, measured between end extremities  15 , may range between 2 and 6 feet. The diametric width of the mat, measured between the outer edges  21  of said rear surfaces, is preferably between 16 and 30 inches. The ratio of the length to diametric width of the mat is preferably between 1.4 and 3.0. The ratio of the thickness of the mat to the diametric width is preferably in the range of 0.06 to 0.12. 
   The rockwool fiber mat  11  has a density preferably between 5 and 9 pounds per cubic foot. The individual rockwool fibers of the mat are interbonded with a bonding agent typically of a thermoset chemical nature. Exemplary bonding agents include: phenol-formaldehyde, urea-formaldehyde and melamine-formaldehyde compositions. During manufacture, such compositions, in low viscosity aqueous formulations, are sprayed onto freshly formed rockwool fibers in a manner to achieve uniform treatment in a conveyor belt operation. The treated fibers are then pressed to the desired degree of compaction and routed through a curing oven where the water solvent is driven off and the bonding agent undergoes chemical cross-linking to a cured thermoset state. Sufficiently small amounts of the bonding agent composition is employed so as to avoid occlusion of the interstitial spaces between fibers. Because of its low viscosity, the formulation merely coats the fibers, and the coating flows along the fiber until it meets a cross contacting fiber. The formulation remains at the cross over site of said contacting fibers until curing occurs. By virtue of such method of interbonding, the intrinsic properties of the rockwool fibers are unaffected, and the collective characteristics of the mat are not compromised. The preferred amount of bonding agent in the rockwool mat is about 3% to 5% based upon the overall weight of the mat. Lesser amounts of bonding agent will not secure adequate integration of the mat, and greater amounts of bonding agent will diminish certain sought properties of the mat. 
   The expression “self-supporting”, as employed herein is intended to denote a structure which will retain its shape unaidedly. As definitive measures of the self-supporting nature of mat  11 , said mat, in flat form, will exhibit a sag of not more than ¼″ in 4 feet when horizontally supported at one end. It will also have a tensile strength of at least 2600 pounds per square foot, and a compressive modulus between about 300 and 500 pounds per square foot, measured at 10% compression. The rockwool fibers of said mat are preferably arranged in layers concentric with said interior and exterior surfaces. Such characteristics of the mat are of critical importance not only in achieving structural stability of the acoustic device, but also in achieving the sought specialized sound-modifying characteristics. 
   Top and bottom end panels  14 , having flat interior and exterior faces  27  and  28 , respectively, are adhesively secured to end surfaces  19 . Said end panels are preferably comprised of the same type of compacted interbonded rockwool composition that constitutes mat  11 . The thickness of said end panels, measured between said interior and exterior surfaces, is preferably similar to the thickness of mat  11 . Said interior and exterior faces have identical perimeters consisting of arcuate forward edges  33  congruent with convex exterior surface  16 , and straight rear edges  34  which define back surfaces  35 . Said back surfaces  35  are disposed in coplanar relationship with rear surfaces  18  of said mat in a rectangular configuration, as shown in  FIG. 6 , and are preferably hardened by way of treatment with a resin composition. Such hardening facilitates securement of the acoustic device to a wall by way of brackets that insert into said rear and/or back surfaces. 
   Fabric facing material  12  is preferably comprised of fiberglass, and may be of woven construction such as square weave, or a scrim or non-woven sheet stabilized by a flexible rear surface coating. Said fabric, with the aid of adhesive bonding, is caused to tautly embrace said convex exterior surface and top and bottom end panels  14 , and extend across said rear surfaces and onto said interior surfaces. The combination of fiberglass facing material disposed upon a rockwool structure causes such embodiment of the acoustic device to be totally fire-resistant. In other embodiments, the facing material may be a plastic film such as perforated polyvinylchloride. 
   The acoustic device of this invention, when tested for sound absorption by way of ASTM Test C423-90a, can provide a noise reduction coefficient (NRC) above, namely better than 1.20 at sound frequencies in the range of 50 Hz–125 Hz, and NRC in the range of 1.7 to 2.59 at sound frequencies above 125 Hz. 
   A further understanding of my invention will be had from a consideration of the following example which illustrates certain preferred embodiments. It is understood that the instant invention is not to be construed as being limited by said example or by the details therein. 
   EXAMPLE 1 
   An acoustic device of the present invention was selected for testing purposes, said device having a length, measured between said opposed end surfaces, of 36 inches, a width, measured between end extremities  15 , of 28 inches, a semi-circularly contoured mat of interbonded rockwool fibers having a density of 96.1 kg/m 3  (6 pounds per cubic foot) and thickness of two inches; top and bottom end panels  14  being fabricated of the same mat material; and an outside covering of Guilford Fabric FR701, Style 2100 adhered to the convex outer face of the mat by way of a thin layer of adhesive at the edges and returned to the rear interior surface of the mat. 
   Ten identical specimens of the aforesaid acoustic device were arranged on the floor  41  of a reverberation chamber  42  as shown in  FIGS. 9 and 10 . The reverberation chamber has a volume of 254 m 3 . Testing was conducted in accordance with Section 9.3 of ASTM C423-90a. 
   The decay rate of sound (which is inversely relative to sound absorption) was measured upon terminating a steady-state broadband pink noise signal within the reverberation chamber. Five ensemble averages containing 32 decays each were measured with both the test specimens inside of and removed from the chamber. The difference between these sound absorptors at a given frequency is defined as the sound absorption of the specimen. The Sound Absorption Coefficient is the sound absorption per unit area of the test specimens. The Noise Reduction Coefficient (NRC) is a four-frequency average of the Sound Absorption Coefficient. A rotation microphone boom and a Norsonic Instruments NI-830 Dual Channel Real Time Analyzer, computer controlled using custom software, were used for all measurements. Measurements were made in the ISO-Preferred one-third octave bands from 100 Hz to 5000 Hz. Data obtained from said testing is displayed in  FIG. 11 . 
   Said data indicate that the NRC of the acoustic device of this invention is better than 1.20 at sound frequencies below 125 Hz, and generally better than 1.70 at frequencies above 125 Hz. 
   The acoustic device of this invention is intended to be mounted upon a flat wall  30 , as shown in  FIG. 5  in a manner such that the long axis of the device is vertically oriented. A plurality of the devices, preferably of identical size, are preferably arranged in a uniformly spaced apart parallel array as an operating assemblage. The particular length and diameter of the devices is dictated by the size of the room and the type of sound modification sought. 
   The second embodiment of acoustic device of this invention, as exemplified in  FIGS. 7 and 8 , differs from the embodiment of  FIGS. 1–6  insofar as a resilient diaphragm  37  is disposed behind interior surface  17  of mat  11 . The exemplified diaphragm is of elongated shape, extending substantially the entire distance between, but not touching, end panels  14 , and is attached at its lateral extremities  38  to interior surface  17 . The manner of attachment is such as to cause the diaphragm to be flexed to an arcuate shape directed toward mat  11 . The separation distance between the diaphragm and interior surface, at the mid point  39  of the diaphragm is preferably between 1 and 5 inches. The diaphragm may be fabricated of stiff, but not rigid plastic sheet stock having a thickness between about 0.3 and 1.3 mm. A particularly suitable sheet stock is a resin-impregnated fiberglass sheet. Such material has a relatively low bending modulus but extremely high tensile modulus, causing it to be non-elastic. Because of its resilient nature, and the fact that it is suspended by its lateral extremities, the diaphragm is capable of undergoing vibration in response to sound energy applied thereto. 
   The diaphragm imparts to the acoustic device greater ability to absorb noise at low frequencies of 125 Hz and below. By way of comparison with the first embodiment, the second embodiment can provide NRC values better than 1.80 at sound frequencies in the range of 50 Hz–125 Hz. 
   While particular examples of the present invention have been shown and described, it is apparent that changes and modifications may be made therein without departing from the invention in its broadest aspects. The aim of the appended claims, therefore, is to cover all such changes and modifications as fall within the true spirit and scope of the invention.