Patent Publication Number: US-2006014455-A1

Title: Sound absorbing article

Description:
RELATED APPLICATIONS  
      This is a continuation in part of PCT Patent Application No. PCT/IL02/01065, filed Dec. 31, 2002, which claims priority from pending U.S. patent application Ser. No. 10/043,336, filed Jan. 14, 2002. All of these applications are hereby incorporated by reference as if fully set forth herein. 
    
    
     FIELD AND BACKGROUND OF THE INVENTION  
      The present invention relates to a sound absorbing article.  
      Sound reverberation in closed spaces, such as classrooms, offices, living areas, and cars is a significant contributor to background noise. Studies in. acoustics and speech intelligibility have shown that as reverberation is reduced, speech intelligibility improves. Thus, controlling reverberant sound is important not only for comfort, but also for improved communication in schools, workplaces, homes and automobiles.  
      Sound reverberation is controlled by incorporating sound absorbers to the interior design of the closed space. The sound absorbers may be acoustic wall panels, ceiling panels, office partitions, rug liners, automotive hood liners and door liners, or liners for air-conditioning systems.  
      There are several methods for evaluating the sound-absorbing characteristics of a sound absorber. Their descriptions may be found, for example, in the web site, “Summary of Acoustic Testing Methods,” Aero-Acoustics Laboratory, www.industrialacoustics.com/RDMETH.htm. A specific example is ASTM C423, “Sound Absorption and Sound Absorption Coefficient, by the Reverberation Room Method,” leading to measured values of sound absorbing coefficients at different sound frequencies.  
      A sabin is a unit of sound absorption. The sabin absorption is defined as the sum of absorption due to objects and surfaces in a room, and due to dissipation of energy in the medium within the room. In a reverberation chamber of a volume V, the speed of sound c, and a reverberation decay rate d, the sabin absorption is computed as A=0.921Vd/c in metric units.  
      The sound absorption of a given material is computed as the difference in sabin absorptions, for each frequency band, with and without the material under test present in the reverberation chamber. The sound absorption coefficient for the given material is its sound absorption, for each frequency band, divided by the surface area of the given material.  
      In general, sound absorbers are evaluated by an overall parameter, a Noise Reduction Coefficient (NRC), which is an arithmetic average of the sound absorption coefficients at 250, 500, 1000, and 2000 Hz. However, for some applications, absorption of a characteristic noise, for example, the noise of a helicopter rotor, requires absorption at a specific range of frequencies, for example, the low range. The sound absorbers are then evaluated at the specific range of frequencies for the application.  
      “Modeling of Hors and Enclosures for Loudspeakers,” by Gavin R. Putland, Department of Electrical and Computer Engineering, University of Queensland, described in http://www.users.bigpond.com/putland/phd/thes.pdf, provides a detailed analogy between an acoustic circuit and an electrical circuit. Accordingly, the sound absorption characteristics of a material are described as acoustic impedance, a complex quantity consisting of frequency dependent components called acoustic resistance and acoustic reactance.  
      ASTM C384, “Impedance and. Absorption of Acoustical Materials by the Impedance Tube Method,” is based on this analogy. It is a relatively simple procedure that measures the sound absorbing properties of small samples of acoustic materials placed inside a long rigid tube. Normal-incidence sound-absorption coefficients are derived from measurements of the standing waves developed when a signal tone is generated in the tube. The method is useful for comparing and evaluating different sound absorbers.  
      According to “The Fridge Architectural Science Lab,” School of Architecture and Fine Arts, The University of Australia, Online Information and Course Note, by Marsh, A., 1999, http://fridge.arch.uwa.edu.au/topics/acoustics/rooms/absorpton.html, a distinction has to be made between sound absorption, that is, the fraction of sound energy that is actually converted to heat, and the absorption coefficient, which is the fraction of sound energy that is not reflected The absorption coefficient describes the fraction of sound energy that is either transmitted or absorbed. This distinction is of concern when the sound source is outside the enclosed space, but is less important for applications wherein the sound source is within the enclosed space, and sound reverberation is of importance.  
      According to Marsh, pervious materials, such as fiberglass, polymeric fiber blankets, and polymeric foams are commonly used as sound absorbers. They are most effective at high frequencies, of short wavelengths, where conversion to heat is produced by friction when vibrating air molecules are forced through and interact with the internal structure of these materials. Sound Absorption may be improved largely by increasing the thickness of the material, or by increasing the resistance to airflow. The latter may be achieved, for example, by increasing the specific weight of the material, or by decreasing the average pore or cell size of foam.  
      U.S. Pat. No. 5,431,996, to Gieseman, describes a composite material of one or more preformed reinforcement materials, co-influencing the final shape and made of tension-resistant organic and/or inorganic material, a second material of alkali water glass and a finely disperse mineralic filler, with hardening having been effected by drying at 80 to 120 degrees C., possibly with subsequent tempering at 400 to 700 degree C. The process for producing the composite material and its use as a fire-proof, bending tension-resistant construction element formed as desired is disclosed Since Giesemann is interested in producing structural elements, which may be used, for example, as paneling, he soaks the fibrous material several times, to achieve maximum strength and water proofing, plugging all the pores in the material.  
      U.S. Pat. Nos. 5,459,291 and 5,824,973, both to Haines et al., describe a method of using a thin, semi-porous film membrane, of controlled airflow resistance, to augment the airflow resistance of an underlying porous insulation. The increased airflow resistance of the laminate results in superior sound absorption properties of the laminate when compared to the porous insulation substrate without the semi-porous membrane.  
      U.S. Pat. No. 4,152,474, to Cook, et al., describes an acoustic absorber and a method for absorbing sound, utilizing a substrate having a plurality of openings therethrough. An organic polymer coating covers the substrate and partially fills the openings in the substrate to form an acoustic absorber having a porosity not greater than 60 CFM per square foot.  
      Abd Technology, whose products may be found at www.abd11c.com/prod01_absorption.htm, offers acoustical foams with different types of film membranes, such as Urethathane film membrane or metalized Mylar film membranes. Unlike the laminate of U.S. Pat. Nos. 5,459,291 and 5,824,973, these are impervious to airflow. Additionally Abd Technology offers a composite, formed of a vinyl barrier, sandwiched between two sheets of foam  
      U.S. Pat. Nos. 5,934,338 and 6,057,378 to Perstev, et al. describe a process for improving the thermal insulation properties of open-cell polymeric foam, by soaking it in a coating solution, which contains particles of a size less than the minimum diametrical length of the passages. The particles, dispersed within the passages, partly block the flow of air between adjacent cells. In this manner, the thermal insulation properties are improved  
      According to “The Fridge Architectural Science Lab,” by Marsh, hereinabove, at low frequencies, membrane absorbers may be used. These may be flexible sheets, stretched over supports or rigid panes, mounted at some distance from a solid wall. Conversion to heat takes place through the resistance of the membrane to rapid flexing and through the resistance of the enclosed air to compression. These, depend on the density of the membrane and on the width of the enclosed space.  
      Polymeric foams, fiberglass and mineral wool are commonly used sound absorbers, and their sound absorption characteristics are continuously being improved. Relevant data are shown in Table 1, for Fibrous Glass 4 and open-cell Polyurethane Foam, based on “Noise Control—Technical Information,” htp://www.tpcdayton.com/NoiseConrol/tech_info/ntech.htm, as follows.  
                       TABLE 1                                      Frequency, Hz                                             Material   125   250   500   1000   2000   4000   NRC               1″ Fibrous   .07   .23   .48   .83   .88   .80   .60       Glass 4       2″ Fibrous   .20   .55   .89   .97   .83   .79   .81       Glass 4       4″ Fibrous   .30   .91   .99   .97   .94   .89   .95       Glass 4       ½″   .05   .12   .25   .57   .89   .98   .46       Polyurethane       Foam       (open cell)       1″   .14   .30   .63   .91   .98   .91   .70       Polyurethane       Foam       (open cell)       2″   .35   .51   .82   .98   .97   .95   .82       Polyurethane       Foam       (open cell)                  
 
      As seen in Table 1, reasonable sound absorption, of NRC values of at least 0.80 may be achieved with a sound absorber that is 5 centimeters in thickness. But when good sound absorption in the low frequency range is also desired, a sound absorber of 10 centimeters in thickness may be needed. These values are rather large for many applications. They present a drawback both in terms of space requirement for the sound absorber and ease of installation  
      Additionally, mineral wool is a synthetic mineral fiber, a fibrous inorganic substance made primarily from rock, clay, slag or glass. Synthetic mineral fibers, such as fiberglass (glasswool and glass filament), mineral wool (rockwool and slagwool), and refractory ceramic fibers (RCF), are believed to cause respiratory cancers and other adverse respiratory effects. Therefore, attempts are made to limit their manufacturing and use.  
      Polymeric foams, on the other hand, may ignite and may produce toxic fines when ignited.  
      There is thus a widely recognized need for, and it would be highly advantageous to have, a sound absorber devoid of the above limitations.  
     SUMMARY OF THE INVENTION  
      According to one aspect of the invention, there is provided a sound absorbing article, comprising:  
      a material which is pervious to air, and which is characterized by proximal and distal surfaces with respect to a sound source, an internal structure, and a specific weight; and  
      an inorganic coating, which is applied in a controlled manner and which adheres to the surfaces and internal structure, increasing the specific weight by a controlled, predetermined factor, the factor being less than 7, so as to maintain a previousness to the material.  
      According to an additional aspect of the invention, the material is a fibrous material.  
      According to an additional aspect of the invention, the fibrous material is fire-proof.  
      According to an additional aspect of the invention, the fibrous material is nonwoven.  
      According to an additional aspect of the invention, the material is a stitch bond.  
      According to an additional aspect of the invention, the material is between 0.4 and 5.0 mm thick.  
      According to an additional aspect of the invention, the material is between 0.7 and 3.0 mm thick.  
      According to an additional aspect of the invention, the material is between 1.0 and 2.0 mm thick.  
      According to an additional aspect of the invention, the coating comprises a silicate compound.  
      According to an additional aspect of the invention, the coating comprises water glass.  
      According to an alternative aspect of the invention, the coating comprises a mixture of silicate compounds.  
      According to an additional aspect of the invention, the coating further comprises a flame-retardant agent mixed therewith.  
      According to an additional aspect of the invention, the flame-retardant agent is water soluble.  
      According to one aspect of the invention, there is provided a method of manufacturing a sound absorbing article, comprising:  
      employing a material, which is pervious to air, and which is characterized by proximal and distal surfaces with respect to a sound source, an internal structure, and a specific weight; and  
      applying to the material, in a controlled manner, an inorganic coating, which adheres to the surfaces and internal structure, increasing the specific weight by a controlled, predetermined factor, the factor being less than 7, so as to maintain a previousness to the material.  
      According to an additional aspect of the invention, the applying for includes applying to the material, in the controlled manner, the inorganic coating, so as to optimize sound absorption properties for a particular frequency range.  
      The present invention successfully addresses the shortcomings of the presently known sound absorbers by providing a sound absorbing article, produced by impregnating a pervious material with a coating, in a controlled manner, so as to increase the specific weight of the material by a controlled, predetermined factor, while maintaining the pervious nature of the material. As a consequence, the resistance to air flow through the material is increased The sound absorbing article is formed of materials which are flame retardant and environmentally friendly. The sound absorbing article may be optimized to a particular application and frequency range.  
      The sound absorbing article of the present invention is advantageous over presently known sound absorbers, because of a unique design which combines at least two physical effects of sound absorption: conversion of sound to friction and heat, on the one hand, as vibrating air molecules are forced through and interact with an internal structure of a pervious material, and conversion of sound to mechanical energy, on the other, as vibrating air causes a flexible sheet, stretched over supports, to vibrate. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The invention is herein described, by way of example only, with reference to the accompanying drawings. With specific reference now to the drawings in detail, it is stressed that the particulars shown are by way of example and for purposes of illustrative discussion of the preferred embodiments of the present invention only, and are presented in the cause of providing what is believed to be the most useful and readily understood description of the principles and conceptual aspects of the invention. In this regard, no attempt is made to show structural details of the invention in more detail than is necessary for a fundamental understanding of the invention, the description taken with the drawings making apparent to those skilled in the art how the several forms of the invention may be embodied in practice.  
      In the drawings:  
       FIGS. 1A-1D  are illustrations of sound-absorbing articles, according to preferred embodiments of the present invention;  
       FIGS. 2A-2B  are illustration of apparatus for applying a coating to a sound absorbing article, according to preferred embodiments of the present invention;  
       FIGS. 3A-3B  are illustrations of apparatus for bonding a membrane to a sound absorbing article, according to preferred embodiments of the present invention;  
       FIG. 4  is an illustration of a sound-absorbing article according to another preferred embodiment of the present invention;  
       FIGS. 5A and 5B  illustrate, in tabular forms, experimental results for sound absorbing articles formed of nonwoven polyester, coated with water glass, according to preferred embodiments of the present invention;  
       FIG. 6  illustrates, in graphical forms, the experimental results of  FIGS. 5A and 5B ;  
       FIGS. 7A and 7B  illustrate, in tabular forms, experimental results for sound absorbing articles formed of nonwoven polyester, coated with a mixture of water glass and hydrated alumni according to other preferred embodiments of the present invention;  
       FIG. 8  illustrates, in graphical forms, the experimental results of  FIGS. 7A and 7B ;  
       FIGS. 9A and 9B  illustrate, in tabular forms, experimental results for sound absorbing articles formed of open-cell foam, coated with a mixture of water glass and hydrated alumina, according to still other preferred embodiments of the present invention;  
       FIG. 10  illustrates, in graphical forms, the experimental results of  FIGS. 9A and 9B ;  
       FIGS. 11A and 11B  illustrate, in tabular forms, experimental results for sound absorbing articles formed of nonwoven polyester, coated with a mixture of water glass and hydrated alumina, bonded to a membrane at varying distances, according to yet other preferred embodiments of the present invention;  
       FIG. 12  illustrates, in graphical forms, the experimental results of  FIGS. 11A and 11B ;  
       FIGS. 13A and 13B  illustrate, in tabular forms, experimental results for sound absorbing articles formed of nonwoven polyester, coated with a mixture of water glass and hydrated alumina, attached to a honeycomb, according to other preferred embodiments of the present invention;  
       FIG. 14  illustrates, in graphical forms, the experimental results of  FIGS. 13A and 13B ;  
       FIGS. 15A and 15B  illustrate, in graphical forms, the experimental results of various types of nonwoven fabrics;  
       FIGS. 16A and 16B  illustrate, in tabular forms, sound-absorption experimental results of  FIGS. 15A-15B ; and  
       FIG. 17  is a photograph of a nonwoven stitch bond material, illustrating the holes formed by the stitches. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
      The present invention is of a sound absorbing article, produced by impregnating a pervious material with a coating, in a controlled manner, so as to increase the specific weight of the material by a controlled, predetermined factor, while maintaining the pervious nature of the material. As a consequence, the resistance to air flow through the material is increased The sound absorbing article is formed of materials which are flame retardant and environmentally friendly. The sound absorbing article may be optimized to a particular application and frequency range.  
      The sound absorbing article of the present invention is advantageous over presently known sound absorbers, because of a unique design which combines at least two physical effects of sound absorption: conversion of sound to friction and heat, as vibrating air molecules are forced through and interact with an internal structure of a pervious material, and conversion of sound to mechanical energy, as vibrating air causes a flexible sheet, stretched over supports, to vibrate.  
      Before explaining at least one embodiment of the invention in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of the components set forth in the following description or illustrated in the drawings. The invention is capable of other preferred embodiments or of being practiced or carried out in various ways. Also, it is to be understood that the phraseology and terminology employed herein is for the purpose of description and should not be regarded as limiting.  
      Referring now to the drawings,  FIG. 1A  illustrates a sound-absorbing article  10 , according to a preferred embodiment of the present invention. Sound absorbing article  10  is formed of a material  12 , which is pervious to air flow, and which is characterized by proximal and distal surfaces  14  and  16 , with respect to a sound source  15 , a width d, an internal structure  18 , and a specific weight W (not shown).  
      According to a preferred embodiment of the present invention, material  12  comprises a fibrous material. Further according to a preferred embodiment of the present invention, material  12  is nonwoven. However, according to other preferred embodiments of the present invention, material  12  may comprise another fibrous material or foam as will be described hereinbelow.  
      According to a preferred embodiment of the present invention, width d is between 1 and 2 mm, for example, 1.6 mm. However, according to other preferred embodiments of the present invention, width d may be less than 1 mm, for example, 0.4 mm, or less. Alternatively, width d may be greater than 2 mm, and may be as large as needed for a specific application. For example, width d may be 3 mm, or 50 mm, or greater than 100 mm.  
      As seen in  FIG. 1A , material  12  firer comprises a coating  20 , which adheres to surfaces  14  and  16  and to surfaces of internal structure  18 , so as to increase specific weight W. Preferably, specific weight W is increased by a factor that yields optimal sound absorption characteristics for a specific application. According to a preferred embodiment of the present invention, specific weight W is increased by a factor between 3 and 9. However, according to other preferred embodiments of the present invention, specific weight W may be increased by a factor of 1.25, or smaller, or by a far greater factor, for example, 10, or 12, or greater.  
      As will be described hereinbelow, in conjunction with  FIG. 2A , coating  20  may be formed by soaking material  12  in a liquid coating solution  48  of a liquid adhesive, so as to impregnate material  12  with coating  20 , then allowing material  12  to dry. Alternatively, as will be described hereinbelow, in conjunction with  FIG. 2B , coating  20  may be formed by spraying material  12  with coating solution  48  of a liquid adhesive, so as to impregnate material  12  with coating  20 , then allowing material  12  to dry.  
      Coating  20  is a novel feature of the present invention. According to “The Fridge Architectural Science Lab,” School of Architecture and Fine Arts, The University of Australia, Online information and Course Note, by Marsh, A., 1999, http://fridge.arch.uwa.edu.au/topics/acoustics/rooms/absorpton.html, sound absorption characteristics of materials, which are pervious to air flow, may be improved by increasing the resistance to air flow. The resistance to airflow, in turn, is increased with increasing specific weight. Coating  20  is operative to increase the specific weight of material  12  by a predetermined factor.  
      According to a preferred embodiment of the present invention, coating  20  is inorganic, and comprises a silicate compound, for example, water glass.  
      Water glass is chiefly produced as sodium silicate. It is a colorless, transparent, glasslike salt, available commercially as a water-soluble powder or as a transparent, viscous solution in water. Chemically it is any one of several compounds containing sodium oxide, Na2O, and silica, Si2O, or a mixture of sodium silicates. The sodium silicates may be, for example, Sodium orthosilicate (Na4SiO4 or 2Na2O.SiO2), sodium metasilicate (Na2SiO3 or Na2O.SiO2), sodium disilicate (Na2Si2O5 or Na2O.2SiO2), and (or) sodium tetrasilicate (Na2Si4O9 or Na2O.4SiO2). All these compounds are transparent, glassy or crystalline solids that have high melting points (above 800° C.) and are water soluble. They are produced chiefly by fusing sand and sodium carbonate in various proportions, or by heating sodium hydroxide with sand under pressure. Sodium silicate is very soluble in water. It hardens to a film of high adhesion, and high resistance to heat, weather, and fire.  
      Water glass is also commercially available as potassium silicate, produced, for example, by fusing sand and potassium carbonate in various proportions, or by heating potassium hydroxide with sand under pressure. Similarly, water glass is commercially available as lithium silicate. These products are also very soluble in water. They too harden to films of high adhesion, and high resistance to heat, weather, and fire.  
      Additionally, water glass is commercially available as a mixture, for example of sodium silicate and potassium silicate.  
      Additionally, other silicate compounds, or mixtures of silicate compounds may be used to form coating  20 . For example, Cesium oxythiomolybdate, Cs2MoOSi3, which is a solid lubricant film at high temperatures, of about 600° C. may be used to form coating  20 . It is a mostly amorphous film with excellent film adhesion Similarly, calcium silicate, which hardens to an amorphous silica film which is heat resistant to temperatures of about 1,500° C. and which is highly weather resistant, may be used to form coating  20 . Additionally, other silicate compound, or a mixture of several silicate compounds may be used to produce coating  20 .  
      According to other preferred embodiments of the present invention, coating  20  may be formed of other substances or mixtures that have adhesive properties, so as to adhere to surfaces  14  and  16  and to surfaces of internal structure  18 , and increase specific weight W. These may be, for example, natural resins, chemically modified natural resins, synthetic resins, and a mixture of these. For example, coating  20  may comprise acrylic adhesives, other polymeric adhesives, or other known adhesives. In particular, an acrylic adhesive known as T1633, which is flame retardant, or another flame retardant resin may be used.  
      Additionally, according to a preferred embodiment of the present invention, coating  20  may be selected based on its heat, fire, and weather resistance for a particular application, or based on its resistance to specific environmental conditions, for example, vapor, or acid fumes.  
      Other features of  FIG. 1A  are described hereinbelow, in conjunction with “Additional Features of FIGS.  1 A- 1 D”.  
      Referring further to the drawings,  FIG. 1B  illustrates a sound-absorbing article  10 , according to a second preferred embodiment of the present invention. Sound absorbing article  10  is formed of a material  12 , which is pervious to air flow, and which is coated with a coating  23 , comprising a mixture of an adhesive and a flame-retardant agent. Coating  23  is operative to adhere to surfaces  14  and  16  and to surfaces of internal structure  18 , and increase specific weight W, while acting as a flame retardant.  
      Preferably, the flame-retardant agent is mixed with an adhesive, in a liquid form, to make coating solution  48  ( FIGS. 2A and 2B , hereinbelow). The mixture composition may be predominantly adhesive, or predominantly flame-retardant agent, but sufficient adhesive is used in the mixture to ensure good adhesion to material  12 , to form a coating. Thus, the flame-retardant agent and the adhesive may be mixed so that the flame-retardant agent forms between 10 and 90% of the mixture. Alternatively, smaller or greater percent values may be used.  
      According to an article by “The National Academies Office of News and Public Information”, edited by Hicks, C., and Roberts, T., and produced online by Solhem S,.www4.nationalacademies.org/news.nsf/isbn, on Apr. 27, 2000, eight flame-retardant chemicals can safely be used on upholstered furniture, while posing little or no health risk to people who may be exposed to them in the home. The eight chemicals include the aforementioned alumina trihydrate and zinc borate and further include, hexabromocyclododecane, decabromodiphenyl oxide, magnesium hydroxide, ammonium polyphosphates, phosphoric acid, and tewalds hydroxymethyl phosphonium chloride. Although toxicity data for some of them are inadequate for certain routes of exposure, these chemicals were found to be safe even under the worst-case exposure assumptions. In accordance with preferred embodiments of the present invention, any of the aforementioned eight chemicals may be used as the flame-retardant agent. Additionally, other flame-retardant agents, or fire and flame-retardant agents that pose little or no health risk may be used.  
      For example, the flame-retardant agent may comprise hydrated alumina, such as aluminum trihydroxides, Al(OH)3. Hydrated alumina is a non-smoking, low toxicity halogen free flame retardant When a plastic, treated with hydrated alumina is exposed to fire, the hydrate begins to decompose endothermically into water and anhydrous alumina. The water acts as a heat sink, cooling the plastic and significantly slowing its degradation into combustible fuel.  
      Alternatively, Zinc Borate, which is non-toxic, flavorless, odorless, non-corrosive, and non-instant, having the molecular formula, 2Zn0.3B2O3.3.5H2O2, or the molecular formula 2Zn0.3B2O3.7H2O, may be used.  
      Alternatively, Seize Fyre 5050, which is a water-soluble co-polymer blend of ammonium polyphosphates may be used It&#39;s supplier is Seize Fyre, www.firenomore.com/flameretardantsapplications.htm.  
      In accordance with other embodiments of the present invention, any known flame retardant or fire and flame-retardant agent may be used.  
      In accordance with some embodiments of the present invention, the flame retardant or fire and flame-retardant agent may be soluble in liquid coating solution  48 , ( FIGS. 2A and 2B , hereinbelow.)  
      Other features of  FIG. 1B  are described hereinbelow, in conjunction with “Additional Features of FIGS.  1 A- 1 D”.  
      Referring now to the drawings,  FIG. 1C  illustrates a sound-absorbing article  10 , according to a another preferred embodiment of the present invention, wherein material  12  is a foam.  12 , which is pervious to air flow. Foam  12  Her includes proximal and distal surfaces  14  and  16 , internal structure  18  and specific weight W (not shown). Foam  12  is coated with a coating  20 , operative to adhere to surfaces  14  and  16  and to surfaces of internal structure  18 , and increase specific weight W. Coating  20  may be formed of a silicate compound, such as water glass, or another adhesive, as has been described hereinabove, in conjunction with  FIG. 1A .  
      Other features of  FIG. 1C  are described hereinbelow, in conjunction with “Additional Features of FIGS.  1 A- 1 D”.  
      Referring now to the drawings,  FIG. 1D  illustrates a sound-absorbing article  10 , according to a another preferred embodiment of the present invention, wherein material  12  is foam  12 , coated with a coating  23 , comprising a mixture of an adhesive and a flame-retardant agent and operative to adhere to surfaces  14  and  16  and to surfaces of internal structure  18 , and increase specific weight W, while acting as a flame retardant, as has been described hereinabove, in conjunction with  FIG. 1B .  
      Additional Features of  FIGS. 1A-1D , in accordance with preferred embodiments of the present invention, are as follows:  
      A membrane  22  is attached to material  12 . Preferably membrane  22  is unpervious to airflow, and is attached only at selected bonding locations  26 . Thus, channels  28  are formed between material  12  and membrane  22 . Additionally, in accordance with the present invention, channels  28  are interconnected, allowing air to pass through them.  
      Furthermore, membrane  22  is preferably attached to distal surface  16 .  
      Membrane  22  is another novel feature of the present invention. As air, flowing through material  12 , strikes membrane  22 , it causes membrane  20  to vibrate as a flexible sheet, thus converting sound energy to mechanical energy and further increasing the sound absorption characteristics article  10 . Additionally, membrane  20  increases the overall resistance of article  10  to airflow, since the air must force its way through interconnected channels  28 , formed between membrane  22  and material  12 , encountering friction so as to add to the conversion of sound absorption energy to heat.  
      According to a preferred embodiment of the present invention, membrane  22  is formed of polyethylene, and has a thickness t of substantially 20μ. According to other preferred embodiments of the present invention, membrane  22  may comprise a natural rubber, a chemically modified natural rubber, a synthetic polymer, a metal foil, Mylar, PVC, a metalized polymer, a laminated sheet of metal and polymer, or another known flexible material, which is impervious to airflow Further according to other preferred embodiments of the present invention, membrane  22  may be formed to a thickness between 5 and 40μ. Alternatively, smaller or greater thickness values may be used.  
      According to other preferred embodiments of the present invention, membrane  22  may be attached to proximal surface  14 . Additionally, membrane  22  may be semipervious.  
      According to a preferred embodiment of the present invention, bonding locations  26 , at which membrane  22  is attached to material  12 , may be formed as bonding points  26 , and may be evenly distributed, with distances X′ between points. Alternatively, bonding points  26  may be distributed unevenly.  
      Additionally, bonding points  26  may be evenly distributed, with distances X′ between points in a first direction (as shown in  FIGS. 1A-1D ) and with distances Y′ between points in a second direction, orthogonal to the first direction (running into the paper in  FIGS. 1A-1D , but shown hereinbelow, in conjunction with  FIG. 3A ).  
      Preferably, both distances X′ and Y′ are substantially 1.5 cm. However, according to other preferred embodiments of the present invention, points  26  may be closer to each other, or farther apart, and distances X′ and Y′ need not be the same. For example, distance X′ may be 0.4 cm, and distance Y′ may be 3 cm. In accordance with the present invention, distances X′ and Y′ may be between 0.1 cm and 20 cm. Alternatively, smaller or greater distances may be used.  
      In accordance with another preferred embodiment of the present invention, bonding locations  26  are formed as bonding lines  26 , with distances X′ between them. Alternatively, any other geometry of bonding membrane  22  to material  12  at selected locations may be employed. For example, broken lines  22 , in a first direction, or a mixture of broken lines in a first direction and an orthogonal direction. Alternatively, bonding locations  26  may be randomly distributed on distal surface  16  or proximal surface  14 .  
      Referring further to the drawings,  FIG. 2A  illustrates apparatus  40  for is applying coating  20  ( FIGS. 1A and 1C ) or coating  23  ( FIGS. 1B and 1D ) to material  12 , according to a preferred embodiment of the present invention. Preferably, uncoated material  12  unravels from a spool  42  onto a conveyer belt  44 , which leads it onto a bath  46  of a coating solution  48 , for soaking, preferably, until material  12  is thoroughly soaked  
      Material  12  exits bath  46 , via conveyer belt  44 , which includes a roller system  50 , having first and second rollers  51  and  53 , set with a spacing r between them, operative to wring out excess solution  48 . According to a preferred embodiment of the present invention, the factor by which specific weight W is increased is predetermined by distance r of roller system  50 . Additionally, distance r may be varied to control the increase in specific weight.  
      Material  12  continues to travel on conveyer belt  44  for a predetermined period of time to air dry. Additionally, an air blower system  54  may be used to speed up the drying process. When dried, coated material  12  may be rolled unto a spool  56 .  
      According to the present invention, coating solution  48  comprises a liquid adhesive, for example, water glass dissolved in water, or a liquid acrylic adhesive, or any other adhesive described in conjunction with  FIGS. 1A and 1C , in its liquid form, to form coating  20 .  
      Alternatively, according to the present invention, coating solution  48  may further comprise the flame-retardant agent, or a fire and flame retardant agent, such as water-soluble Seize Fyre 5050, or hydrated alumina, or any other flame-retardant agent, or a fire and flame retardant agent, described in conjunction with  FIGS. 1B and 1D , to form coating  23 .  
      Referring further to the drawings,  FIG. 2B  illustrates alternative apparatus  41  for applying coating  20  ( FIGS. 1A and 1C ) or coating  23  ( FIGS. 1B and 1D ) to material  12 , according to another preferred embodiment of the present invention.  
      In accordance with the present embodiment, uncoated material  12  unravels from spool  42  onto conveyer belt  44 , which runs under a spray system  49 , for spraying coating  48  onto material  12 , at a predetermined rate. The spraying rate of spray system  49  and the travel rate of conveyer belt  44  together determine the factor by which specific weight W is increased. Material  12  may be air dried by air blower system  54 . When dried, coated material  12  may be rolled unto spool  56 .  
      It will be appreciated that coating  48  may be applied to material  12  at the manufacturing site of material  12 , for example, during the manufacturing process of material  12 , or at a manufacturing site of sound absorbing article  10 .  
      It will be appreciated that another known system for impregnating material  12  with coating solution  48  may be used. Additionally, impregnating may be performed by hand.  
      Referring further to the drawings,  FIG. 3A  illustrates apparatus  60  for attaching membrane  22  to material  12 , according to a preferred embodiment of the present invention.  
      Preferably, material  12  unravels, for example from spool  56  ( FIG. 2A ) onto a conveyer belt  62 . A drip system  64  drips a bonding liquid  66  onto distal surface  16  of material  12 , forming bonding locations  26 , in the form of bonding points  26 .  
      According to a preferred embodiment of the present invention, drip system  64  comprises a plurality of dripping devices  74 , arranged with distance X′ between any two devices  74 . Thus, the dripped points are also arranged with distance X′ between two points, in a first direction. Additionally, dripping devices  74  drip bonding liquid  66  at a specific dripping rate. The dripping rate, together with a travel rate of conveyer belt  62  determine distance Y′ between two points, in a direction orthogonal to the first direction.  
      Thus, the density of points  26  on distal surface  16  may be controlled by varying the number of dripping devices  74  and the distance between them, or by varying the dripping rate, or varying the travel rate of conveyer belt  62 .  
      Membrane  22  is unraveled from a spool  70 , and is pressed against surface  16  of material  12 , by a roller  72 , bonding to material  12  at locations  26 . Thus, channels  28  are formed between material  12  and membrane  22 .  
      Referring further to the drawings,  FIG. 3B  illustrates apparatus  61  for attaching membrane  22  to material  12 , according to another preferred embodiment of the present invention, wherein bonding locations  26 , are formed as parallel bonding lines  26 , arranged with distance X′ between two lines.  
      It will be appreciated that any other geometry of bonding membrane  22  to material  12  at selected locations may be employed For example, dripping system  74  may be arranged to form broken lines  26 , by varying the dripping rate. Additionally, or alternatively, dripping system  74  may be rotated or moved across material  12  to form swirls of bonding locations, or lines or broken lines in a first direction and in another direction. Alternatively, dripping system  74  may be arranged to randomly drip bonding liquid  66  on material  12 .  
      It will be appreciated that another known system for bonding membrane  22  to material  12  may be used. Additionally, bonding may be performed by hand.  
      It will be appreciated that apparatus  60  or  61 , or another system of applying bonding locations to material  12  may similarly be used for applying bonding locations to proximal surface  14 .  
      It will be appreciated that apparatus  40  ( FIG. 2A ) or  41  ( FIG. 2B ) on the one hand, and apparatus  60  ( FIG. 3A ) or  61  ( FIG. 31 ) on the other hand, may be combined into a single apparatus, for coating material  12  and bonding membrane  22  onto material  12  in a single apparatus.  
      Referring further to the drawings,  FIG. 4  illustrates a sound absorbing article  10 , according to a second preferred embodiment of the present invention, wherein sound absorbing article  10  for comprises a rigid honeycomb  30 , arranged between coated material  12  and membrane  22 . Rigid honeycomb  30  comprises a height h and an effective cell diameter c.  
      Rigid honeycomb  30  is another novel feature of the present invention, operative to provide sound absorbing article  10  with stiffness, making it self-supporting.  
      According to the preferred embodiment of the present invention, rigid honeycomb  30  is formed of Kraf paper, for example, of between 80 and 220 gram/m 2 . Alteratively, other weight values may be used. Its effective cell diameter c, may be between 0.5 and 3 cm, preferably, 1.5 cm, and its height h may be between 0.5 and 6 cm, preferably, 1.5 cm. However, according to other preferred embodiments of the present invention, rigid honeycomb  30  may be formed of a rigid plastic, or another rigid material, and may be formed to other dimensions.  
      Additional objects and advantages of the present invention will become apparent to one ordinarily skilled in the art upon examination of the following experimental results of specific examples, presented in tabular and graphical forms, in  FIGS. 5A-14 , without intending to be limiting, as follows:  
       FIGS. 5A and 5B  illustrate, in tabular forms, experimental results for sound absorbing articles  10  ( FIG. 1A ) formed of a nonwoven polyester, with and without membrane  22 . Material  12  has a thickness d of substantially 1.6 mm and is coated with water glass of sodium silicate, to different specific-weight gains, according to preferred embodiments of the present invention. Membrane  22  is formed of polyethylene, to thickness t of substantially 20μ.  
       FIG. 6  illustrates, in graphical forms, the experimental results of  FIGS. 5A and 5B .  
      As seen from  FIGS. 5A-5B  and  6 , coating  20  has an appreciable effect on the NRC values. Whereas the uncoated sound absorbing article has an NRC value of substantially 0.30, that coated to a specific-weight gain factor of 5.2 has an NRC value of substantially 0.59, about twice the uncoated value. The effect of coating  20  reaches a maximum at a specific-weight gain factor of substantially 5.2.  
      Furthermore, membrane  22  has an additional effect, increasing the NRC values from substantially 0.30 to substantially 0.69 for uncoated materials, and from substantially 0.48 to substantially 0.83 for material coated to a specific-weight gain factor of 3. The combined effect of coating  20  and membrane  22  reaches a maximum at a specific-weight gain factor of substantially 3.  
       FIGS. 7A and 7B  illustrate, in tabular forms, experimental results for sound absorbing articles  10  ( FIG. 1B ) formed of a nonwoven polyester, with and without membrane  22 . Material  12  has a thickness d of substantially 1.6 mm and is coated with, a mixture of about 60% water glass of sodium silicate and about 40% hydrated alumina, by weight, to different specific-weight gains, according to preferred embodiments of the present invention. Membrane  22  is formed of polyethylene, to thickness t of substantially 20μ.  
       FIG. 8  illustrates, in graphical forms, the experimental results of  FIGS. 7A and 7B .  
      When comparing  FIGS. 7A-7B  and  8  with  FIGS. 5A-5B  and  6 , it appears that there is a small effect to the composition of the coating, for example, the composition of coating  20  (FIGS.  1 A,  5 A- 5 B and  6 ), compared with that of coating  23  (FIGS.  1 B,  7 A- 7 B and  8 ). Thus for coating  20 , a maximum NRC value of 0.59 is obtained, at a specific-weight-gain factor of 5.2, while for coating  23 , a maximum NRC value of 0.71 is obtained, at a specific-weight-gain factor of 5.7. However, this effect becomes insignificant with the addition of membrane  22 , yielding maximum NRC values of substantially 0.83, for specific-weight-gain factors between 2 and 4 for both coating  20  and coating  23 .  
       FIGS. 9A and 9B  illustrate, in tabular forms, experimental results for sound absorbing articles  10  ( FIG. 1D ) formed of an open-cell polyurethane foam of 18 kg/m 2 , with and without membrane  22 . Material  12  has a thickness d of substantially 4 mm and is coated with a mixture of about 40% water glass of sodium silicate and about 60% hydrated alumina, by weight, to different specific-weight gains, according to preferred embodiments of the present invention. Membrane  22  is formed of polyethylene, to thickness t of substantially 20μ.  
       FIG. 10  illustrates, in graphical forms, the experimental results of  FIGS. 9A and 9B .  
      As seen from  FIGS. 9A-9B  and  10 , coating  23  has little effect on foam. Both the uncoated and the coated sound absorbing articles have NRC values of substantially 0.36. However, the addition of membrane  22  has a significant effect, which increases with the specific-weight-gain factor. Thus, at a specific-weight-gain factor of 8.2 the NRC value of the foam reaches 0.79, compared with 0.30 for uncoated foam with no membrane  22 , and compared with 0.69 for uncoated foam with membrane  22 .  
       FIGS. 11A and 11B  illustrate, in tabular forms, experimental results for sound absorbing articles  10  ( FIG. 1B ) formed of a nonwoven polyester, with membrane  22 , bonded at varying distances X′ between bonding points  26 . Material  12  has a thickness d of substantially 1.6 nm and is coated with a mixture of about 60% water glass of sodium silicate and about 40% hydrated alumina, by weight, according to preferred embodiments of the present invention. Membrane  22  is formed of polyethylene, to thickness t of substantially 20μ.  FIG. 11A  relates to a specific-weight gain of a factor of 3.7, and  FIG. 11B  relates to a specific-weight gain of a factor of 5.3.  
       FIG. 12  illustrates, in graphical forms, the experimental results of  FIGS. 11A and 11B .  
      As seen in  FIGS. 11A-11B  and  12 , the optimal value for X′ is 1.5 cm.  
       FIGS. 13A and 13B  illustrate, in tabular forms, experimental results for sound absorbing articles  10  ( FIG. 4 ) formed of a nonwoven polyester, with and without membrane  22 . Material  12  has a thickness d of substantially 1.6 mm and is coated with a mixture of about 60% water glass of sodium silicate and about 40% hydrated alumina, by weight, to different weight gains, according to preferred embodiments of the present invention Membrane  22  is formed of polyethylene, to thickness t of substantially 20μ. Honeycomb  30  is formed of kraf paper of 147 g/m 2  wherein height h is 2 cm and effective cell diameter c is 1.5 cm.  
       FIG. 14  illustrates, in graphical forms, the experimental results of  FIGS. 13A and 13B .  
      When comparing  FIGS. 13A-13B  and  14  with  FIGS. 7A-7B  and  8 , which have no honeycomb, it appears that honeycomb  30  does not effect the NRC values for the examples without membrane  22  and lowers them somewhat for the example with membrane  22 . The purpose of honeycomb  30  is to give sound absorbing article  10  stiffness and structural strength, while maintaining reasonable NRC values.  
      The present invention further provides for optimizing a sound absorbing article for a particular application and a specific frequency range, by selecting an article of maximum or desired sound absorption coefficient from  FIGS. 5A-14 , or similarly obtained figures. For example, with regard to  FIG. 5A , although a maximum NRC value is obtained at a specific-weight-gain factor of 5.2, for the frequency range of 250 Hz, the maximum sound absorption coefficient is obtained at a specific-weight-gain factor of 6.1. A designer may choose to optimize either the NRC value or the coefficient at a specific frequency, or weigh one against the other.  
      According to a preferred embodiment of the present invention, material  12 , which is pervious to air, may comprise a fibrous material.  
      Further according to a preferred embodiment of the present invention, fibrous material  12  may comprise natural fibers, for example, wool, linen, cotton, canvas, cannabis, reed, weed, straw, stalks, seaweed, another known natural fiber, and a blend thereof.  
      According to another preferred embodiment of the present invention, fibrous material  12  may comprise fibers derived from cellular materials, for example, Rayon, Viscose, another known modified cellular fiber, and a blend thereof. Alternatively, material  12  may comprise fibers derived from cellular materials, such as wood pulp, organic matter, recycled paper, recycled organic waste, recycled cellular fiber, and mixtures thereof.  
      According to yet another preferred embodiment of the present invention, fibrous material,  12  may comprise synthetic polymeric fibers, for example, synthetic polymeric fibers, for example, Polyethylene, Polypropylene, Nylon, Polyester, Kevlar®, Nomex®, Polyacrylonitrile, Polyurethane, another known synthetic polymeric fiber, and a blend thereof.  
      According to still another preferred embodiment of the present invention, fibrous material  12  may comprise polymeric Aramids such as Kevlar®, Nomex®, or blends thereof, so as to produce a fireproof material  12 . Alternatively, another known fiber, which is fireproof, may be used. Additionally or alternatively, fibrous material  12  may comprise fibers, which are fame retardant, or fire and flame retardant.  
      According to yet another preferred embodiment of the present invention, fibrous material  12  may comprise a blend of at least two of the aforementioned fibers, for example, cotton and polyester.  
      According to a preferred embodiment of the present invention, fibrous material  12  is knotted, for example, as a rug.  
      According to another preferred embodiment of the present invention, fibrous material  12  is woven.  
      According to yet another preferred embodiment of the present invention, fibrouss material  12  is nonwoven.  
      Additionally, nonwoven material  12  may be any one of the following: stitch bond, possibly with unidirectional or multiaxial reinforcing, needlepunch, thermo bond, air lay, wet lay, pressed felt, including SAE grade felt, chemically bonded felt, spunbond, spunlace, meltblown, waddings, battings and (or) other known nonwoven materials.  
       FIGS. 15A and 15B  illustrate, in graphical forms, sound-absorption experimental results of different types of nonwoven materials, as follows: stitch bond, needlepunch, and thermo bond Similarly,  FIGS. 16A and 16B  illustrate, in tabular forms, sound-absorption experimental results of  FIGS. 15A-15B .  
      As seen in  FIGS. 15A and 16A , for uncoated materials of varying weight and thickness, the stitch bond nonwoven material is distinctly better than the other types of nonwoven materials, namely, needlepunch, and thermo bond, in two respects.  
      1. Peak NRC value for stitch bond is highest −0.78 for stitch bond, versus 0.63 for needlepunch and 0.48 for thermo bond; and  
      2. Material weight at peak NRC value is lowest, 300 gm for stitch bond, versus 400 for both needlepunch and thermo bond  
      As seen in  FIGS. 15B and 16B , for materials of varying weight and thickness, coated to a weight-gain factor of 2, the stitch bond nonwoven material is again distinctly better than the other types of nonwoven materials, namely, needlepunch, and thermo bond, in three respects.  
      1. Peak NRC value for stitch bond is highest −0.92 for stitch bond, versus 0.72 for needlepunch and 0.61 for thermo bond;  
      2. Material weight at peak NRC value is lowest, 300 gm for stitch bond, versus 400 for both needlepunch and thermo bond;  
      3. Coating weight at peak NRC value is similarly lowest, 300 gm for stitch bond, versus 400 for both needlepunch and thermo bond;  
      In consequence, the use of stitch bond nonwoven material results in highest NRC values, lowest initial material weight and lowest coating weight, so as to be both the highest NRC value and the lowest cost option.  
       FIG. 17  is a photograph of a nonwoven stitch bond material, illustrating the holes formed by the stitches. It may by that the regular hole structure has a beneficial effect on sound absorption.  
      According to still another preferred embodiment of the present invention, fibrous material  12  may comprise fiberglass, for example, glasswool or glass filament.  
      According to yet another preferred embodiment of the present invention, fibrous material  12  may comprise mineral wool, for example, rockwool or slagwool.  
      According to still another preferred embodiment of the present invention, fibrous material  12  may comprise refractory ceramic fibers (RCF).  
      According to yet another preferred embodiment of the present invention, fibrous material  12  may comprise a blend of at least two synthetic wools, selected from fiberglass, mineral wool and RCF.  
      According to a preferred embodiment of the present invention, material  12  may comprise foam.  
      Additionally, according to a preferred embodiment of the present invention, material  12  may comprise an open-cell foam.  
      Further according to a preferred embodiment of the present invention, foam  12  comprises natural rubber.  
      According to another preferred embodiment of the present invention, foam  12  comprises chemically modified natural rubber.  
      According to another preferred embodiment of the present invention; foam  12  comprises synthetic polymeric foam.  
      Further according to a preferred embodiment of the present invention, foam  12  comprises a foam formed of a polymer selected from polyester, polyester, polyethylene, Polyurethane, urethane, polystyrene, latex, Neoprene, Nylon, and any other known polymer.  
      Additionally, according to another preferred embodiment of the present invention, foam  12  comprises an industrial foam, for example, PE foam, EV/VA/EM foam, PPA foam, PU foam EVA foam, EPS foam, PVC foam, and any other known industrial foam.  
      According to preferred embodiments of the present invention, foam  12  may be flame retardant Alternatively, foam  12  may be flame-retardant and flame retardant, to meet FMVSS specifications, For example, foam  12  may comprise expanded polyethylene, expanded polyurethane, or expanded polystyrene, which may be flame retardant or flame-retardant and flame retardant, to meet FMVSS specifications.  
      According to preferred embodiments of the present invention, foam  12  may have different degrees of flexibility, for example, it may be flexible, or semi rigid foam. Additionally, foam  12 , formed of foam, may have a high density of pores, or a low density, and the pore size may be large or small. The foam may have a honeycomb cell structure, or a reticulate cell structure.  
      According to the present invention, membrane  22  may be attached also to uncoated material  12 , such as fibrous material  12  or foam  12 , forming channels  28  between membrane  22  and material  12 . Preferably, channels  28  are interconnected.  
      According to a preferred embodiment of the present invention, sound absorbing article  10  is environmentally friendly, so as to cause little health hazard dung its manufacturing and installation, produce little or no fumes, during use, and little or no toxic fumes when ignited Further according to a preferred embodiment of the present invention, sound absorbing article  10  is flame retardant, or fire and flame retardant, or fireproof.  
      It is appreciated that certain features of the invention, which are, for clarity, described in the context of separate embodiments, may also be provided in combination in a single embodiment. Conversely, various features of the invention, which are, for brevity, described in the context of a single embodiment, may also be provided separately or in any suitable subcombination.  
      Although the invention has been described in conjunction with specific embodiments thereof, it is evident that many alternatives, modifications and variations will be apparent to those skilled in the art. Accordingly, it is intended to embrace all such alternatives, modifications and variations that fill within the spirit and broad scope of the appended claims. All publications, patents and patent applications mentioned in his specification are herein incorporated in their entirety by reference into the specification, to the same extent as if each individual publication, patent or patent application was specifically and individually indicated to be incorporated herein by reference. In addition, citation or identification of any reference in this application shall not be construed as an admission that such reference is available as prior art to the present invention.