Patent Publication Number: US-7913808-B2

Title: Waterproofing loudspeaker cones

Description:
BACKGROUND 
     This disclosure relates to waterproofing loudspeaker cones. 
     Loudspeakers generally include a diaphragm and a linear motor. When driven by an electrical signal, the linear motor moves the diaphragm to cause vibrations in air. The diaphragm may include a cone, surround, and dust cap. Loudspeaker cones are commonly made of paper. Surrounds and dust caps may also be made of paper. In some applications, loudspeakers are used in environments, such as automobiles, where they are exposed to water. Ordinary paper, made of wood pulp, may not perform well as a diaphragm when exposed to water. The paper may absorb water, which increases its mass and reduces its stiffness, which both affect the sound produced when the motor moves the diaphragm. Other materials, such as aluminum and plastic, when used, may be resistant to water but have other disadvantages as loudspeaker components. 
     SUMMARY 
     In general, in one aspect, a loudspeaker component is made from a composition including wood pulp, primary hydrophobic fibers, and stiffening fibers that retain stiffness when wet. The composition is cured into paper forming the component. 
     Implementations may include one or more of the following features. The primary hydrophobic fibers may include fibrillated acrylic fibers. The stiffening fibers may include glass fibers. The composition may also include a binding agent. The binding agent may include secondary hydrophobic fibers. The binding agent may include polypropylene fibrids. The composition may also include fluorocarbon. The relative proportions of materials in the composition may be uniform throughout the loudspeaker component. The loudspeaker component may be a cone. 
     The wood pulp may constitute between 30% and 70% by mass of the composition. The wood pulp may constitute 39% by mass of the composition. The primary hydrophobic fibers may constitute between 10% and 30% by mass of the composition. The primary hydrophobic fibers may constitute 20% by mass of the composition. The stiffening fibers may constitute between 5% and 30% by mass of the composition. The stiffening fibers may constitute 20% by mass of the composition. The binding agent may constitute between 10% and 30% by mass of the composition. The binding agent may constitute 20% by mass of the composition. The fluorocarbon may constitute up to 5% by mass of the composition. The fluorocarbon may constitute 1% by mass of the composition. The wood pulp may have a freeness between 350 and 700 CSF. The fibrillated acrylic fibers may have a freeness between 10 and 600 CSF. The fibrillated acrylic fibers may have a freeness between 40 and 350 CSF. The glass fibers may have an average diameter between 6 and 13 μm. The glass fibers may have an average length between 2 and 8 mm. 
     In general, in one aspect, a loudspeaker component is formed from a composite paper of uniform material composition and having a wet modulus of at least 40% of the paper&#39;s dry modulus and a resistance against surfactant penetration that is significantly higher than that of a cone formed substantially entirely from wood pulp. 
     In general, in one aspect, a composite paper material includes wood pulp, fibrillated acrylic fibers, glass fibers, polypropylene fibrids, and fluorocarbon. 
     In general, in one aspect, a loudspeaker includes a linear motor and a cone formed from a composition comprising wood pulp, fibrillated acrylic fibers, glass fibers, polypropylene fibrids, and fluorocarbon. 
     Advantages include maintaining stiffness and dimensional stability when wet. Wet rub defects in the transducer are reduced. The dry modulus is similar to current cone papers and traditional paper cones. The material has a good resistance against soak-through, low water absorption, and resistance against warping. Good acoustic performance can be achieved, and the cones may be produced on existing cone body manufacturing equipment. The material also has a good heat resistance at high temperatures. 
     Other features and advantages will be apparent from the description and the claims. 
    
    
     
       DESCRIPTION 
         FIG. 1  shows an exploded view of a loudspeaker. 
         FIGS. 2A and 2B  show flow charts. 
     
    
    
     A loudspeaker  10 , shown in  FIG. 1 , includes a cone  12  made of paper, as noted above. In the context of a loudspeaker that will be exposed to water, we refer to the cone as having a wet side  18  and a dry side  16 . Other structures, such as the loudspeaker enclosure (not shown), are expected to prevent moisture from reaching the dry side  16  of the cone  12 . The relationship of the motor  14  (including a magnet  14   a,  voice coil  14   b,  bobbin  14   c,  and pole  14   d  in the example of  FIG. 1 ) to the wet and dry sides of the cone  12  in  FIG. 1  is for illustration only. Other arrangements are possible, for example, the inside of the cone  12  may be the wet side, and the motor  14  may be located inside the volume defined by the cone, independently of which side is wet and which is dry. Other components of the loudspeaker in the example of  FIG. 1  include a basket  20  with ventilation holes  22 , electrical connections  24   a  and  24   b,  and a suspension  26 . 
     To improve the performance of the loudspeaker  10  when the cone  12  is exposed to water, a mixture of wood pulp and synthetic fibers is used to form the cone paper. Standard wood pulp of a soft wood having typically long fibers can be used with a standard wet-chemistry package, known by those skilled in the art. The synthetic fibers are selected to prevent the absorption of water by the paper and to maintain the paper&#39;s material properties if any water is absorbed, such as by stiffening it. Some materials used for the synthetic fibers include acrylics, glass, and polypropylene. The same principles can be applied to other loudspeaker components, such as surrounds, dust caps, or other parts of the diaphragm, and to water-resistant paper products in general. 
     Hydrophobic fibers, including thermoplastic fibers, reduce the absorption of water and have good flexibility. Examples include fibrillated acrylics, such as polyacrylonitrile (PAN) fibers or copolymers containing at least 85% PAN. The fibrillated acrylics also provide good entanglement with the other fibers in the mixture, providing good formation and retention. Other hydrophobic fibers that may be used include polypropylene, polyester, olefin or polyethylene, polyamide (nylon) and polylactide. A number of other synthetic hydrophobic fibers may be useful, such as commercially available specialty fibers, including PVC (vinyon), polyvinylidene chloride (Saran™ resins from Dow Chemical Company), polytetrafluoroethylene (Teflon® fibers from E.I. du Pont de Nemours and Company (DuPont)), polyurethane-polyethylene glycol (PU-PEG) block copolymer (spandex, e.g., Lycra® fibers from Invista), aramids (aromatic polyamide, including Kevlar® and Nomex® fibers from DuPont), polybenzimidazole (PBI), aromatic polyester (vectran fibers from Kuraray Co., Ltd.), thermoset polyurethane (Zylon® fibers from Toyobo Corp.), and polyetheretherketone (PEEK, available from Zyex Ltd.). 
     Glass fibers help to maintain the material properties, such as the stiffness, of the paper when wet. The surface of the glass fibers may be treated with siloxane to further reduce water absorption by the composite material. Polypropylene fibrids, which are also hydrophobic, provide attachment (or binding) of the other fibers in the mixture to each other. This attachment provides a structural stability to the material. Other binding materials may be used, such as polypropylene emulsions, polyurethane (PU) emulsions, reactive epoxy, and phenolic resin powders. In addition to the synthetic fibers, fluorocarbon provides additional resistance to water penetration or absorption. In some examples, a cationic fluoropolymer, positively charged at a pH below 7 imparts both additional water and grease resistance to the fibers. 
     Various ratios of the wood and synthetic fibers may be used, depending on the particular material properties needed in a given application and the relative importance of the different properties. For example, increased glass content improves wet modulus. Wood pulp having a CSF (Canadian Standard Freeness) of between 350 and 700 remains the primary component and may make up 30 to 70 percent of the composition by mass. Hydrophobic fibers in a pulp having a CSF between 10 and 600, and more preferably between 40 and 350, may make up between 10 and 30 percent of the composition by mass. Binding fibers may also constitute between 10 and 30 percent by mass. Stiffening fibers having an average diameter between 6 and 13 μm and an average length between 2 and 8 mm, as defined in manufacturers&#39; specifications, may be as little as 5 percent or as much as 30 percent by mass. The fluorocarbon, if used at all, may be as much as 5 percent of the composition by mass. 
     In one embodiment, the composition includes 39% (by mass) wood fiber having a freeness of 478 CSF, 20% fibrillated acrylic fibers having a freeness of 60 CSF, 20% glass fibers 3 mm long with a diameter of 11 μm, 20% polypropylene fibrids, and 1% fluorocarbon. In some examples, the wood is refined or “beaten” from an initial freeness of ˜600 CSF to the lower freeness used. In some examples, refining or beating the wood fiber is not necessary. This composition demonstrates increased tensile modulus in wet tests when compared to traditional paper cones. The wet modulus of the composite cone (the tensile modulus when the paper is wet) is ˜0.8 GPa, significantly higher than the ˜0.27 GPa of standard cone papers. The composite cone also demonstrates 82% less warping than a traditional paper cone when exposed to water and then dried (95% RH exposure at 65° C. for 65 h, dried at 80° C. for 6 hours). A similar composition having 59% wood fiber, 20% acrylic fibers, 20% bicomponent polyester fibers, and 1% fluorocarbon also has a wet modulus higher than the wet modulus of traditional paper (˜0.37 GPa vs. ˜0.27 GPa). Both compositions demonstrate significantly longer penetration times for mixtures of water with a surfactant, such as soap (5-50 min. vs. &lt;1 min, tested with a soap-to-water ratio of 1:69.5 in a Mini Britt Jar test), with the composition including glass having a shorter time than the composition without glass. Both compositions also demonstrate lower weight gain due to moisture pickup than traditional paper (˜15% vs. ˜35%). Another composition uses phenolics as binders in place of the polyester fibers but is otherwise similar to the second composition (i.e., 59% wood, 20% acrylic fibers; 20% phenolic powder; 1% fluorocarbon) and has a similar wet modulus of 0.4 GPa. 
     Typical paper-making wood fibers, such as such as Q-90 pulp made from black spruce, from Domtar Inc., of Lebel-sur-Quevillon, QC, Canada, or HS400 pulp, made from western red cedar, from Canfor Pulp Limited Partnership, of Vancouver, BC, Canada, or Harmac K10S pulp made from western red cedar, from Pope &amp; Talbot, Inc., of Portland, Oreg., may be used. For the acrylic fibers, examples include CFF 114-3 fibrilated acrylic fibers from EFT/Sterling Fibers of Shelton, Conn. Polypropylene fibrids such as product Y600 from the Functional Fabricated Products Division Mitsui Chemicals, Inc. of Toyko, Japan provide the targeted reduction of water uptake and dimensional stability when wet. Glass fibers having the dimensions noted above are available as EC-11-3-SP from JSC Valmiera Glass of Latvia. Suitable fluorocarbon includes AsahiGuard E60 “C6 environmentally friendly fluorocarbon,” from AGC Chemicals Americas, Inc., of Bayonne, N.J. 
     In some examples, the paper is formed following a process  100  shown in  FIG. 2A . The acrylic fibers are dispersed  102  in a water suspension, using a beater or other method of providing high shear, such as a Hydropulper. The polypropylene fibrids are then added  104  to the acrylic fibers and the mixture is again dispersed  106 . The refined wood pulp and the fluorocarbon are added  108 , and the entire mixture is blended  110 . The glass fibers are added  112  and dispersed in the mixture  114  last to avoid damaging them in the earlier blending steps. 
     In some examples, the paper is formed following a modified process  120  shown in  FIG. 2B . The wood blend is prepared and the fluorocarbon is added  122 . The acrylic fibers and polypropylene fibrids are dispersed and premixed  124 , possibly well in advance of the pulp mixing process. The acrylic/polypropylene mixture is combined  126  with the wood/fluorocarbon mixture and blended  128  in a mixing vessel. The glass is added  130  and dispersed in to the mixture  134  in a mixing vessel. 
     After the mixture is completed, cones are formed and cured using paper molding processes, as is generally known in the art. In the examples described, the overall density of paper formed from the composite material was the same as traditional paper, that is, a cone of the same dimensions as a traditional cone has the same mass. Other paper products can also be formed form the same mixture, using other forming processes, as appropriate. 
     Composite cones made using this composition have been found to have a dry modulus similar to that of typical cone papers. However, the composite cones maintain their stiffness and dimensional stability when wet and through wet-dry cycles much better than traditional papers. Maintaining stiffness and stability when wet reduces wet rub defects (where the voice coil rubs against the pole piece or front plate). The composite material has a good resistance against soap penetration, which improves the durability of other loudspeaker components, low water absorption, which avoids mass loading when wet, and resistance against warping, which decreases variations in performance over time. The composite material also maintains a good resistance to high temperatures. 
     Other implementations are within the scope of the following claims and other claims to which the applicant may be entitled.