Patent Publication Number: US-7708111-B2

Title: Acoustic diaphragm and method for manufacturing an acoustic diaphragm

Description:
CROSS REFERENCES TO RELATED APPLICATIONS 
     The present document contains subject matter related to Japanese Patent Application JP 2005-211525 filed in the Japanese Patent Office on Jul. 21, 2005, the entire contents of which being incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an acoustic diaphragm used in a loudspeaker and others and a method for manufacturing an acoustic diaphragm. 
     2. Description of Related Art 
     Requirements of a diaphragm for loudspeaker having excellent reproduction frequency characteristics are such that the Young&#39;s modulus and internal loss be large and the density be low. For example, the reproduction frequency band can be extended by increasing the Young&#39;s modulus of a diaphragm, and the reproduction frequency characteristics can be flattened by increasing the internal loss of the diaphragm to lower the quality factor. In addition, the reproduction efficiency can be improved by lowering the density of the diaphragm. For meeting the requirements of the diaphragm, the use of a so-called mica paper multi-cellular structure product in a diaphragm for loudspeaker has been proposed wherein the mica paper multi-cellular structure product is prepared by making paper from fine mica flakes and pulp fibers or polyvinyl alcohol fibers and then heating the paper to form a multi-cellular structure. 
     The thus prepared mica paper multi-cellular structure product has an apparent density as small as 0.05 to 0.60 g/cm 3  and hence can achieve a lightweight diaphragm. Further, the fine mica flakes used in the multi-cellular structure product have a large ratio of the area to the thickness (that is, a so-called aspect ratio is large), and therefore the areas with which the mica flakes are stacked on one another are large. For this reason, the mica paper multi-cellular structure product has a large Young&#39;s modulus. 
     However, this multi-cellular structure product has a small internal loss and hence increases the resonance sharpness, making it difficult to obtain flat reproduction frequency characteristics. For solving this problem, a method has been proposed in Examined Japanese Patent Application Publication (KOKOKU) No. 7-28476 (Patent Document 1), in which the mica paper multi-cellular structure product is impregnated with a synthetic resin solution or synthetic resin emulsion to be coated with an extremely thin resin film, lowering the internal loss 
     SUMMARY OF THE INVENTION 
     However, in the technique described in the Patent document 1, the synthetic resin solution or synthetic resin emulsion used in coating the mica paper multi-cellular structure product contains an organic solvent, a surfactant, or the like, and environmental problems of this technique have been pointed out. 
     By the way, a related art mica paper multi-cellular acoustic diaphragm is manufactured by the following method. First, fine mica flakes, pulp fibers, and polyvinyl alcohol fibers are mixed together and uniformly dispersed in water, followed by papermaking using a paper machine or the like. Then, the resultant paper article in a moisture state is placed in a mold and heated to 100° C. or higher to dissolve the polyvinyl alcohol fibers. Finally, the whole of the article is dried to obtain a mica paper multi-cellular acoustic diaphragm. 
     In forming the mica paper multi-cellular structure product from the paper article, first, at a point in time when the temperature of the paper article has reached 80° C. or higher, the polyvinyl alcohol fibers are dissolved in moisture contained in the paper article into liquid and the liquid flows as a binder between the fine mica flakes and the pulp fibers. Then, at a point in time when the temperature has reached 100° C. or higher, the moisture vaporizes into water vapor to generate bubbles, thus forming a multi-cellular structure. In this instance, the water vapor is discharged through drying pores of the mold. 
     However, a problem occurs in that the polyvinyl alcohol which has changed into liquid during the above procedure flows into the drying pores of the mold, making difficult removal of the resultant mica paper multi-cellular structure product from the mold, i.e., so-called release. Accordingly, the present invention provides an acoustic diaphragm which is advantageous not only in that the diaphragm is free of environmental problems and has an internal loss effectively increased to achieve flat reproduction frequency characteristics, but also in that the diaphragm has improved releasability from a mold in the production of the diaphragm, and a method for manufacturing the acoustic diaphragm. 
     For solving the above problems, an acoustic diaphragm according to an embodiment of the present invention includes: a diaphragm base material having a multi-cellular structure obtained from a paper article composed of fine mica flakes, pulp fibers, and polyvinyl alcohol fibers; and a sheet material combined with the paper article or the diaphragm base material. 
     A method for manufacturing an acoustic diaphragm according to an embodiment of the present invention includes the following steps of: forming a paper article; combining a sheet material with the paper article; heating the paper article and the sheet material; and forming a diaphragm base material having a multi-cellular structure from the paper article. In the method, the paper article is composed of fine mica flakes, pulp fibers, and polyvinyl alcohol fibers, and only water vapor contained in the paper article is discharged through the sheet material. 
     In the acoustic diaphragm and the method for manufacturing an acoustic diaphragm of the present invention, a sheet material having an internal loss larger than that of the mica paper multi-cellular diaphragm is combined with the paper article in a mold during the manufacture of the mica paper multi-cellular diaphragm, or a sheet material is combined with the diaphragm base material surface of the thus manufactured mica paper multi-cellular diaphragm, and therefore both flattening the reproduction frequency characteristics and improving the releasability from a mold can be achieved. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an explanatory view of a loudspeaker vibrating portion; 
         FIG. 2  includes diagrammatic views of a diaphragm for loudspeaker, in which  FIG. 2A  is a side view and  FIG. 2B  is a front view thereof; 
         FIG. 3  is a view showing a mold for six pieces; 
         FIG. 4  is a cross-sectional view showing a mold having drying pores formed in one side; 
         FIG. 5  is a cross-sectional view showing a mold having drying pores formed in both sides; 
         FIG. 6  is a front view of drying pores; 
         FIG. 7  is a flowchart of forming a paper article and combining a sheet with the paper article; 
         FIG. 8  is a flowchart of forming a multi-cellular structure diaphragm; and 
         FIG. 9  is a graph of the reproduction frequency characteristics. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Hereinbelow, embodiments of the present invention will be described in detail with reference to the accompanying drawings. 
       FIG. 1  is an explanatory view of a loudspeaker vibrating portion. As shown in  FIG. 1 , a loudspeaker unit has a loudspeaker vibrating portion. In  FIG. 1 , a cone  1  constituting a diaphragm for loudspeaker is made from a material which can be shaped into a thin form for facilitating the movement and is lightweight and stiff, and which provides an appropriate loss called internal loss for suppressing peak dips in the frequency characteristics or transient properties. 
     A center cap  2  is provided for preventing the cone  1  from deforming in the radial direction and preventing iron powder or dust from entering the voids. The center cap  2  has a hole  3  formed in its center, and the hole  3  is covered with open weave  4 . The hole  3  allows air, which is pressed or expanded due to the vibration of the cone  1 , to escape. 
     The open weave  4  prevents dust from entering the inside without inhibiting the flow of air. Voice coils  5  move up and down along the periphery of a pole  6  to vibrate the cone  1 . Dampers  7  keep the voice coils  5  appropriately at the periphery of the pole  6 . A gasket  8  fixes an edge  9  of the cone  1  to a frame  10 . 
       FIG. 2A  and  FIG. 2B  are diagrammatic views of a diaphragm for loudspeaker.  FIG. 2A  is a side view and  FIG. 2B  is a front view. In  FIG. 2A , a diaphragm material is inserted into a mold, and pressed and heated to form a cone  12  constituting a diaphragm for loudspeaker. With only the cone  12 , a diaphragm exhibiting flat reproduction frequency characteristics cannot be obtained. Further, in this case, in the mold for forming the cone  12 , a resin flows into the pores formed in the center to the circumference in  FIG. 2B  for water vapor escape, thus lowering the releasability. 
     For removing the disadvantage, a sheet material having an internal loss larger than that of the cone  12  constituting the below-mentioned mica paper multi-cellular diaphragm is combined, or the sheet material is combined with the paper article in a mold in the manufacture of the mica paper multi-cellular diaphragm, thus flattening the reproduction frequency characteristics and improving the releasability from the mold. 
     First, the method for combining with a film or sheet to improve the mica paper multi-cellular diaphragm in internal loss is described. The mica paper multi-cellular diaphragm used here is obtained by the following preparation method. 
     The procedure for forming a paper article is first described. 
       FIG. 7  is a flowchart for explaining forming a paper article and combining a sheet with the paper article. In  FIG. 7 , first, three materials, i.e., fine mica flakes, pulp fibers, and polyvinyl alcohol fibers are mixed together (step S 1 ). Mica has a high Young&#39;s modulus, pulp has wettability and high strength, and polyvinyl alcohol fibers are water-soluble and have an action such that they are solidified by heating. 
     Next, the mixed material of mica, pulp, and polyvinyl alcohol fibers mixed together in the step S 1  is uniformly dispersed in water (step S 2 ). Then, papermaking is performed using the mixed material of mica, pulp, and polyvinyl alcohol fibers uniformly dispersed in water in the step S 2  (step S 3 ). Papermaking is making paper from a water-soluble mixed material while removing moisture from the material to form a plate-form paper article in a moisture state. In this step, a paper machine or the like may be used. 
     A sheet is combined with the paper article in a moisture state formed in the step S 3  (step S 4 ). Only a sheet may be placed in a mold and then combined with the paper article in a moisture state. Finally, the paper article and sheet combined in the step S 4  are placed in a mold (step S 5 ). 
     A sheet material forming the sheet is composed of a material having an internal loss larger than that of a diaphragm base material formed by heating only the paper article. A sheet material composed of a material in the form of nonwoven fabric or paper having air permeability, a material in the form of woven fabric having air permeability, or a porous material having air permeability can be used. 
     In the step S 1  above, it is preferred to use the fine mica flakes having a particle size of 8 mesh to 400 mesh. Further, it is preferred to use the pulp fibers and polyvinyl alcohol fibers individually having a length of 3 mm to 100 mm. The respective ranges of the amounts of the above three components incorporated, i.e., fine mica flakes, pulp fibers, and polyvinyl alcohol fibers are shown in Table 1. 
     
       
         
           
               
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Fine mica 
                   
                 Polyvinyl 
               
               
                   
                 flakes 
                 Pulp fibers 
                 alcohol fibers 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
            
               
                   
                 Parts by weight 
                 100 
                 5 To 50 
                 5 To 70 
               
               
                   
                   
               
            
           
         
       
     
     The amounts of the pulp fibers and polyvinyl alcohol fibers incorporated vary depending on the physical properties required for the diaphragm manufactured. For example, when the amount of the fine mica flakes is 100% by weight, the amount of the pulp fibers is in the range of from 5% to 50% by weight and the amount of the polyvinyl alcohol fibers is in the range of from 5% to 70% by weight. The mixed material comprised of the three components in the embodiment of the present invention is dispersed in water so that the mixed material concentration becomes 0.1% to 1.0%, and papermaking is performed using the resultant material. 
     Next, the construction of a mold is described. 
       FIG. 3  is a view showing a mold for, e.g., six pieces. In  FIG. 3 , a mold  31  is composed of a top half  38  and a bottom half  39 , and cone forming portions  32  to  37  for forming diaphragms are provided between the top half  38  and the bottom half  39 . The wet paper article and sheet combined in the step S 4  in  FIG. 7  are placed in the cone forming portions  32  to  37 , and the top half  38  and the bottom half  39  are heated while pressing them to form cones constituting mica paper multi-cellular diaphragms. 
       FIG. 4  is a cross-sectional view showing a mold having drying pores formed in one side.  FIG. 4  is a partially sectional view of  FIG. 3 . In  FIG. 4 , a cone-shaped hole  43  for forming a diaphragm is provided between a top half  41  and a bottom half  42 . The wet paper article  47  and the sheet  48  combined in the step S 4  in  FIG. 7  are placed in the cone-shaped hole  43 , and the top half  41  and the bottom half  42  are heated while pressing them, followed by drying. Alternatively, only the sheet  48  may be placed in the mold, and then combined with the paper article  47  in a moisture state. 
     In this instance, the sheet  48  is attached to the bottom half  42 , and the paper article  47  is attached to the top half  41 . Only water vapor contained in the paper article  47  is allowed to pass through a plurality of drying pores  44  formed in the bottom half  42  on the cone-shaped hole  43  side, and the water vapor is discharged as indicated by a reference numeral  46  through an open hole  45  formed in the end to communicate with the drying pores  44  formed in the bottom half  42 , thus forming a cone constituting a mica paper multi-cellular diaphragm. 
       FIG. 5  is a cross-sectional view showing a mold having drying pores formed in both sides.  FIG. 5  is a partially sectional view of  FIG. 3 . In  FIG. 5 , a cone-shaped hole  54  for forming a diaphragm is provided between a top half  51  and a bottom half  55 . Sheets  60 ,  60 ′ combined with both surfaces of a wet paper article  59  in the step S 4  in  FIG. 7  are placed in the cone-shaped hole  54 , and the top half  51  and the bottom half  55  are heated while pressing them, followed by drying. Alternatively, only the sheets  60 ,  60 ′ may be placed in the mold, and then combined with the paper article  59  in a moisture state. 
     In this instance, the respective surfaces of the sheet  60 ′ and the sheet  60  are attached to the top half  51  and the bottom half  55 , and the paper article  59  is attached to the top half  51  and bottom half  55  through the sheet  60 ′ and sheet  60 . Only water vapor contained in the paper article  59  is allowed to pass through a plurality of drying pores  52  and a plurality of drying pores  56  formed respectively in the top half  51  and the bottom half  55  on the cone-shaped hole  54  side, and the water vapor is discharged as indicated by a reference numeral  58  through an open hole  53  and an open hole  57  formed in the ends to respectively communicate with the drying pores  52  and drying pores  56  formed in the top half  51  and bottom half  55 , thus forming a cone constituting a mica paper multi-cellular diaphragm. 
       FIG. 6  is a front view of drying pores.  FIG. 6  shows the form as viewed from the front of the drying pores  44 , or drying pores  52  or drying pores  56  shown in  FIG. 4  or  FIG. 5 . In  FIG. 6 , a plurality of drying pores  62  are formed in a top half or bottom half  61  in a region defined by the cone-shaped hole  43  or cone-shaped hole  54  shown in  FIG. 4  or  FIG. 5 . Water vapor is discharged as indicated by a reference numeral  64  through an open hole  63  formed in the end to communicate with the drying pores  62 . 
     Next, the procedure for forming a multi-cellular structure diaphragm using the mold and the paper article and sheet combined is described. 
       FIG. 8  is a flowchart showing the procedure for forming a multi-cellular structure diaphragm. In  FIG. 8 , the paper article and sheet combined are first placed in a mold (step S 11 ). Then, the mold shown in  FIG. 4  or  5  containing therein the paper article in a moisture state in the step S 11  is heated to 100° C. or higher while applying a pressure to the mold (step S 12 ). 
     In heating in the step S 12 , the heat is transferred from the mold surface through the air-permeable sheet to the paper article in a moisture state, and, at a point in time when the temperature of the paper article has reached 80° C. or higher, the polyvinyl alcohol fibers contained in the paper article are dissolved in water into liquid (step S 13 ). 
     The polyvinyl alcohol which has changed into liquid in the step S 13  flows as a binder between the air-permeable sheet, the fine mica flakes, and the pulp fibers, so that they are bound together (step S 14 ). At this time, the polyvinyl alcohol which has changed into liquid in the step S 13  is absorbed by the air-permeable sheet, and hence does not flow into the drying pores, so that the bonding force between the sheet and the paper article is improved, as compared to that obtained in a related art manufacturing method. 
     When the temperature of the paper article and air-permeable sheet bound by the polyvinyl alcohol in the step S 14  has reached 100° C. or higher by heating in the step S 12 , moisture in the paper article in a moisture state vaporizes into water vapor (step S 15 ). When the moisture in the paper article vaporizes into water vapor in the step S 15 , a number of bubbles are generated in the paper article (step S 16 ). 
     Subsequently, a number of bubbles are generated in the step S 16  to form a multi-cellular structure (step S 17 ). At this time, only water vapor is discharged through the sheet (step S 18 ). In this way, only water vapor is discharged through the air-permeable sheet in the step S 18 , and therefore the constituents of the paper article do not flow into the drying pores, so that the releasability is improved, as compared to that in a related art manufacturing method. 
     As mentioned above, the paper article is dried while dissolving the polyvinyl alcohol fibers contained in the paper article to obtain a mica paper multi-cellular structure article. Further, for increasing the internal loss, a film or sheet material having an internal loss larger than that of a diaphragm composed of the mica paper multi-cellular structure article, for example, a PET (polyethylene terephthalate) film or woven fabric made of Kevlar (registered trademark) is stacked on one side or both sides of the paper article using an adhesive obtained by dissolving the polyvinyl alcohol fibers, thus obtaining a diaphragm composed of a mica paper multi-cellular structure article combined with a film or sheet having an improved internal loss. 
     Next, the method for combining an air-permeable sheet for improving the internal loss and productivity is described. In the stacking method using an air-permeable sheet, an air-permeable sheet is placed on the surface of the mold shown in  FIG. 4  or  5  in which the drying pores are present, and then the paper article in a moisture state is placed in the mold, followed by heating to 100° C. or higher. Thus, both the dissolution of the polyvinyl alcohol fibers and the drying are achieved, obtaining a diaphragm composed of a mica paper multi-cellular structure article. 
     An alternative method for combining an air-permeable sheet with the mica paper multi-cellular article may be employed in which a diaphragm composed of a mica paper multi-cellular structure article having a sheet combined with one surface is first formed as described above, and then an air-permeable sheet is attached using an adhesive to another surface of the mica paper multi-cellular structure article on which no sheet is present. 
     An alternative method may be employed in which a diaphragm composed of a mica paper multi-cellular structure article is formed using a sheet for improving the releasability as mentioned above, and then the sheet is peeled off the mica paper multi-cellular structure article and then another sheet for increasing the internal loss is attached to the mica paper multi-cellular structure article using an adhesive. 
     If, as an air-permeable sheet having an internal loss larger than that of the mica paper multi-cellular diaphragm, for example, nonwoven fabric, paper, or woven fabric made of Kevlar is used, the internal loss can also be improved. Unlike the method using a synthetic resin solution or synthetic resin emulsion proposed related arts, the above method uses neither organic solvent nor surfactant, thus removing environmental problems. 
     Hereinbelow, specific Experimental Examples of the acoustic diaphragm according to the embodiment of the present invention are shown. Internal loss values of the film and sheet used in the Experimental Examples are shown in Table 2. 
     
       
         
           
               
               
               
               
             
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                 PET 
                 Paper 
                 Kevlar woven fabric 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
            
               
                   
                 Internal loss 
                 0.024 
                 0.051 
                 0.034 
               
               
                   
                 tanδ 
               
               
                   
                   
               
            
           
         
       
     
     COMPARATIVE EXAMPLE 
     First, in a Comparative Example, 100 g of fine mica flakes having a particle size of 8 mesh to 150 mesh, 10 g of pulp fibers, and 60 g of polyvinyl alcohol fibers were dispersed in water to prepare a suspension having a concentration of 0.5%. The suspension of 2,000 g prepared was subjected to papermaking into a size of 100 mm×100 mm. The resultant paper article was inserted into a mold shown in  FIG. 4  or  5 , which was preliminarily heated to 150° C., and subjected to press drying for 20 minutes to obtain a mica paper multi-cellular structure article having an apparent density of 0.4 g/cm 3  and a thickness of 5 mm. 
     From the thus obtained mica paper multi-cellular structure article, a sample having a length of 80 mm and a width of 10 mm was prepared, and subjected to measurement of the physical properties (internal loss) by a vibrating reed method, and the result of the measurement and the results in the Experimental Examples below are shown in Table 3. As can be seen from Table 3, the internal loss in the Comparative Example was 0.0134. 
     
       
         
           
               
               
               
               
               
               
             
               
                   
                 TABLE 3 
               
             
            
               
                   
                   
               
               
                   
                   
                 Experimental 
                 Experimental 
                   
                   
               
               
                   
                 Comparative 
                 Example 1 
                 Example 2 
                 Experimental 
                 Experimental 
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                   
                 Example 
                 1-1 
                 1-2 
                 2-1 
                 2-2 
                 Example 3 
                 Example 4 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                 Internal 
                 0.0134 
                 0.0164 
                 0.0182 
                 0.0189 
                 0.0285 
                 0.0175 
                 0.0221 
               
               
                 loss 
               
               
                 tanδ 
               
               
                   
               
            
           
         
       
     
     EXPERIMENTAL EXAMPLE 1 
     A PET film having micropores and having a thickness of 75 μm was stacked on one side (1-1 in Table 3) or both sides (1-2 in Table 3) of the mica paper multi-cellular structure article obtained in the Comparative Example using a hot-melt adhesive by press bonding at a temperature of 150° C. under a pressure of 1 kg/cm 2  for 5 minutes to obtain a PET/mica paper multi-cellular article composite (1-1 in Table 3) or a PET/mica paper multi-cellular article/PET composite (1-2 in Table 3). 
     A sample having a length of 80 mm and a width of 10 mm was prepared in the same manner as in the Comparative Example, and subjected to measurement of the physical properties by a vibrating reed method. In Experimental Example 1, the composite having PET on one side had an internal loss of 0.0164 as shown at the column “1-1” in Table 3, and the composite having PET on both sides had an internal loss of 0.0182 as shown at the column “1-2” in Table 3, which had been improved, as compared to the above value. 
     As a PET film, “LUMIRROR” (registered trademark, trade name), manufactured and sold by Toray Industries Inc., was used, and, as a hot-melt adhesive, “DYNAC (registered trademark) PES140-50” (trade name), manufactured and sold by Kureha Ltd., was used. 
     EXPERIMENTAL EXAMPLE 2 
     Paper having micropores and having a thickness of 100 μm was stacked on one side (2-1 in Table 3) or both sides (2-2 in Table 3) of the mica paper multi-cellular article obtained in the Comparative Example under the same conditions as those in Experimental Example 1 to obtain a paper/mica paper multi-cellular article composite (2-1 in Table 3) or a paper/mica paper multi-cellular article/paper composite (2-2 in Table 3). 
     A sample having a length of 80 mm and a width of 10 mm was prepared in the same manner as in the Comparative Example, and subjected to measurement of the physical properties by a vibrating reed method. In Experimental Example 2, the composite having paper on one side had an internal loss of 0.0189 as shown at the column “2-1” in Table 3, and the composite having paper on both sides had an internal loss of 0.0285 as shown at the column “2-2” in Table 3, which had been improved, as compared to the above value. 
     EXPERIMENTAL EXAMPLE 3 
     The suspension obtained by mixing the fine mica flakes, pulp fibers, and polyvinyl alcohol fibers similar to those used in the Comparative Example was subjected to papermaking under the same conditions as those in the Comparative Example to obtain a paper article. Kevlar woven fabric having micropores was put on one side of the wet paper article so that the woven fabric was present on the drying pores side of the mold, and inserted into a mold (see  FIG. 4 ), which was preliminarily heated to 150° C., and subjected to press drying for 20 minutes to obtain a Kevlar woven fabric/mica paper multi-cellular article composite. The polyvinyl alcohol which had changed into liquid was absorbed by the Kevlar woven fabric, and hence no polyvinyl alcohol flowed into the drying pores in the mold. Therefore, the composite was easily removed from the mold, that is, the releasability from the mold, which had been poor in related arts, was improved, confirming the effect of the embodiment of the present invention. 
     As Kevlar woven fabric, Kevlar Cloth K-281 (trade name), manufactured and sold by Arisawa Mfg. Co. Ltd., was used. A sample for measurement was prepared in the same manner as in the Comparative Example, and subjected to measurement of the physical properties by a vibrating reed method. In Experimental Example 3, the internal loss was 0.0175, which had been improved, as compared to the value in the Comparative Example. 
     EXPERIMENTAL EXAMPLE 4 
     The suspension obtained by mixing the fine mica flakes, pulp fibers, and polyvinyl alcohol fibers similar to those used in the Comparative Example was subjected to papermaking under the same conditions as those in the Comparative Example to obtain a paper article. Kevlar woven fabric having micropores was put on both sides of the wet paper article, and inserted into a mold (see  FIG. 5 ), which was preliminarily heated to 150° C., and subjected to press drying for 20 minutes to obtain a Kevlar woven fabric/mica paper multi-cellular article/Kevlar woven fabric composite. 
     Like in Experimental Example 3, in Experimental Example 4, the polyvinyl alcohol which had changed into liquid did not flow into the drying pores in the mold, and thus the releasability was improved, which confirmed the effect of the embodiment of the present invention. As Kevlar woven fabric, Kevlar Cloth K-281 (trade name), manufactured and sold by Arisawa Mfg. Co. Ltd., was used. 
     A sample for measurement was prepared in the same manner as in the Comparative Example, and subjected to measurement of the physical properties by a vibrating reed method. In Experimental Example 4, the internal loss was 0.0221, which had drastically been improved, as compared to the value in the Comparative Example. As apparent from the results of the measurement, the internal loss in each of the Experimental Examples is larger than that in the Comparative Example, which confirms the effect of the embodiments of the present invention. 
     EXPERIMENTAL EXAMPLE 5 
     Using the suspension obtained by mixing the fine mica flakes, pulp fibers, and polyvinyl alcohol fibers similar to those used in the Comparative Example, there were obtained a diaphragm  1 , a diaphragm  2  and a comparative example diaphragm, each of which was of a cone type having an opening diameter of 10 cm. The diaphragm  1  is composed of a Kevlar woven fabric/mica paper multi-cellular article diaphragm, that is, a diaphragm having microporous Kevlar woven fabric on one side. The diaphragm  2  is composed of a Kevlar woven fabric/mica paper multi-cellular article/Kevlar woven fabric diaphragm, that is, a diaphragm having microporous Kevlar woven fabric on both sides. The comparative example diaphragm is a diaphragm composed of mica paper multi-cellular article only. 
       FIG. 9  is a graph of the reproduction frequency characteristics. Full-range loudspeakers were individually manufactured using the above-obtained diaphragms, and a comparison was made between the reproduction frequency characteristics of them. From  FIG. 9 , it is clear that the loudspeaker using the diaphragm  1  or  2  of the present invention indicated by a reference numeral  92  or  93  has flat characteristics having little peak dips, as compared to the loudspeaker using the comparative example diaphragm  91 .  FIG. 9  has confirmed the increase of the internal loss of the diaphragm. 
     By the present invention, not only can the internal loss of the multi-cellular acoustic diaphragm be effectively increased to improve the vibration-damping properties, thus achieving flat reproduction frequency characteristics, but also the releasability of the diaphragm from a mold in the production of the diaphragm can be improved, thus enhancing the productivity. 
     The amounts of the materials for the mica paper multi-cellular article diaphragm, i.e., fine mica flakes, pulp fibers, and polyvinyl alcohol fibers, the drying temperature, and the conditions for the film or sheet are not limited to those in the Experimental Examples, and can be appropriately selected depending on the desired properties of the diaphragm. 
     The above-described embodiments of the present invention not only can increase the acoustic diaphragm in internal loss to flatten the reproduction frequency characteristics of a loudspeaker but also can improve the releasability of the diaphragm from a mold in the production of the diaphragm, thus effectively enhancing the productivity. 
     The present invention is not limited to the above embodiments, and the present invention can be changed or modified as long as it is within the scope of the present invention.