Patent Publication Number: US-10327085-B2

Title: Loudspeaker diaphragm, manufacturing method for the same, and loudspeaker including the loudspeaker diaphragm

Description:
BACKGROUND 
     1. Technical Field 
     The present disclosure relates to a diaphragm, a method of manufacturing the diaphragm, and a loudspeaker including the diaphragm. 
     2. Description of the Related Art 
     Loudspeaker diaphragms are demanded to have light-weight, high rigidity, and appropriate internal loss. WO2015/011903 discloses a loudspeaker diaphragm including a base layer and a coating layer. The base layer contains natural fibers, such as cellulose fibers. The coating layer, which is composed of cellulose nanofibers, is formed on at least one surface of the base layer. 
     The natural fibers in the base layer can be made from wood or non-wood pulp, or a combination of both. The non-wood pulp is an aggregate of fibers obtained from bamboo or other plants. The cellulose nanofiber in the coating layer is cellulose-containing fiber with nano diameter. The above-mentioned WO2015/011903 discloses, as examples of cellulose nanofiber, nata de coco powder and nano-scale miniaturized bamboo fiber. 
     SUMMARY 
     The present disclosure provides a loudspeaker diaphragm that includes a coating layer on at least one surface of a base layer, thereby having a good balance of physical properties, that is, both high elastic modulus and appropriate internal loss. 
     The loudspeaker diaphragm according to the present disclosure includes a base layer having a first surface and a second surface, and a coating layer on at least one of the first and second surfaces of the base layer. The base layer contains natural fibers. The coating layer is composed of chitin nanofibers each having a higher elastic modulus than that of the base layer. 
     According to the method of manufacturing a loudspeaker diaphragm according to the present disclosure, a chitin nanofiber water dispersion is sprayed onto at least one of first and second surfaces of a base layer containing natural fibers to form an intermediate product, and the intermediate product is hot-pressed into a shape of a diaphragm. 
     The loudspeaker according to the present disclosure includes the above-described diaphragm, an edge, a magnetic circuit, a frame, a voice coil, and a damper. The edge is coupled to the outer periphery of the diaphragm. The magnetic circuit, which is provided with a magnetic gap, is formed of a yoke, a magnet, and a plate. The frame is attached to the magnetic circuit and supports the outer periphery of the diaphragm via the edge. The voice coil has a first end attached to the diaphragm and a second end wound with a coil disposed in the magnetic gap. The damper is coupled to the frame and the voice coil. 
     The molecule of the chitin nanofiber is composed of a fewer number of OH groups than in the molecule of the cellulose nanofiber in the coating layer of loudspeakers known in the art, and acetyl groups, which are less strongly hydrogen-bonded than OH groups. The coating layer composed of such chitin nanofibers has a long intermolecular distance, facilitating the molecular motion. Thus, in the coating layer, the rigid main structure maintains the hardness of the diaphragm, and the molecular motion increases the internal loss of the diaphragm. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a sectional view of a loudspeaker including a loudspeaker diaphragm according to an exemplary embodiment of the present disclosure; 
         FIG. 2  is an enlarged schematic sectional view of the loudspeaker diaphragm according to the exemplary embodiment; 
         FIGS. 3A to 3C  are sectional views showing manufacturing processes of the loudspeaker diaphragm according to the exemplary embodiment; 
         FIG. 4  shows a chemical structure of a molecule of cellulose nanofiber; 
         FIG. 5  shows a chemical structure of a molecule of chitin nanofiber; 
         FIG. 6  is a sectional view of another loudspeaker according to the exemplary embodiment of the present disclosure; and 
         FIG. 7  is a plan view of still another loudspeaker according to the exemplary embodiment of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENT 
     Prior to describing an exemplary embodiment of the present disclosure, problems known in the art will now be described briefly. 
     Loudspeaker diaphragms including a coating layer of cellulose-nanofibers and a base layer have high elastic modulus. As a result, loudspeakers including such a highly rigid diaphragm can have the higher limit frequency as frequency response, and thus produce clearer sound. 
     However, the internal losses of these diaphragms tend to be lower than expected, in spite of their high rigidities. Loudspeakers including a diaphragm with a low internal loss are likely to cause peaks and dips in frequency response. These characteristics can cause reverberation of sound, producing distorted and non-expressive sound. To avoid this, loudspeaker diaphragms are expected to have a better balance of physical properties. 
     The exemplary embodiment of the present disclosure will now be described with reference to drawings. 
       FIG. 1  is a sectional view of a loudspeaker including diaphragm  1  according to the exemplary embodiment of the present disclosure. 
     This loudspeaker includes edge  10 , cone-shaped diaphragm  1 , magnetic circuit  5 , frame  7 , voice coil  9 , and damper  13 . Edge  10  is coupled to the outer periphery of diaphragm  1 . Magnetic circuit  5  includes yoke  2 , magnet  3 , and plate  4 . Magnetic circuit  5  has uniform magnetic gap  6  between the inner periphery of yoke  2  and the outer periphery of plate  4 . Frame  7  is attached to yoke  2  of magnetic circuit  5  near magnetic gap  6  in such a manner as to support the outer periphery of diaphragm  1  via edge  10 . To be more specific, the bottom of frame  7  is coupled to the outer periphery of yoke  2 , and the top of frame  7  is coupled to the outer periphery of diaphragm  1  via edge  10 . Voice coil  9  has a first end, which is attached to the reverse surface of diaphragm  1 , and a second end, which is wound with coil  8  and disposed in magnetic gap  6 . The first end of voice coil  9  is coupled to the center of diaphragm  1 . Damper  13  is coupled to voice coil  9  and frame  7 . Diaphragm  1  may include, in its central region, dust cap  14  to prevent the entry of dust into magnetic gap  6 . 
       FIG. 2  is an enlarged schematic sectional view of diaphragm  1 . 
     Diaphragm  1  includes base layer  1 A mainly composed of natural fibers  11 , and coating layer  1 B formed on the reverse side (surface) of base layer  1 A from magnetic circuit  5 . 
     Natural fibers  11  can be either wood pulp, such as cellulose fiber or non-wood pulp, or a combination of both. Non-wood pulp is an aggregate of fibers obtained from bamboo or other plants. Coating layer  1 B is mainly composed of chitin nanofibers  12  higher in elastic modulus than base layer  1 A. Chitin nanofiber  12  is a polysaccharide composed of linearly-linked acetylglucosamine units. To be more specific, chitin nanofibers  12  are derived from crab shell and have an average diameter in a range from 10 nm to 20 nm, inclusive. 
       FIGS. 3A to 3C  show the manufacturing processes of diaphragm  1 . 
     First, wood or non-wood pulp is beaten into raw paper with a fiber diameter of, for example, 13 μm or so. The raw paper is made into stacked sheets of paper. The stacked sheets are subjected to vacuum extraction to prepare base layer  1 A shown in  FIG. 3A  until the surface of base layer  1 A remains wet to some extent. 
     The above-described beating is performed as follows. The pulp is put into a beater together with at least one of the waterproof agents that are fluorine- and paraffin-based emulsions. Next, the pulp is beaten, with the waterproof agent being adsorbed on the pulp. Further, a resin emulsion may be added to the beater to improve the waterproofness of base layer  1 A. The above-mentioned waterproof agent may be replaced by a silicon- or silane-based waterproof agent. 
     Examples of the resin emulsion include epoxy-, acrylic-, and ester-based synthetic resins, such as vinyl acetate polymers, acrylic ester copolymers, and ethylene-vinyl acetate-acrylic acid copolymers. 
     Next, chitin nanofiber water dispersion  12 A is sprayed onto base layer  1 A to form coating layer  1 B as shown in  FIG. 3B . Each of the chitin nanofibers in coating layer  1 B is a polysaccharide composed of linearly-linked acetylglucosamine units, and have an average diameter in a range from 10 nm to 20 nm, inclusive. 
     Finally, an intermediate product shown in  FIG. 3B , which consists of base layer  1 A and coating layer  1 B formed on one side (surface) of base layer  1 A, is hot-pressed into the shape of a cone diaphragm while being dried. Thus, diaphragm  1  is produced. 
     Subsequently, voice coil  9  and edge  10  are attached to diaphragm  1 , and diaphragm  1  is put into frame  7  to complete the loudspeaker. 
     Diaphragm  1  prepared as Example has the following specifications. The proportion of the waterproof agent with respect to the raw paper is in a range from 5 to 10 wt %, inclusive. The proportion of the chitin nanofibers in dispersion  12 A is 1 wt %. The proportion of coating layer  1 B in the total thickness of diaphragm  1  is in a range from 3.5 to 6%, inclusive. A diaphragm prepared as Comparative Example A includes a base layer, but not a coating layer. In other words, the diaphragm of Comparative Example A is identical to base layer  1 A of Example. A diaphragm prepared as Comparative Example B includes a coating layer composed of cellulose nanofibers. The thickness of the coating layer of Comparative Example B is in a range from 3.5 to 6%, inclusive, of the entire thickness of the diaphragm, as same as Example. The other conditions are common to Example and Comparative Examples A, B. The diaphragms of Example and Comparative Examples A, B are measured for elastic modulus and internal loss. The measurement results are shown in Table 1. 
     
       
         
           
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                   
                 Elastic Modulus (GPa) 
                 Internal Loss 
               
               
                   
               
             
            
               
                 Comparative Example A 
                 2.0 
                 0.040 
               
               
                 (base layer alone) 
                   
                   
               
               
                 Comparative Example B 
                 2.7 
                 0.035 
               
               
                 (base layer + cellulose nanofibers) 
                   
                   
               
               
                 Example 
                 3.5 
                 0.040 
               
               
                 (base layer + chitin nanofibers) 
                   
                   
               
               
                   
               
            
           
         
       
     
     As seen from Table 1, the elastic modulus of Example is 3.5 GPa, which is greater than the elastic modulus (2.7 GPa) of Comparative Example B using the coating layer composed of cellulose nanofibers. Furthermore, the internal loss of Example is 0.040, which is the same as that of Comparative Example A using the base layer alone and is greater than that (0.035) of Comparative Example B. Thus, the diaphragm of Example has both a high rigidity characterized by an elastic modulus of 3.5 GPa and an appropriate internal loss. 
     The diaphragm of Example further has a better waterproofness than that of Comparative Example B because hydrophobic acetyl groups remain on the surface of coating layer  1 B. 
       FIGS. 4 and 5  show the chemical structures of molecules of cellulose nanofiber and chitin nanofiber, respectively. 
     Chitin nanofiber is composed of OH groups and acetyl groups, which are less strongly hydrogen-bonded than OH groups. Chitin nanofiber also contains fewer OH groups than cellulose nanofiber, and thus fewer hydrogen-bonds are formed between the molecules. This seems to be the reason that coating layer  1 B composed of chitin nanofibers  12  used in diaphragm  1  has a longer intermolecular distance, facilitating the molecular motion, and that the rigid main structure of coating layer  1 B maintains the hardness of diaphragm  1 , allowing the molecular motion to increase the internal loss of the diaphragm. 
     In the exemplary embodiment, coating layer  1 B composed of chitin nanofibers is formed only on one surface of base layer  1 A; alternatively however, coating layers  1 B can be formed on both surfaces of base layer  1 A. 
     When coating layers  1 B are formed on both surfaces of base layer  1 A, coating layers  1 B can be more effective, allowing the loudspeaker to have the higher limit frequency and to produce clearer sound. 
     Coating layer  1 B is formed on the entire surface of base layer  1 A in diaphragm  1  in  FIG. 1 ; alternatively, however, coating layer  1 B may be formed only on the central portion of diaphragm  1  as shown in  FIG. 6 .  FIG. 6  is a sectional view of another loudspeaker according to the exemplary embodiment. In this loudspeaker, ring-shaped coating layer  1 B is formed around the central portion of base layer  1 A in diaphragm  1 . 
     In diaphragm  1  of  FIG. 6 , coating layer  1 B is not formed except in the central portion of base layer  1 A. In other words, coating layer  1 B is formed only on the effective portion. This configuration enables the loudspeaker to have a higher sound pressure level as well as the higher limit frequency to produce clearer sound, without a large increase in the entire weight of diaphragm  1 . 
     Alternatively, coating layer  1 B may be formed only on the portion of base layer  1 A that is likely to cause unwanted resonance in diaphragm  1  as shown in  FIG. 7 .  FIG. 7  is a plan view of still another loudspeaker according to the exemplary embodiment. This loudspeaker includes a plurality of separate coating layers  1 B formed on base layer  1 A. These separate coating layers  1 B are near the outer periphery of diaphragm  1  and are equally distant from the center of diaphragm  1 . 
     In diaphragm  1  shown in  FIG. 7 , separate coating layers  1 B are formed only on the effective portion in order to. This configuration enables the loudspeaker to have a higher sound pressure level and to produce clearer sound as well as reducing the unwanted resonance, without a large increase in the entire weight of diaphragm  1 . 
     It is preferable that coating layer  1 B composed of chitin nanofibers has a thickness in a range from 3 to 20%, inclusive, of the entire thickness of diaphragm  1 . 
     Base layer  1 A may contain bamboo cellulose nanofiber. 
     When the natural fibers composing base layer  1 A of diaphragm  1  contains bamboo fibers, the cellulose fibers can be cellulose nanofibers. 
     A combination of these configurations not only makes coating layer  1 B more effective but also makes the fibers of base layer  1 A more entangled with each other. This synergistic effect allows the loudspeaker to have the further higher limit frequency and to produce clearer sound. 
     The loudspeaker according to the present disclosure thus has a good balance of physical properties.