Patent Publication Number: US-2007098207-A1

Title: Structure of ribbon type planar speaker

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
      This application claims the priority benefit of Taiwan application serial no. 94218898, filed Nov. 2, 2005. All disclosure of the Taiwan application is incorporated herein by reference.  
     BACKGROUND OF THE INVENTION  
      1. Field of Invention  
      The present invention relates to a structure of a ribbon type planar speaker.  
      2. Description of Related Art  
      The ribbon type planar speaker has been used and developed for quite a long time. U.S. patents No. 4,273,968, No. 4,480,155, No. 6,104,825, No. 4,471,173 and No. 5,021,613 disclose the structure and related content of the ribbon type planar speaker. The structure of an ordinary ribbon type planar speaker is shown in  FIG. 1 , wherein a metal conductor  14  is directly molded on an insulating thin film  15 , thus forming a diaphragm with an electric circuit. When current signals are led to the speaker, the current that passes through the electric circuit of the diaphragm and the magnetic field of a magnet  11  on the ribbon type planar speaker generate an electromagnetic applied force, such that the diaphragm vibrates and pushes air to make sounds, i.e., converting electric energy into sound energy.  
      Different from an ordinary moving-coil loudspeaker, the electric circuit of the diaphragm of the ribbon type planar speaker has the function of a voice coil which can drive the diaphragm to vibrate. However, the diaphragm of an ordinary moving-coil loudspeaker must be connected with a voice coil and makes sounds via the vibration of the voice coil. As the diaphragm of the ribbon type planar speaker is thin and light, the vibration system thereof is lighter than that of an ordinary moving-coil loudspeaker, and thus the sensitivity of the ribbon type planar speaker is higher.  
      Referring to  FIG. 2 , it is a structure of another ordinary ribbon type planar speaker. Bar magnets  21 ,  22  parallel to the direction of the electric circuit are disposed on both upper and lower sides or either of the above two sides of a diaphragm  25 . The bar magnets  21 ,  22  are symmetrically disposed on the upper and lower sides of the diaphragm  25 , such that the magnetic fields on both sides of the diaphragm are symmetric, thus achieving a wide linear dynamic range and a high sensitivity.  
      The ribbon type planar speaker and the moving-coil loudspeaker both can use the piston motion manner together with the infinite baffle board condition to explain characteristics of the output audio frequency when the vibration system of the speaker vibrates under the electromagnetic force. Referring to Chapter 8, the third edition of “Fundamentals of Acoustics” by Lawrence E. Kinsler, the relationship between the far field sound pressure generated right in front of the speaker and the frequency, equivalent radius of the speaker diaphragm and distance is as follows.
 
Sound pressure  p= (√2*π 2 *ρ 0   *a   2   *f   2 *ξ)/ r  
 
 wherein, π is circumference ratio, ρ 0  is air density, a is equivalent radius of the speaker diaphragm during the piston motion, f is vibration frequency, ξ is the amplitude of vibration of the diaphragm during the piston motion, and r is the distance between the test point and the speaker. 
 
      Therefore, under the same diaphragm area and testing distance, the sound pressure is in direct ratio with the square of the frequency and the first power of the amplitude. To generate the same sound pressure, the diaphragm requires a small amplitude at a high frequency; on the contrary, the diaphragm requires a large amplitude at a low frequency. The low frequency response of the ribbon type planar speaker is poor unless the area of the diaphragm is enlarged.  
     SUMMARY OF THE INVENTION  
      According to one aspect of the present invention, a structure of the ribbon type planar speaker is provided, which comprises a composite diaphragm with a flexible edge fixed on a fixed end; a first magnet and a second magnet, respectively disposed above and below the composite diaphragm, and respectively fixed on an upper frame and a lower frame, wherein both of the upper frame and the lower frame have openings; and a second lower frame, disposed below the lower frame, wherein an air damping absorption layer is disposed between the lower frame and the second lower frame.  
      According to another aspect of the present invention, a structure of the ribbon type planar speaker is provided, wherein the composite diaphragm of the ribbon type planar speaker is of a spatial structure, so as to enhance the rigidity of the composite diaphragm. As such, the composite diaphragm is still approximately in a piston motion with a large amplitude, thus reducing the low frequency distortion.  
      Other advantages and details of the present invention will be further illustrated below by embodiments.  
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a schematic view of a conventional rectangular ribbon type planar speaker.  
       FIG. 2  is a schematic view of another conventional ribbon type planar speaker.  
       FIG. 3  is a schematic view of the ribbon type planar speaker according to an embodiment of the present invention.  
       FIG. 4  is a top view of the composite diaphragm according to an embodiment of the present invention. 
    
    
     DESCRIPTION OF EMBODIMENTS  
      Referring to  FIG. 3 , it is a sectional view of the ribbon type planar speaker according to an embodiment of the present invention. A one-piece integrated composite diaphragm  41  of the speaker comprises an upper ribbon type diaphragm, a planar reinforcement structure  46  and a lower ribbon type diaphragm. The upper ribbon type diaphragm comprises an upper insulating layer  41 A and a metal electrode circuit  43 A above the upper insulating layer. The lower ribbon type diaphragm comprises a low insulating layer  41 B and a metal electrode circuit  43 B below the lower insulating layer. The planar reinforcement structure  46  further comprises a flexible edge structure  47  fixed on a fixed end  48 A and a fixed end  48 B. In the present embodiment, the fixed end  48 A is disposed on an upper frame  42 A and the fixed end  48 B is disposed on a lower frame  42 B. In the present embodiment, the upper ribbon type diaphragm and the lower ribbon type diaphragm are double-layer composite metal thin films formed by metal foils  43 A,  43 B and polymer insulating layers  41 A,  41 B. The double-layer composite metal thin film is fabricated by coating the metal foil with the polyamic acid resin solution and then drying by baking. However, the metal layer can also be formed on the polyimide resin thin film by physical vapor deposition or chemical electroplating. As the double-layer composite metal thin film does not have an adhesive layer as buffer layer, the thermal expansion coefficients of the metal layers  43 A,  43 B and polymer insulating layers  41 A,  41 B had better be the same, so as to prevent the composite metal thin film being curled and the interlaminar stress due to the thermal expansion difference.  
      The planar reinforcement structure  46  with the flexible edge structure  47  is fabricated as follows. After Nomex temperature-resistant fiber paper (from DuPont Company, model: Nomex 411) and impregnated epoxy resin/MEK (methyl-ethyl ketone) solution (Taiwan Kinmen Chemical Industry No. AS1032 single-fluid epoxy resin product) become B-stage epoxy resin pre-preg, the upper and lower ribbon type diaphragms are respectively disposed above or below the one-piece integrated flat plate structure  46  and the conductor circuits  43 A,  43 B of the ribbon type diaphragm have one side facing outside. After that, the composite diaphragm structure  41  with a flexible edge is formed by performing thermal compression molding in a thermal compression mould (not shown).  
      The planar reinforcement structure  46  with the flexible edge structure  47  is made of impregnated fiber, Nomex temperature-resistant fiber paper (from DuPont, model: Nomex 411), or an ordinary cone paper material, such as ordinary cotton/hemp couched cone paper, cotton cloth, synthetic fiber textile such as synthetic paper or cloth made of Conex fiber from Japan Teijin Company or Kevlar fiber from America DuPont Company, or synthetic paper or cloth blended by the above fibers, or even metal thin films. The key point is that the material must be thin, light and can provide reinforcement to the structure of the composite metal thin film, thus achieving the advantages of lightness and sensitivity.  
      The upper and lower ribbon type diaphragms can also be three-layer composite metal thin films clad by metal foils, adhesive layers (not shown) and polymer film layers. The metal foil can be an aluminum foil, aluminum alloy foil or composite metal foil clad by aluminum and other metals, and the thickness thereof had better be smaller than 35 μm. The adhesive layer can be an acrylic resin, epoxy resin or polyimide film layer and the thickness thereof is smaller than 10 μm. The polymer film layer can be polyester adhesive layer such as Mylar thin film produced by America DuPont Company, polyimide resin thin film such as Kapton thin film produced by DuPont Company or PEI plastic thin film produced by America GE, PEN plastic thin film produced by America DuPont Company or other temperature-resistant plastic thin films.  
      Bar magnets  44 A and  44 B are respectively disposed above and below the composite diaphragm  41  and are respectively fixed on the upper frame  42 A and the lower frame  42 B. The longitudinal axis direction of the bar magnet  44 A is perpendicular to the drawing sheet and is parallel to the current direction on the metal electrode circuit  43 A underneath the bar magnet  44 A. The symbols N and S on the bar magnet  44 A represent that the magnetizing direction thereof is parallel to the drawing sheet, wherein the upper side is S pole and the lower side is N pole. The bar magnet  44 A is fixed on the upper frame  42 A of the ribbon type planar speaker. The external sides of the flexible edge structure  47  of the composite diaphragm  41  are fixed by the fixed ends  48 A and  48 B, thus fixing the composite diaphragm  41  between the upper frame  42 A and the lower frame  42 B. When the composite diaphragm  41  vibrates, sound can be transmitted out from openings  42 C in the upper frame  42 A and openings  42 D in the lower frame  42 B simultaneously.  
      A second lower frame  42 E is further disposed underneath the lower frame  42 B. A filled air permeable air damping absorption layer  49  is in the space between the lower frame  42 B and the second lower frame  42 E, which can be an air permeable sound absorption material made of various wool, natural cotton, paper pulp fiber, glass fiber, foam, synthetic fiber and so on. The filled air permeable air damping absorption layer can also be disposed at the external sides of the upper frame  42 A and the lower frame  42 B or at either side. The quantity, size and disposing position of the bar magnets  44 A,  44 B or only disposing the bar magnets either above or below the composite diaphragm  41  can all be appropriately adjusted according to the demands of generating the sound pressure. The spaces above/below the composite diaphragm  41  and inside the ribbon type planar speaker can be filled by sound absorption and damping materials such as various wool, natural cotton, glass fiber and synthetic fiber, so as to reduce unnecessary sound reflection and vibration. The surfaces of the flexible edge structure  47  and the composite diaphragm  41  can also be coated with damping glue to enhance the damping of the flexible edge. The damping glue can be acrylic resin, epoxy resin, polyurethane, silicone or other soft polymer adhesive materials.  
      Referring to  FIG. 4 , it is a top view of the composite diaphragm structure  41 . As an ordinary planar composite diaphragm is of a planar thin structure and the rigidity in the direction perpendicular to the diaphragm plane is very weak, the planar composite diaphragm is easily deformed and it is easy to cause irregular splitting vibration under a big stroke. In the present embodiment, the ordinary planar composite diaphragm is replaced by a spatial composite diaphragm, wherein the spatial diaphragm is formed by compression molding during the composite thermal compression molding or after the planar composite diaphragm is fabricated. Perform plastic deformation to an upper metal electrode layer  53  and a lower metal electrode layer  56  of the compressed composite diaphragm  41 , such that the planar structure is changed into a spatial structure of a compressed plastic deformation region  52  with a composite diaphragm. Therefore, the whole composite diaphragm  41  comprising metal foils  43 A,  43 B and polymer insulating layers  41 A,  41 B is approximately of a “ ”-shaped spatial structure, thus enhancing the rigidity of the composite diaphragm in the direction perpendicularly to the diaphragm plane. In view of the above, the air damping is increased due to the set of the air damping absorption layer  49 , thereby limiting the irregular vibration between the diaphragm and the pushed air. As such, when the ribbon type planar speaker diaphragm is in vibration, the effective vibration region performs uniform synchronous vibration, thus making the frequency response region broader and smoother. Moreover, as the composite diaphragm is formed by compression molding during the composite thermal compression molding, the rigidity of the composite diaphragm perpendicularly to the direction of the diaphragm plane is enhanced and the diaphragm is still approximately in a piston motion with a large amplitude, thus reducing the diaphragm deformation and distortion caused by splitting vibration. Though the present invention is illustrated by the above embodiments, they do not mean that the protecting region of the present invention is limited by the above illustration. For those skilled in the art, various modifications can be made, for example, altering the vibration-absorption materials or magnet conducting materials, altering the shape of the conducting layer on the diaphragm, changing the metal layers into other conducting layers, changing the direction of the components without changing their relative positions, changing the shapes of the magnets and the magnet conducing components or even changing the magnets at both sides of the composite diaphragm into at one side, so as to achieve the same function. However, all those changes must not depart from the spirit of the present invention and still fall in the protecting range of the present invention. Moreover, the protecting range of the present invention falls in the appended claims.