Patent Publication Number: US-2017359656-A1

Title: Electro-acoustic driver and bobbin therefore

Description:
RELATED APPLICATION 
     This application is a continuation-in-part application of U.S. patent application Ser. No. 15/182,055, filed Jun. 14, 2016 and titled “Electro-acoustic Driver having Compliant Diaphragm With Stiffening Element,” the contents of which are incorporated herein by reference. 
    
    
     BACKGROUND 
     This disclosure relates to an electro-acoustic device and a bobbin therefore. 
     SUMMARY 
     In general, in one aspect, an electro-acoustic driver includes a diaphragm formed of a compliant material and having a perimeter, a front surface, a back surface, an inner region and an outer region between the perimeter and the inner region, and a substantially planar shape when the diaphragm is at rest. A bobbin has an inner surface, an outer surface and a bobbin axis. The bobbin is configured to hold a winding of an electrical conductor on the outer surface. A housing has an end and a housing axis that is substantially coaxial with the bobbin axis. The perimeter of the diaphragm is fixed to the end of the housing. A stiffening element is fixed to one or more of the front surface and the back surface at the inner region of the diaphragm. A motion of the bobbin along the bobbin axis generates a movement of the inner region of the diaphragm to thereby generate an acoustic signal that propagates from the front surface of the diaphragm. The bobbin further includes a substantially planar surface at an end of the bobbin which is normal to the bobbin axis and fixed to the back surface of the diaphragm at the inner region. The stiffening element includes the substantially planar surface of the bobbin. The bobbin includes one or more of (a) legs extending from the outer surface to the substantially planar surface, and (b) a plurality of through holes in the substantially planar surface. 
     Implementations may include one or more of the following, in any combination. The substantially planar surface is fixed directly to the back surface of the diaphragm. The driver further includes a layer of adhesive to fix the substantially planar surface of the bobbin to the back surface of the diaphragm at the inner region. The bobbin has an outer diameter and the inner region of the diaphragm has a diameter that is substantially equal to the outer diameter of the bobbin. The outer region has an annular shape. The bobbin includes both of (a) legs extending from the outer surface to the substantially planar surface, and (b) the plurality of through holes in the substantially planar surface. The bobbin includes four legs extending from the outer surface to the substantially planar surface. 
     In general, in another aspect, an electro-acoustic driver includes a housing having a cylindrical shape and a housing axis. A bobbin has an outer surface, a substantially planar surface at an end of the bobbin, and a bobbin axis that is substantially coaxial with the housing axis. The bobbin is disposed inside the housing and configured to move along the bobbin axis. An acoustic diaphragm is secured to the substantially planar surface at the end of the bobbin. A compliant suspension surrounds the acoustic diaphragm and is secured to the acoustic diaphragm and the housing. The bobbin includes one or more of (a) legs extending from the outer surface to the substantially planar surface, and (b) a plurality of through holes in the substantially planar surface. 
     Implementations may include one or more of the features in paragraph 12 above in any combination. 
     In general, in yet another aspect, a bobbin for an electro-acoustic driver includes an outer surface, a substantially planar surface at an end of the bobbin, and a bobbin axis that is substantially coaxial with a housing axis. The bobbin is disposable inside a housing and configured to move along the bobbin axis. The substantially planar surface at the end of the bobbin is securable to an acoustic diaphragm. The bobbin includes one or more of (a) legs extending from the outer surface to the substantially planar surface, (b) a wall which extends about substantially all of a perimeter of the planar surface, and (c) a plurality of through holes in the substantially planar surface. 
     Implementations may include one or more of the following features and features in paragraph 12 above in any combination. The wall stands proud of the planar surface by between about 2 to about 15 microns. The wall has a thickness of between about 5 microns to about 35 microns. The wall is substantially in the shape of an annular ring. The bobbin includes all of (a) legs extending from the outer surface to the substantially planar surface, (b) the wall, and (c) the plurality of through holes in the substantially planar surface. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and further advantages of examples of the present inventive concepts may be better understood by referring to the following description in conjunction with the accompanying drawings, in which like numerals indicate like structural elements and features in various figures. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of features and implementations. 
       FIG. 1 A,  FIG. 1B  and  FIG. 1C  are a perspective view, a cutaway view and an exploded cutaway view, respectively, of an electro-acoustic driver. 
         FIG. 2  is an illustration of the diaphragm of  FIGS. 1A to 1C . 
         FIG. 3  is a cross-sectional side view of the housing, bobbin and coil assembly of  FIGS. 1A to 1C  according to an example in which an inner region of the diaphragm is stiffened by an adhesive. 
         FIG. 4  is an alternative example in which a rigid object is used to stiffen the inner region of the diaphragm. 
         FIG. 5  is another alternative example in which a bobbin includes a planar surface to stiffen the inner region of the diaphragm. 
         FIGS. 6A-C  are perspective, top and side views of another alternative example of a bobbin. 
     
    
    
     DETAILED DESCRIPTION 
     Modern in-ear headphones, or earbuds, typically include microspeakers. The microspeaker may include a coil wound on a bobbin that is attached to an acoustic diaphragm. Motion of the diaphragm due to an electrical signal provided to the coil results in generation of an acoustic signal that is responsive to the electrical signal. The microspeaker may include a housing, such as a sleeve or tube, which encloses the bobbin and coil, and a magnetic structure. As the size of the earbud decreases, it becomes increasingly difficult to fabricate the acoustic diaphragm and surrounding suspension at one end of the bobbin and housing. 
       FIG. 1A ,  FIG. 1B  and  FIG. 1C  are a perspective view, a cutaway perspective view and an exploded cutaway view, respectively, of an example of an electro-acoustic driver  10  (e.g., a microspeaker) that can be used in a miniature earbud. The microspeaker  10  includes a cylindrical housing  12  having an opening at both ends. Inside the housing  12  is a bobbin  14  that is nominally cylindrical in shape and open at both ends. In some examples, the housing  12  is made of stainless steel and the bobbin  14  is made of a polyimide (e.g., KAPTON®) or polyethylene terephthalate (PET) (e.g., MYLAR®). The housing  12  and bobbin  14  are secured at one of their open ends to a diaphragm, or membrane,  16  formed of a compliant material such as an elastomer. A coil assembly  18  is wound onto an outside surface of the bobbin  14 . The coil assembly  18  includes a winding of an electrical conductor and may include a structure to hold the winding is a desired shape and/or to secure the winding on the outer surface of the bobbin  14 . A magnet assembly  20  is secured to a platform  22  at an end of the housing  12  that is opposite to the diaphragm  16 . The magnet assembly  20  includes two magnet pieces  20 A and  20 B that can be, for example neodymium magnets, and an intervening coin  20 C. The magnet assembly  20  extends along a housing axis  24  (i.e., a cylinder axis) and into an open region inside the bobbin  14 . The axis of the bobbin  14  is substantially co-axial with the housing axis  24 . 
     The electro-acoustic driver  10  may be miniaturized such that the outer diameter φH of the housing and the diameter φD of the diaphragm  16  are less than about 4.7 mm. The small dimensions present various fabrication problems, including how to provide a small acoustic diaphragm supported by a compliant surround. 
     In some examples, the housing  12  has an outside diameter φH that is less than about 8 mm. In some examples, the housing  12  has an outside diameter φH that is less than about 4.5 mm. In other examples, the housing  12  has an outside diameter φH that is between about 3.0 mm and 4.5 mm. In other examples, the housing  12  has an outside diameter φH that is between about 3.3 mm and 4.2 mm. In other examples, the housing  12  has an outside diameter φH that is between about 3.6 mm and 3.9 mm. In some examples, the magnet pieces  20  have a diameter φM that is between about 1.5 mm and 4.5 mm. In other examples, the magnet pieces  20  have a diameter φM that is between about 2.0 mm and 4.0 mm. In other examples, the magnet pieces  20  have a diameter φM that is between about 2.5 mm and 3.5 mm. 
     The radiating area is approximately equal to the area of an inner (central) region of the diaphragm  16  that is stiffened in any one of a variety of ways, including those described in detail below, for relatively higher frequencies above where the non-stiffened portion of the diaphragm  16  (i.e. a surround) starts to break up. For relatively lower frequencies below these higher frequencies about half of the surround also contributes to the radiating area of the diaphragm. 
     In some examples, a ratio of the radiating area to the total cross sectional area of the driver  10  is about 0.7. In some examples, a ratio of the radiating area to the total cross sectional area of the driver  10  is between 0.57 and 0.7. In some examples, a ratio of the radiating area to the total cross sectional area of the driver  10  is between 0.6 and 0.67. In some examples, a ration of the radiating area to the total cross sectional area of the driver  10  is between 0.62 and 0.65. 
     Referring also to  FIG. 2 , the diaphragm  16  is shown in isolation with its thickness t exaggerated to simplify identification of various features. The diaphragm  16  may be formed of an elastomeric material such as a volume of liquid silicone rubber that is cured to provide the desired thickness t and to adhere to an end of the bobbin  14  and an end of the housing  12 . The diaphragm  16  has a perimeter, i.e., the circumferential outer edge at a radius Ro, a front surface  32  and a back surface  34 . The diaphragm  16  includes an inner region inside the dashed circular line  36  of radius Ri and an outer region defined by an annular shape that extends from the dashed circular line  36  to the perimeter. The smaller radius Ri is approximately equal to the outer diameter of the cylindrical bobbin  14  and the larger radius Ro is approximately equal to the outer diameter of the housing  12 . By way of non-limiting examples, the diaphragm thickness t can be a few tens of microns to more than 100 μm and the diameter Ro may be less than 4.7 mm. 
     The bobbin  14  moves substantially along its axis, and the housing axis  24 , in response to an electrical current conducted through the winding of the coil assembly  18 . This motion causes the inner region of the diaphragm  16  to move axially and displace air to thereby generate an acoustic signal. 
     The diaphragm  16  has a substantially planar shape when at rest, that is, when no electrical signal is applied to the winding of the coil assembly  18  to generate sound. When the microspeaker  10  is driven by an electrical signal to cause a motion of the bobbin  14  along the housing axis  24 , the compliant nature of the diaphragm  16  results in its deformation. The inner region of the diaphragm  16  acts as an acoustic diaphragm that is used to generate the acoustic signal; however, due to the low value of Young&#39;s modulus for the diaphragm  16 , the inner region can behave similar to a drum head. In particular, the inner region can exhibit unwanted structural resonances with the operating frequency band of the driver  10  and can result in a reduction in driver efficiency. 
     In various examples described below, the inner region of the diaphragm  16  is stiffened, or made rigid, by a stiffening element to substantially reduce or eliminate unwanted resonances during operation. The outer region of the diaphragm  16  is a compliant suspension that surrounds the stiffened inner region. In one example, the stiffening element is a rigid layer of material that is secured to the back surface  34  of the diaphragm  16  over the inner region and which is also secured to the adjacent portion of the inner surface of the bobbin  14 . Alternatively, the stiffening element is a rigid object that is secured to the back surface  34  of the diaphragm  16  within the inner region. The object may be a standalone structure (e.g., a solid disc) or the object may be a structural feature of the bobbin. As a result of the stiffening of the inner region, unwanted resonance frequencies are shifted out of the operating bandwidth of the electro-acoustic driver  10  and/or the displacement of the diaphragm  16  at these resonance frequencies is substantially reduced. Consequently, a smoother acoustical frequency response can be achieved. In addition, stiffening of the inner region has an additional benefit of increasing the effective piston area of the electro-acoustic driver to thereby increase the sound pressure output for a particular bobbin displacement magnitude. 
       FIG. 3  shows a cross-sectional side view of the housing  12 , bobbin  14  and coil assembly  18  according to one example in which the inner region of the diaphragm  16  is stiffened. A small quantity of adhesive is dispensed into the “cup-shaped” structure defined by the bobbin  14  and diaphragm  16  to partially fill the cup. An adequate volume of adhesive is used to ensure that the inner region of the diaphragm  16  is fully covered by the adhesive layer. The adhesive is then cured to form a rigid layer  40  that adheres to a portion of the inner surface  42  of the bobbin  14  and the back surface  34  (see  FIG. 2 ) of the diaphragm  16 . A meniscus  44  may form along the inner wall and improve adhesion to the bobbin  14 . 
       FIG. 4  shows an alternative example in which a rigid object  50  (e.g., disc) is used to stiffen the inner region of the diaphragm  16 . The disc  50  may be a high strength thermoplastic thin film such as a polyetherimide (e.g., ULTEM®). The disc  50  has a diameter that is less than the inner diameter of the bobbin  14  to enable the disc  50  to be inserted into the bobbin  14 ; however, the difference in the diameters is kept small to maximize contact with the inner region of the diaphragm  16 . A thin layer of a bonding agent, or adhesive, may be used to bond the disc  50  to the inner region of the diaphragm  16 . The bonding agent or adhesive may also be used to bond to the inner cylindrical surface of the bobbin  14 . Alternatively, the disc  50  may be placed on top of an uncured layer of an elastomeric material (e.g., liquid silicone rubber) used to create the diaphragm  16 . Subsequent curing of the elastomeric layer results in a bond of the diaphragm  16  directly to the disc  50  and the end of the bobbin  14 . 
       FIG. 5  shows another alternative example in which a bobbin  60  contains structure that is used to stiffen the inner region. The bobbin  60  has a cylindrical portion  60 A similar to the bobbin  14  of  FIG. 3  and  FIG. 4 ; however, the bobbin  60  also includes an end surface  60 B at one end. The end surface  60 B may be integrated with the cylindrical portion  60 A as a single body. In an alternative configuration, the end surface  60 B may be formed independently and then secured to the end of the cylindrical portion  60 A. The end surface  60 B may be fixed to the back surface  34  (see  FIG. 2 ) of the diaphragm  16  along the inner region using a bonding agent or adhesive. Alternatively, the end surface  60 B may be disposed within an uncured layer of an elastomeric material used to create the diaphragm  16  so that subsequent curing of the elastomeric material causes the diaphragm  16  to adhere to the surface  60 B. 
       FIG. 6A-C  show another example of a bobbin  62  having an inner surface  64 , an outer surface  66  and a bobbin axis  68 . The bobbin  62  is configured to hold a winding of an electrical conductor (not shown) on the outer surface  66  of a continuous cylindrical section  67 . The bobbin  62  has a substantially planar surface  70  at an end of the bobbin. The substantially planar surface  70  is substantially normal to the bobbin axis  68  and can be fixed to the back surface of the diaphragm at the inner region (as discussed above). The stiffening element discussed above can be the substantially planar surface  70  of the bobbin  62 . 
     The substantially planar surface  70  and the portion of the bobbin  62  on which the surface  70  resides can have a small radius to it so that this surface is slightly convex or concave. This small radius substantially increases the stiffness of this portion of the bobbin which allows thickness of this portion to be reduced, thereby reducing the mass of the bobbin. A small concavity or convexity provides the benefits of having a somewhat flat surface (to place on a flat elastomer film such as the diaphragm  16  mentioned above)) and provides some of the benefits of a curved surface (e.g. increased stiffness). 
     The bobbin  62  includes legs  72  extending from the outer surface  66  to the substantially planar surface  70 . In this example the bobbin includes four legs, but there could be as few as two legs if they are wide enough to provide sufficient rigidity to the bobbin. A plurality of through holes  74  are provided in the substantially planar surface  70 . The holes  70  (a) provide an escape path for air when the diaphragm is being secured to the bobbin  62 , and (b) reduce the overall mass of the bobbin. The bobbin  62  can be made by a micro injection molding process and is preferably made of plastic. 
     The bobbin  62  also includes a wall (or knife edge)  65  which extends about substantially all of a perimeter of the planar surface  70 . The wall  65  preferably stands proud of the planar surface  70  by between about 2 to about 15 microns and has a thickness of between about 5 microns to about 35 microns. In this example the wall is substantially in the shape of an annular ring, but the wall could be in other shapes such as a square, rectangle, triangle or pentagon. The purpose of the wall  65  is to initiate contact with an adhesive layer that is used to secure the bobbin  62  to a diaphragm. Without the wall  65 , or if the transition from the planar surface  70  to the legs  72  is not sharp, the location of the adhesive coming in contact with the planar surface  70  can see relatively large changes for a small location error, thereby effecting the symmetry of the transducer about the bobbin axis  68 . As shown in  FIG. 6B , in this example the bobbin  62  has an outer diameter of 2.77 mm and a diameter to an outside of the wall  65  of 2.5 mm. As shown in  FIG. 6C , in this example the bobbin  62  has a dimension along the bobbin axis  68  of 1.62 mm. 
     A number of implementations have been described. Nevertheless, it will be understood that the foregoing description is intended to illustrate, and not to limit, the scope of the inventive concepts which are defined by the scope of the claims. Other examples are within the scope of the following claims.