Patent Abstract:
An electromagnetic transducer such as an audio speaker which includes an asymmetric diaphragm to deliver smooth frequency response with reduced distortion by reduction of common modes in the diaphragm. Other benefits such as asymmetric directivity patterns can be realized. The asymmetric cone has a perimeter OD at which a surround may be coupled, and an ID at which a bobbin or spacer may be coupled. The center of the ID is not coincident with the center of the OD. The transducer further includes a stabilization mechanism for reducing rocking of the diaphragm assembly. The stabilization mechanism may include mass balancing of the diaphragm and/or adjustments to the location or symmetry of the suspension components.

Full Description:
RELATED APPLICATION 
   This application is a divisional of application Ser. No. 10/334,752 filed Dec. 31, 2002 by these inventors, and claims benefit of that filing date. 

   BACKGROUND OF THE INVENTION 
   1. Technical Field of the Invention 
   This invention relates generally to electromagnetic transducers such as audio speakers, and more specifically to an electromagnetic transducer having a diaphragm or cone which is asymmetric, meaning that the ID of the cone is not at the geometric center of the OD of the cone. The ID or inner diameter refers to the location, typically but not always a hole, where the bobbin attaches to the diaphragm. The OD or outer diameter refers to the outer perimeter where typically the surround attaches to the diaphragm. 
   2. Background Art 
     FIG. 1  illustrates a conventional audio speaker  10  such as is known in the prior art. The speaker includes a motor assembly  12  coupled to a diaphragm assembly  14  by a frame  16 . The motor assembly includes a magnetic air gap  18  over which magnetic flux flows. The diaphragm assembly includes an electrically conductive voice coil  20  which is rigidly attached to a bobbin or voice coil former  22 . The voice coil is suspended within the magnetic air gap to provide mechanical force to an acoustical radiating member  24 , often termed a diaphragm or cone, which is coupled to the bobbin. When an alternating electric current is passed through the voice coil, the voice coil moves axially in the air gap, causing the diaphragm to generate sound waves. The diaphragm assembly further includes two suspension components which serve to keep the bobbin and diaphragm centered and aligned with respect to the motor assembly, while allowing axial movement. A damper or spider  26  is coupled to the bobbin and the frame, and a surround  28  is coupled to the diaphragm and the frame. A dust cap  30  seals the assembly and protects against infiltration of dust particles and other stray materials which might contaminate the magnetic air gap and thereby interfere with the operation or quality of the speaker. 
   The motor assembly has an axis A m  typically understood to be at the axial center of the magnetic air gap in which the voice coil rides. The diaphragm has an OD or outer perimeter which has a geometric center or axis A od . It is the same distance OD 1  from the axis A od  to a first point on the OD and to a second point on the OD, which two points are radially opposite each other. The diaphragm may be axisymmetric, in the case of e.g. a round 6″ speaker. Alternatively, the diaphragm may be bilaterally symmetric, in the case of e.g. an elliptical 6×9 speaker. Other diaphragm OD shapes are known in the art, as well. The diaphragm also has an ID or inner perimeter which has a geometric center or axis A id . It is the same distance ID 1  from the axis A id  to a first point on the ID and to a second point on the ID, which two points are radially opposite each other. In nearly all cases, speakers use a cylindrical bobbin and a circular ID, but a few exceptions are known. The spider has a center or axis of suspension A sp , and the surround has an center or axis of suspension A su . 
   As shown in  FIG. 1 , virtually all known speakers are constructed such that the motor axis A m , the axis A od  of the OD, the axis A id  of the ID, the axis of suspension A sp  of the spider, and the axis of suspension A su  of the surround, are all coaxial with one another. 
   Ordinarily, in most engineering applications it is desirable to achieve symmetry. However, in audio applications, symmetry has some disadvantages. For example, a symmetric cone exhibits the same breakup modes in all radial segments, as each radial segment has the same shape, size, mass, etc. as the others. As another example, a symmetric speaker exhibits equal diffraction characteristics and cone/edge junction modes at all radial segments. 
     FIGS. 2 and 3  are copied from U.S. Pat. No. 5,022,488 “Transducer Enclosure” issued Jun. 11, 1991 to William House and assigned to Harman International. The House patent teaches a speaker  31  having an asymmetric diaphragm  33 . That inventor was addressing a completely unrelated problem, that of fitting two speakers  35 ,  37  into a single cabinet  39  with separate pressure venting for each. He appears to have moved the woofer&#39;s motor structure  41  away from the center of the woofer&#39;s diaphragm  33  merely for the purpose of providing physical space for the tweeter  37  to fit in front of a portion of the woofer&#39;s diaphragm, and not to have recognized any other benefits from the asymmetry. Indeed, the patent states that “it is not necessary for diaphragm [of the woofer] to be asymmetric, nor for diaphragm [of the tweeter] to be symmetric, nor for either transducer to be a diaphragm type at all.” (col. 3 lines 57–60, reference numbers omitted, bracketed text added). It is noteworthy that the woofer diaphragm is an inverted cone, rather than a conventionally oriented cone with its bell facing outward. This was clearly done to make still more room for the tweeter within the design constraint of gaining “the benefits of shallow loudspeaker mounting” (col. 1 line 11). 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be understood more fully from the detailed description given below and from the accompanying drawings of embodiments of the invention which, however, should not be taken to limit the invention to the specific embodiments described, but are for explanation and understanding only. 
       FIG. 1  shows, in cross-section, a conventional speaker geometry according to the prior art. 
       FIGS. 2 and 3  show a prior art two-speaker arrangement according to U.S. Pat. No. 5,022,488, in which one of the speakers has an asymmetric diaphragm. 
       FIG. 4  shows a front view of a circular OD diaphragm having an off-center ID. 
       FIG. 5  shows a front view of an elliptical OD diaphragm having an ID which is located on one line of symmetry but off the other line of symmetry of the ellipse. 
       FIG. 6  shows a front view of an elliptical OD diaphragm having an ID which is located off both lines of symmetry of the ellipse. 
       FIG. 7  shows, in cross-section, a speaker having an asymmetric diaphragm and an on-motor-axis symmetric spider. 
       FIG. 8  shows, in cross-section, a speaker having an asymmetric diaphragm and an asymmetric spider. 
       FIG. 9  shows, in cross-section, a speaker having an asymmetric diaphragm and an asymmetric spider. 
       FIG. 10  shows, in cross-section, a speaker having an asymmetric diaphragm and an off-motor-axis symmetric spider. 
       FIG. 11  shows, in cross-section, a speaker having an asymmetric diaphragm and an on-motor-axis symmetric spider, frame, and surround. 
       FIG. 12  shows, in cross-section, a speaker having an asymmetric diaphragm and an on-motor-axis symmetric spider, frame, and surround. 
       FIG. 13  shows, in partial cutaway top view, a speaker having an asymmetric diaphragm. 
       FIG. 14  shows, in cross-section, a portion of an asymmetric diaphragm assembly with mass balancing of the diaphragm by making the shorter side thicker than the longer side. 
       FIG. 15  shows, in cross-section, a portion of an asymmetric diaphragm assembly with mass balancing of the diaphragm by tapering the diaphragm thickness and weighting each side toward the outer edge of the short side. 
       FIG. 16  shows, in cross-section, a portion of an asymmetric diaphragm assembly with mass balancing after differential foaming of the diaphragm material to increase the stiffness of the long side. 
       FIG. 17  shows, in cross-section, a portion of an asymmetric diaphragm assembly with mass balancing assisted by suitably making the inactive attachment portion of the surround longer, and thus heavier, at the short side of the diaphragm. 
       FIG. 18  shows, in cross-section, a portion of an asymmetric diaphragm assembly with mass balancing assisted by making the inactive attachment portion of the surround thicker, and thus heavier, at the short side of the diaphragm. For convenience, a rear-attach surround is used. 
   

   DETAILED DESCRIPTION 
   The invention may be utilized in a variety of magnetic transducer applications, including but not limited to audio speakers, microphones, and the like. For the sake of convenience, the invention will be described with reference to audio speaker embodiments, but this should be considered illustrative and not limiting. For ease of illustration only, the invention will be illustrated with reference to an external magnet geometry speaker, but is not so limited. 
     FIG. 4  illustrates a front view of an axisymmetric-OD (round) diaphragm  24 A in which the ID is not concentric with the OD. The axis A id  is not coaxial with the axis A od . 
     FIG. 5  illustrates a front view of an elliptical diaphragm  24 B in which the ID center is not at the center of the OD. The elliptical diaphragm is bilaterally symmetric about a vertical line and a horizontal line. The ID is located on one of these lines but not on the other. The axis A id  is not coaxial with the axis A od . 
     FIG. 6  illustrates a front view of an elliptical diaphragm  24 C in which the ID center is not at the center of the OD. The elliptical diaphragm is bilaterally symmetric about a vertical line and a horizontal line. The ID center is not located on either of these lines. The figure further suggests that it is not necessarily the case that an asymmetric cone have its ID completely on or off a particular bilateral symmetry line; in other words, zero, one, or both of those lines may pass through the ID, so long as they are not both coincident with the center of the ID. The axis A id  is not coaxial with the axis A od . 
   The reader will readily appreciate that, while round and elliptical asymmetric diaphragms have been shown, the invention is not thus limited. The reader will further appreciate that the asymmetric diaphragm may be practiced with conventional, concave cones, or with inverted cones, or with flat diaphragms, or with other diaphragm configurations. 
     FIG. 7  illustrates one embodiment of a speaker  40  in which the axis A od  of the OD of the diaphragm is not coaxial with the axis A id  of the ID of the diaphragm. In this example, the A id  is coaxial with the axis A m  of the motor assembly. With the ID off-center from the OD, the diaphragm  24  includes a short side  24 S and a long side  24 L on opposite sides of the ID. The frame includes a first portion  16 A adapted to hold the short side  24 S and a second portion  16 B adapted to hold the long side  24 L. A symmetric spider  26  is coupled to and centered about the bobbin  22 , which is coaxial with the axis A m  of the motor assembly. 
   The axis of suspension A su  of the symmetric surround  28  is generally coaxial with the A od , but the axis of suspension A sp  of the symmetric spider  26  is generally coaxial with the A id , and the A od  and A id  are not coaxial. This may tend to cause rocking of the diaphragm assembly during operation of the speaker. 
   The short side  24 S and long side  24 L of the diaphragm have respected projected chords SS and LS. 
   The high frequency dispersion pattern of the speaker will be asymmetrically controlled by the resultant angle, from the primary motor axis, of the long side with respect to that of the short side. By employing an asymmetric diaphragm, the speaker designer can control the dispersion by modifying the ratio of the long side to short side, which in turn affects the respective angles of the diaphragm at those locations. 
     FIG. 8  illustrates a speaker  42 , demonstrating one possibility for making the A sp  more coaxial with the A su . The spider  26  is formed so as to not be axisymmetric about the bobbin  22 . The asymmetric spider includes a short side  26 A and a long side  26 C. The frame includes a first portion  16 A adapted to hold the short side  24 S of the diaphragm and the short side  26 A of the spider, and a second portion  16 C adapted to hold the long side  24 L of the diaphragm and the long side  26 C of the spider. Typically, the frame and spider may each be constructed with a continuously varying shape to provide a smooth transition from its first portion to its second portion. The geometric center of the spider has been moved from the axis A m  of the motor assembly toward the geometric center of the diaphragm A od , but at the cost of having larger, more compliant rolls of material in the longer side of the spider. 
   This embodiment has the disadvantage that the softer portion of the spider suspension (with larger, more compliant rolls) is supporting the heavier portion of the diaphragm on the diaphragm&#39;s longer side. The long side of the diaphragm may have a greater moment of rotational inertia about the A id  than does the short side, which may cause rocking in response to acceleration of the diaphragm assembly. This may be exacerbated by the spider being softer on the long side of the diaphragm. In other words, while the A sp  has been moved off of the A m , it has moved toward the A su  rather than away from it, and both suspension components have their axis of suspension on the same side of A m . 
     FIG. 9  illustrates one embodiment of a speaker  44  in which the axes of suspension have been adjusted to reduce rocking. The frame includes a first portion  16 A adapted to hold the short side  24 S of the diaphragm, and a second portion  16 D adapted to hold the long side  24 L of the diaphragm. A first portion  26 A of the spider is adapted to secure the bobbin to the first portion  16 A of the frame, and a second portion  26 D of the spider is adapted to secure the bobbin to the second portion  16 D of the frame. The A sp  of the asymmetric spider has been moved from the A m  of the motor assembly farther away from the A od  of the diaphragm, with the result of having smaller, and therefore stiffer, rolls of material in the shorter side of the spider which is suspending the bobbin on the long side of the diaphragm. Thus, the stiffer portion of the spider is suspending the longer, heavier side of the diaphragm, in order to balance the diaphragm displacement of both sides of the speaker, at resonance, which will in turn minimize the tendency for rocking to occur. In addition, the A sp  is moved to the opposite side of A m  from the A su  and thus the average of A sp  and A su  more closely coincides with A m  with the result that, if rocking occurs, the rotational center of the rocking will more closely coincide with the center of the voice coil, minimizing the chances of the voice coil striking or rubbing the motor structure. 
   In either of these embodiments, one could reduce the rocking tendency by altering the shape or compliance of the surround instead the spider. Or, one could alter both the spider and the surround. The skilled designer will need to take into account the relative stiffnesses of the surround and the spider, and the relative mass and balance of the diaphragm, as well as the relative mass and balance of the rest of the moving components including the spider and the surround, in determining where to place the axes of suspension of the surround and spider in order to achieve a balanced, non-rocking speaker. 
     FIG. 10  illustrates another embodiment of a speaker  46  which uses a symmetric spider  27 , but which moves the axis A sp  of the spider&#39;s suspension to be substantially coaxial with the axis A su  of the surround&#39;s suspension. The frame includes a first portion  16 A adapted to hold the short side  24 S of the diaphragm, and a second portion  16 E adapted to hold the long side  24 L of the diaphragm. A rigid, eccentric spacer  48  is coupled between the bobbin and the spider. The spacer includes a short side  48 S located with the short side of the diaphragm, and a long side  48 L located with the long side of the diaphragm. The geometric center of the spacer, as measured by distance R os  from points along its outer perimeter where it mates with the spider, is substantially coaxial with the A od  of the diaphragm and, thus, coaxial with the A su  of the surround. With the spacer rigidly coupled to the bobbin, the bobbin is effectively suspended by the spider about the axis A od  of the diaphragm, although the voice coils and bobbin themselves remain centered about the axis A m  of the motor assembly. The A sp  and A su  are substantially coaxial with A od , to reduce rocking. 
     FIG. 11  illustrates another embodiment of a speaker  50  which uses a similar arrangement, except that an eccentric diaphragm spacer  52  is coupled between the bobbin and the diaphragm, rather than between the bobbin and the spider. This speaker has the further advantage that, except for its asymmetric cone and the eccentric spacer, the rest of its components can be conventional, symmetric parts, including the frame  16 , spider  26 , and surround  28 . With a symmetric frame, the A od  is coaxial with the A m , the A su  is coaxial with the A sp , and, in fact, all four of those may be coaxial, with only the A id  being at a different location, which makes balancing the diaphragm assembly relatively simple. 
     FIG. 12  illustrates another embodiment of a speaker  54 , in which the eccentric diaphragm spacer  56  has an OD which is coupled to the bobbin ID, meaning that the spacer is disposed within the bobbin with a short side  56 S of the spacer adjacent the short side  24 S of the diaphragm, and a long side  56 L of the spacer adjacent the long side  24 L of the diaphragm. The off-center cone ID is coupled to the ID of the eccentric spacer. 
     FIG. 13  illustrates the speaker  40  of  FIG. 7 , in partial cutaway top view with some of the components removed for better visibility of underlying components. The speaker includes a motor assembly having a magnet  60  and a top plate  62  surrounding a pole piece  64 . The diaphragm assembly includes a voice coil  20  coupled to a bobbin  22  within the magnetic air gap  18  between the pole piece and the top plate. An asymmetric diaphragm  24  is coupled to the bobbin and includes a short side  24 S, a long side  24 L opposite the short side, and an intermediate portion  24 M providing a size transition between the short side and the long side. The A id  is not coincident with the A od . 
     FIG. 14  illustrates a partial diaphragm assembly in which the diaphragm has been balanced by adding mass to the short side  24 S by making it thicker, and/or by removing mass from the long side  24 S by making it thinner. Typically, but not necessarily, the thickness transition may be continuous around the diaphragm thickness from the thick side to the thin side. 
     FIG. 15  illustrates a partial diaphragm assembly in which the balancing has further been accomplished by tapering the diaphragm to bias one or both sides  24 S,  24 L of the diaphragm toward the outer edge of the short side  24 S. The short side is thicker at its OD edge (at the surround  28 ) than it is at its ID edge (at the bobbin  22 ), and the long side is thicker at its ID edge than it is at its OD edge. Typically, but not necessarily, the taper transition may be continuous around the diaphragm from one side to the other. 
     FIGS. 15 and 16  together also illustrate one particularly advantageous method of forming the diaphragm. The diaphragm is formed from a plastic such as polypropylene, or any other suitable material, in a mould having a taper as shown in  FIG. 15 . Then, at the correct time during the moulding and curing process, the mould halves for the top and bottom surfaces of the diaphragm are hinged partially open, with the hinge at or near the OD edge of the short side  24 S of the diaphragm, such that the mould opens more at the OD edge of the long side  24 L of the diaphragm than it does in the middle near the bobbin, and more in the middle than at the hinge. When the mould is hinged open, the material (typically in the presence of an activating agent) will foam to fill the newly enlarged space. Thus, the OD edge of the long side  24 L will foam to a more increased thickness than will the other portions of the diaphragm. The mass in each locality will stay the same as before the differential foaming, but the density will change in relationship to the locality&#39;s distance and angle from the hinge. The longer side will be less dense than the shorter side. In general, the more the foaming increases the thickness, the stiffer that locality will be. By appropriately selecting the diaphragm material, shaping the mould halves, locating the hinge, and hinging the mould halves open to induce foaming, the designer can achieve a diaphragm having any desired stiffness, thickness, and mass profile. In particular, it may be desirable to create a diaphragm which demonstrates equal stiffness along each chord, in every angle, to minimize cone breakup modes and other undesirable effects which may distort the sound produced by the speaker, and at the same time, achieve mass balancing in order to reduce rocking modes. 
   Alternatively, rather than shifting the mass of the diaphragm material, balancing may be accomplished by simply affixing a weight to the diaphragm in a suitable location. 
     FIG. 17  illustrates a different balancing mechanism, in which the mass of the surround  28  is used to balance the diaphragm  24 . On the short side, the portion  70  of the surround which is affixed to the diaphragm (and therefore is simply moving mass, and not an active part of the suspension) is cut or formed so as to be longer than that portion  72  which is affixed to the long side of the diaphragm. 
     FIG. 18  illustrates a similar balancing mechanism, in which the portion  74  of a rear-attached surround which is affixed to the short side of the diaphragm is made thicker than the portion  76  which is affixed to the long side of the diaphragm. 
   With reference now to any of the figures describing the invention, in order to achieve desired acoustic results, the dimensional ratio between the short side and the long side may be adjusted by moving the ID relative to the OD. Below are given example formulas which can be used in selecting ratios for round speakers. 
   Table 1 gives the formula for Phi, the value upon which the Fibonacci sequence and other natural phenomena are built. 
   
     
       
             
           
         
             
               TABLE 1 
             
             
                 
             
             
               Phi 
             
             
                 
             
           
           
             
               
                 
                   
                     
                       Phi 
                       = 
                       
                         
                           1 
                           + 
                           
                             
                               1 
                               + 
                               
                                 
                                   1 
                                   + 
                                   
                                     … 
                                   
                                 
                               
                             
                           
                         
                       
                     
                   
                 
               
             
             
                 
             
           
        
       
     
   
   Table 2 gives a simpler formula for approximating Phi, which may also be termed the golden ratio GR. Having an LS:SS ratio of approximately Phi or Phi 2  may, in many applications, produce good results. In some applications, having a ratio of the LS or the SS versus the intermediately sized portions of Phi or Phi 2  may be advantageous. 
   
     
       
             
           
         
             
               TABLE 2 
             
             
                 
             
             
               Golden Ratio aka Phi 
             
             
                 
             
           
           
             
               
                 
                   
                     
                       GR 
                       = 
                       
                         
                           0.5 
                           + 
                           
                             
                               5 
                             
                             2 
                           
                         
                         = 
                         1.618034 
                       
                     
                   
                 
               
             
             
                 
             
           
        
       
     
   
   Table 3 gives a formula for calculating the functional diameter FOD of the diaphragm, which is the overall diameter minus the distance which is occupied by the voice coil. 
   
     
       
             
           
         
             
               TABLE 3 
             
             
                 
             
           
           
             
               Functional OD 
             
             
                 
             
             
               FOD = OD − ID 
             
             
                 
             
           
        
       
     
   
   Table 4 gives a formula for calculating the length of the projected chord LS on the longer side of the diaphragm, measured from the bobbin to the surround. Bdepth is the depth of the cone or diaphragm, or, in other words, the distance between the diaphragm&#39;s OD plane and the diaphragm&#39;s ID plane. 
   
     
       
             
           
         
             
               TABLE 4 
             
             
                 
             
             
               Length of Projected Long Chord 
             
             
                 
             
           
           
             
               
                 
                   
                     
                       LS 
                       = 
                       
                         
                           ⌊ 
                           
                             
                               
                                 GR 
                                 2 
                               
                               · 
                               FOD 
                             
                             - 
                             
                               
                                 
                                   
                                     
                                       ( 
                                       
                                         
                                           - 
                                           
                                             Bdepth 
                                             2 
                                           
                                         
                                         + 
                                         
                                           2 
                                           · 
                                           
                                             GR 
                                             2 
                                           
                                           · 
                                           
                                             Bdepth 
                                             2 
                                           
                                         
                                         + 
                                         
                                           
                                             GR 
                                             2 
                                           
                                           · 
                                           
                                             FOD 
                                             2 
                                           
                                         
                                         - 
                                       
                                     
                                   
                                 
                                 
                                   
                                     
                                       
                                         
                                           GR 
                                           4 
                                         
                                         ⁣ 
                                         
                                           · 
                                           
                                             BDepth 
                                             2 
                                           
                                         
                                       
                                       ) 
                                     
                                   
                                 
                               
                             
                           
                           ⌋ 
                         
                         
                           
                             GR 
                             2 
                           
                           - 
                           1 
                         
                       
                     
                   
                 
               
             
             
                 
             
           
        
       
     
   
   Table 5 gives a formula for calculating the distance which the geometric center A od  of the diaphragm is offset from the axis A m  of the motor assembly. 
   
     
       
             
           
         
             
               TABLE 5 
             
             
                 
             
             
               Offset from A m  to A od   
             
             
                 
             
           
           
             
               
                 
                   
                     
                       Offset 
                       = 
                       
                         LS 
                         - 
                         
                           FOD 
                           2 
                         
                       
                     
                   
                 
               
             
             
                 
             
           
        
       
     
   
   Table 6 gives a formula for calculating the length of the projected chord on the shorter side of the diaphragm, measured from the bobbin to the surround. 
   
     
       
             
           
         
             
               TABLE 6 
             
             
                 
             
           
           
             
               Length of Projected Short Chord 
             
             
                 
             
             
               SS = FOD − LS 
             
             
                 
             
           
        
       
     
   
   Table 7 gives a formula for calculating the centeredness ratio of the speaker, which is the ratio of the lengths of the short and long projected chords. 
   
     
       
             
           
         
             
               TABLE 7 
             
             
                 
             
             
               Centeredness Ratio 
             
             
                 
             
           
           
             
               
                 
                   
                     
                       CenterRatio 
                       = 
                       
                         SS 
                         LS 
                       
                     
                   
                 
               
             
             
                 
             
           
        
       
     
   
   Table 8 gives the value of rho, the density of air. 
   
     
       
             
           
         
             
               TABLE 8 
             
             
                 
             
           
           
             
               Density of Air 
             
             
                 
             
             
               ρ = 1.18 
             
             
                 
             
           
        
       
     
   
   Table 9 gives a formula for calculating the air load mass on the diaphragm, ignoring the air load mass that will be on the dust cap, or, more precisely, the portion of the dust cap which overlies the bobbin. 
   
     
       
             
           
         
             
               TABLE 9 
             
             
                 
             
             
               Air Load Mass, Excluding Voice Coil Area 
             
             
                 
             
           
           
             
               
                 
                   
                     
                       
                         M 
                         al 
                       
                       = 
                       
                         
                           ρ 
                           ⁣ 
                           
                             · 
                             
                               FOD 
                               3 
                             
                           
                         
                         
                           3,000,000 
                         
                       
                     
                   
                 
               
             
             
                 
             
           
        
       
     
   
   In order to prevent rocking of the diaphragm, which may distort the sound or, if it becomes exaggerated enough, may even cause the bobbin to impact the pole piece or plate, it is desirable to balance the diaphragm. The diaphragm may be balanced, to a first order of approximation, by forming the diaphragm such that any two opposing chord cross-sections are of equal area; in other words, opposite strips of diaphragm will have equal mass. 
   Table 10 gives a formula for calculating how much the mass of the short chord side of the diaphragm should be adjusted upward, and the mass of the long chord side of the diaphragm should be adjusted downward from this equal mass configuration, in order to balance the diaphragm over the axis of the bobbin to a next order of approximation, which includes the air load mass difference. 
   
     
       
             
           
         
             
               TABLE 10 
             
             
                 
             
             
               Diaphragm Mass Adjustment 
             
             
                 
             
           
           
             
               
                 
                   
                     
                       Delta 
                       = 
                       
                         
                           
                             M 
                             al 
                           
                           · 
                           CenterRatio 
                         
                         
                           2 
                           · 
                           ConeMass 
                         
                       
                     
                   
                 
               
             
             
                 
             
           
        
       
     
   
   CONCLUSION  
   In order to achieve desired acoustic results, the dimensional ratio between the short side and the long side may be adjusted by moving the ID relative to the OD. In some applications, the speaker designer may elect to design a speaker in which the ratio is determined as between one of the long side and short side versus a midpoint side (e.g.  24 M in  FIG. 13 ). 
   In order to prevent rocking of the diaphragm, which may distort the sound or, if it becomes exaggerated enough, may even cause the bobbin to impact the pole piece or plate, it is desirable to balance the diaphragm. The diaphragm may be balanced, to a first order of approximation, by forming the diaphragm such that any two opposing chord cross-sections are of equal area; in other words, opposite strips of diaphragm will have equal mass. 
   Further improvements may be made by making further adjustments for the relative moments of rotational inertia of the respective chords, to further reduce the tendency of the diaphragm assembly to rock as it accelerates in and out of the motor assembly. 
   The invention may be practiced with diaphragms of any suitable shape, such as but not limited to circular, elliptical, oval, egg-shaped, rectangular, or any polygon. In some implementations, a conical diaphragm may be used. A conical diaphragm may be said to have an apex at its “deepest” point; this is typically where the bobbin is mounted. 
   Mass may be added to portions of the diaphragm, to balance it, either by adding actual diaphragm material, or by adding some other material or fixture. Suspension stiffness may be adjusted asymmetrically in order to compensate for mass imbalances or differences in rotational moments. 
   Reference in the specification to “an embodiment,” “one embodiment,” “some embodiments,” or “other embodiments” means that a particular feature, structure, or characteristic described in connection with the embodiments is included in at least some embodiments, but not necessarily all embodiments, of the invention. The various appearances “an embodiment,” “one embodiment,” or “some embodiments” are not necessarily all referring to the same embodiments. 
   If the specification states a component, feature, structure, or characteristic “may”, “might”, or “could” be included, that particular component, feature, structure, or characteristic is not required to be included. If the specification or claim refers to “a” or “an” element, that does not mean there is only one of the element. If the specification or claims refer to “an additional” element, that does not preclude there being more than one of the additional element. 
   When one component is said to be “adjacent” another component, it should not be interpreted to mean that there is absolutely nothing between the two components, only that they are in the order indicated. 
   The several features illustrated in the various figures may be combined in many ways, and should not be interpreted as though limited to the specific embodiments in which they were explained and shown. 
   Those skilled in the art having the benefit of this disclosure will appreciate that many other variations from the foregoing description and drawings may be made within the scope of the present invention. Indeed, the invention is not limited to the details described above. Rather, it is the following claims including any amendments thereto that define the scope of the invention.

Technology Classification (CPC): 7