Abstract:
There is provided multiple embodiments of a micro-speaker that reproduces audio signals at a low resonant frequency, which correspondingly allows for low frequency reproduction of audio signals by the micro-speaker. The various embodiments of the micro-speaker may enable usage of spiders with larger areas such that mechanical compliance of the spider is increased.

Description:
FIELD OF INVENTION 
       [0001]    This invention relates to a micro-speaker that is able to reproduce audio signals at low frequencies using several different setups. 
       BACKGROUND 
       [0002]    Micro-speakers are commonly used in portable audio and multimedia systems for their small size, light weight and aesthetic appeal. However, the small size of such speakers is the primary reason why the acoustic performance of such speakers is not ideal. The small size of such speakers generally causes high resonant frequency f s , and low acoustic sensitivity. 
         [0003]    Theile-Small theory states that the low frequency cutoff of loudspeakers is determined by its resonant frequency f s  as per the following equation, 
         [0000]    
       
         
           
             
               f 
               s 
             
             = 
             
               
                 1 
                 
                   2 
                    
                   
                       
                   
                    
                   π 
                 
               
                
               
                 1 
                 
                   
                     
                       M 
                       ms 
                     
                      
                     
                       C 
                       ms 
                     
                   
                 
               
             
           
         
       
     
         [0000]    where M ms =total mass of vibration system (includes diaphragm, dustcap, voice coil, surround, spider, adhesive and additional radiation mass), and C ms =total mechanical compliance from surround and spider. C ms  is determined as per the following equation, 
         [0000]    
       
         
           
             
               1 
               
                 C 
                 ms 
               
             
             = 
             
               
                 1 
                 
                   C 
                   surr 
                 
               
               + 
               
                 1 
                 
                   C 
                   spider 
                 
               
             
           
         
       
     
         [0000]    where C surr =surround compliance, and C spider =spider compliance. 
         [0004]    Referring to the aforementioned equations, it can be seen that resonant frequency f s  may be lowered if the total compliance C ms  is larger. 
       SUMMARY OF INVENTION 
       [0005]    In a first embodiment of the present invention, there is provided a micro-speaker that includes a cylindrical diaphragm with a coil coupled to a first end of the diaphragm; and a horn coupled to a second end of the diaphragm, the horn mounted upon an upper part of a speaker frame. It is advantageous that induced vibration of the coil reproduces audio signals at a low resonant frequency, which correspondingly allows for low frequency reproduction of audio signals by the micro-speaker. A shape of the horn may affect either radiation area or radiation resistance. 
         [0006]    The micro-speaker may further include a spider surrounding the cylindrical diaphragm, with the spider mounted in-between the upper part and a lower part of the speaker frame. The diaphragm may be made of either a metal or a metal alloy. A dustcap may be used to cover the second end of the diaphragm. 
         [0007]    The reproduction of audio signals at low resonant frequency may be dependent upon a mechanical compliance of the spider, with the spider preferable being made of a material such as, for example, meta-aramid fibres, cotton or linen. The mechanical compliance of the spider may be dependent upon an area of the spider. 
         [0008]    In a second embodiment of the present invention, there is provided a micro-speaker including: a cylindrical diaphragm with a coil coupled to a first end of the diaphragm; and a spider surrounding the diaphragm, the spider being mounted in-between an upper part and a lower part of a speaker frame. It is advantageous that induced vibration of the coil reproduces audio signals at a low resonant frequency, allowing for low frequency reproduction of audio signals by the micro-speaker. The reproduction of audio signals at low resonance frequency may be dependent upon a mechanical compliance of the spider. The mechanical compliance of the spider may be dependent upon an area of the spider. 
         [0009]    The micro-speaker may further include a surround around a second end of the diaphragm with the diaphragm being made of either a metal or a metal alloy. A dustcap may cover the second end of the diaphragm. The surround may preferably be made of a material of high mechanical compliance such as, for example, rubber, cloth-immersed rubber, polymers, or foam cotton. 
     
    
     
       DESCRIPTION OF DRAWINGS 
         [0010]    In order that the present invention may be fully understood and readily put into practical effect, there shall now be described by way of non-limitative example only preferred embodiments of the present invention, the description being with reference to the accompanying illustrative drawings. 
           [0011]      FIG. 1  shows a cross-sectional view of a first embodiment of the present invention. 
           [0012]      FIG. 2  shows a perspective view of the first embodiment of the present invention. 
           [0013]      FIG. 3  shows a cross-sectional view of a conventional micro-speaker. 
           [0014]      FIG. 4  shows a resonance mode representation of a conventional micro-speaker using finite element analysis at low frequencies. 
           [0015]      FIG. 5  shows a resonance mode representation of the first embodiment of the present invention using finite element analysis at low frequencies. 
           [0016]      FIG. 6  shows a cross-sectional view of a second embodiment of the present invention. 
           [0017]      FIG. 7  shows a perspective view of the second embodiment of the present invention. 
           [0018]      FIG. 8  shows a resonance mode representation of the second embodiment of the present invention using finite element analysis at low frequencies. 
       
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0019]    Referring to  FIGS. 1 and 2 , there is provided a cross-sectional and perspective view respectively of a first embodiment of the present invention in the form of a micro-speaker  20 . The micro-speaker  20  includes a cylindrical diaphragm  22  with a coil  24  coupled to a first end  26  of the diaphragm  22 . The diaphragm  22  may be made of either a metal or a metal alloy. 
         [0020]    A horn  28  may be coupled to a second end  30  of the diaphragm  22 . The coupling of the horn  28  to the second end  30  of the diaphragm  22  may be at or around an edge of the second end  30  of the diaphragm  22 . The horn  28  may be securely mounted upon an upper part  32  of a speaker frame. There may be a dustcap  38  covering the second end  30  of the diaphragm  22 . Radiation area and radiation resistance may be dependent upon a shape of the horn  28 . The horn  28  may increase both radiation area and diaphragm radiation resistance, correspondingly enhancing sensitivity of the micro-speaker  20  in relation to sound reproduction. 
         [0021]    The micro-speaker  20  may have a spider  34  surrounding the cylindrical diaphragm  22 , with the spider  34  being mounted in-between the upper part of the speaker frame  32  and a lower part of the speaker frame  36 . If the spider  34  has a larger outside diameter, the mechanical compliance of the spider  34  increases. The spider  34  may be made of materials such as, for example, meta-aramid fibres, cotton, linen and the like. The spider  34  may keep the coil  24  in a central position of the micro-speaker  20  by providing a restoring force if the coil  24  is displaced from the central position where the coil  24  surrounds a magnet  21 . 
         [0022]    When induced vibration of the coil  24  due to the magnet  21  in the micro-speaker  20  causes reproduction of audio signals, the micro-speaker  20  is able to reproduce audio signals at a low resonant frequency, allowing for low frequency reproduction of audio signals by the micro-speaker  20 . The reproduction of audio signals at low resonant frequency is dependent upon the mechanical compliance of the spider  34 , where the mechanical compliance of the spider  34  is dependent upon an area of the spider  34 . The greater the area of the spider  34 , the greater the mechanical compliance of the spider  34 . 
         [0023]    Referring to  FIG. 3 , there is shown a cross-sectional view of a conventional micro-speaker  40 . It should be noted that a diameter of a coil  42  is nearly identical to a diameter of a diaphragm  44 . An inner diameter  46  of a spider  48  is nearly identical to an inner diameter  49  of a surround  50 , with the surround  50  being located at an exterior facing end  47  of the diaphragm  44 . The surround  50  may be made of materials with high mechanical compliance, such as, for example, rubber, cloth-immersed rubber, polymers, foam cotton and the like. Due to considerations pertaining to assembly of the micro-speaker  40  during manufacture of the micro-speaker  40 , an outside diameter  52  of the spider  48  is preferably less than an outside diameter  54  of the surround  50 . 
         [0024]      FIG. 4  shows a resonance mode representation of the conventional micro-speaker  40  using finite element analysis at low frequencies. Similarly,  FIG. 5  shows a resonance mode representation of the first embodiment of the present invention using finite element analysis at low frequencies. It should be noted that the conventional micro-speaker  40  and the micro-speaker  20  of the first embodiment of the present invention may be dimensionally similar such that one may be used to replace the other if necessary. It should be observed that the micro-speaker  20  of the first embodiment of the present invention which includes the horn  28  rather than a surround does not significantly vibrate at a flared end  39  of the horn  28  compared to when the surround is used like in conventional micro-speakers. The micro-speaker  20  of the first embodiment of the present invention also includes the spider  34  that has a larger area than the spider  48  of the conventional micro-speaker  40 . In this regard, the spider  34  has a higher mechanical compliance than the spider  48  leading to lower resonant frequency and correspondingly, extended low frequency reproduction. When comparing  FIGS. 5 and 4 , it should be noted that the flared end  39  of the horn  28  has less vibration compared to the second end  49  of the diaphragm  44 . This results in less distortion of the reproduced audio signals. 
         [0025]    Referring to  FIGS. 6 and 7 , there is provided a cross-sectional and perspective view respectively of a second embodiment of the present invention in the form of a micro-speaker  60 . The micro-speaker  60  includes a cylindrical diaphragm  62  with a coil  64  coupled to a first end  66  of the diaphragm  62 . The diaphragm  62  may be made of either a metal or a metal alloy. 
         [0026]    There may be a spider  68  surrounding the diaphragm  62 . The spider  68  may be mounted between an upper part  70  of a speaker frame and a lower part  72  of the speaker frame. If the spider  68  has a larger outside diameter, the mechanical compliance of the spider  68  increases. The spider  68  may be made of materials such as, for example, meta-aramid fibres, cotton, linen and the like. The spider  68  may keep the coil  64  in a central position of the micro-speaker  60  by providing a restoring force if the coil  64  is displaced from the central position where the coil  64  surrounds a magnet  61 . 
         [0027]    The speaker  60  may include a surround  74  around a second end  76  of the diaphragm  62 . The surround  74  may be located at or around an edge of the second end  76  of the diaphragm  62 . The surround  50  may be made of materials with high mechanical compliance, such as, for example, rubber, cloth-immersed rubber, polymers, foam cotton and the like. There may be a dustcap  78  covering the second end  76  of the diaphragm  62 . The micro-speaker  60  has an assembly rather similar to the conventional micro-speaker  40  as shown in  FIG. 3 , with the primary difference being the use of a multi-part speaker frame. The multi-part speaker frame facilitates a use of a bigger spider  68  with a larger outside diameter, resulting in a higher mechanical compliance of the spider  68 . 
         [0028]    When induced vibration of the coil  64  due to the magnet  61  in the micro-speaker  60  causes reproduction of audio signals, the micro-speaker  60  is able to reproduce audio signals at a low resonant frequency, allowing for low frequency reproduction of audio signals by the micro-speaker  60 . The reproduction of audio signals at low resonant frequency is dependent upon the mechanical compliance of the spider  68  and the mechanical compliance of the spider  68  is dependent upon an area of the spider  68 . The greater the area of the spider  68 , the greater the mechanical compliance of the spider  68 . 
         [0029]      FIG. 8  shows a resonance mode representation of the second embodiment of the present invention using finite element analysis at low frequencies. It should be noted that the conventional micro-speaker  40  and the micro-speaker  60  of the second embodiment of the present invention may be dimensionally similar such that one may be used to replace the other if necessary. The micro-speaker  60  has an assembly rather similar to conventional micro-speaker  40  as shown in  FIG. 3 , with the primary difference being the use of a multi-part speaker frame. The micro-speaker  20  of the first embodiment of the present invention also includes the spider  34  that has a larger area than the spider  48  of the conventional micro-speaker  40 . In this regard, the spider  68  has a higher mechanical compliance than the spider  48  leading to lower resonant frequency and correspondingly, extended low frequency reproduction. When comparing  FIGS. 8 and 4 , it should be noted that the second end  76  of the diaphragm  62  has less vibration compared to the second end  49  of the diaphragm  44 . This results in less distortion of the reproduced audio signals. 
         [0030]    Whilst there has been described in the foregoing description preferred embodiments of the present invention, it will be understood by those skilled in the technology concerned that many variations or modifications in details of design or construction may be made without departing from the present invention.