Abstract:
An exemplary earphone module includes a faceplate, a bottom cover connected to the top cover, and a microphone received between the faceplate and the bottom cover. The faceplate defines a sound hole therein. The microphone defines a Helmholtz resonance chamber therein. A washer is placed between the faceplate and the microphone. The washer has a sound chamber communicating the sound hole with the Helmholtz resonance chamber. The Helmholtz resonance chamber has a volume V, the sound hole has a diameter d and a length l, and the sound chamber has a diameter D. The diameter D of the sound chamber meets the equation D=d or the formula 
     
       
         
           
             D 
             ≥ 
             
               
                 
                   
                     4 
                      
                     V 
                   
                   
                     π 
                      
                     
                       ( 
                       
                         l 
                         + 
                         
                           0.8 
                            
                           d 
                         
                       
                       ) 
                     
                   
                 
               
               .

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present application is a continuation-in-part (CIP) application of patent application Ser. No. 13/272,175 entitled “MICROPHONE MODULE WITH HELMHOLTZ RESONANCE CHAMBER” and filed on Oct. 12, 2011, and which in turn is a continuation-in-part (CIP) application of patent application Ser. No. 12/758,805 entitled “MICROPHONE MODULE WITH HELMHOLTZ RESONANCE CHAMBER” and filed on Apr. 13, 2010, now abandoned. The disclosures of the parent applications are incorporated herein by reference in their entireties. 
     
    
     BACKGROUND 
       [0002]    1. Technical Field 
         [0003]    The disclosure generally relates to microphones and, particularly, to a microphone module with a Helmholtz resonance chamber. 
         [0004]    2. Description of Related Art 
         [0005]    With the continuing development of audio and sound technology, microphones have been widely used in electronic devices such as headsets, mobile phones, computers and other devices providing audio capabilities. 
         [0006]    A typical microphone defines a resonance chamber therein. The size of the resonance chamber determines the amount of a corresponding mass of air therein, and the quality of low frequency sound transmitted is commensurate with the amount of air. If the microphone is reduced in size, the size of the resonance chamber of the microphone and the maximum power the microphone can handle are accordingly reduced, resulting in both a reduction in loudness as well as a poorer overall quality of sound. On the other hand, increasing the size of the microphone to increase the size of the resonance chamber is not feasible in many portable device applications. 
         [0007]    What is needed, therefore, is a means which can address the limitations described. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]    Many aspects of the present embodiments can be better understood with reference to the following drawings. The components in the drawings are not necessarily drawn to scale, the emphasis instead being placed upon clearly illustrating the principles of the present embodiments. Moreover, in the drawings, like reference numerals designate corresponding parts throughout the various views. 
           [0009]      FIG. 1  is an assembled, isometric view of a microphone module in accordance with a first embodiment of the disclosure. 
           [0010]      FIG. 2  is an exploded, isometric view of the microphone module of  FIG. 1 . 
           [0011]      FIG. 3  is similar to  FIG. 2 , but viewed from an inverted aspect thereof. 
           [0012]      FIG. 4  is a cross section of the microphone module of  FIG. 1 , taken along line IV-IV thereof. 
           [0013]      FIG. 5  is a cross section of a standard Helmholtz resonance chamber. 
           [0014]      FIG. 6  is similar to  FIG. 4 , but showing a cross section of a microphone module in accordance with a second embodiment of the present disclosure. 
           [0015]      FIG. 7  is similar to  FIG. 4 , but showing a cross section of a microphone module in accordance with a third embodiment of the present disclosure. 
           [0016]      FIG. 8  is similar to  FIG. 4 , but showing a cross section of a microphone module in accordance with a fourth embodiment of the present disclosure. 
       
    
    
     DETAILED DESCRIPTION 
       [0017]    Referring to  FIGS. 1 and 2 , a microphone module in accordance with a first embodiment of the present disclosure is shown. The microphone module is configured for use in electronic devices such as headsets, mobile phones, computers, and others. The microphone module includes a shell  10 , a circuit board  20  located in the shell  10 , and a microphone  30  located on the circuit board  20  and received in the shell  10 . 
         [0018]    Referring also to  FIGS. 3 and 4 , the shell  10  includes a bottom cover  11 , a top cover  12  engaging the bottom cover  11 , a pair of vertical plates  13  respectively disposed at opposite ends of the bottom and top covers  11 ,  12 , and a faceplate  14  located on the top cover  12 . 
         [0019]    The bottom cover  11  is semi-enclosed, and includes a bottom wall  111 , two sidewalls  112  extending upwardly from two opposite sides of the bottom wall  111 , respectively, and an engaging wall  116  extending upwardly from an end of the bottom wall  111 . The bottom wall  111  and the sidewalls  112  cooperatively define a receiving chamber  113  of the bottom cover  11  (see  FIG. 4 ). The bottom wall  111  is substantially rectangular. A pair of supporting ribs  114  and a pair of elastically deformable buckles  115  extend upwardly from the two sidewalls  112 , respectively. The supporting ribs  114  support the circuit board  20  thereon, and the buckles  115  press the circuit board  20  downwardly towards the supporting ribs  114 , thereby fixing the circuit board  20  within the bottom cover  11 . Each of the sidewalls  112  defines a mounting groove  117  in an inner surface thereof. The mounting grooves  117  communicate with the receiving chamber  113 . Each of the sidewalls  112  forms a step  118  at a top face thereof. An outer side of the step  118  is lower than an inner side of the step  118 . The engaging wall  116  interconnects the two sidewalls  112 . The engaging wall  116  has a height less than that of the sidewalls  112 . The engaging wall  116  defines a recess  119  in a top face thereof, for engagingly receiving one of the vertical plates  13 . 
         [0020]    The top cover  12  is also semi-enclosed. The top cover  12  includes a top wall  121 , and two sidewalls  122  depending downwardly from two opposite sides of the top wall  121 , respectively. The top wall  121  and the sidewalls  122  cooperatively define a receiving chamber  123  in the top cover  12  (see  FIG. 4 ). 
         [0021]    The top wall  121  is substantially rectangular, and defines two rectangular holes  124  in two adjacent corners thereof, respectively. The top wall  121  further defines a through hole  127  in a central area thereof. The top wall  121  has an annular flange  128  extending downwardly therefrom at a circumferential edge of the through hole  127 . That is, the flange  128  extends towards the bottom cover  11  (see  FIG. 3 ). 
         [0022]    A distance between outer surfaces of the two sidewalls  122  of the top cover  12  is equal to or slightly less than a distance between inner surfaces of the two sidewalls  112  of the bottom cover  11 . A mounting hook  125  extends downwardly from a bottom face of each sidewall  122  of the top cover  12 . Each mounting hook  125  is received in the mounting groove  117  of a corresponding sidewall  112  of the bottom cover  11 , thereby locking the top cover  12  with the bottom cover  11 . 
         [0023]    The vertical plates  13  are made of elastic material, such as rubber. Each of the vertical plates  13  includes a base  131 , and a protrusion  132  protruding inwardly from a central area of the base  131 . The base  131  is rectangular, and is joined to lateral sides of the top wall  121  of the top cover  12  and the bottom wall  111  of the bottom cover  11 . The protrusion  132  of one vertical plate  13  is received in the recess  119  of the bottom cover  11  in a manner that the protrusion  132  of the one vertical plate  13  is pressed downwardly by a bottom face of the top wall  121  of the top cover  12  and abuts against an outer circumferential face of the flange  128  of the top cover  12 . The protrusion  132  of the other vertical plate  13  is pressed downwardly by the bottom face of the top wall  121  of the top cover  12 , and is spaced from the flange  128  of the top cover  12 . 
         [0024]    The faceplate  14  includes a top plate  141 , two side plates  142  extending downwardly towards the bottom cover  11  from two opposite sides of the top plate  141 , respectively, and a washer  143  attached to the top plate  141 . 
         [0025]    The top plate  141  is substantially rectangular, and has a pair of engaging hooks  144 , which depend downwardly toward the bottom cover  11  from a bottom face of the top plate  141 . The engaging hooks  144  of the top plate  141  are engaged in the rectangular holes  124  of the top cover  12 , so that the faceplate  14  is fixed to the top cover  12 . 
         [0026]    The top plate  141  defines a sound hole  147  in a center thereof. The sound hole  147  extends perpendicularly through the top plate  141 , and is aligned with the through hole  127  of the top cover  12 . The sound hole  147  is circular, and has a diameter far less than that of the through hole  127  of the top cover  12 . The top plate  141  has an annular flange  148  extending down towards the top cover  12 . The annular flange  148  surrounds the sound hole  147 . 
         [0027]    The washer  143  is annular (hollow), and made of elastic material such as sponge, rubber, or another suitable material. An outer diameter of the washer  143  is less than an inner diameter of the annular flange  148 . The washer  143  is adhered to the top plate  141 , and is surrounded by the annular flange  148  and a top face of the microphone  30 . In a further or alternative embodiment, the washer  143  is restricted by the annular flange  148  that surrounds it. The washer  143  has a sound chamber  149  therein. An inner diameter of the washer  143 , namely, a diameter of the sound chamber  149 , exceeds that of the sound hole  147 . 
         [0028]    Each of the side plates  142  forms a step  146  at a bottom face thereof. An outer side of the step  146  is lower than an inner side of the step  146 . The steps  146  are matched with the steps  118  of the sidewalls  112  of the bottom cover  11 , so that the faceplate  14  can be fittingly engaged with the bottom cover  11 . 
         [0029]    The circuit board  20  is received in the receiving chamber  113  of the bottom cover  11  of the shell  10 . The circuit board  20  forms a pair of holes  21  therein. 
         [0030]    The microphone  30  is disposed on the top surface of the circuit board  20 , and electrically connects to the circuit board  20 . In this embodiment, the microphone  30  is an electret condenser microphone (ECM). The microphone  30  is cylindrical, with two pins  300  extending downwardly into the two holes  21  of the circuit board  20 . The microphone  30  has an outer diameter less than an inner diameter of the through hole  127  of the top cover  12  of the shell  10 . The microphone  30  defines an acoustic chamber  31  in an interior thereof, and an acoustic hole  37  in a top end thereof. The acoustic hole  37  communicates the acoustic chamber  31  with an exterior of the microphone  30 . The acoustic hole  37  and the acoustic chamber  31  cooperatively form a first Helmholtz resonance chamber  38  in the microphone  30 . A tuning cloth  39 , made of unwoven cloth, is arranged on the acoustic hole  37 . A bottom surface of the washer  143  is fixed to the tuning cloth  39 . The tuning cloth  39  cooperates with the acoustic hole  37  to improve the sound quality factor and adjust the sound sharpness of the microphone  30 . 
         [0031]    In the microphone module, the washer  143  with the sound chamber  149  therein is provided between the microphone  30  and the faceplate  14 , and the sound chamber  149  of the washer  143  and the sound hole  147  of the top plate  141  of the faceplate  14  cooperatively form a second Helmholtz resonance chamber  50  outside of the microphone  30 . The two Helmholtz resonance chambers  38 ,  50  work together to improve the sound quantity of the microphone module, i.e., widening the frequency bandwidth of the sound generated by the microphone module, and lowering the lowest resonance frequency of the sound generated by the microphone module. On the other hand, an interior space of the microphone module is adequately used without increasing a volume of the microphone module. 
         [0032]    The factors of the sound chamber  149  of the washer  143 , such as volume, diameter, and depth, may affect the lowest resonance frequency of the microphone module, and this directly affects the quality of the sound captured by the microphone module. Generally, the smaller the lowest resonance frequency, the better the quality of the sound captured by the microphone module. Therefore in order to choose a suitable washer  143  for the microphone module and obtain a smallest lowest resonance frequency, the factors of the sound chamber  149  must be calculated beforehand. Referring to  FIG. 5 , a standard Helmholtz resonance chamber  40  is introduced for reference. The standard Helmholtz resonance chamber  40  consists of a chamber  42  and a passage  41  communicating with the chamber  42 . The standard Helmholtz resonance chamber  40  has a lowest resonance frequency that satisfies the formula: 
         [0000]    
       
         
           
             
               
                 
                   
                     f 
                     0 
                   
                   = 
                   
                     
                       C 
                       
                         2 
                          
                         π 
                       
                     
                      
                     
                       
                         S 
                         
                           
                             ( 
                             
                               l 
                               + 
                               
                                 0.8 
                                  
                                 d 
                               
                             
                             ) 
                           
                            
                           V 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
         [0033]    In the formula (1), f 0  represents the lowest resonance frequency, C represents the sound speed (i.e., 340 meters/second), S represents a horizontal cross-sectional area of the passage  41 , l represents a length (or depth) of the passage  41 , d represents a diameter of the passage  41 , and V represents a volume of the chamber  42 . 
         [0034]    According to the formula (1), in addition to the volume V of the chamber  42 , the lowest resonance frequency f 0  is also related to the horizontal cross-sectional area S, the length l, and the diameter d of the passage  41 . That is, an influence of the factors of l, d, and S with respect to f 0  may not be less than an influence of the factor of V with respect to f 0 . Different situations of the microphone module of this embodiment are discussed below in light of the formula (1). 
         [0035]    Firstly, factors of the microphone module of this embodiment are defined as follows: the first Helmholtz resonance chamber  68  has a volume V; the sound chamber  149  of the washer  143  has a volume V 1 , a diameter D, and a height h; and the sound hole  147  has a horizontal cross-sectional area S, a diameter d, and a length (or depth) l. 
         [0036]    In an extreme situation, the inner diameter of the washer  143  is reduced to make the diameter D of the sound chamber  149  equal to the diameter d of the sound hole  147 . In this situation, the sound chamber  149  and the sound hole  147  can be cooperatively regarded as the passage  41  of the standard Helmholtz resonance chamber  40 , and the first Helmholtz resonance chamber  38  can be regarded as the chamber  42  of the standard Helmholtz resonance chamber  40 . The lowest resonance frequency f 1  of the microphone module of this embodiment in this situation is calculated as: 
         [0000]    
       
         
           
             
               
                 
                   
                     f 
                     1 
                   
                   = 
                   
                     
                       C 
                       
                         2 
                          
                         π 
                       
                     
                      
                     
                       
                         S 
                         
                           
                             ( 
                             
                               l 
                               + 
                               h 
                               + 
                               
                                 0.8 
                                  
                                 d 
                               
                             
                             ) 
                           
                            
                           V 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
           
         
       
     
         [0037]    In an ordinary situation, the diameter D of the sound chamber  149  is larger than the diameter d of the sound hole  147 . In this situation, only the sound hole  147  is regarded as the passage  41  of the standard Helmholtz resonance chamber  40 , and the sound chamber  149  and the first Helmholtz resonance chamber  38  are cooperatively regarded as the chamber  42  of the standard Helmholtz resonance chamber  40 . The lowest resonance frequency f 2  of the microphone module of this embodiment in this situation is calculated as: 
         [0000]    
       
         
           
             
               
                 
                   
                     f 
                     2 
                   
                   = 
                   
                     
                       C 
                       
                         2 
                          
                         π 
                       
                     
                      
                     
                       
                         S 
                         
                           
                             ( 
                             
                               l 
                               + 
                               
                                 0.8 
                                  
                                 d 
                               
                             
                             ) 
                           
                            
                           
                             ( 
                             
                               V 
                               + 
                               
                                 V 
                                 1 
                               
                             
                             ) 
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   3 
                   ) 
                 
               
             
           
         
       
     
         [0038]    In order to get the result of f 2 &lt;f 1 , the two formulas (2), (3) can be associated as: 
         [0000]      ( l+ 0.8 d )( V+V   1 )&gt;( l+h+ 0.8 d ) V   (4)
 
         [0039]    The formula (4) can be further concluded as: 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       V 
                       1 
                     
                     V 
                   
                   &gt; 
                   
                     h 
                     
                       l 
                       + 
                       
                         0.8 
                          
                         d 
                       
                     
                   
                 
               
               
                 
                   ( 
                   5 
                   ) 
                 
               
             
           
         
       
     
         [0040]    Therefore, according to the formula (5) given above, the ratio of the volume V 1  of the sound chamber  149  to the volume V of the first Helmholtz resonance chamber  38  should be larger than h/(l+0.8d), whereby the lowest resonance frequency f 2  of the ordinary situation can be ensured to be lower than the lowest resonance frequency f 1  of the extreme situation. 
         [0041]    For a practical application of the microphone module of this embodiment, the diameter d of the sound hole  147  is generally equal to the length l of the sound hole  147 , and the height h of the sound chamber  149  is about 1.31 (or 1.3d). As a result, the formula (5) can be calculated to V 1 /V&gt;0.7. Therefore, one condition to choose the washer  143  for the microphone module of this embodiment is to make V 1 /V&gt;0.7 (i.e., f 2 &lt;f 1 ), with the diameter D of the sound chamber  149  being larger than the diameter d of the sound hole  147 . An alternative condition to choose the washer  143  is to make V 1 /V&lt;0.7 (i.e., f 1 &lt;f 2 ), with the diameter D of the sound chamber  149  being equal to the diameter d of the sound hole  147 . 
         [0042]    The washer  143  used in this embodiment is annular, whereby the sound chamber  149  of the washer  143  is correspondingly cylindrical. The volume V 1  of the cylindrical sound chamber  149  is expressed as 
         [0000]    
       
         
           
             
               V 
               1 
             
             = 
             
               
                 
                   π 
                    
                   
                     ( 
                     
                       D 
                       2 
                     
                     ) 
                   
                 
                 2 
               
                
               
                 h 
                 . 
               
             
           
         
       
     
         [0000]    Accordingly, the formula (5) can be varied as: 
         [0000]    
       
         
           
             
               
                 
                   D 
                   &gt; 
                   
                     
                       
                         4 
                          
                         V 
                       
                       
                         π 
                          
                         
                           ( 
                           
                             l 
                             + 
                             
                               0.8 
                                
                               d 
                             
                           
                           ) 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   6 
                   ) 
                 
               
             
           
         
       
     
         [0043]    Thus the value of the diameter D of the sound chamber  149  is selected to be equal to the diameter d of the sound hole  147  (in the extreme situation), or larger than or identical to 
         [0000]    
       
         
           
             
               
                 4 
                  
                 V 
               
               
                 π 
                  
                 
                   ( 
                   
                     l 
                     + 
                     
                       0.8 
                        
                       d 
                     
                   
                   ) 
                 
               
             
           
         
       
     
         [0000]    (in the ordinary situation). That is, D=d or 
         [0000]    
       
         
           
             D 
             ≥ 
             
               
                 
                   
                     4 
                      
                     V 
                   
                   
                     π 
                      
                     
                       ( 
                       
                         l 
                         + 
                         
                           0.8 
                            
                           d 
                         
                       
                       ) 
                     
                   
                 
               
               . 
             
           
         
       
     
         [0000]    Any value of the diameter D of the sound chamber  149 , which does not belong to such range, cannot obtain the smallest lowest resonance frequency. 
         [0044]    Further, if the diameter D of the sound chamber  149  already meets the formula (6), it is known that the volume V 1  of the sound chamber  149  is in direct proportion to the lowest resonance frequency according to the formula (3). Therefore, a method for lowering the lowest resonance frequency is to increase the volume V 1  of the sound chamber  149 . 
         [0045]      FIGS. 6-8  show various methods for increasing volumes V 1  of sound chambers  149   a ,  149   b ,  149   c , without increasing spaces that washers  143   a ,  143   b ,  143   c  occupy. The washer  143   a  of  FIG. 6  defines a groove  140   a  in an inner face thereof, the groove  140   a  communicating with the sound chamber  149   a . The groove  140   a  is annular, and has a diameter gradually increasing along a bottom-to-top direction of the washer  143   a . An inner face of the groove  140   a  is curved. The washer  143   b  of  FIG. 7  defines a groove  140   b  in an inner face thereof, the groove  140   b  communicating with the sound chamber  149   b . The groove  140   b  is annular, and has a diameter gradually decreasing along a bottom-to-top direction of the washer  143   b . An inner face of the groove  140   b  is curved. The washer  143   c  of  FIG. 8  defines a groove  140   c  in an inner face thereof, the groove  140   c  communicating with the sound chamber  149   c . The groove  140   c  is annular, and has a diameter firstly increasing and then decreasing along a bottom-to-top direction of the washer  143   c . An inner face of the groove  140   c  is curved. 
         [0046]    It is to be understood, however, that even though numerous characteristics and advantages of the present embodiments have been set forth in the foregoing description, together with details of the structures and functions of the embodiments, the disclosure is illustrative only, and changes may be made in detail, especially in matters of shape, size, and arrangement of parts within the principles of the invention to the full extent indicated by the broad general meaning of the terms in which the appended claims are expressed.