Abstract:
An artificial mouth includes a front cover, a loudspeaker, and an acoustic tube. The front cover has a plurality of holes which is coplanar. The loudspeaker generates sound waves which pass through the acoustic tube, and turn into plane waves when arriving at the plurality of holes of the front cover.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of U.S. Provisional Application No. 61/019,859. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The invention relates to an artificial mouth for testing at least one microphone, and more particularly to an artificial mouth capable of being used for a phase matching test, sensitivity test, microphone categorization test, etc. for a plurality of microphones. 
         [0004]    2. Description of the Related Art 
         [0005]    An artificial mouth can be used for testing the sensitivity of a microphone. Referring to  FIG. 1 , a conventional artificial mouth  10  includes a loudspeaker  11 , a front cover  12  affixed to the loudspeaker  11 , and an acoustic tube  13  provided in the front cover  12 . The front cover  12  has a hole  121 . In a test, a microphone is disposed in the hole  121  of the front cover  12  to receive sound waves from the loudspeaker  11  through the acoustic tube  13 . The test, however, is not efficient because only one microphone is tested by the artificial mouth  10  which is provided with only one hole  121 . Further, the artificial mouth  10  can not be used for a phase matching test for a plurality of microphones due to the same reason. 
       BRIEF SUMMARY OF THE INVENTION 
       [0006]    The invention provides an artificial mouth capable of being used to test a plurality of microphones. 
         [0007]    The invention also provides an artificial mouth capable of being used for a phase matching test, a sensitivity test, a microphone categorization test, etc. for a plurality of microphones. 
         [0008]    The artificial mouth in accordance with an exemplary embodiment of the invention includes a front cover, a loudspeaker, and an acoustic tube. The front cover has a plurality of holes which is coplanar. The loudspeaker generates sound waves which pass through the acoustic tube, and turn into plane waves when arriving at the plurality of holes of the front cover. 
         [0009]    In another exemplary embodiment, the acoustic tube is a round tube. 
         [0010]    In yet another exemplary embodiment, the acoustic tube is a square tube. 
         [0011]    In another exemplary embodiment, the artificial mouth further includes an anti-dust screen disposed between the acoustic tube and the loudspeaker. 
         [0012]    In yet another exemplary embodiment, the acoustic tube is made of brass. 
         [0013]    In another exemplary embodiment, the acoustic tube is made of marble. 
         [0014]    In yet another exemplary embodiment, the acoustic tube is made of stainless steel. 
         [0015]    The invention also provides a process for testing at least one microphone. The process in accordance with an exemplary embodiment comprises the steps of providing the above artificial mouth, locating the microphone and a standard microphone in the plurality of holes, and turning the loudspeaker to a work frequency less than a cut-off frequency of the acoustic tube. 
         [0016]    In another exemplary embodiment, the acoustic tube is a round tube, and the cut-off frequency of the acoustic tube 
         [0000]    
       
         
           
             
               f 
               = 
               
                 
                   1.84 
                    
                   c 
                 
                 
                   π 
                    
                   
                       
                   
                    
                   D 
                 
               
             
             , 
           
         
       
     
         [0000]    wherein c is a speed of the sound waves in air, and D is a diameter of the acoustic tube. 
         [0017]    In yet another exemplary embodiment, the acoustic tube is a square tube, and the cut-off frequency of the acoustic tube 
         [0000]    
       
         
           
             
               f 
               = 
               
                 c 
                 
                   2 
                    
                   D 
                 
               
             
             , 
           
         
       
     
         [0000]    wherein c is a speed of the sound waves in air, and D is a side length of the acoustic tube. 
         [0018]    In another exemplary embodiment, the cut-off frequency of the acoustic tube 
         [0000]    
       
         
           
             
               f 
               = 
               
                 c 
                 
                   2 
                    
                   L 
                 
               
             
             , 
           
         
       
     
         [0000]    wherein c is a speed of the sound waves in air, and L is an effective length of the acoustic tube. 
         [0019]    A detailed description is given in the following embodiments with reference to the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0020]    The invention can be more fully understood by reading the subsequent detailed description and examples with references made to the accompanying drawings, wherein: 
           [0021]      FIG. 1  is a schematic diagram of a conventional artificial mouth; 
           [0022]      FIG. 2  is a schematic diagram of an artificial mouth in accordance with a first embodiment of the invention; 
           [0023]      FIG. 3A  is a perspective diagram of an artificial mouth in accordance with a second embodiment of the invention; 
           [0024]      FIG. 3B  is a sectional view of the artificial mouth in accordance with the second embodiment of the invention; 
           [0025]      FIG. 4A  is a perspective diagram of an artificial mouth in accordance with a second embodiment of the invention; and 
           [0026]      FIG. 4B  is a sectional view of the artificial mouth in accordance with the second embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0027]    The following description is of the best-contemplated mode of carrying out the invention. This description is made for the purpose of illustrating the general principles of the invention and should not be taken in a limiting sense. The scope of the invention is best determined by reference to the appended claims. 
         [0028]    Referring to  FIG. 2 , in a first embodiment of the invention, an artificial mouth  20  comprises a loudspeaker  21 , a front cover  22  affixed to the loudspeaker  21 , and an acoustic tube  23  provided in the front cover  22 . The front cover  22  has two holes  221  and  222  which are arranged in a plane. The acoustic tube  23  may be made of brass, marble, stainless steel, and others. 
         [0029]    During a test, two microphones  28  and  29  are disposed in the holes  221  and  222  of the front cover  22 , respectively. Sound waves generated by the loudspeaker  21  pass through the acoustic tube  23  to the microphones  28  and  29 . The two microphones  28  and  29  are coplanar because the holes  221  and  222  are coplanar. Additionally, the acoustic tube  23  is implemented in such a way that the output sound waves are “plane waves”. Thus, the phase, amplitude, and frequency response obtained from the two microphones  28  and  29  are consistent, which enables the utilization of the artificial mouth  20  to be applied to a phase matching test, sensitivity test, microphone categorization test, etc. 
         [0030]    In the first embodiment, the two microphones  28  and  29  are a standard microphone and a test microphone, respectively. 
         [0031]    Referring to  FIGS. 3A and 3B , in a second embodiment of the invention, an artificial mouth  30  comprises a loudspeaker  31 , a front cover  32  affixed to the loudspeaker  31 , an anti-dust screen  35  affixed to the front cover  32  by a fixing ring  34 , and an acoustic tube  33  provided in the front cover  32 . The front cover  32  has two holes  321  which are arranged in a plane. The anti-dust screen  35  is disposed between the acoustic tube  33  and the loudspeaker  31  and prevents the loudspeaker  31  from dust or foreign objects. The acoustic tube  33  may be made of brass, marble, stainless steel, and others. 
         [0032]    During a test, two microphones are disposed in the holes  321  of the front cover  32 , respectively. Sound waves generated by the loudspeaker  31  pass through the acoustic tube  33  to the microphones. The two microphones are coplanar because the holes  321  are coplanar. Additionally, the acoustic tube  33  is implemented in such a way that the output sound waves are “plane waves”. Thus, the phase, amplitude, and frequency response obtained from the two microphones are consistent, which enables the utilization of the artificial mouth  30  to be applied to a phase matching test, sensitivity test, microphone categorization test, etc. 
         [0033]    Referring to  FIGS. 4A and 4B , in a third embodiment of the invention, an artificial mouth  40  comprises a loudspeaker  41 , a front cover  42  affixed to the loudspeaker  41 , an anti-dust screen  45  affixed to the front cover  42  by a fixing ring  44 , and an acoustic tube  43  provided in the front cover  42 . The front cover  42  has more than two holes  421  which are arranged in a plane. The anti-dust screen  45  is disposed between the acoustic tube  43  and the loudspeaker  41  and prevents the loudspeaker  41  from dust or foreign objects. The acoustic tube  43  may be made of brass, marble, stainless steel, and others. 
         [0034]    During a test, a plurality of microphones is disposed in the holes  421  of the front cover  42 , respectively. Sound waves generated by the loudspeaker  41  pass through the acoustic tube  43  to the microphones. The microphones are coplanar because the holes  421  are coplanar. Additionally, the acoustic tube  43  is implemented in such a way that the output sound waves are “plane waves”. Thus, the phase, amplitude, and frequency response obtained from the microphones are consistent, which enables the utilization of the artificial mouth  40  to be applied to a phase matching test, sensitivity test, microphone categorization test, etc. 
         [0035]    The sound waves output from the acoustic tube will be plane waves if the work frequency of the loudspeaker is less than the cut-off frequency of the acoustic tube. The cut-off frequency is determined by the shape and the sizes of the acoustic tube: 
         [0036]    For a round tube, the cut-off frequency 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       f 
                       1 
                     
                     = 
                     
                       
                         1.84 
                          
                         c 
                       
                       
                         π 
                          
                         
                             
                         
                          
                         D 
                       
                     
                   
                   , 
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
         [0037]    wherein c is the speed of sound in the air, and
       D is the inner diameter of the round tube.       
 
         [0039]    For a square tube, the cut-off frequency 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       f 
                       1 
                     
                     = 
                     
                       c 
                       
                         2 
                          
                         D 
                       
                     
                   
                   , 
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
           
         
       
     
         [0040]    wherein c is the speed of sound in the air, and
       D is the side length (or width) of the square tube.       
 
         [0042]    Furthermore, to avoid the maximum and minimum sound pressure generated in the acoustic tube, the cut-off frequency f 2  is determined as follows: 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       f 
                       2 
                     
                     = 
                     
                       c 
                       
                         2 
                          
                         l 
                       
                     
                   
                   , 
                 
               
               
                 
                   ( 
                   3 
                   ) 
                 
               
             
           
         
       
     
         [0043]    wherein c is the speed of sound in the air, and
       l is the effective length of the acoustic tube.       
 
       PRACTICAL EXAMPLE 
       [0045]    Referring to  FIG. 3B  or  4 B, the acoustic tube is a round tube with an inner diameter D=0.04 m and an effective length L=0.04 m. The loudspeaker was turned to a work frequency of 4 KHz (or more). The speed of sound in the air was 343 m/s. 
         [0046]    According to formula (1), the cut-off frequency 
         [0000]    
       
         
           
             
               f 
               1 
             
             = 
             
               
                 
                   1.84 
                   × 
                   343 
                 
                 
                   3.14 
                   × 
                   0.04 
                 
               
               = 
               
                 
                   5025 
                    
                   
                       
                   
                    
                   Hz 
                 
                 &gt; 
               
             
           
         
       
     
         [0000]    than the work frequency 4 KHz. It was therefore understood that the sound waves output from the acoustic tube are plane waves. 
         [0047]    According to formula (3), the cut-off frequency 
         [0000]    
       
         
           
             
               f 
               2 
             
             = 
             
               
                 343 
                 
                   2 
                   × 
                   0.04 
                 
               
               = 
               
                 
                   4288 
                    
                   
                       
                   
                    
                   Hz 
                 
                 &gt; 
                 than 
               
             
           
         
       
     
         [0000]    the work frequency 4 KHz. It was therefore understood that no wave peak and trough occurred in the frequency response when the frequency was less than 4 KHz. 
         [0048]    While the invention has been described by way of example and in terms of preferred embodiment, it is to be understood that the invention is not limited thereto. To the contrary, it is intended to cover various modifications and similar arrangements (as would be apparent to those skilled in the art). Therefore, the scope of the appended claims should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements.