Abstract:
There is provided a variable directional microphone including dynamic microphone units that is small in size and has good directional frequency response. In a variable directional microphone  1 A in which a unidirectional first dynamic microphone unit (front-side unit)  1 F and a second dynamic microphone unit (rear-side unit)  1 R, which has substantially the same configuration as that of the front-side unit  1 F and is provided with an output adjusting means of sound signal, are provided as a pair; the front-side unit  1 F and the rear-side unit  1 R are arranged coaxially so that the directivity axes thereof are directed to the directions 180° opposite to each other; and the output signals of the front-side unit  1 F and the rear-side unit  1 R are generated via a signal synthesis circuit, one rear air chamber  1   b  is used in common by the front-side unit  1 F and the rear-side unit  1 R.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    The present application is based on, and claims priority from, Japanese Application Serial Number JP2010-066093, filed Mar. 23, 2010, the disclosure of which is hereby incorporated by reference herein in its entirety. 
       TECHNICAL FIELD 
       [0002]    The present invention relates to a variable directional microphone. More particularly, it relates to a variable directional microphone configured by two unidirectional dynamic microphone units. 
       BACKGROUND ART 
       [0003]    By synthesizing sound signals generated from two microphone units by using a variable directional microphone configured by the two microphone units, directivity such as omnidirectivity, cardioid, hypercardioid, or bidirectivity can be obtained selectively. 
         [0004]    In this case, as both of the two microphone units, unidirectional microphone units are used, and the microphone units are arranged coaxially so that the directivity axes thereof are directed to the directions opposite to each other (180° directions) (for example, refer to Patent Document 1 (Japanese Patent Application Publication No. 2005-184347)). 
         [0005]    Therefore, as the microphone unit, a small-size unidirectional condenser microphone has been used frequently, and a dynamic microphone unit has scarcely been used because of its large size. 
         [0006]    The reason why the dynamic microphone unit is large in size is that a rear air chamber is needed to obtain an omnidirectional component regardless of whether it is omnidirectional or unidirectional.  FIG. 10A  is a sectional view showing the schematic configuration of the dynamic microphone unit, and  FIG. 10B  is an equivalent circuit diagram of the dynamic microphone unit shown in  FIG. 10A . 
         [0007]    As shown in  FIG. 10A , a dynamic microphone unit  1  includes, as a basic configuration, an electrokinetic acousto-electric converter  1   a  and a rear air chamber  1   b.    
         [0008]    The electrokinetic acousto-electric converter  1   a  has a diaphragm  10  having a voice coil  11  and a magnetic circuit section  20  having a magnetic gap  21  in which the voice coil  11  are oscillatably arranged. The magnetic circuit section  20  is housed in a cylindrical unit holder  30 . The diaphragm  10  is supported on a peripheral edge portion of an enlarged-diameter flange part  31  of a unit holder  20 . 
         [0009]    Since this dynamic microphone unit  1  is unidirectional, the flange part  31  is provided with a bidirectional component intake port (rear acoustic terminal)  32  communicating with a front air chamber  12  existing on the back surface side of the diaphragm  10 . In the case where the dynamic microphone unit  1  is omnidirectional, the bidirectional component intake port  32  is not provided. 
         [0010]    The rear air chamber  1   b  is formed by a substantially enclosed unit case  40  mounted on the rear end side of the unit holder  30 . The front air chamber  12  on the diaphragm  10  side and the rear air chamber  1   b  are connected acoustically to each other via a sound wave passage in the unit holder  30 . In the sound wave passage, a predetermined acoustic resistance material  33  is provided. 
         [0011]    In the equivalent circuit diagram of  FIG. 10B , P denotes a front sound source, Pe -jkd cos θ  denotes a rear sound source, m 0  and S 0  denote the mass and stiffness of the diaphragm  10 , respectively, S 1  denotes the stiffness of the front air chamber  12 , r 0  and m 1  denote the resistance and mass of the bidirectional component intake port  32 , respectively, r 1  denotes the braking resistance of the acoustic resistance material  33 , and S 2  denotes the stiffness of the rear air chamber  1   b.    
         [0012]    The low frequency limit in the frequency characteristics is mainly determined by the mass and compliance (1/S 0 ) of the diaphragm  10 . However, in the case where the capacity Ca of the rear air chamber  1   b  is low, the low frequency limit is affected. Therefore, in the dynamic microphone unit  1 , the capacity Ca of the rear air chamber  1   b  must be increased. Accordingly, the external dimensions of the dynamic microphone unit  1  become far larger than those of the condenser microphone unit. The large capacity Ca of the rear air chamber  1   b  exerts an influence on a low frequency (omnidirectional component) only, and scarcely exerts an influence on the frequency band (bidirectional component) in which the unidirectivity is obtained. 
         [0013]    In the case where the variable directional microphone is configured by a pair of above-described dynamic microphone units  1 , a series mode in which the two dynamic microphone units  1  are arranged coaxially in a back-to-back form as shown in  FIG. 11A  and a parallel mode in which the rear air chambers  1   b  of the two dynamic microphone units  1  are lapped on each other as shown in  FIG. 11B  are conceivable. 
         [0014]    In the series mode shown in  FIG. 11A , unfortunately, the overall length becomes double the length of the dynamic microphone unit  1 , and accordingly the distance between the acoustic terminals of the dynamic microphone units  1  also increases. Therefore, there arises a problem that the difference (phase difference) between arrival times of sound waves to the acoustic terminals from the sound source increases, so that turbulence is easily produced especially in a high sound range. 
         [0015]    In contrast, according to the parallel mode shown in  FIG. 11B , the overall length can be shortened as compared with the series mode. However, the acoustic terminals of the dynamic microphone units  1  are arranged asymmetrically in the right-and-left direction. Therefore, there arises a problem of deteriorated directional frequency response. 
         [0016]    Accordingly, an object of the present invention is to provide a variable directional microphone including dynamic microphone units that is small in size and has good directional frequency response. 
       SUMMARY OF THE INVENTION 
       [0017]    To achieve the above object, the present invention provides a variable directional microphone in which a unidirectional first dynamic microphone unit and a second dynamic microphone unit, which has substantially the same configuration as that of the first dynamic microphone unit and is provided with an output adjusting means of sound signal, are provided as a pair; the first and second dynamic microphone units are arranged coaxially so that the directivity axes thereof are directed to directions 180° opposite to each other; and the output signals of the dynamic microphone units are generated via a signal synthesis circuit, wherein one rear air chamber that is used in common by the first and second dynamic microphone units is provided between the first and second dynamic microphone units. 
         [0018]    According to the present invention, in arranging the first and second dynamic microphone units coaxially so that the directivity axes thereof are directed to the directions 180° opposite to each other, one rear air chamber that is used in common by these microphone units is provided between the first and second dynamic microphone units. Thereby, the length of the microphone can be shortened by at least the length of one rear air chamber, and good directional frequency response can be obtained. 
         [0019]    The present invention also embraces a mode in which the rear air chamber is arranged on an outside between the dynamic microphone units via a predetermined tube member. 
         [0020]    By arranging the rear air chamber on the outside between the dynamic microphone units via the predetermined tube member, the distance between acoustic terminals (distance between diaphragms of the units) is shortened, so that better directional frequency response can be obtained. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0021]      FIG. 1  is a schematic sectional view showing a first embodiment of the present invention; 
           [0022]      FIG. 2  is a schematic sectional view showing a second embodiment of the present invention; 
           [0023]      FIG. 3  is a circuit diagram for making the directivity variable; 
           [0024]      FIGS. 4A to 4E  are polar pattern diagrams illustrating the directivities obtained by the present invention; 
           [0025]      FIG. 5A  is a polar pattern diagram in accordance with actual measurement data in the case where directivity is made bidirectivity in the present invention; 
           [0026]      FIG. 5B  is a graph showing the directional frequency response of  FIG. 5A ; 
           [0027]      FIG. 6A  is a polar pattern diagram in accordance with actual measurement data in the case where directivity is made hypercardioid in the present invention; 
           [0028]      FIG. 6B  is a graph showing the directional frequency response of  FIG. 6A ; 
           [0029]      FIG. 7A  is a polar pattern diagram in accordance with actual measurement data in the case where directivity is made cardioid in the present invention; 
           [0030]      FIG. 7B  is a graph showing the directional frequency response of  FIG. 7A ; 
           [0031]      FIG. 8A  is a polar pattern diagram in accordance with actual measurement data in the case where directivity is made subcardioid in the present invention; 
           [0032]      FIG. 8B  is a graph showing the directional frequency response of  FIG. 8A ; 
           [0033]      FIG. 9A  is a polar pattern diagram in accordance with actual measurement data in the case where directivity is made omnidirectivity in the present invention; 
           [0034]      FIG. 9B  is a graph showing the directional frequency response of  FIG. 9A ; 
           [0035]      FIG. 10A  is a schematic sectional view showing a basic configuration of a conventional unidirectional dynamic microphone unit; 
           [0036]      FIG. 10B  is an equivalent circuit diagram of the dynamic microphone unit shown in  FIG. 10A ; 
           [0037]      FIG. 11A  is a schematic view showing a first imaginary mode of a variable directional microphone using the dynamic microphone units shown in  FIG. 10A  as a pair; and 
           [0038]      FIG. 11B  is a schematic view showing a second imaginary mode of  FIG. 11A . 
       
    
    
     DETAILED DESCRIPTION 
       [0039]    Embodiments of the present invention will now be described with reference to  FIGS. 1 to 3 . The present invention is not limited to the embodiments described below. 
         [0040]    First, a variable directional microphone  1 A in accordance with a first embodiment of the present invention is explained with reference to  FIG. 1 . This variable directional microphone  1 A includes two unidirectional dynamic microphone units  1 F and  1 R. 
         [0041]    In this embodiment, one dynamic microphone unit  1 F is a front-side unit that is directed to the sound source side when sound is picked up. In contrast, the other dynamic microphone unit  1 R is a rear-side unit that is directed to the rear with respect to the sound source. In the following explanation, one dynamic microphone unit IF is sometimes referred simply to as a “front-side unit  1 F”, and the other dynamic microphone unit  1 R is sometimes referred simply to as a “rear-side unit  1 R”. 
         [0042]    The front-side unit  1 F and the rear-side unit  1 R have substantially the same configuration, and each are provided with an electrokinetic acousto-electric converter  1   a  that is similar to that explained before with reference to  FIG. 10A . 
         [0043]    That is, referring to  FIG. 10A , the electrokinetic acousto-electric converter  1   a  is configured so that a diaphragm  10  having a voice coil  11  and a magnetic circuit section  20  having a magnetic gap  21  are supported by the unit holder  30 , and a flange part  31  of the unit holder  30  is provided with a bidirectional component intake port (rear acoustic terminal)  32  communicating with a front air chamber  12  existing on the back surface side of the diaphragm  10 . 
         [0044]    The electrokinetic acousto-electric converter  1   a  of the front-side unit  1 F and the electrokinetic acousto-electric converter  1   a  of the rear-side unit  1 R are arranged coaxially so that the directivity axes thereof are directed to the directions 180° opposite to each other. In the variable directional microphone  1 A in accordance with the first embodiment, the electrokinetic acousto-electric converters  1   a  are connected coaxially to each other via a cylindrical connecting cylinder  41  consisting of a straight tube, and a space in the connecting cylinder  41  is used in common as a rear air chamber  1   b  of the front-side unit  1 F and the rear-side unit  1 R. 
         [0045]    The capacity Ca of the rear air chamber  1   b  in the connecting cylinder  41  may be approximately equal to the capacity Ca of the rear air chamber  1   b  explained before with reference to  FIG. 10A  considering the low frequency limit required by per one dynamic microphone unit. The connecting cylinder  41  is formed of a metallic material or synthetic resin material that is less liable to be deformed by an external force. 
         [0046]    According to the variable directional microphone  1 A in accordance with the first embodiment, the rear air chamber  1   b  required by the front-side unit  1 F and the rear-side unit  1 R is used in common by the front-side unit  1 F and the rear-side unit  1 R. Therefore, the distance between the acoustic terminals (the distance between the diaphragms) of the front-side unit  1 F and the rear-side unit  1 R can be shortened by at least the length of one rear air chamber as compared with the first imaginary mode of series mode shown in  FIG. 11A . 
         [0047]    Next, a variable directional microphone  1 B in accordance with a second embodiment is explained with reference to  FIG. 2 . In this variable directional microphone  1 B, to further shorten the distance between the acoustic terminals of the front-side unit  1 F and the rear-side unit  1 R, the rear air chamber  1   b  used in common by the front-side unit  1 F and the rear-side unit  1 R is disposed on the outside between the units. 
         [0048]    In this second embodiment, therefore, as a connecting cylinder for coaxially connecting the electrokinetic acousto-electric converters  1   a  of the front-side unit  1 F and the rear-side unit  1 R to each other, a connecting cylinder  42  that is shorter than the connecting cylinder  41  in the first embodiment is used. 
         [0049]    The connecting cylinder  42  is integrally formed with an air chamber housing  44  connected to the connecting cylinder  42  between the electrokinetic acousto-electric converters  1   a  via a tube part  43 . In this case, the sum of the capacity in the air chamber housing  44 , the capacity in the tube part  43 , and the capacity between the electrokinetic acousto-electric converters  1   a  is made equal to the capacity Ca of the rear air chamber  1   b  in the first embodiment. 
         [0050]    According to the configuration of the variable directional microphone  1 B of the second embodiment, the distance between the acoustic terminals of the front-side unit  1 F and the rear-side unit  1 R can be shortened further while the electrokinetic acousto-electric converters  1   a  of the front-side unit  1 F and the rear-side unit  1 R are arranged coaxially. 
         [0051]    The above-described variable directional microphones  1 A and  1 B each include an output level adjustment circuit  110  and a signal synthesis circuit  120  shown in  FIG. 3 . The output level adjustment circuit  110  consists of a variable resistor, and is provided in the rear-side unit  1 R. 
         [0052]    The signal synthesis circuit  120  is an addition/subtraction switching switch having first and second movable elements  121  and  122  and first and second fixed contacts  123  and  124 . 
         [0053]    The proximal end of the first movable element  121  is connected to the (−) side of the front-side unit  1 F, and the proximal end of the second movable element  122  is connected to the minus-side output terminal OUT(−) of the signal synthesis circuit  120 . 
         [0054]    Also, the first fixed contact  123  is connected to the (−) side of the rear-side unit  1 R, and the second fixed contact  124  is connected to the (+) side of the rear-side unit  1 R. The (+) side of the front-side unit  1 F is connected to the plus-side output terminal OUT(+) of the signal synthesis circuit  120 . 
         [0055]    If a connecting state shown in  FIG. 3 , in which the first movable element  121  is connected to the first fixed contact  123  side, and the second movable element  122  is connected to the second fixed contact  124  side, is formed, the sound signal of the front-side unit  1 F and the sound signal of the rear-side unit  1 R are subtracted from each other. In this state, by making the resistance value (level attenuation factor) of the output level adjustment circuit  110  substantially zero, the bidirectivity as shown in  FIG. 4A  is obtained. 
         [0056]    In the connecting state shown in  FIG. 3 , if the level of sound signal of the rear-side unit  1 R is attenuated with the resistance value of the output level adjustment circuit  110  being a predetermined value, the directivity of hypercardioid as shown in  FIG. 4B  is obtained. 
         [0057]    Also, from the connecting state shown in  FIG. 3 , the first movable element  121  is switched to the second fixed contact  124  side, whereby both the first movable element  121  and the second movable element  122  are connected to the second fixed contact  124 . Thereby, the sound signal of the rear-side unit  1 R is made zero. Therefore, by the sound signal of the front-side unit  1 F only, the directivity of cardioid as shown in  FIG. 4C  is obtained. In the connecting state shown in  FIG. 3 , if the resistance value of the output level adjustment circuit  110  is raised, and thereby the sound signal of the rear-side unit  1 R is made substantially zero, too, the directivity of cardioid as shown in  FIG. 4C  is obtained. 
         [0058]    Also, the first movable element  121  is switched to the second fixed contact  124  side, and the second movable element  122  is switched to the first fixed contact  123  side. Thereby, the sound signal of the front-side unit  1 F and the sound signal of the rear-side unit  1 R are added to each other. If the level of sound signal of the rear-side unit  1 R is attenuated with the resistance value of the output level adjustment circuit  110  being a predetermined value in this state, the directivity of subcardioid as shown in  FIG. 4D  is obtained. 
         [0059]    In this adding state, by making the resistance value (level attenuation factor) of the output level adjustment circuit  110  substantially zero, the omnidirectivity as shown in  FIG. 4E  is obtained. 
         [0060]    An actual machine of the variable directional microphone in accordance with the mode of the first embodiment shown in  FIG. 1  was prepared, and the output level adjustment circuit  110  and the signal synthesis circuit  120  were operated, whereby the directivities shown in  FIGS. 4A to 4E  were observed by polar pattern diagrams in accordance with the actual measurement data and graphs showing directional frequency response. The results are shown in  FIGS. 5 to 9 . 
         [0061]      FIGS. 5A and 5B  are graphs showing the polar pattern and the directional frequency response thereof in the case of bidirectivity.  FIGS. 6A and 6B  are graphs showing the polar pattern and the directional frequency response thereof in the case of hypercardioid.  FIGS. 7A and 7B  are graphs showing the polar pattern and the directional frequency response thereof in the case of cardioid.  FIGS. 8A and 8B  are graphs showing the polar pattern and the directional frequency response thereof in the case of subcardioid.  FIGS. 9A and 9B  are graphs showing the polar pattern and the directional frequency response thereof in the case of omnidirectivity. 
         [0062]    As seen from these graphs, in the present invention, in which the rear air chamber  1   b  is used in common by the front-side unit  1 F and the rear-side unit  1 R, it is recognized that even if any directivity is selected, as a peculiar effect, the frequency characteristics of the front (0-degree direction) do not change greatly.