Patent Publication Number: US-8983079-B2

Title: Stereo microphone

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a stereo microphone having stable directional frequency response in a high-frequency range without generation of intrinsic noise. 
     2. Related Background Art 
     As disclosed in Japanese Unexamined Patent Application Publication No. H06-303691, a stereo microphone includes two microphone units, which output signals from left and right channels respectively. 
     Such a stereo microphone has, for example, two bidirectional microphone units, one of which has a directional axis directed at an angle of −45° to the left relative to the front of the microphone and the other has a directional axis directed at an angle of +45° to the right. The stereo microphone employs a Blumlein array for sound collection. The bidirectional microphone units configuring the Blumlein array include electrostatic condenser microphone units and electrodynamic ribbon microphone units. 
     The two microphone units included in the stereo microphone are generally disposed such that their directional axes are provided in the same horizontal plane. For the bidirectional microphone units used in the Blumlein array, however, it is not preferred in view of the performance that the left and right channel units be adjacently disposed since the properties are adversely affected unless a proximate construction is designed front/back symmetrically relative to the sound center. Thus, the two bidirectional units of the Blumlein stereo microphone are vertically stacked. 
     In such a Blumlein stereo microphone, the directional axes of the two microphone units are present in different horizontal planes, which configuration is not preferred for stereo sound collection. Since the directional axes of the two vertically stacked microphone units do not reside in the same horizontal plane, vertical (upper and lower) imbalance of a sound source relative to a proximate sound source, in particular, is picked up separately in the upper and lower microphone units, thus resulting in horizontally (left and right) unbalanced output. 
     It is thus desired for the stereo microphone to have compatibility between prevention of impact on acoustic properties due to a proximate construction and satisfactory stereo sound collection by disposing microphone units in the same plane. 
     The Blumlein stereo microphone may include bidirectional condenser microphone units. The bidirectional microphone units each have sound terminals in the front and back. A diaphragm vibrates in response to a sound pressure gradient determined by the distance between the front and back sound terminals. In order to achieve desired sensitivity, it is necessary to easily generate the sound pressure gradient, thus requiring a certain distance between the sound terminals. A long distance between the sound terminals, however, lowers a high-frequency sound collection limit. 
     In the case of using the bidirectional condenser microphone units, each of which generally includes a circular diaphragm, a large diameter of the unit is required for higher sensitivity. 
     One method of stereo sound collection is to combine two unidirectional condenser microphone units at a 180° direction to each other such that output sound signals are subtracted to achieve bidirectivity. Disposing the bidirectional sound collection axes configured as above in the same horizontal plane can solve the problem caused by vertical arrangement of the bidirectional microphone units described above, thus allowing stereo sound collection similar to the case of using the bidirectional condenser microphone units. 
     A similar configuration is provided in a four-channel one-point pickup microphone. Such a stereo microphone includes four unidirectional condenser microphone units disposed at different directions by 90° in the horizontal plane and back sound terminals sonically combined. Subtracting sound signals of the two unidirectional condenser microphone units disposed in the 180° direction provides bidirectivity. 
     Since the back sound terminals of the two unidirectional condenser microphone units disposed in the 180° direction are sonically combined, the distance between the sound terminals is inevitably long. Such a long distance between the sound terminals lowers the high-frequency sound collection limit, similar to the case of using the two bidirectional condenser microphone units described above. 
     With reference to  FIGS. 4A to 4C , in a layout, four circular unidirectional condenser microphone units  10   a ,  10   b ,  10   c , and  10   d  are disposed in the same horizontal plane such that external diameter portions thereof are in contact with each other by rotating directional axes of adjacent condenser microphone units  10  by 90°. A microphone having such a unit layout is referred to as a “four-channel one-point microphone,” which collects sounds from four directions and converts the sounds separately into sound signals for output. 
     In the unit layout shown in  FIGS. 4A to 4C , collaboration of the two diagonally-positioned condenser microphone units  10   a  and  10   c  or  10   b  and  10   d , each having directional axes disposed at 180° to each other, provides a pair of bidirectional microphone units. The two pairs of bidirectional microphone units are disposed such that the directional axes are disposed at 90° to each other, and thereby a stereo microphone is provided. 
     In this case, a distance W 2  between sound terminals of the pair of microphone units is defined by the diameter of each of the condenser microphone units  10 . In the condenser microphone unit, as the effective capacitance increases between a diaphragm  101  and a fixed electrode  102 , the sensitivity increases while the effective noise decreases. In order to increase the effective capacitance, it is necessary to increase the area S 2  of the diaphragm  101  of the condenser microphone unit  10 . 
     In order to increase the area S 2  of the diaphragm  101  of the condenser microphone unit  10 , it is necessary to increase the diameters of the diaphragm  101  and the condenser microphone unit  10 . The increased diameters thereof, however, lead to a large distance W 2  between the sound terminals of the pair of microphone units, thus reducing the high-frequency sound collection limit, as shown in  FIG. 5 . 
     SUMMARY OF THE INVENTION 
     In view of the circumstances above, an object of the present invention is to provide a stereo microphone having stable directional frequency response in a high-frequency range without generation of intrinsic noise by maintaining a short distance between sound terminals even with an increase in diaphragm size. 
     A main aspect of the present invention provides a stereo microphone including four condenser microphone units having directional axes disposed in a single horizontal plane, the condenser microphone units each having unidirectivity, the condenser microphone units each having a quadrangular shape viewed from a direction of each directional axis, the condenser microphone units being disposed such that adjacent condenser microphone units have directional axes directed in different directions by 90° from one another, two of the condenser microphone units diagonally positioned and having the directional axes directed at 180° to each other collaborating with each other to form a pair of bidirectional microphone units. The two pairs of bidirectional microphone units are disposed such that two directional axes of the pairs are disposed at 90° to each other. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a partial cross-sectional front view illustrating a stereo microphone according to an embodiment of the present invention; 
         FIGS. 2A ,  2 B, and  2 C are a plan view, a front view, and a cross-sectional view, respectively, of a condenser microphone unit included in the stereo microphone; 
         FIG. 3  is a graph illustrating the acoustic performance of the stereo microphone; 
         FIGS. 4A ,  4 B, and  4 C are a plan view, a front view, and a cross-sectional view, respectively, of a condenser microphone unit included in a conventional stereo microphone; and 
         FIG. 5  is a graph illustrating the acoustic performance of the conventional stereo microphone. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A stereo microphone according to an embodiment of the present invention is described with reference to  FIGS. 1 to 3 . Configurations the same as those in a conventional stereo microphone shown in  FIGS. 4A to 4C  are denoted with the same reference numerals. 
     With reference to  FIG. 1 , a stereo microphone  1  according to the embodiment of the present invention has a microphone casing  2  that accommodates four unidirectional condenser microphone units  10   a ,  10   b ,  10   c , and  10   d  each having a substantially square shape in a plan view such that directional axes of the microphone units are disposed in the same horizontal plane. External sound reaches the condenser microphone units  10   a ,  10   b ,  10   c , and  10   d  through a shield mesh (not shown in the drawing) provided in the microphone casing  2 . 
     The microphone casing  2  includes a circuit board  3  on which electronic components, including a step-up transformer, are mounted. The output from the condenser microphone units  10   a ,  10   b ,  10   c , and  10   d  passes through an output transformer so as to be supplied to an external device through a three-pin connector  4 . The three-pin connector  4  includes hot and cold terminals of the output transformer and a ground terminal. 
     With reference to  FIGS. 2A to 2C , four unidirectional condenser microphone units  10   a ,  10   b ,  10   c , and  10   d  are disposed in the same horizontal plane such that peripheral portions thereof are in contact with each other by rotating directional axes of adjacent condenser microphone units  10  by 90°. The microphone units  10   a ,  10   b ,  10   c , and  10   d  each have a square shape viewed from the front as described above and are common in size, electroacoustic transduction, and other specifications. 
     In the plan view of  FIG. 2A , collaboration of the two diagonally-positioned condenser microphone units  10   a  and  10   c  or  10   b  and  10   d , each having directional axes disposed at 180° to each other, provides a pair of bidirectional microphone units for right and left channels. The condenser microphone units  10   a  and  10   c  that serve as a pair of microphone units for the right channel are connected in series, while the condenser microphone units  10   b  and  10   d  that serve as a pair of microphone units for the left channel are connected in series. Thus, the two pairs of the microphone units provided as above are disposed having the directional axes at 90° to each other. 
     In the stereo microphone configured as above, a distance W 1  between sound terminals of one pair of microphone units is defined by the length of the side of the substantially planar square of the condenser microphone unit  10  as the effective capacitance increases between a diaphragm  101  and a fixed electrode  102 , the sensitivity increases while the effective noise decreases. In order to increase the effective capacitance, it is necessary to increase the area S 1  of the diaphragm  101  of the condenser microphone unit  10 . 
     In the following description, the area S 1  of the diaphragm  101  in the embodiment is the same as the area S 2  of the circular diaphragm  101  of the conventional condenser microphone unit  1  shown in  FIGS. 4A to 4C . 
     With the area S 1  of the diaphragm  101  in the embodiment same as the area S 2  of the conventional circular diaphragm  101 , the effective capacitance is also the same, thus similarly preventing generation of intrinsic noise. 
     In addition, the distance W 1  between the sound terminals of the condenser microphone unit  10  in the embodiment can be shorter than the distance W 2  between the sound terminals of the conventional stereo microphone. This is achieved with the square diaphragm  101  in the embodiment to eliminate an extra space S 3  (refer to  FIG. 4B ) associated with the use of the circular diaphragm  101  in the conventional stereo microphone, thus increasing the volume ratio of the microphone unit in a predetermined volume space. 
     Thus, as shown in  FIG. 3 , the stereo microphone according to the present invention achieves stable directional frequency response in a high-frequency range, which is not achieved by the conventional stereo microphone (refer to  FIG. 5 ). 
     The object of the present invention can be achieved by increasing the ratio of the microphone unit in a predetermined space. To this end, the planar shape of the microphone unit may be rectangular.