Abstract:
To obtain a composite type microphone, the microphone preventing an increase in size and weight and thereby improving the freedom for installation and handling while keeping the phases of signals output from respective microphone units the same. A composite type microphone that incorporates microphone units of different electroacoustic conversion methods into a common microphone body is provided. Here, in a front acoustic terminal portion of a first microphone unit based on one electroacoustic conversion method, a second microphone unit based on another electroacoustic conversion method is disposed, and in the front acoustic terminal portion of the first microphone unit, an air that vibrates in the same phase with that of a vibrating plate of the first microphone unit exists, and within the air that vibrates in the same phase with that of the vibrating plate of the first microphone unit, the second microphone unit is disposed.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a composite type microphone that incorporates a plurality of microphone units of different electroacoustic conversion methods into a common microphone body, e.g., the composite type microphone capable of disposing a condenser microphone unit in a front acoustic terminal portion of a dynamic microphone unit. 
     2. Related Background of the Invention 
     In recording a sound generated from the same musical instrument or the like, microphones of different electroacoustic conversion methods, e.g., a dynamic microphone and a condenser microphone, are sometimes disposed side by side for use. This is intended to record a sound from the same sound source with a plurality of microphones of different conversion methods and mix the output signals of the respective microphones, thereby taking advantage of the characteristics of the respective microphones, because the microphones of different electroacoustic conversion methods have mutually different sound quality. In particular, such recording form is often employed in recording a bass drum. 
     In case of attempting to record a sound coming from the same sound source with a plurality of microphones of different conversion methods as described above, since a phase difference occurs in the output signals of both microphones depending on the relative installation position relation between both microphones, the relative installation position of the plurality of microphones is carefully determined so as not to cause a phase difference. When a player plays a musical instrument, however, there is a drawback that a stand supporting the microphone vibrates, the installation position of microphone will shift with this vibration and the initial installation position can not be maintained, resulting in a phase difference in the output signals of the mutual microphones. 
     In order to solve such positional shift problem, composite type microphones are put in practical use. The conventional composite type microphone is made by disposing microphone units of mutually different electroacoustic conversion methods, e.g., a dynamic microphone unit and a condenser microphone unit, in parallel within a single microphone body to position vibrating plates of both microphone units on the same plane. According to such conventional composite type microphone, even if the installation position of the microphone has shifted due to playing of a musical instrument and the like, the mutual positional relation between two microphone units incorporated in the microphone body will not shift and thus the mutual phase of the output signals from two microphone units will not shift. 
     However, since the conventional composite type microphone is made by disposing two microphone units in parallel to integrate, a drawback is that the microphone is increased in size in the diameter direction. Since the upsized microphone is difficult to install and increases its weight, a drawback is that the microphone stand also needs to have a robust structure, thus increasing its weight and causing a difficulty in handling. 
     In addition, as a known art related to the composite type microphone, there is a microphone made by combining a bone-conduction microphone and an air-conduction microphone, which is used under noise environment. Such type of composite type microphone includes a combining control circuit for combining the output component of bone conduction from the bone-conduction microphone and the output component of air conduction from the air-conduction microphone. A combining control circuit is proposed, which includes a noise-level measurement unit for measuring the external noise level, and is configured to carry out a control of increasing a ratio of the output component of air conduction to the output component of bone conduction when the external noise level measured by this measurement unit is small, and decreasing the ratio of the output component of air conduction to the output component of bone conduction when the external noise level is high, so that the mixing ratio of the output component of bone conduction and the output component of air conduction is maintained appropriately even under fluctuation of the external noise (e.g., see Japanese Patent Application Laid-Open No. 8-214391). 
     The invention described in Japanese Patent Application Laid-Open No. 8-214391 however differs from the present invention in the problem to be solved and in the means for solving the problem. 
     As a known art related to the composite type microphone, there is also a composite type microphone made by integrating an FM wireless microphone circuitry and an optical wireless microphone circuitry. This is a wireless microphone device which accommodates the FM wireless microphone circuitry and the optical wireless microphone circuitry in the same housing, wherein an optical system of a high sound quality and a radio wave system capable of providing a long propagation distance are shared by means of a combination of a multiplication circuitry, a frequency conversion circuitry, and a PLL circuitry (e.g., see Japanese Patent Application Laid-Open No. 10-75497). 
     The invention described in Japanese Patent Application Laid-Open No. 10-75497 however differs from the present invention in the problems to be solved and in the means for solving the problems. 
     SUMMARY OF THE INVENTION 
     Problem to be Solved by the Invention 
     The present invention is intended to provide a composite type microphone that incorporates microphone units of different electroacoustic conversion methods into a common microphone body, and is capable of preventing an increase in size and weight while keeping the phases of signals output from the respective microphones the same and is capable of improving the freedom for installation and handling. 
     Means for Solving the Problem 
     A main feature of the present invention is a composite type microphone that incorporates microphone units of different electroacoustic conversion methods into a common microphone body, wherein in a front acoustic terminal portion of a first microphone unit based on one electroacoustic conversion method, a second microphone unit based on another electroacoustic conversion method is disposed. 
     The second microphone unit is preferably disposed such that the center thereof is aligned with the acoustic center of a front acoustic terminal of the first microphone unit. 
     Since in the front acoustic terminal portion of first microphone unit, an air that vibrates in the same phase with that of a vibrating plate of the first microphone unit exists, the second microphone unit is more preferably disposed within the air that vibrates in the same phase with that of the vibrating plate of the first microphone unit. 
     Preferably the first microphone unit is a dynamic microphone unit and the second microphone unit is a condenser microphone unit. 
     Advantages of the Invention 
     In a front acoustic terminal portion of a first microphone unit based on one electroacoustic conversion method, a second microphone unit based on another electroacoustic conversion method is disposed, and therefore the first and second microphone units are disposed in series in the front-back direction and it is thus possible to prevent a microphone from increasing in size in the diameter direction and to improve freedom for installation and handling. 
     If the second microphone unit is disposed within the air that vibrates in the same phase with that of the vibrating plate of the first microphone unit in the front acoustic terminal portion of the first microphone unit, it is possible to vibrate the vibrating plates of the first and second microphone units in the same phase and to eliminate a phase shift of the signals output from the first and second microphone units. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a vertical cross-sectional view showing an embodiment of a microphone concerning the present invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     An embodiment of a microphone concerning the present invention will be described hereinafter with reference to the accompanying drawing. 
     In  FIG. 1 , in a front end portion (at the left end portion in  FIG. 1 ) of a case  30  serving as a microphone body that is formed in a cylindrical shape, a dynamic microphone unit  10  is incorporated as a first microphone unit. Also to the microphone body case  30 , a condenser microphone unit  20  projecting forwardly from the front end of the microphone body case  30  is attached as a second microphone unit. Accordingly, two microphone units of different electroacoustic conversion methods are incorporated in the common microphone body case  30  to constitute a composite type microphone. 
     The dynamic microphone unit  10  includes a diaphragm  11  as a vibrating plate disposed at the front end portion of the microphone body case  30 , a coil  12  projectingly secured to the back face side of the diaphragm  11 , a permanent magnet  13 , a back yoke  14 , a front yoke  15 , and a front side outer peripheral yoke  16 . The diaphragm  11  has a relatively large dome-shaped portion as its main body, wherein the periphery thereof forms a dome-shaped edge with a small cross section and the peripheral portion of the edge is secured to the front end of the microphone body case  30 . To a boundary between the dome-shaped portion in the center and the dome-shaped edge, one end of the coil  12  wound up in a cylindrical shape is secured. When the diaphragm  11  receives a sound wave, it vibrates with the above-described secured portion being as a supporting point and the coil  12  also vibrates in the front-back direction integrally with the diaphragm  11 . 
     The permanent magnet  13 , the back yoke  14 , the front yoke  15 , and the front side outer peripheral yoke  16  are members constituting a magnetic circuit, and the back yoke  14  and the front yoke  15  are stacked across the permanent magnet  13 . The back yoke  14  is formed in the shape of closed-end cylinder by its outer peripheral portion being cylindrically formed, and the front end face of the cylindrical outer peripheral portion and the back end face of the front side outer peripheral yoke  16  are secured to each other. The front yoke  15  and the front side outer peripheral yoke  16  are positioned at the inner peripheral side and at the outer peripheral side, respectively, in the same plane, and a cylindrical gap is formed between the outer peripheral face of the front yoke  15  and the inner peripheral face of the front side outer peripheral yoke  16 , and the coil  12  passes through this gap. The permanent magnet  13 , the back yoke  14 , the front yoke  15 , the front side outer peripheral yoke  16 , and the above-described gap constitute a magnetic circuit, thus forming a magnetic field in the gap. The coil  12  exists within this magnetic field. When receiving a sound wave, the diaphragm  11  vibrates and the coil  12  moves with the diaphragm  11  and crosses the magnetic field, thereby generating an electric signal in the coil  12  and this electric signal is output as a sound signal. In this way, the microphone unit is constituted by the diaphragm  11 , the coil  12 , the permanent magnet  13  forming the magnetic circuit, and the like. 
     To the front end of the microphone body case  30 , an end plate  31  is fixed covering the diaphragm  11  and forming an appropriate gap between the same and the diaphragm  11 . In the end plate  31 , there are formed an appropriate number of holes for connecting the front side of the interior space, in which the diaphragm  11  exists, with the exterior space, and these holes constitute a front side acoustic terminal T 1 - 1  of the dynamic microphone unit  10 . Near the outer periphery of the microphone body case  30 , there are formed an appropriate number of holes for connecting the back side of the interior space, in which the diaphragm  11  exists, with the exterior space, and these holes constitute a back side acoustic terminal T 1 - 2  of the dynamic microphone unit  10 . 
     The condenser microphone unit  20  is fixed to a support medium  32  that integrally extends from the front end face of the end plate  31 . The condenser microphone unit  20  includes, in a cylindrical unit case  28 , a diaphragm  21  as a vibrating plate, a back plate  22  that is fixed with an appropriate gap being formed behind the diaphragm  21 , an insulator  24  disposed behind the back plate  22 , and an end plate  27  disposed behind the insulator  24 . Since the output impedance of the condenser microphone unit  20  is extremely high, an impedance conversion circuit including an FET (field effect transistor)  25  as a basic component is incorporated therein. The FET  25  is disposed so as to be buried in the insulator  24 , and an output terminal of the FET  25  is passed through a hole of the end plate  27  and is brought out backward as an output terminal  26  of the condenser microphone unit  20 . 
     In the center of the front end face of the unit case  28 , a hole for releasing the front face of the diaphragm  21  to the exterior space is formed and this hole serves as a front side acoustic terminal T 2 - 1  of the condenser microphone unit  20 . In the back plate  22 , the insulator  24 , and the end plate  27 , there is formed a hole for connecting the back face of the diaphragm  21  with the exterior space and this hole serves as a back side acoustic terminal T 2 - 2  of the condenser microphone unit  20 . An acoustic resistor  23  is disposed in the middle of the back side acoustic terminal T 2 - 2 . The outer diameter of the condenser microphone unit  20  is small relative to the outer diameter of the dynamic microphone unit  10  and is on the order of approximately ½. 
     In the description of the prior art, a phase difference in the output signals at the time of using a plurality of microphones with respect to one sound source was described. Further, it was also described that the phase difference problem does not occur if the diaphragms of the respective units exist on the same plane in the composite type microphone in which a plurality of microphone units are incorporated in a common microphone body. 
     However, according to the illustrated embodiment, the diaphragms  11  and  21  of the first and second microphone units are positioned as shifted back and forth, and thus this embodiment seems to have factors that cause a phase difference in the output signals of the respective microphone units. Under a certain condition, however, even if the diaphragms  11  and  21  of the first and second microphone units are positioned as shifted back and forth, it is possible to align the phases of the output signals of the respective microphone units to each other, and the illustrated embodiment satisfies this condition. This condition will be described hereinafter. 
     In the microphone, an air that vibrates in the same phase with that of the diaphragm exists near the acoustic terminal. The acoustic center of the front acoustic terminal exists in the portion of the air that vibrates in the same phase with that of the diaphragm. Now, assuming that the outer diameter of the dynamic microphone unit  10 , which is the first microphone unit, is approximately 28 mm, then the radius ad is 1.4 (cm). Let ρ denote the density of air, then ρ=1.22×10 −3  (g/cm 3 ), and the mass M of the air that moves with the diaphragm  11  due to the vibration of the diaphragm  11  is given as follows. 
                   M   =     0.61   ⁢   Πρ   ⁢           ⁢     ad   3     ⁢           ⁢     (     g     )                   =     0.61   ×   3.14   ×   1.22   ×     10     -   3       ×     1.4   3                   =     6.41   ⁢           ⁢     (   mg   )                   
In other words, there exists an air corresponding to the mass (additional mass) 6.41 (mg) that vibrates in the same phase with that of the diaphragm  11  due to the vibration of the diaphragm  11 . For this reason, the acoustic center of the front acoustic terminal of the microphone exists forward of the microphone unit  10  itself. Moreover, the larger the diameter of the microphone unit, the further forward of the acoustic center is positioned. In  FIG. 1 , a broken line depicted in a dome shape in front of the dynamic microphone  10  indicates a borderline of the air that vibrates in the same phase with that of the diaphragm  11 . In a space AM inside this borderline, the air that vibrates in the same phase with that of the diaphragm  11  exists. Accordingly, if the second microphone unit is disposed in the space AM, it is possible to vibrate the diaphragms of the first and second microphone units in the same phase with respect to the same sound source and thereby obtain the output signals of the same phase.
 
     The condenser microphone unit can be produced with a small diameter and size as compared with the dynamic microphone unit. Then, in the illustrated embodiment, the dynamic microphone unit  10  with a relatively large diameter is the first microphone unit, and the condenser microphone unit  20  as the second microphone unit is disposed with its center being aligned with an acoustic center S of the front acoustic terminal. In this way, the condenser microphone unit  20 , which is the second microphone unit, is disposed in the space AM, in which the air exists that vibrates in the same phase with that of the vibrating plate  11  of the dynamic microphone unit  10  of the front acoustic terminal portion of the dynamic microphone unit  10 , which is the first microphone unit, and therefore the phases of the output signals of the dynamic microphone unit  10  and the condenser microphone unit  20  with respect to the same sound source can be aligned although the dynamic microphone unit  10  and the condenser microphone unit  20  are disposed in series as shifted in the front-back direction. 
     Also, according to the above-described embodiment, since the first and second microphone units are disposed in the front-back direction of the common microphone body and with the respective axis lines being the same, the diameter of the composite type microphone can be reduced. By setting the second microphone unit as a condenser microphone unit capable of being miniaturized, it is possible to set the length in the front-back direction of the composite type microphone to almost the same length as that of a single dynamic microphone. In this way, miniaturization of the composite type microphone is possible, and thus the weight of the composite type microphone also can be reduced, allowing an easily handled composite type microphone to be provided. 
     The electroacoustic conversion methods of the first microphone unit and the second microphone unit just need to differ from each other. Although the electroacoustic conversion method of each unit is not limited in particular, the second microphone unit is preferably as compact as possible as shown in the illustrated embodiment, and thus a condenser microphone unit is suitable.