Patent Publication Number: US-7903829-B2

Title: Microphone

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a capacitor microphone capable of providing favorable directivity and frequency characteristic. 
     2. Description of the Related Art 
     An output voltage of a capacitor microphone unit is proportional to a displacement of a vibrating membrane. If the capacitor microphone unit is configured as a non-directional microphone unit, the capacitor microphone unit has no frequency dependence at a frequency lower than a resonance point of a vibrating system. However, once directivity is provided for the capacitor microphone unit, an electromotive force applied to the vibrating membrane has frequency dependence. 
       FIG. 8  shows the most standard configuration of a unidirectional microphone unit.  FIG. 8  shows an internal structure of the directional microphone unit. In a cylindrical outer casing  11 , a case  12  having a rear plate  12 R on one end of a cylindrical body in an axial direction is housed. On the other end of the case  12  in the axial direction, a diaphragm (for example, a thin vibrating membrane; a vibrating section)  13  for closing the open end of the case  12  is provided. 
     At the location separated from the diaphragm  13  by a predetermined distance toward the rear plate  12 R, a back plate  14  including a plurality of holes perforated therethrough is provided. In the approximate center of the back plate  14  in a radial direction, an electrode bar  14   a  having a predetermined diameter is formed. The electrode bar  14   a  is provided to extend toward the rear plate  12 R so that its end penetrates through the rear plate  12 R to form an electrically-conductive terminal  14   aa.    
     A plurality of through holes  15  are provided in the rear plate  12 R. An acoustic resistor  16  made of, for example, cloth is provided on an end of each of the through holes  15 , the end being on the outer side of the rear plate  12 R. A thin fluid layer  17  is formed by a gap between the diaphragm  13  and the back plate  14 . An air resistance of the thin fluid layer  17  is set high enough to prevent a resonance of the diaphragm  13  at a high frequency and not to greatly affect the directivity and the frequency characteristic. 
     An area on the back side of the back plate  14 , which is surrounded by an inner circumference of the case  12 , is a hollow space  18 . Together with the through holes  15  and the acoustic resistors  16 , the hollow space  18  forms a phase-shift circuit to obtain directivity. The frequency dependence of the unidirectional microphone unit controls the vibration of the diaphragm  13  by the acoustic resistors  16 . If a resistance value of the acoustic resistors  16  is increased, the directivity is decreased to finally provide no directivity. 
     In a DC-bias capacitor microphone, a DC-bias voltage is applied through a high resistance between a vibrating membrane and a back plate. In the case shown in  FIG. 8 , a DC power source for biasing is connected between the diaphragm  13  and the outer casing  11  (not shown) so that the outer casing  11  and the case  12  function as the high resistance. 
     By a change in capacitance of the microphone unit shown in  FIG. 8 , a change in voltage occurred between both ends of the high resistance (that is, between the outer casing  11  and the terminal  14   aa ) is obtained as an electric signal. 
     Conventionally, as described in Japanese Patent Application Publication No. Sho 60-22897, a so-called MS microphone, which uses a unidirectional microphone unit and a bidirectional microphone unit to obtain a stereophonic sound, has been proposed. 
     SUMMARY OF THE INVENTION 
     If the diaphragm  13  of the unidirectional microphone unit shown in  FIG. 8  is oriented toward a sound source situated on the front, sound on the rear side (behind the microphone unit) is cancelled by the offset between sound entering the microphone unit through the acoustic resistors  16  and the through holes  15  of the rear plate  12 R and sound taking a detour from the back to the front to be introduced through the diaphragm  13 . 
     However, the unidirectional microphone unit has a so-called proximity effect; when a sound source located behind is close to the microphone unit, a bass sound is enhanced in a sound coming from the sound source. 
     Since the MS microphone described in Japanese Patent Application Publication No. Sho 60-22897 includes the unidirectional microphone unit, the microphone unit is difficult to have a broad frequency characteristic to a bass sound range as in the case of a non-directional microphone unit, due to a structural characteristic of the unidirectional microphone unit. Furthermore, a directional characteristic in the bass sound range is inferior to that of a midrange. 
     The present invention is devised in view of the above circumstances and aims to provide a microphone capable of providing a two-channel stereo output and a four-channel stereo output with favorable directivity and frequency characteristic. 
     A microphone according to an embodiment of the present invention has: 
     (1) a first bidirectional microphone unit formed by connecting two unidirectional microphone units, each having a vibrating section on a front side, back to back; and a second bidirectional microphone unit formed by connecting two unidirectional microphone units, each having a vibrating section on a front side, back to back, the second bidirectional microphone unit being arranged so that its directional axis is shifted by 90 degrees with respect to that of the first bidirectional microphone unit. 
     In the unidirectional microphone, when a front side having a vibrating section (a diaphragm, a vibrating membrane or the like) is oriented toward a sound source, a rear sound is cancelled by the offset between a sound entering the microphone from the back and a sound taking a detour to enter from the front. As a result, the unidirectional microphone has unidirectionality to capture only a front sound. 
     The unidirectional microphone has a so-called proximity effect; when a sound source situated behind is close to the microphone unit, a bass sound is enhanced in a sound coming from the back. In the bidirectional microphone unit according to an embodiment of the present invention, however, among the unidirectional microphones connected to each other back to back, the vibrating section of one of the unidirectional microphones is situated on the rear face side of the other unidirectional microphone and the vibrating section of the other unidirectional microphone is situated on the rear face side of one unidirectional microphone. Therefore, each of the vibrating sections acts as a compliance for correcting the proximity effect (an acoustic compliance; cm 3 /μbar) to keep the unidirectionality even in a relatively low frequency. 
     Since the compliance has an increasing reactance as a frequency becomes lower, an acoustic pressure of an acoustic wave introduced from the back of the microphone becomes gradually smaller. Specifically, the characteristic of the microphone becomes closer to that of a non-directional microphone whose back is closed. Therefore, a frequency characteristic on the front side becomes flat to a bass sound range. As a result, favorable directivity and a broad frequency characteristic to a bass sound range can be obtained, which are not affected by a difference in frequency. 
     (2) The first bidirectional microphone unit inputs an acoustic signal corresponding to a front and rear channel, the second bidirectional microphone unit inputs an acoustic signal corresponding to a left and right channel, and the microphone includes a two-channel signal circuit for generating and outputting a two-channel acoustic signal of front left and front right channel, formed from the acoustic signal corresponding to the front channel output from the first bidirectional microphone unit and from the acoustic signal corresponding to the left and right channel output from the second bidirectional microphone unit. 
     With the first and the second bidirectional microphone units and the two-channel signal circuit, a two-channel stereo microphone having favorable directivity that is not varied by a difference in frequency can be configured. 
     (3) The first-bidirectional microphone unit inputs an acoustic signal corresponding to a front and rear channel, the second bidirectional microphone unit inputs an acoustic signal corresponding to a left and right channel, and the microphone includes a four-channel signal circuit for generating and outputting four-channel acoustic signals of front left, front right, rear left and rear right channel, formed from the acoustic signal corresponding to the front and rear channel output from the first bidirectional microphone unit and from the acoustic signal corresponding to the left and right channel output from the second bidirectional microphone unit. 
     With the first and the second bidirectional microphone units and the four-channel signal circuit, a four-channel stereo microphone having favorable directivity that is not varied by a difference in frequency can be configured. 
     (1) According to the present invention, a characteristic expanding even in a bass sound range can be obtained in terms of frequency characteristic as compared with a stereo microphone configured by a general directional microphone. 
     (2) The microphone according to the present invention does not have the proximity effect, which is a characteristic of a directional microphone. Therefore, a phenomenon that an output in a bass sound range increases if a distance to a sound source is small does not occur. 
     (3) Furthermore, the directivity of the stereo microphone consisting of a general directional microphone varies depending on a frequency. In particular, in a bass sound range, a characteristic becomes closer not to a cardioid characteristic but to bidirectionality. According to the present invention, however, a cardioid characteristic can be realized even in a bass sound range. 
     (4) According to the invention recited in claim  3 , by adding a circuit to the configuration of the two-channel stereo microphone, the four-channel stereo microphone can be realized. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a simplified schematic internal configuration diagram showing an embodiment of a bidirectional microphone unit according to the present invention; 
         FIG. 2  is a perspective view of a main part, showing an embodiment in which a first bidirectional microphone unit and a second bidirectional microphone unit according to the present invention are vertically arranged; 
         FIG. 3  is a perspective view of a main part, showing an embodiment in which the first and the second bidirectional microphone units according to the present invention are horizontally arranged; 
         FIG. 4  is a circuit diagram of an embodiment in which a two-channel stereo microphone is realized by the present invention; 
         FIG. 5  is a circuit diagram of an embodiment in which a four-channel stereo microphone is realized by the present invention; 
         FIG. 6  is a characteristic view showing a polar pattern of the two-channel stereo microphone according to an embodiment of the present invention; 
         FIG. 7  is a characteristic view showing a polar pattern of the four-channel stereo microphone according to an embodiment of the present invention; and 
         FIG. 8  is a simplified schematic internal configuration diagram of a directional microphone unit. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereinafter, an embodiment of the present invention will be described with reference to the accompanying drawings. However, the present invention is not limited to the following embodiment.  FIG. 1  shows a bidirectional microphone unit obtained by connecting a pair of the unidirectional microphone units shown in  FIG. 8  back to back so as to be opposed to each other. In  FIG. 1 , the same components are denoted by the same reference numerals as those in  FIG. 8 . 
     Each of the reference numerals  21  and  22  denotes a unidirectional microphone unit configured in the same manner as the microphone unit shown in  FIG. 8 . The rear plates  12 R of the unidirectional microphone units  21  and  22  are arranged to be opposed to each other with a predetermined gap there between. The outer casings  11  of the microphone units  21  and  22  are connected to each other with a metal connection ring  23  having an axial dimension which allows the connection of the two microphone units  21  and  22 . 
     In the bidirectional microphone unit shown in  FIG. 1 , an output from the unidirectional microphone unit  21  is obtained as an output  1  which is output from between the terminal  14   aa  of the microphone unit  21  and the metal connection ring  23 , whereas an output from the unidirectional microphone unit  22  is obtained as an output  2  which is output from between the terminal  14   aa  of the microphone unit  22  and the metal connection ring  23 . 
     The structure of each of the components of the unidirectional microphone units  21  and  22  shown in  FIG. 1  is the same as that of  FIG. 8 . A bias voltage is applied to the unidirectional microphone units  21  and  22  as in the case of  FIG. 8 . However, if bias voltages of the same polarity are applied to the diaphragms  13  of the microphone units  21  and  22 , respectively, output voltages from the diaphragms  13  exhibit cardioid characteristics in the opposite directions. A non-directional microphone is realized by summing these outputs. If bias voltages of the opposite polarities are applied to the diaphragms  13  of the microphone units  21  and  22  to obtain the sum of the outputs, a bidirectional microphone is realized. 
     In the microphone shown in  FIG. 1 , on the back side of one of the microphone units, for example, the unidirectional microphone unit  21 , the diaphragm  13  of the other unidirectional microphone unit  22  is present. On the back side of the other unidirectional microphone unit  22 , the diaphragm  13  of the unidirectional microphone unit  21  is present. Therefore, the two diaphragms  13  function as a compliance for correction of the proximity effect and the unidirectionality is kept even in a relatively low frequency. 
     Since the connected compliance has an increasing reactance as a frequency becomes lower, an acoustic pressure of an acoustic wave introduced from the back of the microphone becomes gradually smaller. Specifically, the characteristic of the microphone becomes closer to that of a non-directional microphone having a closed back. For this reason, a frequency characteristic on the front side becomes flat to a bass sound range. 
     Each of  FIGS. 2 and 3  shows the arrangement of bidirectional microphone units forming the microphone according to the embodiment of the present invention. In  FIG. 2 , bidirectional microphone units  20   mid  and  20   side , each being configured in the same manner as in  FIG. 1 , are arranged in a vertical direction with respect to a sound source with a predetermined gap therebetween. The microphone units  20   mid  and  20   side  are provided so that their directional axes are shifted by 90 degrees each other. 
     More specifically, a 0-degree direction is a recorded sound source direction. The bidirectional microphone unit  20   mid  is provided so as to be oriented in the 0-degree direction and a 180-degree direction at the upper position of a vertical axis, whereas the bidirectional microphone unit  20   side  is provided so as to be oriented in a 90-degree direction and a 270-degree direction at the lower position of the same vertical axis. 
     In  FIG. 3 , the bidirectional microphone units  20   mid  and  20   side , each being configured in the same manner as in  FIG. 1 , are arranged in a horizontal direction with respect to a sound source with a predetermined gap therebetween. The directional axes of the microphone units  20   mid  and  20   side  are provided to form 90 degrees therebetween. 
     More specifically, a 0-degree direction is a recorded sound source direction. The bidirectional microphone unit  20   mid  is provided so as to be oriented in the 0-degree direction and a 180-degree direction at a predetermined position on a horizontal axis, whereas the bidirectional microphone unit  20   side  is provided so as to be oriented in a 90-degree direction and a 270-degree at a position which is a predetermined distance away from the microphone unit  20   mid  in the horizontal direction. 
     The microphone units are formed to have a circuit configuration as shown in  FIG. 4  or  FIG. 5  to constitute a two-channel stereo microphone or a four-channel stereo microphone. 
     Specifically,  FIG. 4  shows an example of a two-channel signal circuit for obtaining two-channel acoustic signals, i.e., front right and front left (two-channel stereo outputs) from outputs of the microphone units arranged as shown in  FIG. 2  or  FIG. 3 . 
     In  FIG. 4 , an output Front of the bidirectional microphone unit  20   mid  corresponds to an output of the directional microphone unit oriented in the 0-degree direction with respect to the sound source as described with reference to  FIGS. 2 and 3 , specifically, for example, the output  1  in  FIG. 1 . An output Rear corresponds to an output of the directional microphone unit oriented in the 180-degree direction with respect to the sound source described with reference to  FIGS. 2 and 3 , specifically, for example, the output  2  in  FIG. 1 . 
     An output L-side of the bidirectional microphone unit  20   side  corresponds to an output of the directional microphone unit oriented in the 270-degree direction with respect to the sound source as described with reference to  FIGS. 2 and 3 , specifically, for example, the output  1  in  FIG. 1 . An output R-side corresponds to an output of the directional microphone unit oriented in the 90-degree direction with respect to the sound source described with reference to  FIGS. 2 and 3 , specifically, for example, the output  2  in  FIG. 1 . 
     The output Front of the bidirectional microphone unit  20   mid  passes through an amplifier  31  to be input to one of input terminals of an adder  32  and a non-inverting input terminal of a differential amplifier  33 . The output L-side of the bidirectional microphone unit  20   side  is input to a non-inverting input terminal of a differential amplifier  34 , whereas the output R-side is input to an inverting input terminal of the differential amplifier  34 . 
     An output of the differential amplifier  34  is input to the other input terminal of the adder  32  and an inverting input terminal of the differential amplifier  33 . An output of the adder  32  is output as an L-channel, whereas an output of the differential amplifier  33  is output as an R-channel. In  FIG. 4 , the output Rear of the bidirectional microphone unit  20   mid  is not used. 
       FIG. 5  shows an example of a four-channel signal circuit for obtaining four-channel acoustic signals (four-channel stereo outputs), i.e., front left, right, rear left and right, from outputs of the microphone units arranged as shown in  FIG. 2  or  3 . 
     The output Front of the bidirectional microphone unit  20   mid  passes through an amplifier  35  to be input to one of input terminals of an adder  36  and a non-inverting input terminal of a differential amplifier  37 . The output L-side of the bidirectional microphone unit  20   side  is input to a non-inverting input terminal of a differential amplifier  38  and a non-inverting input terminal of a differential amplifier  39 , whereas the output R-side is input to an inverting input terminal of the differential amplifier  38  and an inverting input terminal of the differential amplifier  39 . 
     An output of the differential amplifier  38  is input to an inverting input terminal of the differential amplifier  37  and the other input terminal of the adder  36 . An output of the adder  36  is output as a Front-L-channel, whereas an output of the differential amplifier  37  is output as a Front-R-channel. An output of the differential amplifier  39  is input to an inverting input terminal of a differential amplifier  40  and one of input terminals of an adder  41 . 
     The output Rear of the bidirectional microphone unit  20   mid  passes through an amplifier  42  to be input to a non-inverting input terminal of the differential amplifier  40  and the other input terminal of the adder  41 . An output of the differential amplifier  40  is output as a Rear-R-channel, whereas an output of the adder  41  is output as a Rear-L-channel. 
     In any of the cases of the two-channel stereo microphone shown in  FIG. 4  and the four-channel stereo microphone shown in  FIG. 5  above, favorable directivity and frequency characteristic as described with reference to  FIG. 1  are obtained. 
       FIG. 6  shows an example of a directional angle pattern of the two-channel stereo microphone shown in  FIG. 4 . According to the embodiment of the present invention, even in a low-frequency band, the directional characteristic shown in  FIG. 6  is obtained. 
       FIG. 7  shows an example of a directional angle pattern of the four-channel stereo microphone shown in  FIG. 5 . According to the embodiment of the present invention, even in a low-frequency band, the directional characteristic shown in  FIG. 7  is obtained. 
     The arrangement of the first and the second bidirectional microphone units ( 20   mid  and  20   side ) according to the embodiment of the present invention is not limited to those shown in  FIGS. 2 and 3 . Any arrangement can be used as long as their directional axes are shifted by 90 degrees. 
     The two-channel signal circuit according to the embodiment of the present invention is not limited to that shown in  FIG. 4 , and the four-channel signal circuit according to the present invention is not limited to that shown in  FIG. 5 . Each of the two-channel signal circuit and the four-channel signal circuit may be configured with another circuit having a similar function. 
     It should be understood by those skilled in the art that various modifications, combinations, sub-combinations and alterations may occur depending on design requirements and other factors insofar as they are within the scope of the appended claims or the equivalents thereof. 
     The present invention contains subject matter related to Japanese Patent Application JP2005-124767, filed in the Japanese Patent Office on Apr. 22, 2005, the entire contents of which being incorporated herein by reference.