Patent Publication Number: US-9838801-B2

Title: Unidirectional condenser microphone

Description:
TECHNICAL FIELD 
     The present invention relates to a unidirectional condenser microphone. 
     BACKGROUND ART 
     Sensitivity and a low-band limit of a unidirectional microphone are changed according to tension of a diaphragm. In the unidirectional microphone, the sensitivity of the microphone is increased as the tension of the diaphragm becomes higher. However, in this case, a low-band sound collection limit is shifted to a higher frequency, and thus a sound wave having a low frequency is not collected. In contrast, if the tension of the diaphragm becomes lower, the low-band sound collection limit is shifted to a lower frequency, and the sound wave having a lower frequency becomes able to be collected. However, in this case, the sensitivity of the microphone is decreased. Further, in a condenser microphone, the diaphragm is easily stuck to a fixed electrode due to electrostatic absorption force in a case where the tension of the diaphragm is low. The condenser microphone cannot collect sounds if the diaphragm is stuck to the fixed electrode. 
     As described above, in unidirectional condenser microphones, frequency response of the diaphragm and the sensitivity are in a trade-off relationship. 
     JP 2013-46194 A describes a unidirectional microphone that includes a cylindrical acoustic resistance tube in a front surface of a diaphragm to obtain favorable directional frequency response and high sensitivity. 
     The conventional unidirectional microphone described in JP 2013-46194 A collects a sound wave of a sound source of a low-band frequency, if the sound source exists in a position proximity to the microphone in a direction of 180° with respect to a sound collecting axis. As described above, if the sound source exists in the position proximity to the microphone, the microphone collects the low-band sound wave with emphasis. Such a phenomenon is typically referred to as proximity effect. 
     Directivity of the unidirectional microphone is controlled by a sound pressure difference between two points. Therefore, a typical unidirectional microphone includes two opening portions through which the sound waves are taken in, in the front and rear of the microphone. In the conventional unidirectional microphone, when the sound source exists near the rear opening portion, a sound in a low frequency deviating from the sound collecting axis is emphatically collected due to the proximity effect. Therefore, the emphasized unnecessary low sound overlaps with a sound to be collected and deteriorates sound quality. 
     SUMMARY OF INVENTION 
     An object of the present invention is to provide a unidirectional condenser microphone that can realize favorable directivity regardless of a frequency of a sound wave. 
     According to the present invention, there is provided a unidirectional condenser microphone having a front opening portion and a rear opening portion for respectively passing sound waves to a front surface and a back surface of a diaphragm of a microphone unit, the unidirectional condenser microphone including: an acoustic tube provided in the front opening portion; a first air chamber provided between the rear opening portion and the back surface of the diaphragm of the microphone unit, and having a predetermined acoustic capacity; and a second air chamber communicating into the first air chamber, and having an acoustic capacity larger than the predetermined acoustic capacity, wherein sensitivity to a direction of 0° with respect to a directional axis is improved by the first air chamber and the acoustic tube, and a proximity effect due to the sound wave from a direction of 180° with respect to the directional axis is prevented by the second air chamber. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a sectional side view illustrating an embodiment of a unidirectional condenser microphone according to the present invention; 
         FIG. 2  is an enlarged sectional side view illustrating the unidirectional condenser microphone of  FIG. 1 ; 
         FIG. 3  is a circuit diagram illustrating an acoustic equivalent circuit of the unidirectional condenser microphone of  FIG. 1 ; 
         FIG. 4  is a sectional side view of a unidirectional condenser microphone of a reference example; 
         FIG. 5  is a circuit diagram illustrating an acoustic equivalent circuit of the unidirectional condenser microphone of the reference example; 
         FIG. 6A  is a characteristic diagram illustrating a directivity pattern of the unidirectional condenser microphone of  FIG. 1 ; 
         FIG. 6B  is a graph illustrating directional frequency characteristics of the unidirectional condenser microphone of  FIG. 1 ; 
         FIG. 7A  is a characteristic diagram illustrating a directivity pattern of the unidirectional condenser microphone as a reference example; 
         FIG. 7B  is a graph illustrating directional frequency characteristics of the unidirectional condenser microphone of the reference example; 
         FIG. 8A  is a characteristic diagram illustrating a directivity pattern of a unidirectional condenser microphone as another reference example; and 
         FIG. 8B  is a graph illustrating directional frequency characteristics of a unidirectional condenser microphone of another reference example. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereinafter, an embodiment of a unidirectional condenser microphone according to the present invention will be described with reference to the drawings. 
     &lt;Schematic Configuration of Unidirectional Condenser Microphone&gt; 
     As illustrated in  FIG. 1 , a unidirectional condenser microphone  10  includes a microphone unit  11  and an acoustic tube  12  provided in front (at the left side on the sheet of  FIG. 1 ) of the microphone unit  11 . Further, the unidirectional condenser microphone  10  includes a first air chamber  13  provided in the rear (at the right side on the sheet of  FIG. 1 ) of a diaphragm of the microphone unit  11  and a second air chamber  15  provided in the rear of the first air chamber  13  and in the rear of the microphone unit  11 . 
     The unidirectional condenser microphone  10  includes a first acoustic resistance  14  that serves as an acoustic resistance of a sound wave that reaches the rear of the microphone unit  11 , and a communicating path  18  that allow the first air chamber  13  and the second air chamber  15  to communicate into each other. Further, the unidirectional condenser microphone  10  includes a second acoustic resistance  16  provided between the first air chamber  13  and the second air chamber  15 , and a rear opening portion  17  for passing the sound wave from an outside to the rear of the microphone unit  11 . 
     The unidirectional condenser microphone  10  includes a cylindrical metal-made case  21 , a circuit board  22  stored in the case  21 , and an output connector  23  electrically connected with the circuit board  22 , in the rear of the second air chamber  15 . In the circuit board  22 , a field effect transistor (FET) as an impedance converter, an amplifier circuit, a low-cut circuit, and the like are mounted. 
     &lt;Specific Configuration of Unidirectional Condenser Microphone&gt; 
     A specific configuration example of the unidirectional condenser microphone  10  will be described using an enlarged sectional view of a side surface illustrated in  FIG. 2 . The microphone unit  11  includes a diaphragm  111  that vibrates by the sound wave from an outside and a fixed electrode  112  that configures a condenser together with the diaphragm  111 . Further, the microphone unit  11  includes an insulating holder  113  that holds the diaphragm  111  and the fixed electrode  112 , and a unit case  115  that holds the diaphragm  111 , the fixed electrode  112 , and the like. The insulating holder  113  has a through path  114  that passes the sound wave taken in through the rear opening portion  17  to the first air chamber  13 . 
     The acoustic tube  12  is at a front surface side of the diaphragm  111 , and is a hollow tubular member provided in a front opening portion of the microphone unit  11 . The acoustic tube  12  includes a front surface opening portion  121  provided in a position facing the front surface of the diaphragm  111 , and a tube wall opening portion  122  provided in a tube wall of a side surface of the acoustic tube  12 . The front opening portion is configured from the front surface opening portion  121  and the tube wall opening portion  122 . The acoustic tube  12  passes the sound wave from a front of the unidirectional microphone  10  to the front surface of the diaphragm  111  through the front opening portion, that is, the front surface opening portion  121  and the tube wall opening portion  122 . 
     The first air chamber  13  is formed of the fixed electrode  112  and the unit case  115  at aback surface side of the diaphragm  111 , as described above. The first air chamber  13  is a space having a predetermined acoustic capacity. The first air chamber  13  communicates into the back surface of the diaphragm  111  through an opening portion provided in the fixed electrode  112 . 
     The first acoustic resistance  14  is provided on a path of the sound wave that passes through the through path  114  and is introduced into the first air chamber  13 , and serves as an acoustic resistance of the sound wave introduced through the rear opening portion  17  into the first air chamber  13 . 
     The second air chamber  15  communicates into the first air chamber  13  through the communicating path  18 . The second air chamber  15  communicates into the first air chamber  13 , thereby the second air chamber  15  communicates into the back surface of the diaphragm  111  of the microphone unit  11 . The second air chamber  15  is a space having a larger acoustic capacity than the predetermined acoustic capacity of the first air chamber  13 . 
     The second acoustic resistance  16  is provided between the first air chamber  13  and the second air chamber  15 , to be specific, at a front side of the communicating path  18  as viewed from the second air chamber  15 . The second acoustic resistance  16  serves as an acoustic resistance of the sound wave that passes between the first air chamber  13  and the second air chamber  15 . The second acoustic resistance  16  allows the sound wave having a frequency in a lower frequency range than a predetermined frequency to pass between the first air chamber  13  and the second air chamber  15 . 
     As described above, the unidirectional condenser microphone  10  includes the two air chambers including the first air chamber  13  and the second air chamber  15  in the rear of the diaphragm  111 . Respective functions of the first air chamber  13  and the second air chamber  15  will be described below. The sound waves taken in through the rear opening portion  17  where a rear acoustic terminal is positioned are divided into two paths like below according to its frequency, and reach the back surface of the diaphragm  111 . 
     The sound wave having a higher frequency in a middle and high range than the predetermined frequency is divided at the first acoustic resistance  14  and the first air chamber  13 , and applies a pressure to the back surface of the diaphragm  111 , so that this configuration realizes unidirectivity. That is, the sound wave having the frequency in the middle and high range reaches the back surface of the diaphragm  111  only through the first air chamber  13 . 
     Meanwhile, the sound wave having a lower-band frequency than the predetermined frequency is divided at the first acoustic resistance  14  and the second acoustic resistance  16 , and reaches the back surface of the diaphragm  111 . That is, in the sound wave having the low-band frequency, an effect of the second air chamber  15  larger than the first air chamber  13  is dominant. As a result, a non-directional component, of components that configure the unidirectivity, is enhanced by the second air chamber  15 , and an increase in a low-pitched range due to the proximity effect can be prevented even if there is a sound source near the rear opening portion. 
     &lt;Acoustic Equivalent Circuit&gt; 
       FIG. 3  illustrates an acoustic equivalent circuit of the unidirectional condenser microphone  10 . In  FIG. 3 , the sound wave taken in from the front of the acoustic tube  12  is P 1 , an acoustic mass of the diaphragm  111  is m 0 , stiffness of the diaphragm  111  is s 0 , a damping resistance of the diaphragm  111  is r 0 . Further, the sound wave taken in through the rear opening portion  17 , of the sound waves transmitted from the front of the acoustic tube  12 , is P 2 . Further, the acoustic resistance of the first acoustic resistance  14  is r 1 , and the acoustic resistance of the second acoustic resistance  16  is r 2 . Further, the acoustic capacity of the first air chamber  13  is s 1 , and the acoustic capacity of the second air chamber  15  is s 2 . For easy understanding, these reference numerals are appropriately added to  FIG. 2 . 
     The unidirectional condenser microphone  10  obtains the non-directional component as the sound wave P 1  reaches the front surface of the diaphragm  111  and obtains a bi-directional component as the sound wave P 2  reaches the back surface of the diaphragm  111 , thereby to realize the unidirectivity. To be specific, the unidirectional condenser microphone  10  connects the acoustic capacity s 1  of the first air chamber  13  and the second air chamber  15  with the acoustic resistance r 2 . Therefore, the sound wave having a low-band frequency is divided at the acoustic resistance r 1  and the acoustic resistance r 2 , and reaches the back surface of the diaphragm  111 . Therefore, the configuration with an acoustic capacity s 2  becomes equivalent to a configuration operated with a large air chamber, and drive force of the non-directional component is increased. 
     As described above, the acoustic capacity s 2  of the second air chamber  15  is larger than the acoustic capacity s 1  of the first air chamber  13 , and thus the acoustic capacity s 2  of the second air chamber  15  dominantly functions in the low-band frequency. Further, the unidirectional condenser microphone  10  is operated with the acoustic capacity S 1  of only the first air chamber  13  in the sound wave having a frequency in a middle and high range, and thus similarly functions to typical unidirectional microphones. 
       FIG. 4  illustrates a unidirectional condenser microphone  100  as a reference example. The unidirectional condenser microphone  100  includes a diaphragm  111 , a fixed electrode  112 , an insulating holder  113 , and a through path  116 , and has a similar configuration to the microphone unit  11  of the unidirectional condenser microphone  10 . Meanwhile, the unidirectional condenser microphone  100  is not provided with a second air chamber  15  and is provided with an insulating cap  110  in the rear of an air chamber  130  formed of the fixed electrode  112  and the insulating holder  113 . 
     As illustrated in an acoustic equivalent circuit of  FIG. 5 , the unidirectional microphone  100  does not include an acoustic capacity s 2  of a second air chamber  15  and an acoustic resistance r 2  of a second acoustic resistance  16 , and thus a non-directional component by a sound wave having a low-band frequency is not increased. That is, the unidirectional microphone  100  of the reference example is easily subject to proximity effect. 
     &lt;Directivity Pattern and Directional Frequency Characteristics&gt; 
     A characteristic diagram of a directivity pattern of the unidirectional microphone  10  according to the present embodiment is illustrated in  FIG. 6A , and a graph of directional frequency characteristics of the unidirectional microphone  10  is illustrated in  FIG. 6B , respectively. As illustrated in  FIG. 6A , the unidirectional microphone  10  obtains excellent unidirectivity. Further, as illustrated in  FIG. 6B , in the unidirectional microphone  10 , sound collection of the low-band frequency is suppressed in a direction of 180° with respect to a sound collecting axis. Further, frequency response in a direction of 90° with respect to the sound collecting axis becomes flat. Thus, frequency directional characteristics of the unidirectional microphone  10  is stable from the low band to the high band. 
     A characteristic diagram of a directivity pattern of the unidirectional microphone  100  of the reference example is illustrated in  FIG. 7A , and a graph of directional frequency characteristics of the unidirectional microphone  100  is illustrated in  FIG. 7B , respectively. As illustrated in  FIG. 7A , it can be seen that the unidirectional microphone  100  collects sounds even in a direction of 180° with respect to a sound collecting axis, compared with the unidirectional microphone  10  according to the present embodiment illustrated in  FIG. 6A . Further, as illustrated in  FIG. 7B , it can be seen that the unidirectional microphone  100  collects sounds having a low-band frequency in the direction of 180° with respect to the sound collecting axis. 
     That is, compared with the unidirectional microphone  100  of the reference example, the proximity effect is decreased and the favorable directivity is obtained regardless of the frequency in the unidirectional microphone  10  according to the present embodiment. 
     A characteristic diagram of a directivity pattern of a unidirectional microphone of another reference example is illustrated in  FIG. 8A , and a graph of directional frequency characteristics of the reference example is illustrated in  FIG. 8B , respectively. A unidirectional microphone of another reference example is obtained by attaching an acoustic tube to the front surface of the diaphragm  111  of the unidirectional microphone  100  of the above-described reference example. 
     As illustrated in  FIGS. 8A and 8B , it can be seen that the unidirectional microphone of another reference example also collects sounds having a low-band frequency in a direction of 180° with respect to a sound collecting axis. 
     That is, as can be seen from comparison with the unidirectional microphones of the reference examples, the proximity effect is decreased and the favorable directivity is obtained regardless of the frequency in the unidirectional microphone  10  according to the present embodiment. 
     As described above, according to the unidirectional microphone  10  according to the present embodiment, the first air chamber  13  and the second air chamber  15  having a larger acoustic capacity than the first air chamber are included, whereby the favorable directivity can be realized regardless of the frequency of the sound wave. 
     Especially, according to the unidirectional microphone  10 , the proximity effect of the sound wave having the low-band frequency in the direction of 180° with respect to the sound collecting axis is decreased, and the excellent directional characteristics can be obtained.