Abstract:
A unidirectional condenser microphone includes a first and second condenser elements each having a diaphragm and a fixed electrode disposed opposite the diaphragm, an insulating base having an opening at the center thereof and supporting the respective fixed electrodes of the first and second condenser elements at opposite sides of the insulating base, acoustic resistance materials covering both ends of the opening, and air chambers formed respectively between each of the fixed electrodes and the insulating base, such that respective back sides of the diaphragms of the first and the second condenser elements are acoustically in communication with each other, wherein the diaphragm of the second condenser elements is formed to be an annular-shape having a central opening, and the second condenser element has a rear acoustic terminal hole communicating with the central opening.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a unidirectional condenser microphone unit, more particularly to a unidirectional condenser microphone unit that can collect sounds in a lower frequency range by proximity effect depending on situations, even in the case where a diaphragm is placed on a rear acoustic terminal side of the condenser microphone unit. 
         [0003]    2. Description of the Related Art 
         [0004]    Generally, a unidirectional condenser microphone exhibits proximity effect. The proximity effect is an effect of rise of output level in a low sound range when a sound source, such as speakers&#39; mouth, becomes close to the microphone. Sound collection using the proximity effect is often performed because a bass-rich voice can be collected therewith. 
         [0005]    The proximity effect, however, causes a change of a lower frequency response depending on a distance from a sound source, and therefore, the proximity effect is undesirable when a sound quality should not be changed depending on the distance. 
         [0006]    In order to solve the above problem, employed is a unidirectional condenser microphone having a diaphragm on a rear acoustic terminal, as shown in  FIG. 9 . 
         [0007]    A condenser microphone unit  50  shown in  FIG. 9  includes an insulating base  9  made of an electric insulator such as synthetic resin or ceramics, and a first and a second condenser element  51  and  52  supported by opposite sides of the insulating base  9 . The condenser elements  51  and  52  are identical except being disposed laterally symmetrically. The condenser elements  51  and  52  include diaphragms  15  and  16  stretched with a predetermined tension over supporting rings  13  and  14  which are integrally formed on peripheral areas of resonators  11  and  12  made of metal plates. 
         [0008]    Further, the diaphragms  15  and  16  are disposed opposite to fixed electrodes  19  and  20  through spacer rings  17  and  18 . And peripheral portions on both sides of the insulating base  9  and peripheral portions of the resonators  11  and  12  are integrally assembled by connection rings  21  and  22 . 
         [0009]    The diaphragms  15  and  16  use synthetic resin thin films having a metal-, preferably gold, evaporated films on one side thereof. The fixed electrodes  19  and  20  are made of perforated metal plate having a large number of sound holes (not shown). Additionally, electret dielectric films may be provided on the fixed electrodes  19  and  20 . 
         [0010]    The resonator  11  and  12  include acoustic terminal holes  11   a  and  12   a  for collecting sound waves. A communication hole  9   a  is provided at the center of the insulating base  9 , and both ends of the communication hole  9   a  are covered with acoustic resistance materials  23  and  24 . 
         [0011]    Further, air chambers A 1  and A 2  are provided between the fixed electrodes  19 ,  20  and the insulating base  9  in order to acquire velocity components through acoustic resistance materials  23  and  24 . In an example shown in  FIG. 9 , tapered members  25  and  26  are disposed on the both side faces of the insulating base  9 . The tapered members  25 ,  26  form conical surfaces with the acoustic resistance materials  23  and  24  as apex parts and the peripheral portions of the insulating base  9  as peripheries, and the air chambers A 1  and A 2  function as acoustic transducers having a speaker-cone-like shape. 
         [0012]      FIG. 10  shows an equivalent circuit for a condenser microphone unit  50 . Let the first condenser element  51  be on a front side, then P 1  denotes a sound source on a side of the acoustic terminal hole  11   a,  m 0 f denotes mass of the diaphragm  15 , s 0 f denotes stiffness of the diaphragm  15 , r 0 f denotes an acoustic resistance, and s 1  denotes acoustic mass of the air chamber A 1 . And P 2  denotes a sound source on a side of the acoustic terminal hole  12   a,  m 0 a denotes mass of the diaphragm  16 , s 0 a denotes stiffness of the diaphragm  16 , r 0 a denotes an acoustic resistance, and s 2  denotes acoustic mass of the air chamber A 2 . A resultant acoustic resistance of the acoustic resistance materials  23  and  24  is shown as r 1 . 
         [0013]    In the condenser microphone unit  50  of this structure, in the case where the sound source P 1  is on a side of the first condenser microphone element  51 , for example, sound waves P 2  from the rear acoustic terminal  12   a  toward the first condenser element  51  comes through the diaphragm  16  (m 0 a, s 0 a, r 0 a) of the second condenser element  52 . 
         [0014]    Therefore, because sound waves in low frequency range do not enter the first condenser element  51  side, the condenser microphone unit  50  performs as being omnidirectional and the proximity effect is hardly obtained. That is, this configuration is preferable for keeping the sound quality unchanged even though a distance from a sound source is changed. 
         [0015]    A condenser microphone unit of this configuration was disclosed in Japanese Patent Application Laid-Open (JP-A) No. 2011-55062. 
         [0016]    However, collection of bass-rich sound using the proximity effect is not achievable with the condenser microphone  50  shown in  FIG. 9 . Thus, depending on the situation, it is necessary to prepare other types of microphones with which the proximity effect is available. 
       SUMMARY OF THE INVENTION 
       [0017]    The present invention has been made in view of the above-mentioned problems and an object of the invention is to provide a unidirectional microphone unit that can collect sounds in a lower frequency range by proximity effect, even in the case where a diaphragm is placed on a rear acoustic terminal side of the condenser microphone unit. 
         [0018]    In order to solve the above-mentioned problem, the present invention provides a unidirectional microphone unit including: a first condenser element and a second condenser element, each including a diaphragm that is vibrated by sound waves and a fixed electrode having sound holes and disposed opposite the diaphragm; an insulating base having an opening at a center thereof and configured to support the respective fixed electrodes of the first and second condenser elements at opposite sides of the insulating base; acoustic resistance materials covering both ends of the opening of the insulating base; and air chambers formed respectively between each of the fixed electrodes and the insulating base, such that respective back sides of the diaphragms of the first and the second condenser elements are acoustically in communication with each other, wherein the diaphragm of the second condenser element is formed to be an annular-shape having a central opening, and the second condenser element has a rear acoustic terminal hole communicating with the central opening. 
         [0019]    Additionally, the second condenser element is preferably provided with an opening/closing device for opening/closing the rear acoustic terminal hole communicating with the central opening of the diaphragm. Further, the opening/closing device is preferably configured such that a degree of opening of the rear acoustic terminal hole can be changed. 
         [0020]    Alternatively, it is preferable that an acoustic resistance member having a predetermined acoustic resistance is detachably mounted on the rear acoustic terminal hole communicating with the central opening of the diaphragm. 
         [0021]    Further, an additional acoustic terminal hole for introducing sound waves is preferably arranged around the rear acoustic terminal hole of the second condenser element. 
         [0022]    Thus, with this configuration of the unidirectional condenser microphone unit according to the present invention, a diaphragm is annular in shape and placed on a rear acoustic terminal side, and the rear acoustic terminal communicates with the central opening. 
         [0023]    That is, by closing/opening the rear acoustic terminal communicating with the central opening, sound waves from the rear side are collected via the diaphragm, or collected directly. Thus, depending on the sound collecting situation, it becomes possible to control occurrence of the proximity effect in collecting sound in a low sound range. 
         [0024]    Additionally, degree of the proximity effect can also be controlled by varying materials of acoustic resistance materials covering the rear acoustic terminal communicating with the central opening. 
         [0025]    Thus, it becomes possible to obtain a unidirectional microphone unit that can collect sounds in a lower frequency range by the proximity effect, even in the case where a diaphragm is placed on a rear acoustic terminal side of the condenser microphone unit. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0026]      FIG. 1  is a cross-sectional view illustrating a unidirectional microphone unit of a first embodiment of the present invention; 
           [0027]      FIG. 2  is a cross-sectional view illustrating a unidirectional microphone unit of a second embodiment of the present invention; 
           [0028]      FIG. 3  is an equivalent circuit diagram of the unidirectional microphone unit of  FIG. 2  where an acoustic resistance member is detachably mounted thereon; 
           [0029]      FIG. 4  is an equivalent circuit diagram of the unidirectional microphone unit of  FIG. 2  where the acoustic resistance member is detached; 
           [0030]      FIG. 5  is a graph showing a result of directional characteristics measurement of the first embodiment of the present invention; 
           [0031]      FIG. 6  is a graph showing a result of directional characteristics measurement of the second embodiment of the present invention; 
           [0032]      FIG. 7  is a graph showing a result of directional characteristics measurement of comparative example  1 ; 
           [0033]      FIG. 8  is a graph showing a result of directional characteristics measurement of comparative example  2 ; 
           [0034]      FIG. 9  is a cross-sectional view of a conventional unidirectional microphone unit; and 
           [0035]      FIG. 10  is an equivalent circuit diagram of the conventional unidirectional microphone unit shown in  FIG. 9 . 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0036]    Hereinafter, an embodiment of the invention will be described with reference to the accompanying drawings.  FIG. 1  shows a cross-sectional view illustrating a first embodiment of a unidirectional microphone unit of the present invention. In  FIG. 1 , reference numbers of component members in the unidirectional condenser microphone unit  1  are the same as those of the corresponding members in the conventional condenser microphone unit already explained with  FIG. 9 . 
         [0037]    The unidirectional condenser microphone unit  1  shown in  FIG. 1  includes an electrically insulating base  9  made of synthetic resin or ceramics or the like, and a first and a second condenser elements  2 ,  3  supported by opposite sides of the insulating base  9 . 
         [0038]    The condenser elements  2 ,  3  are disposed laterally symmetrically, but are partly different in configuration. Firstly, the first condenser element  2  includes a diaphragm  15  stretched with a predetermined tension over a support ring  13  formed on a peripheral portion of a resonator  11  made of a metal plate. 
         [0039]    The diaphragm  15  is disposed opposite a fixed electrode  19  via a spacer ring  17 , and the peripheral portion of the resonator  11  and a peripheral portion of the insulating base  9  are integrally assembled by a connection ring  21 . 
         [0040]    The diaphragm  15  uses a synthetic resin thin film having a metal-, preferably gold, evaporated film on one side. The fixed electrode  19  is made of a perforated metal plate having a large number of sound holes (not shown). Additionally, an electret dielectric film may be formed on the fixed electrode  19 . 
         [0041]    The resonator  11  includes an acoustic terminal hole  11   a  for introducing sound waves. A communication hole  9   a  is formed at the center of the insulating base  9 , and both sides of the communication hole  9   a  are covered with acoustic resistance materials  23 ,  24 . 
         [0042]    An air chamber A 1  is formed between the fixed electrode  19  and the insulating base  9  in order to acquire velocity components through the acoustic resistance material  23 . In the example shown in  FIG. 1 , a tapered member  25  is provided on the air chamber side of the insulating base  9 . The tapered member  25  forms a conical surface with the acoustic resistance material  23  as an apex part and the peripheral portion of the insulating base  9  as a hem, and consequently the air chamber A 1  functions as an acoustic transducer having a speaker-cone-like shape. 
         [0043]    On the other hand, the second condenser element  3  has a resonator  4  made of a metal plate, and a cylinder-shaped rear central acoustic terminal hole  7  is formed at the center part of the second condenser element  3 . A diaphragm  6  having a donut-like shape (annular diaphragm) is stretch with a predetermined tension over a distal portion of a cylinder-shaped small diameter supporting ring  8  that forms the rear central acoustic terminal hole  7 , and a distal portion of a supporting ring  5  formed on a periphery of the resonator  4 . That is, the diaphragm  6  has a central opening  6   a  communicating with the rear central acoustic terminal hole  7 . 
         [0044]    The diaphragm  6  is disposed opposite a fixed electrode  20  via the spacer ring  18 . And the peripheral portion of the insulating base  9  and a peripheral portion of the resonator  4  are integrally assembled by a connection ring  22 . 
         [0045]    The diaphragm  6  uses a synthetic resin thin film having a metal-, preferably gold, evaporated film on one side. The fixed electrode  20  is made of a perforated metal plate having a large number of sound holes. Additionally, an electret dielectric film may be formed on the fixed electrode  20 . 
         [0046]    The resonator  4  includes an acoustic terminal hole  4   a  around the rear central acoustic terminal hole  7  for introducing sound waves. 
         [0047]    Further, an air chambers A 2  is provided between the fixed electrode  20  and the insulating base  9  in order to acquire a velocity component through the acoustic resistance material  24 . In the example shown in  FIG. 1  or  2 , a tapered member  26  is disposed on a side face of the air chamber side of the insulating base  9 . The tapered member  26  forms a conical surface with the acoustic resistance material  24  as an apex part and the peripheral portion of the insulating base  9  as a periphery, and the air chamber A 2  functions as an acoustic transducer having a speaker-cone-like shape. 
         [0048]    In thus configured condenser microphone unit  1 , when sound source is positioned on a side of the first condenser element  2 , sound waves enter to the first condenser unit  2  directly from the rear central acoustic terminal hole  7  without passing through the diaphragm  6  of the second condenser element  3 . Consequently, a lower frequency sound waves enter into the first condenser element  2 , and proximity effect can be obtained. 
         [0049]    The acoustic terminal represents a position of air which effectively provides a sound pressure to the microphone unit  1 . In other words, the acoustic terminal is a center position of the air moving through both the acoustic terminal hole  11   a  and the rear central acoustic terminal hole  7  simultaneously (together) with the diaphragm provided in the microphone unit  1 . Because the microphone unit  1  is unidirectional, the acoustic terminal includes a front acoustic terminal and a rear acoustic terminal, and the front acoustic terminal is located in front of the diaphragm  15  and the rear acoustic terminal is located behind a back of the diaphragm  6 . 
         [0050]      FIG. 2  is a cross-sectional view illustrating a second embodiment of the unidirectional microphone unit according to the present invention. 
         [0051]    The second embodiment is different from the first embodiment only in that an acoustic resistance member  10  having a predetermined acoustic resistance is detachably mounted in the supporting ring  8  that forms the rear central acoustic terminal hole  7 . 
         [0052]    With this configuration, acoustic resistance value of the rear central acoustic terminal hole  7  can be varied by opening/closing the rear central acoustic terminal hole  7  or changing materials of the acoustic resistance member  10 . 
         [0053]      FIGS. 3 and 4  show equivalent circuit diagrams of the condenser microphone unit  1  shown in  FIGS. 1 and 2 . Let the first condenser element  2 , shown in  FIGS. 3 and 4 , be on a front side, then P 1  denotes a sound source on a side of the front acoustic terminal hole  11   a,  m  0 f denotes mass of the diaphragm  15 , s 0 f denotes stiffness of the diaphragm  15 , r 0 f denotes acoustic resistance, and s 1  denotes acoustic mass of the air chamber A 1 . And P 2  denotes a sound source on a side of the rear acoustic terminal hole  4   a  of the second condenser element  3 , m 0 b denotes mass of the diaphragm  6 , s 0 b denotes stiffness of the diaphragm  6 , r 0 b denotes acoustic resistance, and s 2  denotes acoustic mass of the air chamber A 2 . A resultant acoustic resistance of combination of the acoustic resistance materials  23  and  24  is denoted as r 1 . And the rear central acoustic terminal hole  7  is openable by detaching the acoustic resistance member  10 ; this open/close mechanism is shown as a switch and denoted as SW. This switch SW is connected in parallel with a series circuit of m 0 b, s 0 b and r 0 b. The switch is in an open state (OFF state) when the acoustic resistance member  10  is mounted in the rear central acoustic terminal hole  7 , and is in a closed state (ON state) when the acoustic resistance member  10  is detached. 
         [0054]    In the thus configured condenser microphone unit  1  where the switch SW is closed; that is the acoustic resistance member  10  is detached as shown in  FIG. 1 , the equivalent circuit becomes as shown in  FIG. 4 . That is, if the sound source P 1  is on the side of the first condenser element  2 , and the switch SW is ON, a serial circuit consisting of m 0 b, s 0 b and r 0 b is short-circuited, sound waves on the side of the first condenser unit  2  enter directly from the rear central acoustic terminal hole  7  without passing through the diaphragm  6 , corresponding to the serial circuit consisting of m 0 b, s 0 b and r 0 b, of the second condenser element  3 . 
         [0055]    Therefore, low frequency sound waves come into a side of the first condenser element  2 , and the proximity effect can be obtained thereby. 
         [0056]    On the other hand, the equivalent circuit becomes as shown in  FIG. 3 , when the switch SW is open; that is, the acoustic resistance member  10  is mounted and the rear central acoustic terminal hole  7  is lidded therewith as shown in  FIG. 2 . That is, if the sound source P 1  is on the side of the first condenser element  2 , and the switch SW is opened; then, sound waves P 2  coming from the rear acoustic terminal holes  4   a  toward the first condenser unit  2  enter through the diaphragm  6 , corresponding to the serial circuit consisting of m 0 b, s 0 b and r 0 b, of the second condenser element  3 . 
         [0057]    Therefore, low frequency sound waves do not go into the side of the first condenser element  2 , and the microphone unit operates as omnidirectional in a lower frequency range. Thus, the proximity effect is hardly obtained. 
         [0058]    As described the above, according to the first and second embodiments of the present invention, in the condenser microphone unit having a diaphragm on the rear acoustic terminal side, the diaphragm  6  having an annular-shape is disposed on the rear acoustic terminal side so that the rear central acoustic terminal hole  7  is provided without the diaphragm  6 . 
         [0059]    That is, by closing/opening the rear central acoustic terminal hole  7 , sound waves from the rear side are collected via the diaphragm, or collected directly. Thus, depending on the sound collecting situation, it becomes possible to control occurrence of the proximity effect in collecting sounds in a low sound range. 
         [0060]    Additionally, degree of the proximity effect can also be controlled by varying the acoustic resistance by selecting materials of the acoustic resistance member  10  covering the rear central acoustic terminal hole  7 . 
         [0061]    In the embodiments described the above, although a configuration that the acoustic resistant member  10  is mounted in the rear central acoustic terminal hole  7  is given, the invention is not limited to the above embodiments. A lid made of a sound insulating plate member having an opening/closing device may be employed to open/close the rear central acoustic terminal hole  7 . In that case, the lid may preferably be capable of changing degree of opening of the rear central acoustic terminal hole  7 . 
       EXAMPLES 
       [0062]    The unidirectional condenser microphone unit according to the invention will be described in more detail with reference to examples. 
         [0063]    In the examples, unidirectional microphone units as described in the description of the preferred embodiments were manufactured and the characteristics of the microphone units were confirmed through the experiments for measurement. 
       Examples 1 
       [0064]    Directionality of a condenser microphone unit having a configuration shown in  FIG. 1  was measured under conditions where the rear central acoustic terminal hole was opened, and the measurement distance was 0.5 m (condition  1 ) and 0.3 m (condition  2 ).  FIG. 5  is a graph showing directionality characteristics measured at directions of 0 degree, 90 degree, and 180 degree under the condition  1 , and  FIG. 6  is a graph showing directionality characteristics measured at directions of 0 degree, 90 degree, and 180 degree under the condition  2 . 
         [0065]    According to the graphs shown in  FIGS. 5 and 6 , it was confirmed that when sound was collected at a short distance, the condenser microphone unit operates as omnidirectional at lower frequencies and the proximity effect was obtained. 
       Examples 2 
       [0066]    Directionality of a condenser microphone having a configuration shown in  FIG. 2  was measured under conditions that the rear central acoustic terminal hole was closed, and the measurement distance was 0.5 m (condition  3 ) and 0.3 m (condition  4 ).  FIG. 7  is a graph showing directionality characteristics measured at directions of 0 degree, 90 degree, and 180 degree under the condition  3 , and  FIG. 8  is a graph showing directionality characteristics measured at directions of 0 degree, 90 degree, and 180 degree under the condition  4 . 
         [0067]    According to the graphs shown in  FIGS. 7 and 8 , it was confirmed that when sound was collected at a short distance, the sound level at 180 degree direction is low at lower frequencies and the proximity effect was not obtained. 
         [0068]    Effects of the invention were confirmed from the above results.