Patent Publication Number: US-8983106-B2

Title: Narrow directional microphone

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     The present application is based on, and claims priority from, Japanese Application Serial Number JP2011-169038, filed Aug. 2, 2011, the disclosure of which is hereby incorporated by reference herein in its entirety. 
     TECHNICAL FIELD 
     The present invention relates to a narrow directional microphone using an acoustic tube, and more particularly to a technique that reduces degradation in directional frequency response caused by covering a front end opening of an acoustic tube with a film in order to suppress wind noise and the like. 
     BACKGROUND ART 
     In a narrow directional microphone using an acoustic tube, a narrow cylindrical acoustic tube having a prescribed axial length is attached to a front acoustic terminal side, which is a front side of a diaphragm of a unidirectional microphone unit. 
     Air existing in an acoustic tube serves as an acoustic mass in a low frequency band. The acoustic mass in the low frequency band operates in a manner equivalent to the mass being added to a diaphragm (equivalent to an additional weight to the diaphragm). This facilitates capturing vibration noise. 
     The acoustic tube is provided with an opening which many sound waves enter (the front end opening or a slit-like opening formed on the wall of the tube). Accordingly, the tube is susceptible to wind noise. Vibration noise and wind noise mainly include low frequency components. 
     For the purpose of reference,  FIG. 4  shows a graph of directional frequency response of a narrow directional microphone using a conventional acoustic tube. This graph indicates that an output level at a low frequency band increases and vibration noise and wind noise largely appear. 
     Thus, the present assignee has proposed a technique that covers the front end opening of an acoustic tube with a film capable of being displaced by sound waves, mainly for reducing wind noise, in Japanese Patent No. 4684012. 
     This technique allows the film to prevent low frequency sound waves from passing. Accordingly, wind noise can be reduced. However, in the case where the film is planar, bending of the film by a wind or the like sometimes makes noise that is specific to the film. The invention described in Japanese Patent No. 4684012 uses a film preferably formed into a corrugated shape. 
     However, even in the case where the film is formed into a corrugated shape, the film has a mass and a stiffness to restore the film to the original position. Accordingly, in a narrow directional microphone having the configuration described in Japanese Patent No. 4684012, resonance occurs owing to the stiffness of the film and the acoustic mass of an air column in the acoustic tube. In an equivalent circuit, the stiffness is represented by a capacitance C, and the acoustic mass is represented by an inductance L. 
       FIG. 5  shows a graph of directional frequency response measured by the narrow directional microphone having the configuration described in Japanese Patent No. 4684012. In this measurement example, resonance due to the C and L occurs around 200 Hz, showing degradation in directional frequency response. This also means degradation in sound quality. 
     It is thus an object of the present invention to provide a narrow directional microphone including an acoustic tube having a front end opening covered with a film mainly for reducing wind noise wherein degradation is reduced in directional frequency response due to resonance between the stiffness of the film and the acoustic mass of the air column in the acoustic tube. 
     SUMMARY OF THE INVENTION 
     In order to achieve the object, the present invention provides a narrow directional microphone including: a unidirectional microphone unit; and a cylindrical acoustic tube having a prescribed axial length, wherein a rear end of the acoustic tube is coupled to a side of a front acoustic terminal of the unidirectional microphone unit, and a front end opening of the acoustic tube is covered with a film capable of being displaced by a wind pressure, and the microphone further includes an acoustic resistor disposed at a position which is on an outward side of the film and at which the resistor does not come into contact with the film even when the film is displaced by the wind pressure. 
     According to a preferable mode of the present invention, the microphone further includes an annular spacer disposed between the film and the acoustic resistor to prevent the film and the resistor from being in contact with each other. 
     Furthermore, any of a nonwoven fabric, a thin metal plate having many pores, and a sponge material having open-cell foam is preferably adopted as the acoustic resistor. 
     Moreover, it is preferred that the film be made of a thermoplastic resin, and a first irregularity pattern having rough irregularities with a long period and a second irregularity pattern having fine irregularities with a short period be formed over an entire area of the film. 
     According to the present invention, the acoustic resistor is arranged on the outward side of the film covering the front end opening of the acoustic tube. This arrangement allows the acoustic resistor to suppress series resonance of the film. Accordingly, degradation in directional frequency response due to resonance can be reduced. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  is a sectional view showing an embodiment of a narrow directional microphone according to the present invention; 
         FIG. 1B  is a sectional view of an exterior casing applied to the embodiment; 
         FIG. 1C  is a sectional view showing an acoustic tube and a unit section that are accommodated in the exterior casing; 
         FIG. 2A  is a plan view of a film applied to the embodiment; 
         FIG. 2B  is a partially enlarged plan view of  FIG. 2A ; 
         FIG. 2C  is a partially enlarged sectional view of  FIG. 2A ; 
         FIG. 3  is a graph showing directional frequency response measured in the embodiment; 
         FIG. 4  is a graph showing directional frequency response of a conventional, typical narrow directional microphone; and 
         FIG. 5  is a graph showing directional frequency response of a narrow directional microphone having a film over a front end opening of an acoustic tube. 
     
    
    
     DETAILED DESCRIPTION 
     An embodiment of the present invention will now be described with reference to  FIGS. 1 to 3 . However, the present invention is not limited thereto. 
     As shown in  FIG. 1A , a narrow directional microphone  1  according to an embodiment of the present invention is assembled by accommodating an acoustic tube  20  and a unit section  30  shown in  FIG. 1C  in an exterior casing  10  shown in  FIG. 1B . 
     The exterior casing  10  has a long cylindrical shape with a length capable of accommodating the acoustic tube  20  and the unit section  30  that are coupled to each other. The material of the exterior casing  10  may be a metal or a plastic. 
     Slits  11  are formed, at parts of the tube wall of the exterior casing  10  where the acoustic tube  20  is accommodated, along the axial direction thereof. In this example, three slits  11  are formed. However, a continuous slit in which these slits are connected to each other may be adopted. 
     A stopper ring  13  for preventing the acoustic tube  20  from being separated out is attached to a front end opening of the exterior casing  10 . This attachment is made by a female screw  10   a  formed in the front end opening of the exterior casing  10  and a male screw  13   a  formed on the outer periphery of the stopper ring  13  mating with each other. 
     A rear sound wave intake  12  for a rear acoustic terminal of a unidirectional microphone unit  33  included in the unit section  30  is perforated at a part of the tube wall of the exterior casing  10  where the unit section  30  is accommodated. 
     The acoustic tube  20  has a cylindrical shape that has an axial length shorter than that of the exterior casing  10  and can be fitted into the exterior casing  10  substantially without a clearance. At the tube wall thereof, slits  21  that are for capturing sound waves and match with the respective slits  11  of the exterior casing  10  are perforated. The slits  11  and  21  may be formed, for instance, at positions opposite to each other and separated by 180° on the tube wall, or at four sets of positions separated by 90°. 
     In this embodiment, the unit section  30  includes a cylinder  31  and a unit casing  32  that are formed into cylinders having substantially the same diameter. Although not shown, the cylinder  31  accommodates a circuit board on which an audio signal output circuit and the like are mounted. A tripolar (three-pin) output connector to be connected to a phantom power source is provided at the rear end of the cylinder  31 . 
     The unit casing  32  is coupled to the front end of the cylinder  31  and internally includes a unidirectional microphone unit  33 . In the narrow directional microphone, a capacitor microphone unit is often adopted as the microphone unit  33 . 
     As shown in  FIG. 1 , the acoustic tube  20  and the unit section  30  are accommodated in the exterior casing  10  in a state where a rear end  20   b  of the acoustic tube  20  is coupled to the front end of the unit casing  32 , that is, on a side of a front acoustic terminal  33   a  of the microphone unit  33 . A film  22  that is displaced by a wind pressure is provided at a front end opening  20   a  of the acoustic tube  20  in order to reduce wind noise. 
     The film  22  is made of thermoplastic resin, such as polyethylene terephthalate and polyphenylene sulfide, and may be a film formed into a corrugated shape. More preferably, a film is adopted that is analogous to a three-dimensionally formed diaphragm described in Japanese Patent Application No. 2008-142067 (Japanese Patent Laid-Open No. 2009-290638) filed by the present assignee. 
     This film will be described with reference to  FIG. 2 . As shown in  FIG. 2A , the film  22  is stretched over a support ring  221  in a state where a prescribed tension is applied. A first irregularity pattern  222  with rough irregularities having a long period, and a second irregularity pattern  223  with fine irregularities having a period shorter than the long period are formed over the entire area. 
     As shown in  FIG. 2B , it is preferred that the first irregularity pattern  222  have a hexagonal honeycomb pattern. The second irregularity pattern  223 , in conjunction with the irregularities of the first irregularity pattern  222 , is formed across the entire area of the film  22 . 
     Referring to  FIG. 2C , the first irregularity pattern  222  has irregularities with a large difference in height. Meanwhile, the second irregularity pattern  223  has many continuous fine irregularities with a height from the bottom to the top of the irregularities equal to or larger than the thickness of the film. 
     Here, provided that the period (one pitch) of the first irregularity pattern  222  is denoted by T 1  and the period (one pitch) of the second irregularity pattern  223  is denoted by T 2 , it is preferred that the period T 1  be at least ten times as long as the period T 2 . 
     That is, it is preferred that at least ten irregularities of the second irregularity pattern  223  exist between the irregularities of the first irregularity pattern  222 . Such a film  22  can be easily formed by a press machine including a heated die. 
     As shown in  FIGS. 1A and 1C , the film  22  in the state of being stretched over the support ring  221  is attached to the acoustic tube  20  so as to cover the front end opening  20   a  where the support ring  221  is oriented outwardly (on a side opposite to the acoustic tube). In the present invention, an acoustic resistor  23  is further arranged on the outward side of the film  22  (on the side opposite to the acoustic tube). 
     A nonwoven fabric formed into a flat plate, a thin metal plate having many pores, or a sponge material having open-cell foam (porous material having open-cell foam) is preferably adopted as the acoustic resistor  23 . The acoustic resistor  23  is disposed at a position that does not come into contact with the film  22  even when the film  22  is displaced by a wind pressure. 
     In this embodiment, the support ring  221  of the film  22  serves as a spacer ring for holding the acoustic resistor  23  so as not to be contact with the film  22 . The film  22  and the acoustic resistor  23  are fixed to the front end opening  20   a  of the acoustic tube  20  with an annular holder ring  24 . As shown in  FIG. 1A , the holder ring  24  is finally held by the stopper ring  13 . 
     The acoustic resistor  23  is thus arranged on the outward side of the film  22  for reducing wind noise in a non-contact state. This arrangement suppresses series resonance between the stiffness of the film  22  and the acoustic mass of the air column in the acoustic tube  20 . Accordingly, degradation in directional frequency response due to resonance can be reduced. 
       FIG. 3  exemplifies a graph of directional frequency response measured by a narrow directional microphone adopting an acoustic resistor plate that has an outer diameter of 24.2 mm made of a nonwoven fabric JH-1007 (made of polyester fiber and having a surface density of 70 g/m 2  and a thickness of 0.13 mm) manufactured by Japan Vilene as the acoustic resistor  23  and has a separation from the film  22  of 0.8 mm, the film  22  having a effective vibration diameter of about 20 mm. 
     This graph shows that the series resonance of the film  22  is sufficiently suppressed and, more specifically, the output level in a low frequency band is flattened and acoustic capacity according to the capacity of an air room in the acoustic tube  20  does not degrade acoustic movement in medium and high frequencies. 
     The embodiment adopts the configuration accommodating the acoustic tube  20  and the unit section  30  in the exterior casing  10 . However, the exterior casing  10  is not necessarily adopted. In the case without the exterior casing  10 , the unit casing  32  of the unit section  30  may be provided with the rear sound wave intake  12  for the rear acoustic terminal of the unidirectional microphone unit  33 .