Abstract:
A variable directional microphone includes a unidirectional microphone unit having a front acoustic terminal and a rear acoustic terminal; an acoustic tube which accommodates the microphone unit therein and having a plurality of acoustic resistance openings in a circumferential wall thereof; a supporting member mounted within the acoustic tube to be movable along an axis of the acoustic tube and supporting the microphone unit, the supporting member having a predetermined acoustic resistance and allowing transmission of sound waves; and further includes a gap formed between an outer circumferential wall of the microphone unit and an inner circumferential wall of the acoustic tube for providing communication between the front acoustic terminal and the rear acoustic terminal. Directionality of the variable directional microphone changes depending on the position of the microphone unit in the acoustic tube.

Description:
RELATED APPLICATIONS 
     The present application is based on, and claims priority from, Japanese Application No. JP2014-145026 filed Jul. 15, 2014, the disclosure of which is hereby incorporated by reference herein in its entirety. 
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to a variable directional microphone, the directionality of which can be switched from unidirectional to narrow directional and vice versa depending on an ambient noise level. 
     Description of the Related Art 
     In case of sound recording in an open-air environment, when an ambient noise level surrounding a targeted sound source is low, a unidirectional (such as a cardioid or a sub-cardioid directional) microphone is preferably used. In this case, high quality sound recording is possible while suppressing to record undesirable ambient sounds, even though there might be quality changes of sound depending on distance or the direction from the sound source. 
     In contrast, when the ambient noise level is high, use of a narrow directional microphone is preferred. In this case, the targeted sound wave can be surely recorded, even though there is considerable changes of sound quality depending on the distance or the direction from the sound source. 
     Thus, preferable directional characteristics of the microphones to be used are different depending on the surrounding condition. Therefore, a microphone the directionality of which can be switched from unidirectional to narrow directional and vice versa has been required; the microphone is operated as a unidirectional microphone when the ambient noise level is low, and operated as a narrow directional microphone when the ambient noise level is high. Particularly in news media, broadcasting with simple devices in comparison with the former ones becomes possible due to development of communication technologies. Therefore, live broadcasting can be more easily performed, which conventionally required large-scaled techniques. As for microphones in addition, it is convenient that the directionality of the microphone is easily switchable from unidirectional (a handy microphone case) to narrow directional (a shotgun microphone case) and vice versa, since consequently there is no need to prepare many kinds of microphones. 
     Japanese Patent Application Laid-Open (JP-A) No. 2000-50385 discloses a line microphone in which a unidirectional microphone unit is placed inside of a rear end side of an acoustic tube, and the microphone unit divides the acoustic tube into a front acoustic capacity room and a rear acoustic capacity room, and a rear sound-wave inlet-opening for a rear acoustic terminal of the microphone unit is additionally provided in the rear acoustic capacity room side. 
     In the above-said microphone, there is provided, on the area of the rear sound wave inlet-opening, a switching cover capable of varying the acoustic resistance of the rear sound wave inlet-opening, and switching the directionality to either hyper cardioid or cardioid, for example. 
     According to the microphone disclosed in the above referred patent application, the directionality can be switched between cardioid and hyper cardioid by changes of the acoustic resistance caused by variation of the area of the rear sound wave inlet-opening covered with the switching cover. 
     That is, by fully opening or fully closing the rear sound wave inlet-opening, predetermined directionality of cardioid or hyper-cardioid is instantaneously available, respectively. 
     To obtain intermediate directionality between the directionalities above, however, it is necessary to vary the area of the small rear sound wave inlet-opening by partially covering with the switching cover. Thus, it is difficult for users to vary the acoustic resistance of the rear acoustic terminal finely, adjustment of directionality is not easy. 
     SUMMARY OF THE INVENTION 
     The present invention has been made in view of the above-mentioned problems and an object of the invention is to provide a variable directional microphone, the directionality of which can be varied steplessly and easily by users themselves from unidirectional to narrow directional according to status of recording sound. 
     In order to solve the above problems, a variable directional microphone according to an aspect of the present invention includes a unidirectional microphone unit having a front acoustic terminal and a rear acoustic terminal; an acoustic tube accommodating the microphone unit therein and having a plurality of acoustic resistance openings in a circumferential wall thereof; a supporting member mounted within the acoustic tube to be movable along an axis of the acoustic tube and supporting the microphone unit, the supporting member having a predetermined acoustic resistance and allowing transmission of sound waves; and further includes a gap formed between an outer circumferential wall of the microphone unit and an inner circumferential wall of the acoustic tube for providing communication between the front acoustic terminal and the rear acoustic terminal. 
     The directionality of the variable directional microphone preferably changes depending on the position of the microphone unit in the acoustic tube. 
     In addition, the supporting member is preferably made of foamed material having air-permeability. And the supporting member is preferably elastic. 
     With this configuration, a user can move the microphone unit along the axis of the acoustic tube. With such operation, the directionality of the microphone can be changed steplessly from unidirectional to narrow directional, depending on the sound recording situation. The directionality of this structure is determined by the position of the microphone unit in the long acoustic tube and varied moderately from cardioid, that is, unidirectional to hyper-cardioid. Thus, the variable directional microphone of this structure enables users to obtain desired directionality of the microphone. For example, when recording sound, users can select the directionality of the microphone to be unidirectional if the ambient noise level is low, and select narrow directional if the ambient noise level is high. 
     In addition, the acoustic tube preferably includes a slit-like slide groove along the axial direction, and the microphone unit is movable along the slide groove. 
     Further, the microphone unit preferably includes a protrusion protruding from the slide groove, and the protrusion is configured to be movable so as to change the position of the microphone unit. 
     Further, elastic material members are preferably provided on both sides of the slide groove and sealed together, and the protrusion protrudes outward through the sealed portion of the elastic material members. 
     Thus, by moving the protrusion protruding from the slide groove of the acoustic tube, the position of the microphone unit can be changed, and accordingly the directionality can be easily varied from cardioid, that is, unidirectional to hyper cardioid. 
     There can be obtained a variable directional microphone wherein directionality of which can be varied steplessly and easily by users themselves from unidirectional to narrow directional according to status of recording sound. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional view illustrating the structure of a variable directional microphone according to the present invention; 
         FIG. 2  is a cross-sectional view illustrating the structure of a variable directional microphone according to the present invention, showing a different setting of directionality from that shown in  FIG. 1 ; 
         FIG. 3  is a cross-sectional view illustrating the structure of a variable directional microphone according to the present invention, showing a different setting of directionality from that shown in  FIG. 1  or  FIG. 2 ; 
         FIG. 4  is a disassembling view illustrating an example of a modified structure of a variable directional microphone according to the present invention; 
         FIG. 5A  is a cross-sectional views illustrating the structure of a variable directional microphone shown in  FIG. 4 , the directionality is set in the same way as that shown in  FIG. 1 ; 
         FIG. 5B  is a cross-sectional views illustrating the structure of a variable directional microphone shown in  FIG. 4 , the directionality is set in the same way as that shown in  FIG. 2 ; 
         FIG. 6  is a graph showing directional characteristics measured at directions of 0 degree, 90 degree, 135 degree and 180 degree, for a setting of the variable directional microphone shown in  FIG. 1 ; 
         FIG. 7  is a polar pattern diagram for the directional characteristics shown in  FIG. 6 ; 
         FIG. 8  is a graph showing directional characteristics measured at directions of 0 degree, 90 degree, 135 degree and 180 degree, for a setting of the variable directional microphone shown in  FIG. 2 ; 
         FIG. 9  is a polar pattern diagram for the directional characteristics shown in  FIG. 8 ; 
         FIG. 10  is a graph showing directional characteristics measured at directions of 0 degree, 90 degree, 135 degree and 180 degree, for a setting of the variable directional microphone shown in  FIG. 3 ; and 
         FIG. 11  is a polar pattern diagram for the directional characteristics shown in  FIG. 10 . 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereinafter, embodiments of the invention will be described with reference to the accompanying drawings.  FIGS. 1 through 3  are cross-sectional views illustrating a variable directional microphone according to the present invention.  FIGS. 1 through 3  illustrate settings for different directionalities, a position of a microphone unit is different in each figure. A variable directional microphone  1  illustrated in  FIG. 1  includes a cylindrical acoustic tube  2 , a microphone unit  10  accommodated in the acoustic tube  2 , and a supporting member  15  which supports the microphone unit  10  movably along the axial direction in the acoustic tube  2 . The microphone unit  10  is a unidirectional microphone unit. 
     The acoustic tube  2  is a cylindrical body having a front end opening  2   a  and a rear end opening  2   b . A plurality of acoustic resistance slit-based openings  3  are formed in the circumferential wall of the acoustic tube  2 . Acoustic resistant material (not illustrated in the  FIGS. 1 through 3 ), such as nylon cloth #508 manufactured by NBC Meshtec Inc., is attached to the acoustic resistant openings  3 . 
     In addition, a front acoustic terminal  10   a  and a rear acoustic terminal  10   b  are provided on the front end side and on the rear end side of the microphone unit  10 , respectively. Further, an electrical cable  11  is provided on the central part of the rear surface of the microphone unit  10  in order to take out sound signal, and is laid through the acoustic tube  2  and drawn out from the rear opening  2   b.    
     The supporting member  15  also functions simultaneously as acoustic resistant material which allows sound waves to pass through, and therefore the supporting member is made of foamed material having air-permeability, such as sponge having continuous air bubbles. A predetermined air gap S is formed between the outer surface of the microphone unit  10  and inner circumferential wall of the acoustic tube  2 , when the supporting member  15 , for example, supports the microphone unit  10  in a manner illustrated in  FIG. 2 . Therefore, among sound waves coming into the acoustic tube  2 , a sound wave coming from the rear side of the microphone unit  10  is collected from the rear acoustic terminal  10   b  after passing through the supporting member  15  and the gap S. 
     The supporting member  15  as set forth above is made of elastic material, such as spongy material. By applying an external force higher than a predetermined value to the supporting member  15 , the supporting member  15  can be moved along the axis direction in the acoustic tube  2 , so that the microphone unit  10  can be moved with the supporting member  15 . The movement of the microphone unit  10  with the supporting member  15  can also be achieved by pulling out or pushing into the cable  11  along the axis direction of the acoustic tube. Such a moving method, however, may cause breaking of the cable. A method for pushing into the microphone unit  10  from the front end opening  2   a  or the rear end opening  2   b  of the acoustic tube  2  is desirably employed by using a pushing member or a thin rod, for example, (not illustrated). 
     Further, the slide groove  4  may be formed separately from the acoustic hole  3  on the circumferential wall of the acoustic tube  2  as shown in  FIG. 4 . The slide groove  4  is formed along the axis direction all over the acoustic tube  2 . The microphone unit  10  has a protrusion  10   c  which is adapted to the slide groove  4  and is movable along the slide groove  4 . On both sides of the slide groove  4 , a sound insulating member  5  is provided, which is made of elastic material such as rubber, and sealed from both sides so as to hold the protrusion  10   c . Consequently the protrusion  10   c  protrudes outward from the slide groove  4 . The microphone unit  10  can be moved by holding the protrusion  10   c  as a knob. The sound insulating member  5  may be composed of a pair of pieces or one piece of elastic material. In case of one piece of elastic material, a slit is provided so that the protrusion  10   c  is movable. The slit is illustrated as an opening for explanation in  FIG. 4 , however, the actual slit is configured such that the sides of elastic member  5  on both sides of the groove are sealed together and insulates sound waves. 
     Thus, in the microphone  1 , position of the microphone unit  10  can be changed by moving it together with the supporting member  15  in the acoustic tube  2 . 
     As illustrated in  FIG. 1 , the front and rear acoustic terminals of the microphone unit  10  are in a state where the both terminals are exposed in the free space from the front opening  2   a  of the acoustic tube  2 . For the configuration illustrated in  FIG. 4 , the usage state of the position of the microphone unit is as illustrated in  FIG. 5A . 
     In this case, the microphone  1  works as a unidirectional microphone and sound recording is achieved with a cardioid directional characteristics when the ambient noise level is low. 
     Additionally, the directionality can be changed to unidirectional close to narrow directionality when the microphone unit  10  is moved to nearly the center of the acoustic tube  2  as illustrated in  FIG. 2  or  FIG. 5B . 
     Further, the directionality becomes narrow directional when the microphone  1  is moved to the rear end of the acoustic tube  2 , as illustrated in  FIG. 3 , and then sound recording is achieved with a hyper cardioid directional characteristics. 
     Thus, according to embodiments of the present invention, users can move the microphone unit  10  along the axis in the acoustic tube  2 . With this operation, the directionality can be varied steplessly from unidirectional to narrow directional in consideration of the situation of recording sound. The directionality which is determined by the position of the microphone unit  10  in the long acoustic tube  2  varies slowly from cardioid, that is, unidirectional to hyper cardioid. This enables users to set the microphone to desired directionality easily. For example, users can record sound with a unidirectional microphone when the ambient noise level is low, and a narrow directional microphone when the ambient noise level is high. 
     Moreover, in the above embodiment, supporting member  15  is formed of sponge having continuous air bubbles, but without limiting to sponge, other air-permeable material, such as non-woven fabric, can be used. 
     EXAMPLES 
     The microphone according to this invention is described in more detail with reference to examples. 
     In the examples, microphones illustrated in the description of the preferred embodiments were manufactured and the characteristics of the microphones were confirmed through the experiments for measurement. 
     Example 1 
     The directionality of the microphone according to the embodiments at the state shown in  FIG. 1  is described. 
     As shown in  FIG. 1 , a microphone unit was placed at the front end of an acoustic tube with the front and rear acoustic terminals of the microphone unit being exposed, and the directional characteristics was measured.  FIG. 6  is a graph showing directional characteristics measured at directions of 0 degree, 90 degree, 135 degree and 180 degree, for a setting of the variable directional microphone.  FIG. 7  is a polar pattern of the directional characteristics for the same setting as above. The directionality of the microphone was confirmed to be unidirectional, as shown in  FIGS. 6 and 7 . 
     Example 2 
     The directionality of the microphone according to the embodiments at the state shown in  FIG. 2  is described. As shown in  FIG. 2 , a microphone unit was placed nearly at the center of an acoustic tube and the directional characteristics was measured.  FIG. 8  is a graph showing directional characteristics measured at directions of 0 degree, 90 degree, 135 degree and 180 degree, for a setting of the variable directional microphone shown in  FIG. 2 .  FIG. 9  is a polar pattern of the directional characteristics for the above setting. The directionality of the microphone was confirmed to be narrower directional compared with the Example 1, as shown in  FIGS. 8 and 9 . 
     Example 3 
     The directionality of the microphone according to the embodiments at the state shown in  FIG. 3  is described. 
     As shown in  FIG. 3 , a microphone unit was placed at the rear end of an acoustic tube and the directional characteristics was measured. 
       FIG. 10  is a graph showing directional characteristics measured at directions of 0 degree, 90 degree, 135 degree and 180 degree, for the setting, shown in  FIG. 3 .  FIG. 11  is a polar pattern of the directional characteristics for the above setting. The directionality of the microphone was confirmed to be further narrower directional (hyper-cardioid) compared with the Example 2, as shown in  FIGS. 10 and 11 . 
     With the results of Examples 1 through 3, according to the present invention, the directionality of the microphone was confirmed to be easily changeable by users&#39; adjusting the position of the microphone unit in the acoustic tube, depending on the situation of sound recording.