Patent Publication Number: US-8538057-B2

Title: Highly directional microphone

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a highly directional microphone that includes an acoustic tube, and specifically, a highly directional microphone of which the directivity can be fine-tuned by a user. 
     2. Related Background Art 
     In a highly directional microphone that includes an acoustic tube, a microphone unit is disposed inside one end in a longitudinal direction or middle of the acoustic tube. Sound waves from directions other than a target direction, that is, a front end of the acoustic tube, interfere with and are cancelled by sound waves from openings on the side wall of the acoustic tube due to a time lag therebetween. The highly directional microphone, thus, has high sensitivity to the sound waves from the front end of the acoustic tube to obtain narrow directivity. Directivity of the highly directional microphone, therefore, depends on the wavelength of sound and the length of the acoustic tube. A long acoustic tube exhibits narrow directivity over a wide frequency range up to low frequency, while a short acoustic tube exhibits narrow directivity only in a high frequency region. 
     In general, a highly directional microphone including an acoustic tube and a unidirectional condenser microphone unit which are combined with each other, is designed to operate in a unidirectional mode at a frequency band equal to or lower than a band in which the acoustic tube exhibits narrow directivity. 
     An example of the highly directional microphone including acoustic tube and a unidirectional condenser microphone unit which are combined with each other is disclosed in Japanese Unexamined Patent Application Publication 2000-083292. 
     In general, a polar pattern at a low frequency band of the highly directional microphone including a combination of the condenser microphone unit and acoustic tube is designed to be hypercardioid for reducing the sound waves from the side direction. If a noise source is present at 180-degree direction, that is, at the rear end of the acoustic tube, the sound waves of an extremely low frequency band are disadvantageously picked up. Within the frequency band in which the highly directional microphone unidirectionally operates, it is preferable to adjust the angle in order to avoid a reduction in the sensitivity in response to the direction of the noise source, that is, it is preferable to adjust the directivity. 
     A possible measure to adjust the directivity of the highly directional microphone is adjustment of the acoustic resistance of the microphone unit.  FIG. 13  illustrates an exemplary conventional highly directional microphone of which the directivity can be adjusted by adjusting the acoustic resistance of the microphone unit incorporated in the acoustic tube.  FIG. 13  illustrates an elongated cylindrical acoustic tube  110  one end of which is connected to a tubular microphone unit holder  120 . A microphone unit  130  is disposed inside the tubular microphone unit holder  120 . Hereinafter, the end of the acoustic tube  110  at which the microphone unit  130  is disposed is referred to as a rear end and the opposite end thereof as a front end. In this example, the microphone unit  130  is a condenser microphone unit and, as is well known, includes a diaphragm composed of a thin film and a fixed electrode that faces the diaphragm with a slight gap therebetween. The microphone unit  130  itself has unidirectional directivity and includes the diaphragm that is disposed so as to face the front end of an acoustic tube  110 . The diaphragm and the fixed electrode constitute the condenser. Vibration of the diaphragm receiving the sound waves varies the capacitance of the condenser, and the variable capacitance is output as a change in electric signal. A front cap  160  is attached to the front end of the acoustic tube  110 . 
     Slits (not shown) are formed on the peripheral surface of and parallel to the central axis of the acoustic tube  110 . The sound waves from directions other than the target direction, that is, other than the front-end direction of the acoustic tube  110  enter the acoustic tube  110  through the slits and the front end of the acoustic tube  110 . The sound waves that enter the acoustic tube  110  through the slits and the sound waves that enter the acoustic tube  110  through the front end thereof interfere with and cancelled by each other inside the acoustic tube  110  because they enter the acoustic tube  110  at a certain time lag. Accordingly, the sound pressure that reaches the microphone unit  130  decreases. In contrast, the sound pressure of the sound waves from the front end direction of the acoustic tube  110  does not decrease. Thus, the sound waves from the front end direction are dominantly electro-acoustically converted. This achieves narrow directivity. 
     As explained above, in the highly directional microphone including a combination of the acoustic tube and the highly directional microphone, the acoustic resistance is adjusted for adjustment of the directivity. The conventional narrow directivity microphone in  FIG. 13  includes an acoustic resistive material  133  that is disposed behind the diaphragm of the microphone unit  130  and determines the acoustic resistance of the rear acoustic terminal, and an adjustable nut  135  which adjusts the acoustic resistance by adjusting the urging force of the acoustic resistive material  133 . The acoustic resistance of the acoustic resistive material  133  varies with the extent of tightening of the adjustable nut  135  to adjust the directivity. 
     As is shown by the conventional highly directional microphone in  FIG. 13 , the directivity of the highly directional microphone including a combination of the acoustic tube and a highly directional microphone can be adjusted. The adjustment of the directivity of the conventional highly directional microphone, however, requires skillful adjustment of the adjustable nut  135  of the microphone unit  130  disposed in the acoustic tube  110  or the tubular microphone unit holder  120 . Since the microphone unit  130  must be directly adjusted, improper adjustment creates various problems, such as damage of the diaphragm and an increase in noise due to decreased insulation. As matters now stand, therefore, it is difficult to adjust the directivity by a user without asking a manufacture to adjust the directivity. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to resolve the problems of the above-explained conventional highly directional microphone and to provide a highly directional microphone having a simple structure that enables a user to adjust the directivity by a simple operation. 
     According to an aspect of the present invention, a highly directional microphone includes an acoustic tube and a microphone unit disposed inside the base end of the acoustic tube. The acoustic tube is composed of an elastic material. An adjustable member elongates and contracts the distance between the microphone unit and the front end of the acoustic tube. 
     The acoustic tube is composed of an elastic material and can adjust the distance between the microphone unit and the front end of the acoustic tube. Thereby, the directivity can be adjusted by elongating and contracting the acoustic tube. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  is a partially-abbreviated vertical cross-sectional view of a highly directional microphone according to an embodiment of the present invention; 
         FIG. 1B  is a partially-enlarged vertical cross-sectional view of the embodiment; 
         FIG. 2A  is a main part of the vertical cross-sectional view illustrating the original state where pull strength is not applied to an acoustic tube of the embodiment; 
         FIG. 2B  is a main part of the vertical cross-sectional view illustrating a state where pull strength is applied to the acoustic tube of the embodiment; 
         FIG. 3  is a directional characteristic diagram when the acoustic tube of the embodiment is elongated by 2.5 mm; 
         FIG. 4  is a frequency response characteristic diagram when the acoustic tube of the embodiment is elongated by 2.5 mm; 
         FIG. 5  is a directional characteristic diagram when the acoustic tube of the embodiment is elongated by 5.0 mm; 
         FIG. 6  is a frequency response characteristic diagram when the acoustic tube of the embodiment is elongated by 5.0 mm; 
         FIG. 7  is a directional characteristic diagram when the acoustic tube of the embodiment is elongated by 7.5 mm; 
         FIG. 8  is a frequency response characteristic diagram when the acoustic tube of the embodiment is elongated by 7.5 mm; 
         FIG. 9  is a directional characteristic diagram when the acoustic tube of the embodiment is elongated by 10.0 mm; 
         FIG. 10  is a frequency response characteristic diagram when the acoustic tube of the embodiment is elongated by 10.0 mm; 
         FIG. 11  is a directional characteristic diagram when the acoustic tube of the embodiment is elongated by 12.5 mm; 
         FIG. 12  is a frequency response characteristic diagram when the acoustic tube of the embodiment is elongated by 12.5 mm; and 
         FIG. 13  is a vertical cross-sectional view of a typical conventional highly directional microphone. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A highly directional microphone according to the embodiment of the present invention will be described below with reference to the accompanying drawings. 
       FIGS. 1A ,  1 B,  2 A and  2 B illustrate a highly directional microphone according to the embodiment of the present invention.  FIGS. 1A and 1B  depict the overall structure, and  FIGS. 2A and 2B  depict only the concept or principle of the present invention which is excerpted from the above embodiment. The concept of the present invention will be described below.  FIGS. 2A and 2B  illustrate an acoustic tube  10  and a microphone unit  30 . The microphone unit  30  is assembled to the inside of a tubular microphone holder  20  that is fitted to the base end of the acoustic tube  10 . The microphone unit  30 , thus, is substantially disposed inside the base end of the acoustic tube  10 . The acoustic tube  10  is composed of an elastic material, such as porous rubber having numerous openings  11  through which sound waves travel. A condenser microphone unit is used as the microphone unit  30  and the microphone unit  30  itself has unidirectional directivity and hypercardioid characteristics. 
       FIG. 2A  illustrates the original state where pull strength is not applied to the acoustic tube  10 , while  FIG. 2B  illustrates a state where the acoustic tube  10  is elongated by the pull strength applied to the front end thereof. The numerous openings  11  are also elongated with the elongation of the acoustic tube  10 . The elongation of the acoustic tube  10  reduces the interference frequency inside the acoustic tube  10  to enhance the narrow directivity at a low-frequency range. Furthermore, the elongation of the numerous openings  11  with the elongation of the acoustic tube  10  results in a reduction in acoustic resistance of the peripheral wall of the acoustic tube  10 . This reduction in the acoustic resistance changes the directivity of the microphone unit  30  from hypercardioid directivity to omnidirectional directivity. The directivity, thus, can be changed. When a noise source is lateral to the acoustic tube  10  (90-degree direction), the hypercardioid directivity is applied without elongation of the acoustic tube  10 , while the noise source is behind the acoustic tube  10  (180-degree direction), the cardioid directivity is applied with elongation of the acoustic tube  10 , thereby preventing noise pickup. 
       FIGS. 1A and 1B  illustrate an embodiment in which the above-explained principle of variable directivity is developed to a practical level.  FIGS. 1A and 1B  depict the elastic acoustic tube  10  that is fitted into the inner periphery of an acoustic-tube protector  40 . For example, the acoustic-tube protector  40  is formed by partly removing the peripheral wall of a cylindrical member other than a base end  41  (adjacent to the microphone unit  30 ) and a front end  42  in the axis direction to make openings  43  through which the sound waves freely travel. The acoustic-tube protector  40  can keep its stiffness as a whole. A slight gap is provided between the outer periphery of the acoustic tube  10  and the inner periphery of the acoustic-tube protector  40 , and the acoustic tube  10  can be elongated or contracted relative to the acoustic-tube protector  40 . As explained above, the acoustic tube  10  is composed of an elastic material, such as porous rubber having numerous openings  11  through which sound waves travel. The base end of the acoustic tube  10  is integrated to the inner periphery of the base end  41  of the acoustic-tube protector  40 . 
     A cylindrical acoustic-tube holder  22  of a tubular microphone unit holder  20  is fitted to the outer periphery at the base end  41  of the acoustic-tube protector  40  to be integrated with the acoustic-tube protector  40 . A cylindrical microphone unit holder  21  is integrated to the rear end of the tubular microphone unit holder  20  and accommodates the microphone unit  30  therein. As is well known, the microphone unit  30  of this embodiment according to the present invention is a condenser microphone unit comprising a diaphragm  31  composed of a thin film; a fixed electrode  32  that faces the diaphragm  31  with a slight gap therebetween; a rear acoustic terminal that conducts external air therethrough to an air chamber formed at the back surface of the diaphragm  31 ; and an acoustic resistor  33  disposed so as to cover the rear acoustic terminal. The rear acoustic terminal urges the acoustic resistor  33  by an appropriate urging force with a nut  35  to generate an appropriate acoustic resistance. The microphone unit  30  is assembled such that the diaphragm  31  therein faces the front end of the acoustic tube  10 . 
     A sliding cylinder  50  is slidably fitted inside the inner periphery at the front end  42  of the acoustic-tube protector  40  in the axis direction thereof, that is, the axis direction of the acoustic tube  10 , with being guided by the inner periphery at the front end  42 . An appropriate number of thread holes  51  is aligned on the sliding cylinder  50  parallel to the axis of the acoustic tube  10 . As shown in the example of  FIG. 1B , the two thread holes  51  are symmetrically formed on opposite sides of the central axis of the sliding cylinder  50 . A front cap  60  is fitted to the front end of the acoustic-tube protector  40 . Two adjustable threads  70  are inserted into the front cap  60  parallel to the axis of the acoustic-tube protector  40 . The two adjustable threads  70  are respectively screwed into the thread holes  51  on the sliding cylinder  50  through the acoustic-tube protector  40 . The sliding cylinder  50  moves along the acoustic-tube protector  40  by adjusting with adjustable threads  70  so as to elongate and contract the acoustic tube  10  that is connected to the acoustic-tube protector  40 . 
     Since the expansion and contraction of the acoustic tube can be adjusted by controlling the pull strength at the front end of the acoustic tube without adjusting at the end adjacent to the microphone unit, the directivity can be adjusted without damaging the microphone unit by the user. Fine adjustment of the directivity can be achieved by elongating and contracting the acoustic tube. 
     A gist of the present invention is to provide a highly directional microphone including the acoustic tube  10 ; and the microphone unit  30  disposed inside the base end of the acoustic tube  10 , in which the acoustic tube  10  is composed of an elastic material and an adjustable member (adjustable threads  70  in the embodiment in  FIG. 1B ) increases or decreases the distance between the microphone unit  30  and the front end of the acoustic tube  10 . Practically, the acoustic tube  10  can be maintained at a predetermined elongated or contracted position by holding or protecting the acoustic tube  10  by a rigid member. In the example illustrated in  FIGS. 1A and 1B , the acoustic tube  10  can be elongated or contracted by the operation from the outside of the acoustic-tube protector  40 . With this configuration, the directivity of the highly directional microphone can be adjusted by the user to achieve fine adjustments to the directivity. 
     For example, the elastic acoustic tube  10  having numerous holes  11  may be composed of a sponge member similar to that for generating bubbles in the water in an aquarium. An exemplary process for manufacturing the member involves shaping of a rubber mixed with water-soluble particles into a tube and dissolution of the particles with water. Accordingly, holes through which sound waves travel are formed in the portions corresponding to the dissolved particles in the rubber. 
     While the length of the acoustic tube  10  of the highly directional microphone according to the above-explained embodiment was adjusted, the directional characteristics and the frequency response characteristics were measured at each length under a standardized condition. The length of the acoustic tube  10  in the original state where pull strength was not applied thereto was 100 mm.  FIGS. 3 ,  5 ,  7 ,  9 , and  11  depict the directional characteristics when the acoustic tube  10  is elongated by 2.5 mm, 5.0 mm, 7.5 mm, 10.0 mm, and 12.5 mm, respectively, from 100 mm in the original state.  FIGS. 4 ,  6 ,  8 ,  10  and  12  depict the frequency response characteristics when the acoustic tube  10  is elongated by 2.5 mm, 5.0 mm, 7.5 mm, 10.0 mm, and 12.5 mm, respectively, from 100 mm in the original state. With respect to the frequency response characteristics, a heavy line, a middle-thick line, a thin line represent a sound source at the front (0-degree direction), a sound source at the side (90-degree direction), and a sound source at the rear (180-degree direction) of the acoustic tube  10 . 
     As is obvious from  FIGS. 3 ,  5 ,  7 ,  9 , and  11 , the directional characteristics vary from hypercardioid to cardioid as the length of the acoustic tube  10  increases.  FIGS. 4 ,  6 ,  8 ,  10 , and  12  show no substantial variation in the frequency response characteristics, and in particular, little variation in the sound source at the front direction. 
     With the highly directional microphone according to the present invention, even a general user who does not get used to handle microphones can readily adjust the directivity. Demand for the highly directional microphone, therefore, can be expected not only by professional sound technicians but also by general users.