JP5080197B2 - Narrow directivity condenser microphone unit - Google Patents

Description

  The present invention relates to a narrow directivity condenser microphone unit, and more particularly to a structure for fixing a built-in component.

  Capacitor microphones are suitable for reducing the size and thickness of microphone units due to their characteristics, and are used for pin type microphones, shotgun microphones, and so on, which are also called lavalier microphones. A lavalier microphone can be used by fastening it to clothes with a pin or the like, and can collect sound even if it is not held in the hand. On the other hand, the shotgun microphone is set to have a narrow directivity in order to reduce the collection of ambient sounds as much as possible and collect only the target sound.

  A generally known condenser microphone unit has a cylindrical shape, and is provided with a circular diaphragm, a back electrode plate, a ring-shaped spacer, etc. in a unit case having a diameter slightly smaller than the diameter. Inside the unit case, a spacer is interposed between the diaphragm and the back electrode plate so that the diaphragm is not attracted to the back electrode plate, and there is a capacitor between the diaphragm and the back electrode plate. It is configured. The sound from the outside vibrates the diaphragm in the unit case, and the capacitance of the capacitor changes as the distance from the back plate is mechanically displaced. This change in capacitance is converted into an audio signal as an electrical signal and output. However, since the output impedance of the capacitor is extremely high, an impedance conversion element for reducing the impedance is essential. The impedance conversion element is usually installed in the unit case. In addition, the unit case requires an acoustic space for sound to pass, and depending on the built-in component fixing structure inside the unit case, a space for interposing a joining member for fixing the built-in component is required. Become.

  The condenser microphone can obtain a better sound signal as the value of the S / N ratio (also referred to as signal-to-noise ratio) is larger. In order to increase the S / N ratio, it is necessary to increase the area in which the back electrode plate acts effectively. For example, since an electrical signal cannot be obtained at a portion where the spacer contacts the back electrode plate, the capacitance at this portion becomes an invalid capacitance (also referred to as “stray capacitance”). On the other hand, the capacitance of the part where the back plate and the spacer are not in contact with each other, and the part where the diaphragm and the back plate face each other with a slight spatial margin, changes due to the vibration of the diaphragm and generates an audio signal. Therefore, effective capacitance is obtained. By increasing the effective capacitance with respect to the invalid capacitance, it is possible to obtain a condenser microphone with less noise and a good S / N ratio. In order to increase the S / N ratio, it is necessary to increase the effective capacity inside the condenser microphone. Conventional condenser microphones have been devised to increase the S / N ratio by various methods.

  A first conventional example of a condenser microphone unit will be described with reference to the drawings. In FIG. 8, the code | symbol 10 shows a cylindrical unit case. The unit case 10 includes a back plate 20, a diaphragm 30, a diaphragm holding ring 32, a spacer 40, an insulating block 120, a circuit board 130, and the like. This conventional example employs a structure in which the unit case 10 is made of a plastic workable material, and the open end (lower side in FIG. 8) of the unit case 10 is plastically deformed by curling to fix the built-in components. Yes. That is, the curling portion of the case presses the circuit board 130, the insulating block 120, the back plate 20, the spacer 40, the diaphragm 30, and the diaphragm holding ring 32 in this order toward the ceiling surface of the case 10. These built-in components are fixed in the case 10. However, since the unit case 10 and the unit built-in component have individual differences such as dimensional errors, the structure in which the unit built-in component is fixed by curling the open end of the unit case 10 as described above is used as an interior component. It is difficult to keep the internal stress applied to the uniform. For example, when the axial stacking tolerance of the unit built-in components is large, the internal stress becomes high, sometimes the diaphragm ring is deformed, and the tension of the diaphragm changes. Further, when the stacking tolerance is small, the internal stress is small, and sometimes the unit built-in component rattles.

  FIG. 9 shows a second conventional example of a condenser microphone unit. Reference numeral 10 denotes a unit case, and reference numeral 12 denotes a lock ring. In the bottomed cylindrical unit case 10, a back electrode plate 20, a diaphragm 30, a diaphragm holding ring 32, a spacer 40, an insulator 121, and the like are incorporated. A female screw 10-5 is formed on the inner periphery of the unit case 10 from the opening end to the center. A male screw is formed on the outer periphery of the lock ring 12. A circular jetty formed on the outer peripheral edge is formed on the surface of the insulator 121 on the back electrode plate 20 side, and the back electrode plate 20 is fitted in a recess surrounded by the jetty. One surface side of the back electrode plate 20 protrudes from the jetty of the insulator 121, and a spacer 40 is interposed between the outer peripheral edge portion of the protruding surface and the outer peripheral edge portion of the diaphragm 30. These built-in components are housed in the unit case 10, and the lock ring 12 is screwed into the inner screw 10-5 of the unit case 10. The lock ring 12 presses the insulator 121, the back plate 20, the spacer 40, the diaphragm 30, and the diaphragm holding ring 32 in this order toward the ceiling surface of the case 10, and these built-in components are placed in the case 10. It is fixed. In this conventional example, the built-in component tolerance is absorbed and stable internal stress can be applied to the unit built-in component. However, a screw allowance for screwing the lock ring 12 is required in the radial direction of the unit case 10, and the thickness of the unit case must be increased by that amount, thereby reducing the effective capacity inside the unit case.

  Patent Document 1 describes another example of a condenser microphone. The condenser microphone described in Patent Document 1 is designed to reduce the thickness of the microphone while maintaining the desired acoustic characteristics. By making the outer diameter of the diaphragm (diaphragm) elliptical with respect to the cylindrical metal housing, the area occupied by the capacitor structure in the housing can be made larger than before. That's it.

  The condenser microphone unit has an effective capacitance that contributes to audio signal output and an invalid capacitance that does not contribute to audio signal output. As described above, the effective capacitance is larger than the invalid capacitance. A good condenser microphone with a large S / N ratio can be obtained. The reactive capacitance increases in proportion to the area where the spacer contacts the back electrode plate. The invention described in Patent Document 2 relates to a technology for reducing the reactive capacitance as much as possible in a condenser microphone. The spacer in contact with the back electrode plate has a cylindrical shape, and its upper end portion forms a plurality of protrusions at equal intervals, and each protrusion is bent at a right angle in the central direction. The protrusions are interposed between the diaphragm and the back electrode plate to reduce the contact area between the back electrode plate and the spacer as much as possible. By designing the reactive capacitance to be relatively small with respect to the effective capacitance, it is possible to obtain a condenser microphone configuration having a large S / N ratio and low noise.

  The device described in Patent Document 3 aims to reduce the reactive capacitance by reducing the contact area between the back electrode plate and the spacer by devising the shape of the back electrode plate of the condenser microphone. The shape of the back electrode plate used in the condenser microphone unit is a multi-valve plum flower shape, and this petal-shaped portion abuts on a jetty made of resin provided inside the camber cylinder. The recess with respect to the petal-shaped portion plays a role of communicating the space with the rear side of the back electrode plate, and it is not necessary to form a new hole. In addition, a leaf spring for pushing up the back electrode plate toward the diaphragm is provided on the back side of the petal-shaped part, and an effective capacitance is applied by applying pressure evenly to the back plate. It has gained.

JP 2006-19884 A JP 2003-209899 A Utility Model Registration No. 3103711

  In the conventional example shown in FIG. 8, as described above, when the stacking tolerance of the built-in parts increases, the tension of the diaphragm changes and a high-quality S / N ratio cannot be obtained. In addition, if the stacking tolerance is reduced, the built-in components in the unit case may rattle, which may impair the desired acoustic characteristics. Since the conventional example of FIG. 9 forms an internal thread on the inner periphery of the unit case, it is necessary to increase the thickness of the peripheral wall. For this reason, there is a limit to the miniaturization of the unit case, and the effective capacity in the unit case is reduced.

  The invention described in Patent Document 1 is intended to reduce the thickness of the condenser microphone unit case, and is different from the gist of the present invention. The inventions described in Patent Documents 2 and 3 focus on the shapes of the spacer and the back electrode plate, respectively, increase the effective capacitance with respect to the reactive capacitance, and obtain a condenser microphone with a good S / N ratio. Is the purpose. However, as in the present invention, no attention is paid to a technique for fixing the built-in components in the unit case by equal pressing.

  In view of the prior art as described above, the present invention efficiently secures a built-in component housing space in the unit case by devising a component fixing structure in the unit case, and To provide a condenser microphone unit that can increase the ratio of effective capacitance to invalid capacitance by devising the configuration so that the stress on the back electrode plate can be adjusted and the stress is applied evenly. It is an object.

The present invention relates to a condenser microphone including a diaphragm that vibrates by sound, a back electrode plate fixed to face the diaphragm, and a spacer interposed between the diaphragm and the back plate inside a cylindrical unit case. A unit, which is a member for fixing a back plate support integrally coupled with the back plate and a built-in component including the diaphragm, spacer, and back plate support in the unit case. Te a fixing member in which a plurality of the indenter is formed extending in a direction parallel to the central axis from the outer peripheral portion, and a washer which is attached on the opposite surface to the pressure element formation side of the fixing member, the fixing member and the washer a first cushion ring interposed between the second cushion ring interposed between the end of the indenter and the back plate support the fixing member, and a plurality of threaded members tightening the said fixed member and the washer , it has a The first cushion ring is elastically deformed by being compressed between the fixing member and the washer, protrudes toward the outer periphery, and is crimped to the inner wall of the unit case so that the built-in component is placed in the unit case. The indenter of the fixing member is pressed along with the compression of the first cushion ring to press the second cushion ring, and the second cushion ring places the back electrode plate support on the outer peripheral portion of the diaphragm. The most important feature is that it is pushing toward.

  According to the present invention, in the back electrode plate support, the first cushion ring is elastically deformed and protrudes in the outer peripheral direction by fastening the plurality of screw members, and presses and fixes the inner wall of the unit case. . Therefore, the inner wall of the unit case only needs to have a thickness that can withstand the pressure caused by the compression of the first cushion ring. The compressed first cushion ring tilts the indenter formed on the back electrode plate support in the direction of the central axis, and the end of the indenter distributes the built-in components in the unit case evenly via the second cushion ring. The built-in components are fixed by pressing in the direction of the diaphragm with force. Furthermore, an invalid electrostatic capacity can be reduced by fixing a built-in component with a uniform force by a plurality of indenters, and an effective electrostatic capacity can be obtained.

  Embodiments of the present invention will be described below with reference to the drawings. In FIG. 1, the code | symbol 10 has shown the unit case. The unit case 10 is formed in a cylindrical shape, and an inward flange 10-1 is formed on the inner peripheral side of the upper end portion. The inner peripheral edge portion of the inward flange 10-1 projects upward in parallel with the central axis direction. Yes. In the unit case 10, in order from the flange 10-1 side, the diaphragm holding ring 32, the diaphragm 30 having an outer peripheral edge fixed to one end face of the diaphragm holding ring 32, the spacer 40, and the back electrode plate 20. The back electrode plate support 50, the electrode cover 72, the second cushion ring 80-2, the first cushion ring 80-1, the fixing member 90, and the like that are integrally coupled with the back electrode plate 20 are incorporated. The diaphragm 30 is circular and is attached to the diaphragm holding ring 32 while maintaining an appropriate tension. The diaphragm holding ring 32 is accommodated in the upper part of the unit case 10 with the sticking surface of the diaphragm 30 facing down. The diaphragm holding ring 32 is configured so as not to come out of the unit case 10 from above by being supported by the flange 10-1 of the unit case 10. On the lower side of the diaphragm 30, a spacer 40 made of an insulator having a ring shape substantially the same diameter as the diaphragm holding ring 32 and having a slight thickness is provided. Below the spacer 40, a flat disk-shaped back electrode plate 20 and a back electrode plate support 50 that supports the back electrode plate 20 are disposed.

  The back electrode plate support 50 is a substantially disk-shaped member made of an electrical insulator, and a circular jetty 50-1 is formed along the peripheral edge on the end surface on the back electrode plate 20 side. The back electrode plate 20 is dropped into the enclosed depression. A central hole is formed in the back electrode plate support 50, and a cylindrical protrusion 50-2 is formed on the formation surface side of the jetty 50-1 along the periphery of the central hole. The back electrode plate support 50 further has a jetty 50-3 on the inner circumference side of the jetty 50-1 and on the formation surface side of the jetty 50-1, on a circle concentric with the jetty 50-1 and the projection 50-2. Is formed. The upper surface of the jetty 50-1 is in contact with the spacer 40 in FIG. The back electrode plate 20 has a center hole, and an electrode 60 is inserted into the center hole. The electrode 60 has a shape similar to that of a screw or a bolt, and has a head 60-1 and a male screw 60-2 formed on the outer periphery of the main body. The male screw 60-2 is the center of the back electrode plate support 50. It is inserted through the hole.

  The electrode 60 penetrates the back electrode plate support 50 and projects downward from the lower surface of the back electrode plate support 50 in FIG. An electrode terminal 65 is screwed into the protruding portion of the electrode 60. The electrode terminal 65 is rod-shaped and made of a conductor, and the upper part has a large outer diameter, and has a vertical hole in the upper part. A female screw 65-1 is formed in the vertical hole, and the female screw 65-1 is screwed into the male screw 60-2 of the electrode 60, whereby the back electrode plate 20 and the back electrode are formed by the head 60-1 and the electrode terminal 65 of the electrode 60. The plate support 50 is sandwiched. As a result, the back electrode plate 20 is supported by the back electrode plate support 50, and the back electrode plate support 50 and the electrode terminal 65 are integrally coupled. The outer diameter of the back electrode plate 20 is smaller than the inner diameter of the jetty 50-1, and the back electrode plate 20 is in contact with the projection 50-2 of the back plate support 50 and the upper surface of the jetty 50-3. 50. The back plate 20 supported in this way is dropped into the recess of the back plate support 50 so that a slight gap is formed between the back plate 20 and the vibration plate 30. A capacitor is formed between them. A plurality of through holes 50-4 are formed in the back electrode plate support 50 between the jetty 50-2 and the jetty 50-3.

  A jetty 50-5 is formed on the lower surface side of the back electrode plate support 50 in FIG. 1 along a circle concentric with the outer peripheral surface thereof. The acoustic resistor 70 is attached to the lower surface of the back electrode plate support 50 by closing the through hole 50-4 on the inner peripheral side of the jetty 50-5, and the second cushion on the outer peripheral side of the jetty 50-5. A ring 80-2 is arranged. A male screw 65-2 is formed at a substantially central portion in the length direction of the electrode terminal 65, and the lower portion of the electrode terminal 65 extends further to the outside than the lower end of the unit case 10. The electrode terminal 65 is connected to an external circuit, for example, an impedance conversion circuit, as an output of the microphone unit.

  An electrode cover 72 is fitted on the outer periphery of the electrode terminal 65 from the lower end side. The electrode cover 72 is made of an insulator, has a substantially cylindrical shape, and has a flange 72-1 at the top. A female screw 72-2 is formed in the lower half of the electrode cover 72, and the female screw 72-2 is screwed into the male screw 65-2 of the electrode terminal 65. The acoustic resistor 70 is in contact with the upper surface of the flange 72-1 of the electrode cover.

  As described above, the back electrode plate 20 and the back plate support 50 are sandwiched between the head 60-1 of the electrode 60 and the electrode terminal 65 by fastening the electrode terminal 65 and the electrode 60 to each other. The plate 20 is fixed to the back electrode plate support 50. Further, the acoustic resistor 70 is compressed between the flange 72-1 of the electrode cover 72 and the back electrode plate support 50 by tightening the electrode cover 72 with respect to the electrode terminal 65. By adjusting this compressive force, the acoustic resistance value can be adjusted from the outside of the unit case 10.

  A more characteristic component of the present embodiment is a structure in which a built-in component including the components described so far is fixed to the unit case 10. Hereinafter, this fixing structure will be described. 4 (a) and 4 (b), reference numeral 90 denotes a fixing member. The fixing member 90 has a cylindrical portion 90-4 as a main part, and the fixing member 90 is inserted into the unit case 10 by inserting the electrode cover 72 into the center hole 90-1. In the cylindrical portion 90-4 of the fixing member 90, a plurality of screw holes 90-3 are formed at equal intervals in the circumferential direction in a direction parallel to the central axis. As shown in FIG. 4B, the upper portion of the cylindrical portion 90-4 of the fixing member 90 is formed in a tapered shape whose diameter continuously increases downward, and the screw hole 90-3 has the tapered surface. It is formed through. A plurality of arms 90-5 extend outward in the radial direction from the outer peripheral surface of the cylindrical portion 90-4. These arms 90-5 extend in parallel with the central axis of the cylindrical portion 90-4 and are bent upward at a right angle in FIG. 4B, and extend in parallel with the central axis. Is an indenter 90-2. The function of the indenter 90-2 will be described later.

  FIG. 3 shows a state in which the second cushion ring, the fixing member 90, and other components among the components built in the unit case 10 according to the embodiment are separated. In FIG. 3, the tip of the indenter 90-2 of the fixing member 90 is in contact with the second cushion ring 80-2. A first cushion ring 80-1 is disposed on the lower surface side of the protrusion 90-5 of the fixing member 90, and a washer 100 is disposed so as to support the first cushion ring 80-1 from below. As shown in FIG. 5, the washer 100 is circular, a center hole 100-1 having the same diameter as the center hole 90-1 of the fixing member 90 is formed, and a plurality of holes 100-2 are formed at equal intervals in the circumferential direction. Is formed. The member denoted by reference numeral 80 in FIG. 6 is a cushion ring, and this cushion ring 80 is used as a first cushion ring 80-1 and a second cushion ring 80-2. That is, in this embodiment, the first cushion ring 80-1 and the second cushion ring 80-2 are assumed to have the same dimensions. In FIG. 3, reference numeral 110 denotes a screw, and the screw 110 passes through the hole 100-2 of the washer 100 and is screwed into the screw hole 90-3 of the fixing member 90. The first cushion ring 80-1 is sandwiched between the washer 100 and the lower surface of the fixing member 90 (more specifically, the lower surface of the protrusion 90-5 of the fixing member 90).

  As can be seen from FIG. 3, the first cushion ring 80-1 and the washer 100 are assembled with the fixing member 90 as a main body by screws 110, thereby forming a fixing member assembly. As shown in FIG. 1, the fixing member assembly is inserted into the unit case 10 while inserting an electrode cover 72, which is an insulator, into the central hole of the fixing member 90. The fixing member assembly is inserted into the unit case 10 according to the procedure described later. Fixed. Therefore, a fixing member is fitted between the outer periphery of the electrode cover 72 as an insulator and the inner periphery of the unit case 10.

  7A, 7 </ b> B, and 7 </ b> C illustrate the operation of the first cushion ring 80-1 and the second cushion ring 80-2 when the screw 110 is tightened to the fixing member 90 and the indenter 90-2 of the fixing member 90. The movement of the is shown continuously. 7A, when the screw 110 is tightened, the washer 100 approaches the jetty 90-5 of the fixing member 90, and the first cushion ring 80-1 interposed therebetween is compressed and elastically deformed. In the space where the first cushion ring 80-1 is disposed, the outer peripheral side of the fixing member 90 and the washer 100 is open. Therefore, as shown by reference numeral 81 in FIG. 80-1 tends to protrude further toward the outer peripheral side than the outer peripheral surfaces of the fixing member 90 and the washer 100. The portion 81 where the first cushion ring 80-1 is about to protrude from the outer periphery of the fixing member 90 presses the inner wall of the unit case 10 outward, and the fixing member 90 is fixed in the unit case 10 by this pressing force. To do.

  Thus, the first cushion ring 80-1 is elastically deformed by being compressed and attempts to release the stored energy by protruding in the outer peripheral direction, but the protrusion is restricted by the inner peripheral wall of the unit case 10, The repulsive force is stored in the first cushion ring 80-1. The stored repulsive force acts as a force that pushes up each protrusion 90-5 of the fixing member 90, as shown in FIG. By this pushing-up force, each protrusion 90-5 bends, and an indenter 90-2 formed integrally with each protrusion 90-5 is inclined toward the center direction of the fixing member 90. The end surfaces of the indenter 90-2 compress the second cushion ring 80-2 as the protrusions 90-5 are bent upward and the indenter 90-2 is inclined. The second cushion ring 80-2 presses the back electrode plate support 50 by the compressive force of the second cushion ring 80-2. This pressing force acts as a force for fixing built-in components such as the diaphragm 30, the back plate 20, and the back plate support 50 within the unit case 10. A plurality of the protrusions 90-5 and the indenters 90-2 are provided at equal intervals in the circumferential direction, and press the back electrode plate support 50 through the plurality of indenters 90-2 and the second cushion ring 80-2. Therefore, the back electrode plate support 50 is pressed evenly.

  The screws 110 are individually adjusted for tightening force, whereby the pressing force against the inner wall of the unit case 10 by the first cushion ring 80-1 and the second cushion ring 80- by the indenter 90-2 of the fixing member 90 are adjusted. The pressing force of 2 can be adjusted.

  FIGS. 2A and 2B show an embodiment of a narrow directivity condenser microphone in which the condenser microphone unit according to the present invention is installed in an acoustic tube. Reference numeral 5 denotes an acoustic tube. The microphone unit configured as described above is installed at a substantially central portion in the length direction of the acoustic tube 5. In FIG. 2A, the opening of the microphone unit faces the right side, and sound is collected from the right end of the acoustic tube, and the sound reaches the microphone unit.

  As described above, according to the present invention, the cushion ring is compressed and deformed and stored, and the built-in component is fixed in the unit case using this stored force, so that an excessive force is applied to the built-in component. The built-in components can be pressed evenly and stably. Moreover, since the built-in components can be fixed even if the thickness of the unit case is reduced, the ratio of the effective capacitance to the invalid capacitance can be increased, and a good S / N ratio can be obtained. it can.

It is sectional drawing which shows the Example of the condenser microphone unit concerning this invention. (A) which shows the example of the narrow directivity condenser microphone using the said condenser microphone unit is a schematic sectional drawing, (b) is a schematic diagram. It is an exploded sectional view showing the example of the above-mentioned condenser microphone unit. (A) which shows the fixing member in the said Example is a top view, (b) is sectional drawing. (A) which shows the washer in the said Example is a top view, (b) is sectional drawing. (A) which shows the cushion ring in the said Example is a top view, (b) is sectional drawing. It is a longitudinal cross-sectional view which shows the internal component fixing operation | movement by the fixing member and cushion ring in the said Example in order of (a) (b) (c). It is sectional drawing which shows the example of the conventional capacitor | condenser microphone unit. It is sectional drawing which shows another example of the conventional capacitor | condenser microphone unit.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Unit case 12 Lock ring 20 Back electrode plate 30 Diaphragm 32 Diaphragm ring 40 Spacer 50 Back electrode support body 60 Electrode 65 Electrode terminal 70 Acoustic resistor 72 Electrode cover 90 Fixing member 100 Washer 110 Screw member 80-1 1st Cushion ring 80-2 Second cushion ring 90-2 Indenter

Claims (6)

A condenser microphone unit including a diaphragm that vibrates by sound, a back electrode plate fixed to face the diaphragm, and a spacer interposed between the diaphragm and the back electrode plate inside a cylindrical unit case. ,
A back plate support that integrally supports the back plate;
A member for fixing the built-in components including the diaphragm, spacer, and back electrode plate support in the unit case, and a fixing member formed with a plurality of indenters extending in the direction parallel to the central axis from the outer peripheral portion; ,
A washer which is attached on the opposite surface to the pressure element formation side of the fixing member,
A first cushion ring interposed between the fixing member and the washer;
A second cushion ring interposed between an end of the indenter of the fixing member and the back electrode plate support;
Has a plurality of screw tightening and the fixing member and the washer,
The first cushion ring is elastically deformed by being compressed between the fixing member and the washer, protrudes toward the outer periphery, and is crimped to the inner wall of the unit case to fix the built-in component in the unit case. ,
The indenter of the fixing member is pressed along with the compression of the first cushion ring to press the second cushion ring, and the second cushion ring presses the back electrode plate support toward the outer periphery of the diaphragm. Capacitor microphone unit.   2. The condenser microphone unit according to claim 1, wherein each of the indenters of the fixing member is inclined toward the center in the radial direction by being pressed by the first cushion ring and presses the second cushion ring. The screw can adjust the pressing force of the first cushion ring against the inner wall of the unit case and the pressing force to the second cushion ring by the indenter of the fixing member by adjusting the tightening force. 3. The condenser microphone unit according to 1 or 2 . The back electrode plate support includes a back electrode plate, a screw, and a conductor, and the screws penetrate the back electrode plate and are screwed into the conductor so that these components are fastened together. Item 1. A condenser microphone according to item 2, 2 or 3 .   5. The condenser microphone according to claim 4, wherein an insulator is screwed into the outer periphery of the conductor, and the insulator is integrally coupled to the back electrode plate support. 6. The condenser microphone according to claim 5, wherein a fixing member is fitted between the outer periphery of the insulator and the inner periphery of the unit case.

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