Condenser microphone unit and method of manufacturing the same

A condenser microphone unit includes a diaphragm vibrated by acoustic waves, a fixed electrode disposed to face the diaphragm, and an insulation base making contact with a rim portion of the fixed electrode to support the fixed electrode, wherein a ring-shaped protrusion is provided on a rim portion of the insulation base, the ring-shaped protrusion protruding toward the fixed electrode with a radially inward taper and having a ring-shaped distal face to oppose the rim portion of the fixed electrode, the distal face of the ring-shaped protrusion supports the rim portion of the fixed electrode, and an adhesive is provided on a tapered surface of the ring-shaped protrusion positioned between the insulation base and the fixed electrode, the adhesive having property to shrink by curing. When the adhesive is cured, contact portions of the insulation base and the fixed electrode are tightly bonded together.

RELATED APPLICATIONS

The present application is based on, and claims priority from, Japanese Application No. JP2014-200337 filed Sep. 30, 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 condenser microphone unit that can be used for, e.g., a narrow directional microphone equipped with an acoustic tube, and to a method of manufacturing the condenser microphone unit.

Description of the Related Art

FIG. 7is a cross sectional view of a typical unidirectional condenser microphone unit. The condenser microphone unit illustrated inFIG. 7includes a unit case10including a plurality of front acoustic terminal holes10aon the front end surface thereof, an electroacoustic transducer20contained in the unit case10, and a circuit board30disposed on the rear end opening of the unit case10.

The electroacoustic transducer20includes a diaphragm22stretched across a supporting ring21with a predetermined tension, a disk shaped fixed electrode24supported on a face side of an insulation base25, and a spacer ring23having electrical insulating property disposed between the diaphragm22and the fixed electrode24.

As illustrated in the drawing, the diaphragm22and the fixed electrode24are disposed to oppose each other with the spacer ring23therebetween to form an electrostatic electroacoustic transducer20.

A field-effect transistor (FET)40serving as an impedance converter is mounted on the circuit board30.

The circuit board30includes a rear acoustic terminal32. Acoustic holes (acoustic wave introducing holes)25aand24aare drilled in the insulation base25and the fixed electrode24, respectively.

This configuration allows acoustic waves traveling from the rear acoustic terminal32to have effect on the back side of the diaphragm22via the acoustic holes25aand24a.

A predetermined acoustic resistance material26is disposed in the air chamber50provided between the fixed electrode24and the acoustic hole25a.

By connecting an acoustic tube (not shown) to the front face side of the microphone unit, the microphone unit can be used as a microphone having narrow directional property.

However, the condenser microphone equipped with an acoustic tube having narrow directional property has disadvantage that narrow directional property cannot be provided by using the acoustic tube at low frequency because of the dimensional relationship between the length of the acoustic tube and the wavelength of acoustic waves. So that, for low frequencies where the acoustic tube does not work, an acoustic tube is connected to a front acoustic terminal of a unidirectional unit to operate the microphone as a unidirectional microphone. A microphone having narrow directional property equipped with an acoustic tube is disclosed in JP 2000-050386 A.

In the narrow directional microphone as described above, the effective distance between acoustic terminals at low frequency band is long, so that the acoustic mass of the acoustic tube is connected to the front side of the diaphragm22of the unidirectional condenser microphone unit. Thus the directional property of the unit should be adjusted to have directional frequency response almost identical to omnidirectional property when measured in a free space.

The air chamber50in the rear side of the fixed electrode24drives omnidirectional elements to the diaphragm22and determines the equivalent mechanical mass of the diaphragm22and a resonance frequency of the stiffness of the air chamber50. To achieve the design providing a resonance frequency at a high limit of a sound collection band, the air chamber50should be designed to have a small volume to increase its stiffness.

In addition, to obtain directional frequency response almost identical to omnidirectional property, the acoustic resistance of the rear acoustic terminal32should be increased to reduce the force that drives bidirectional elements to the rear side of the diaphragm22from the rear acoustic terminal32. Since the air chamber50has high stiffness, the acoustic resistance of the rear acoustic terminal32is designed to be very high.

However, when leakage occurs between the rear side of the diaphragm22and the acoustic resistance of the rear acoustic terminal32, the effective acoustic resistance during operation is reduced and a problem arises that the intended directional property cannot be achieved.

Specifically, as illustrated inFIG. 8, the problem is the leakage from the contact portion (contact portion at the rim portion) between the insulation base25and the fixed electrode24.

The fixed electrode24is usually punched out from a metal plate having an electret material (FEP film) thermally bonded thereto, so that the fixed electrode24has a rough end surface with a sheared surface61and a sharp edge62.

The insulation base25is usually manufactured by injection molding of polycarbonate (PC). For the insulation base25, shrinking of material during cooling produces roughness on the surface which is to make contact with the fixed electrode24. These rough surfaces produced during manufacturing disadvantageously create a leak passage63between parts.

Moreover, variation in dimensions of the leak passage63disadvantageously causes difference in directional frequency response at low range among manufactured microphones. In particular, for a condenser microphone equipped with a long acoustic tube having narrow directional property, the leakage causes disadvantageous effects and has become a serious problem.

SUMMARY OF THE INVENTION

The present invention is made in view of the aforementioned problem. For a condenser microphone unit in which an insulation base supports the rim portion of a fixed electrode disposed to face a diaphragm, an object of the present invention is to provide a condenser microphone with no leakage from contact portions of the insulation base and the fixed electrode and a method of manufacturing the condenser microphone.

To solve the aforementioned problem, a condenser microphone unit according to the present invention includes a diaphragm vibrated by acoustic waves, a fixed electrode disposed to face the diaphragm, and an insulation base making contact with a rim portion of the fixed electrode to support the fixed electrode, wherein a ring-shaped protrusion is provided on a rim portion of the insulation base, the ring-shaped protrusion protruding toward the fixed electrode with a radially inward taper and having a ring-shaped distal face to oppose the rim portion of the fixed electrode, the distal face of the ring-shaped protrusion supports the rim portion of the fixed electrode, and an adhesive is provided on a tapered surface of the ring-shaped protrusion positioned between the insulation base and the fixed electrode, the adhesive having property to shrink by curing.

Preferably, contact portions of the insulation base and the fixed electrode are tightly bonded together when the adhesive is cured.

Preferably, the insulation base has on the tapered surface of the ring-shaped protrusion a plurality of positioning projections which makes contact with an outer circumferential surface of the fixed electrode to position the fixed electrode on the insulation base.

In such a configuration, the insulation base and the fixed electrode can tightly be bonded together with no gap therebetween when the adhesive is cured.

Consequently, with no leak passage between the insulation base and the fixed electrode, a condenser microphone equipped with a long acoustic tube having narrow directional property can be manufactured without variation in property among products.

To solve the aforementioned problem, a method of manufacturing a condenser microphone unit according to the present invention is a method of manufacturing a condenser microphone unit including a diaphragm vibrated by acoustic waves, a fixed electrode disposed to face the diaphragm, and an insulation base making contact with a rim portion of the fixed electrode to support the fixed electrode, and the method includes a step of forming a ring-shaped protrusion on a rim portion of the insulation base, the ring-shaped protrusion protruding toward the fixed electrode with a radially inward taper and having a ring-shaped distal face to oppose the rim portion of the fixed electrode, a step of supporting the rim portion of the fixed electrode by the distal face of the ring-shaped protrusion, a step of providing an adhesive on a tapered surface of the ring-shaped protrusion positioned between the insulation base and the fixed electrode, the adhesive having property to shrink by curing, and a step of curing the adhesive.

Preferably, in the step of supporting a rim portion of the fixed electrode by the distal face of the ring-shaped protrusion, the fixed electrode is positioned on the insulation base by a plurality of positioning projections provided on the tapered surface of the ring-shaped protrusion when fixed electrode makes contact with the insulation base.

Preferably, in the step of providing an adhesive, having property to shrink by curing, on the tapered surface of the ring-shaped protrusion positioned between the insulation base and the fixed electrode, the adhesive is provided in an inner side of the positioning projection.

Preferably, in addition, the distal face of the ring-shaped protrusion and the contact surface of the fixed electrode are both flat.

In the condenser microphone unit manufactured by such a method, the insulation base and the fixed electrode can tightly be bonded together with no gap therebetween when the adhesive is cured.

Consequently, with no leak passage between the insulation base and the fixed electrode, a condenser microphone equipped with a long acoustic tube having narrow directional property can be manufactured without variation in property among products.

Thus, a condenser microphone with no leakage from contact portions of the insulation base and the fixed electrode and a method of manufacturing the condenser microphone can be provided for a condenser microphone unit in which the insulation base supports the rim portion of the fixed electrode disposed to face a diaphragm.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

An embodiment of the present invention will now be described referring to the drawings.FIG. 1is a cross sectional view of a condenser microphone unit according to an embodiment of the present invention. For a condenser microphone unit1illustrated inFIG. 1, the component equivalent to that of the condenser microphone unit already described usingFIG. 7is appended with the same reference sign.

The illustrated condenser microphone unit1includes a unit case10including a plurality of front acoustic terminal holes10aon the front end surface thereof, an electroacoustic transducer20contained in the unit case10, and a circuit board30disposed on the rear end opening of the unit case10.

The electroacoustic transducer20includes a diaphragm22stretched across a supporting ring21with a predetermined tension, a disk shaped fixed electrode24disposed to face the rear side of the diaphragm22, and an insulation base2supporting the rim portion of the fixed electrode24. A spacer ring23having electric insulating property is provided between the fixed electrode24and the diaphragm22at rim portions thereof with a predetermined gap therebetween. An electrostatic electroacoustic transducer20is thus configured.

A field-effect transistor (FET)40serving as an impedance converter is mounted on the circuit board30. Agate electrode, one of three electrodes of the FET40is connected to the fixed electrode24via predetermined electrically connecting means.

To achieve unidirectional characteristics of the condenser microphone unit1, a circuit board30includes a rear acoustic terminal32, and acoustic holes (acoustic wave introducing holes)2aand24aare drilled in the insulation base2and the fixed electrode24, respectively.

This configuration allows acoustic waves traveling from the rear acoustic terminal32to have effect on the back side of the diaphragm22via the acoustic holes2aand24a. A predetermined acoustic resistance material26is disposed in the air chamber50provided between the fixed electrode24and the acoustic hole2a.

The condenser microphone unit1according to the embodiment is characterized by the configuration of the insulation base2supporting the fixed electrode24.FIG. 2is a plan view of the insulation base2.

As illustrated inFIGS. 1 and 2, the insulation base2is provided with a small-diameter-ring-shaped protrusion3in the central portion and a large-diameter-ring-shaped protrusion4in the rim portion to support the rim portion of the fixed electrode24.

The small-diameter-ring-shaped protrusion3protrudes to forma sleeve with a constant inner diameter and a constant outer diameter respectively. The outer circumferential surface of the small-diameter-ring-shaped protrusion3and the inner circumferential surface of the large-diameter-ring-shaped protrusion4forms an air chamber50in which the acoustic resistance material26is provided as illustrated inFIG. 1.

The large-diameter-ring-shaped protrusion4has an inner circumferential surface with a constant diameter. The outer circumferential surface of the large-diameter-ring-shaped protrusion4is a tapered surface4bwhich is tapered radially inward and protrudes toward the fixed electrode24. A distal face4acontinuing from the tapered surface4bof the ring-shaped protrusion4is formed flat. The distal face4asupports the bottom face of the rim portion of the fixed electrode24.

Bar-like positioning projections5are provided at a plurality of circumferential positions (four positions inFIG. 2) on the tapered surface4bof the large-diameter-ring-shaped protrusion4. The positioning projections5extend upright to support the outer circumferential surface of the fixed electrode24. The circumferential width of the positioning projection5is not particularly limited.

The height of the positioning projection5is such that the inner circumferential surface5aof the positioning projection5makes contact with the bottom half section, approximately, of the outer circumferential surface of the fixed electrode24when the fixed electrode24is placed on the distal face4aof the ring-shaped protrusion4. The positioning projection5has a tapered surface5bon the top front portion thereof so that the fixed electrode24can easily be placed on a predetermined position on the insulation base2.

An adhesive8which shrinks by curing (e.g., rubber-based adhesive) is provided on the tapered surface4bof the ring-shaped protrusion4in the space in the inner side of the positioning projection5and underneath the bottom face of the rim portion of the fixed electrode24. When the adhesive8is cured, the fixed electrode24is supported on the insulation base2with no gap.

A step of placing the fixed electrode24on the insulation base2will now be described referring toFIGS. 3 to 6.FIGS. 3 to 6are cross sectional views each partially illustrating the rim portion of the insulation base2.

First, as illustrated inFIG. 4, an uncured adhesive8is provided in the inner side of the positioning projection5on the tapered surface4bof the ring-shaped protrusion4of the insulation base2illustrated inFIG. 3.

Then the rim portion of the fixed electrode24is placed on the distal face4aof the ring-shaped protrusion4. Since the distal face4aof the ring-shaped protrusion4and the contact surface of the fixed electrode24are both flat, a sheared surface61or a sharp edge62is allowed to exist on the end face of the fixed electrode24as illustrated inFIG. 4.

As illustrated inFIG. 5, the space which is in the inner side of the positioning projection5and surrounded by the bottom face of the rim portion of the fixed electrode24and the tapered surface4bis filled with the adhesive8.

When thinner of the adhesive8volatilizes, the adhesive8shrinks, reducing its volume. As a result, as illustrated inFIG. 6, a force F is produced to pull the fixed electrode24toward the insulation base2to tightly bond together the bottom face of the rim portion of the fixed electrode24and the distal face4aof the ring-shaped protrusion4of the insulation base2. Since contact portions of the insulation base2and the fixed electrode24are flat, namely with no shrinkage recess on the insulation base2and no sharp edge produced by press forming on the fixed electrode24, no gap exists between the contact portions, and therefore air leakage is prevented.

To prevent air leakage from between the insulation base2and the fixed electrode24, the embodiment according to the present invention is configured that the insulation base2and the fixed electrode24are positioned to make contact with each other via flat contact portions and then tightly bonded together with no gap therebetween when the adhesive8, which shrinks by curing, is cured.

Consequently, with no leak passage, a condenser microphone equipped with a long acoustic tube having narrow directional property can be manufactured without variation in property among products.

Although the embodiment is illustrated to have four positioning projections5on the rim portion of the insulation base2, the number of positioning projections5is not limited. Note that, preferably at least three positioning projections5are circumferentially provided at an even pitch to make positioning of the fixed electrode24easy.

Although the embodiment is illustrated to provide the adhesive8before the insulation base2makes contact with the fixed electrode24, other procedure can be used. The adhesive8may be provided after the insulation base2makes contact with the fixed electrode24.