Capacitor microphone unit and capacitor microphone

Electro-acoustic converters each include a diaphragm, and a fixed electrode apart from the diaphragm for a certain distance and facing the diaphragm. The electro-acoustic converters are anteroposteriorly disposed on the same axis in a single casing, and are electrically connected in series. The front and rear converters each include impedance converters, and are serially connected with each other together with the impedance converters.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a capacitor microphone unit that can have improved sensitivity while maintaining its excellent directional frequency response characteristics up to a high tone range and a capacitor microphone using such a capacitor microphone unit.

2. Description of the Related Art

A capacitor microphone unit is an electro-acoustic converter including a diaphragm and a fixed electrode facing each other with a certain space provided therebetween and utilizing a mechanism in which the capacity of a capacitor formed of the diaphragm and the fixed electrode changes when the diaphragm vibrates upon receiving sound wave.FIGS. 10A,10B and11exemplary illustrate a conventional capacitor microphone unit.

As illustrated inFIGS. 10A,10B, and11, a cylindrical casing101incorporates a diaphragm ring102, a diaphragm103, a spacer104, a fixed electrode105, an acoustic resistor106, a terminal107, an insulating substrate108, and a ring screw109in this order. The diaphragm103is made of a thin resin film having a surface on which a piece of metal is vapor deposited. The peripheral portion of the diaphragm103is fixedly adhered to the diaphragm ring102. The casing101has a flange directed inward on one end, i.e., the front end that is on the left side inFIG. 11. The diaphragm ring102is in contact with the peripheral portion of the flanged portion. The spacer104of a thin ring-shaped plate is interposed between the diaphragm103and the fixed electrode105, thereby forming a gap corresponding to the thickness of the spacer104between the diaphragm103and the fixed electrode105. With this structure, an electret capacitor microphone can be provided by forming an electret layer on either of the surfaces of the diaphragm103and the fixed electrode105that are facing each other.

The terminal107penetrates the center hole of the insulating substrate108to have its rear end protrude towards the rear side of the microphone unit while the head portion on the front side of the terminal107is in contact with the fixed electrode105. The acoustic resistor106is held by the insulating substrate108and defines an acoustic resistance in a space reaching the rear surface of the diaphragm103through a hole in the fixed electrode105from an acoustic terminal formed of a space provided in the insulating substrate108. The ring screw109is screwed into the inner periphery at the rear end of the casing101to press the insulating substrate108towards the front side of the casing101. With the above described elements being pressed with this pressing force, the diaphragm ring102is in contact with the inward-directed flange of the casing101and the elements are held in the casing101in a mutually pressed state.

The diaphragm ring102is electrically connected to the diaphragm103and the casing101. Thus, a sound signal as a result of electro-acoustic conversion can be output from the casing101and through the terminal107electrically connected to the fixed electrode105. Generally, an impedance converter such as a field electric transistor (FET) is provided to lower the impedance of the sound signal that is weak but has high impedance. An output circuit of a capacitor microphone using the above described capacitor microphone unit is exemplary illustrated inFIG. 12. In the output circuit, the fixed electrode105is connected to the input terminal of an impedance converter110, the diaphragm plate103is connected to the ground side, and the primary coil of a transformer111is connected between an output terminal of the impedance converter110and the ground. The ends of the secondary coil of the transformer111, respectively serving as a hot-side and a cold-side output terminals for a balanced output, are each connected to a microphone cable via a connector. The ground side is connected to a shielding cable of the microphone cable. Thus, balanced sound signal can be output.

Directional frequency response characteristics of a conventional capacitor microphone unit having the above described structure are depicted inFIG. 13. The thickest characteristic line represents the directional frequency response characteristic measured at the front of the microphone unit, i.e., the position that is not offset from the central axis of the microphone unit. The second thickest characteristic line represents the directional frequency response characteristic measured at a side of the microphone unit, i.e., the position offset from the central axis of the microphone unit by 90 degrees. The thin characteristic line represents the directional frequency response characteristic measured at the rear of the microphone unit, i.e., the position offset from the central axis of the microphone unit by 180 degrees. The characteristics were measured in accordance with EIAJ standard. Capacitor microphones are demanded to have improved sensitivity without degrading directional frequency response characteristics especially in a high frequency domain.

It is desirable that sensitivity of a microphone is high. Higher sensitivity can be provided to a capacitor microphone with the following possible measures:1. increasing the driving force;2. lowering the impedance of the microphone unit; and3. providing the microphone unit with a diaphragm plate having a larger area.

It is most practical to provide higher sensitivity by providing the microphone unit with a diaphragm plate having a larger area among the measures. Unfortunately, this degrades the directional frequency response characteristics in a high frequency domain, i.e., sensitivity in a high frequency domain is degraded. Therefore, the inventors of the present invention have proposed an invention disclosed in Japanese Patent Application Publication 2006-5710 that relates to a capacitor microphone with which intrinsic noise can be reduced without degrading directional frequency response characteristics in a frequency domain including a high frequency domain. In the capacitor microphone according to such an invention, a plurality of small-diameter unidirectional capacitor microphone capsules (microphone units) is apposed, connected in parallel, and is connected to a single impedance converter.

The capacitor microphone described in Japanese Patent Application Publication 2006-5710 can solve the problem only to a certain level. More specifically, sensitivity over an expected level cannot be obtained because multiple microphone capsules are connected in parallel.

Therefore, the assignee filed a patent application, Japanese Patent Application Publication 2009-151768, on a capacitor microphone in which multiple microphone units are connected in series in an arrangement in which diaphragms of the respective microphone units are arranged to be on the same plane and an output from an impedance converter connected to one of the microphone units drives the ground side of another microphone unit. This application (hereinafter, referred to as prior invention) is not yet published at the point of the application of the present invention.

The capacitor microphone according to the prior application can improve the directional frequency response characteristics up to a high frequency domain while improving the sensitivity.

On the other hand, with the capacitor microphone according to the prior application, a new technical problem to be solved arises. Specifically, the size of the microphone applying this configuration is large because multiple microphone units are physically arranged in series.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a capacitor microphone unit and a capacitor microphone that can have excellent directional frequency response characteristics in a frequency domain including a high frequency domain without having a large size (i.e., while solving the new technical problem), and a higher sensitivity without degrading the directional frequency response characteristics.

A capacitor microphone unit according to an aspect of the present invention includes: a casing; a plurality of electro-acoustic converters anteroposteriorly disposed on the same axis in the casing; and a spacer dividing the electro-acoustic converters. The electro-acoustic converters each includes: a diaphragm; a fixed electrode apart from the diaphragm for a certain distance and facing the diaphragm; an impedance converter; and terminals that are respectively connected to the diaphragm and the fixed electrode. The diaphragm and the fixed electrode of each of the electro-acoustic converters are electrically insulated from the casing. An output from the impedance converter connected to one of the electro-acoustic converters drives another one of the electro-acoustic converters.

In the capacitor microphone unit according to another aspect of the present invention, the electro-acoustic converters are electret capacitor microphone units.

In the capacitor microphone unit according to still another aspect of the present invention, the diaphragm ring of each of the electro-acoustic converters except for one of the electro-acoustic converters disposed at front most position is thinner than the diaphragm ring of the one of the electro-acoustic converters disposed at the front most position, the diaphragm ring being fixed to the diaphragm.

In the capacitor microphone unit according to yet still another aspect of the present invention, the capacitor microphone unit includes two capacitor microphone units described above. A plurality of electro-acoustic converters in one of the two capacitor microphone units and a plurality of electro-acoustic converters in the other one of the two capacitor microphone units are connected in series while being electrically opposite from each other. A balanced output is taken from the capacitor microphone unit with one of the two capacitor microphone units serving as a hot-side and the other one of the two capacitor microphone units serving as a cold-side.

A capacitor microphone according to an aspect of the present invention includes a casing; and the capacitor microphone unit described above incorporated in the casing.

The structure is such that in which a plurality of electro-acoustic converters is anteroposteriorly disposed on the same axis in the casing and each of the electro-acoustic converters includes the diaphragm and the fixed electrode apart from the diaphragm for a certain distance and facing the diaphragm. Therefore, the unit as a whole can be downsized with a diameter being the same as that of a general conventional capacitor microphone unit and the size in the axial direction being only slightly larger than a general conventional capacitor microphone unit.

Furthermore, the structure is such that the electro-acoustic converters are electrically separated by the spacer, the diaphragm and the fixed electrode of each of the electro-acoustic converters are electrically insulated from the casing, the terminals are respectively provided for the diaphragm and the fixed electrode, each of the electro-acoustic converters includes the impedance converter, and the output from the impedance converter connected to one of the electro-acoustic converters drives another one of the electro-acoustic converters. Thus, excellent directional frequency response characteristics up to a high frequency domain can be obtained and sensitivity can be improved with out degrading the directional frequency response characteristics.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

An embodiment of a capacitor microphone unit and a capacitor microphone is described below with reference to some of the accompanying drawings.

As illustrated inFIGS. 1A,1B, and2, a casing1has a cylindrical shape, is made of an insulating material, and sequentially incorporates a terminal plate2, a terminal3, a diaphragm ring4, a diaphragm5, a spacer6, a fixed electrode7, a terminal plate8, a spacer9, a diaphragm plate10, a diaphragm11, a spacer12, a fixed electrode13, an acoustic resistor14, a terminal15, an insulating substrate16, and a ring screw17. The diaphragms5and11, each of which is made of a thin resin film, are fixed to the diaphragm rings4and10, respectively, at their peripheral portions. The casing1has an inward-directed flange on one end (front end, which is the left end as viewed inFIG. 2). A peripheral portion of the terminal plate2is in contact with the inner surface of the flange. The spacer6is a thin ring shaped plate and is provided between the diaphragm5and the fixed electrode7. As a result, a gap defined by the thickness of the spacer6is provided between the diaphragm5and the fixed electrode7. Similarly, the spacer12is a thin ring shaped plate and is provided between the diaphragm11and the fixed electrode13. As a result, a gap defined by the thickness of the spacer12is provided between the diaphragm11and the fixed electrode13. An electret capacitor microphone unit is formed by providing an electret layer on either of the respective opposing surfaces of the diaphragm5and the fixed electrode7and on either of the respective opposing surfaces of the diaphragm11and the fixed electrode13.

The terminal3having a circular rod shape penetrates the terminal plate2through the center hole thereof from the rear side to the front side of the terminal plate2. The front end of the terminal3protrudes forward from the front end of the casing1and the rear end of the terminal3is in contact with the terminal plate2at its large diameter portion. The terminal plate2is electrically connected with the diaphragm5via the diaphragm ring4, whereby the diaphragm5is electrically connected to the terminal3. The terminal plate8having a ring shape is connected to the rear side of the fixed electrode7, whereby the terminal plate8is electrically connected to the fixed electrode7. The periphery of the terminal plate8partly protrudes radially outward through a notch formed on the casing1. The protruding portion serves as an output terminal connected to the fixed electrode7. A capacitor type electro-acoustic converter18is formed of the terminal plate2, the terminal3, the diaphragm ring4, the diaphragm5, the spacer6, the fixed electrode7, and the terminal plate8. Hereinafter, this electro-acoustic converter may also be referred to as the front unit18.

The terminal15having a round rod shape penetrates the insulating substrate16positioned on the right side as viewed inFIG. 2, through the center hole of the insulating substrate16from the front side toward the rear side. Thus, a tip of the terminal15protrudes from the rear side of the insulating substrate16while a large diameter portion of the terminal15contacts and presses the fixed electrode13. As a result, the fixed electrode13and the terminal15are electrically connected to each other. The periphery of the diaphragm ring10partly protrudes radially outward through a notch formed on the casing1. The protruding portion serves as an output terminal connected to the diaphragm11. The acoustic resistor14is provided between the fixed electrode13and the insulating substrate16and the acoustic resistor14provides an acoustic resistance for an acoustic terminal formed of a hole on the insulating substrate16.

A capacitor type electro-acoustic converter19is formed of the diaphragm ring10, the diaphragm11, the spacer12, the fixed electrode13, the acoustic resistor14, and the terminal15. Hereinafter, this electro-acoustic converter may also be referred to as the rear unit19.

The spacer9is provided between the front unit18and the rear unit19to divide the units on the front side and the back side. The ring screw17is screwed into the inner surface of the rear end (right side as viewed inFIG. 2) of the casing1to press the insulating substrate16towards the front. With the pressing force thus applied, the above described elements in the casing1are pressed towards the front and the terminal plate2contacts the inward-directed flange of the casing1. Thus, the elements are positioned and held in the casing1in a mutually contacted state. Accordingly, the front and the rear electro-acoustic converters18and19are incorporated in the casing1while being disposed anteroposteriorly on the same axis and being is serial connection. An output signal from the front unit18is taken from the terminal3and the terminal plate8. An output signal from the rear unit19is taken from the terminal15and the diaphragm ring10. Accordingly, the structure is such that the diaphragms5and11and the fixed electrodes7and13of the respective electro-acoustic converters18and19are insulated from the casing1and terminals connected to the respective diaphragms5and11and the respective fixed electrodes7and13are separately provided.

FIG. 3illustrates an acoustic equivalent circuit according to the embodiment. Sbrepresents the stiffness of a space formed of a space between the front diaphragm5and the fixed electrode7, a hole provided in the fixed electrode7, and a space between the fixed electrode7and the rear diaphragm11. The stiffness Sbserves as a border line between the front and the rear units18and19connected with each other. P1represents sound pressure of the front unit18. mOA, SOA, and rOArespectively represent a mass, stiffness, and an acoustic resistance of an air chamber of the front unit18. P2represents sound pressure of the front unit19. mOB, SOB, and rOBrespectively represents a mass, stiffness, and an acoustic resistance of an air chamber of the front unit19. S1represents a stiffness of the hole provided on the fixed electrode13and a rear air chamber communicating with the hole. r1represents an acoustic resistance value of the acoustic resistor14.

An example of an electrical connection of the embodiment is described with reference toFIG. 4. In the example illustrated inFIG. 4, two above described capacitor microphone units are used for balanced output. First, an example of an output circuit of one of the microphone units that is on the upper side inFIG. 4is described. InFIG. 4, the diaphragm5and the fixed electrode7form the main portion of the front unit18while the diaphragm11and the fixed electrode13form the main portion of the rear unit19. As described above, the diaphragm5of the front unit18is connected to the outside of the unit via the diaphragm ring4, terminal plate2, and the terminal3and is grounded as illustrated inFIG. 4. The fixed electrode7is connected to outside of the unit through the terminal plate8and is connected to the input terminal of the impedance converter21. The output terminal of the impedance converter21is connected to the diaphragm11through the diaphragm ring10of the rear unit19. The fixed electrode13of the rear unit19, which is connected to the outside of the unit via the terminal15, is connected to the input terminal of the impedance converter22. The output terminal of the impedance converter22serves as a signal output terminal for one capacitor microphone unit in which the front and the rear units18and19are connected in series.

As described above, the front and the rear units18and19are incorporated in a single casing. The front and the rear units18and19are disposed on the same axis in a physical sense, include the impedance converters21and22, respectively in an electrical sense, and are serially connected together with the impedance converters21and22. In other words, multiple diaphragms and respective multiple fixed electrodes facing the diaphragms and are insulated from one another and anteroposteriorly arranged on the same axis in a single casing. Electro-acoustic converters are divided by a spacer. The diaphragm and the fixed electrode of each of the electro-acoustic converter are electrically insulated from the casing and respective terminals connected to the diaphragm and the fixed electrode are separately provided. The electro-acoustic converters each includes an impedance converter and an output from the impedance converter connected to one of the electro-acoustic converters drives the other electro-acoustic converter. By connecting multiple electro-acoustic converters electrically in series as described above, an output voltage of N fold, i.e., 20 logN (N=2, 3, . . . ), can be obtained where N is the number of units connected in series. This means the increase for 10 logN because intrinsic noise is uncorrelated, thereby improving the S/N ratio.

The fixed electrodes7and13of the respective front and rear units18and19are provided with multiple holes through which the front and the rear units18and19are acoustically connected in series. The diaphragm ring10of the rear unit19is thinner than the diaphragm ring4of the front unit18so that the space between the front fixed electrode7and the rear diaphragm11is small to have higher stiffness (SbinFIG. 3). Not only in the structure in which two units are anteroposteriorly disposed as in the illustrated embodiment, but also in a general structure in which multiple electro-acoustic converters are anteroposteriorly disposed, the electro-acoustic converter provided behind the one at the front most position may have a diaphragm ring thinner than the electro-acoustic converter of the front most one.

The thickness of the diaphragm ring of the electro-acoustic converter except for the front most one preferably is at the smallest possible limit for maintaining certain strength to prevent response characteristics for high frequencies from degrading. Diaphragm rings manufactured in a conventional method, i.e., machining, cannot have a thickness below a certain level while maintaining its strength. Therefore, a diaphragm ring haying small thickness as much as possible while maintaining certain strength to be suitable for the electro-acoustic converters except for the front most one should be manufactured by etching a metal plate. With the diaphragm ring10of the rear unit19having as small thickness as much as possible, the response characteristics of the rear unit19for high frequency domain are prevented from degrading. The diaphragm ring10of the rear unit19has a thickness of about 200 micrometers while a general diaphragm ring has a thickness of about 800 micrometers. In the embodiment, the diaphragm ring10of the rear unit19has a thickness of 200 micrometers and is made by etching a brass plate and gold-plating the resultant object.

For the diaphragm11of the rear unit19, the space between the fixed electrode7and the diaphragm5of the front unit18and the space between the fixed electrode7and the diaphragm11of the rear unit19serve as front acoustic resistors. Accordingly, directionality of the rear unit19is more bidirectional compared with that of the front unit18.FIGS. 5 and 7depict the directionalities of the front and the rear units18and19of the embodiment, respectively. As described above, the directionality of the rear unit19depicted inFIG. 7is more bidirectional compared with that of the front unit18depicted inFIG. 5. The front and the rear units18and19are connected in series and the output signals are synthesized. Thus, resistance of the acoustic resistor14provided at the rear side is appropriately adjusted to provide the synthesized output signal with desired directionality, e.g.; cardioide.

Furthermore, outputs from the units18and19can be independently taken. Thus, directional characteristics such as wide cardioide and cardioide can be obtained by selecting the either of the outputs from units18and19or mixing the outputs. By further adjusting the resistance of the acoustic resister14, directionalities such as cardioide and super cardioide can be obtained.

FIGS. 6 and 8depict the frequency responses of the front and the rear units18and19, respectively.FIG. 9depicts the frequency response of the synthesized output obtained from the front and the rear units18and19connected in series as illustrated in the upper part ofFIG. 4. InFIGS. 6,8, and9, the thickest characteristic line represents the frequency response measured at the front of the microphone unit, i.e., the position that is not offset from the central axis of the microphone unit. The second thickest characteristic line represents the frequency response measured at a side of the microphone unit, i.e., the position offset from the central axis of the microphone unit by 90 degrees. The thin characteristic line represents the frequency response measured at the rear of the microphone unit, i.e., the position offset from the central axis of the microphone unit by 180 degrees. The frequency responses were measured in accordance with EIAJ standard as in the case of the frequency responses of the conventional example depicted inFIG. 13. Through comparison betweenFIGS. 9 and 13, improved sensitivity of the embodiment of the present invention can be confirmed.

Returning toFIG. 4, this structure has one capacitor microphone unit made by incorporating the front and the rear units18and19in a single casing1on the upper part as described above, and has another capacitor microphone unit on the lower side. With the capacitor microphone units in a pair, balanced output can be taken from the system. The upper and the lower capacitor microphone units inFIG. 4have the same physical configuration. Therefore, the elements in the lower capacitor microphone units are given the same reference numerals for those in the upper side counterpart except for that a character “A” is provided behind each of the numerals.

As described above, the units18and19of the upper capacitor microphone unit is connected in series together with their impedance converters. The units of the lower capacitor microphone unit are also connected in series together with their impedance converters but in a reversed manner from the upper counterpart. Specifically, a fixed electrode7A of the front unit is grounded and a diaphragm5A of the front unit is connected to an input terminal of the impedance converter21A of the front unit. An output terminal of the impedance converter21A is connected to a fixed electrode13A of the rear unit and a diaphragm11A of the rear unit is connected to an input terminal of the impedance converter22A of the rear unit.

InFIG. 4, an output terminal of the impedance converter22of the upper capacitor microphone unit serves as a hot-side output terminal for balanced output while an output terminal of the impedance converter22A of the lower capacitor microphone unit serves as a cold-side output terminal for balanced output. The hot-side and the cold-side output terminals are each connected to a microphone cable via a connector and a ground side is connected to a shielding cable of the microphone cable. Thus, a balanced acoustic signal is output. As described above, the example illustrated inFIG. 4includes two capacitor microphone units having the described structures in a pair. The electro-acoustic converter in the respective capacitor microphone units are connected in series and in an electrically opposite manner. Thus, balanced output can be taken from the pair of capacitor microphone units.

To achieve balanced output using the capacitor microphone unit according to the present invention, two capacitor microphone units in a pair as illustrated inFIG. 4are not necessarily required. Instead, the balanced output can be achieved by connecting a transformer to an output circuit as illustrated inFIG. 12.

Although in the illustrated embodiment, two electro-acoustic converters are anteroposteriorly disposed on a single axis, the electro-acoustic converter is required to be provided in a plurality and the number thereof can be three or more. Still, two electro-acoustic converters can provide sufficient effect and is preferable in terms of downsizing.

An innovative capacitor microphone can be obtained by incorporating the above described capacitor microphone units according to the present invention in a microphone casing.

With the present invention providing such an effect, a user-friendly capacitor microphone can be obtained and the application of capacitor microphones can be expanded.