Dynamic microphone unit and dynamic microphone

A dynamic microphone unit includes: a diaphragm that vibrates in response to received sound waves; a voice coil that is fixed to the diaphragm and vibrates together with the diaphragm; a magnetic circuit that generates a magnetic field in a magnetic gap, the voice coil being disposed in the magnetic gap; a resonator that is disposed adjacent to the obverse of the diaphragm; and a noise canceling coil that is fixed to a surface of the resonator so as to face a position of fixing the voice coil, the surface facing the diaphragm. The noise canceling coil is connected in series with the voice coil and has a winding direction different from that of the voice coil.

TECHNICAL FIELD

The present invention relates to a dynamic microphone unit and a dynamic microphone including a coil for reducing noise.

BACKGROUND ART

In a dynamic microphone, a voice coil fixed to a diaphragm is disposed in a magnetic gap defined by magnetic circuit components. In the dynamic microphone, the voice coil vibrates together with the diaphragm in the magnetic gap in response to received sound waves to generate audio signals in response to the vibration velocity due to electromagnetic conversion between the voice coil and a magnetic field. The voice coil then outputs the audio signals. At this time, if the magnetic circuit is affected by some external factor to vary a magnetic flux in the magnetic gap, the voice coil generates signals independent of sound waves. The signals independent of sound waves are noise.

A factor generating noise in the dynamic microphone is, for example, an alternating magnetic field applied to the magnetic circuit. When the alternating magnetic field is generated from a commercial AC power supply, the resulting noise has a relatively low frequency corresponding to the frequency of the commercial AC power. The noise caused by the commercial AC power supply is called hum noise.

Hum noise flows into a microphone through various paths. For example, hum noise is generated by contact of a finger with a switch knob for the switching operation of a microphone. Japanese Unexamined Patent Application Publication No. 2010-68364 discloses a switch knob connected through a metal wire to a shield cover of a switch body. This configuration eliminates electrostatic coupling between the switch knob and a switch contact point to prevent hum noise from occurring at contact of a finger with the switch knob.

Japanese Unexamined Patent Application Publication No. 2009-200869 discloses a narrow-directivity condenser microphone for reducing, for example, hum noise. In more detail, this configuration includes an acoustic tube in electrical contact with a built-in microphone unit through an electrically conductive intermediate disposed therebetween.

Both the above patents disclose condenser microphones for reducing hum noise. In general, dynamic microphones employ electromagnetic conversion as described above and thus generate larger hum noise than condenser microphones.

Dynamic microphones illustrated inFIGS. 4 and 5are proposed in order to cancel hum noise based on unique configurations utilizing such characteristics of the dynamic microphones. Techniques related thereto will be described below.

FIG. 4illustrates a unit frame101which is a base of a microphone unit. The substantially cylindrical unit frame101composed of magnetic material functions as an outer yoke. A disk yoke102is fixed in the center hole of the unit frame101. A disk magnet103is fixed above the yoke102. A disk pole piece104is fixed above the magnet103. The yoke102, the magnet103, and the pole piece104are bonded to each other. While the unit frame101functions as an outer yoke as described above, the yoke102functions as an inner yoke.

The outer circumferential surface of the yoke102is in close contact with the inner circumferential surface of the unit frame101. A round gap is provided between the outer circumferential surface of the pole piece104and the inner circumferential surface of the unit frame101. A magnetic circuit is defined by the yoke102, the unit frame101, the round gap, and the pole piece104. A magnetic flux from the magnet103returns to the magnet103through the magnetic circuit. In this configuration, the round gap functions as a magnetic gap.

A cylindrical member135is fixed to the upper outer circumference of the unit frame101. The cylindrical member135has an inward flange136at the upper inner circumference. The flange136is fixed to the upper outer circumference of the unit frame101. The flange136has multiple through holes137in the vertical direction. A cylindrical space between the inner circumferential surface of the cylindrical member135and the outer circumferential surface of the unit frame101communicates with a space above the cylindrical member135through the through holes137.

The circumference of a diaphragm105is fixed to the upper end of the cylindrical member135. The diaphragm105is made by shaping a material such as synthetic resin or metal. The diaphragm105includes a center dome and a sub-dome surrounding the center dome. The outer circumference of the sub-dome is fixed to the outer circumference of the cylindrical member135. The diaphragm105can vibrate in response to the sound pressure from received sound waves, in the anteroposterior direction (the vertical direction inFIG. 4) around the outer circumference of the sub-dome as a supporting node.

A voice coil106is fixed along the boundary between the center dome and the sub-dome in the diaphragm105. The voice coil106has an air-cored cylindrical shape formed by winding a thin conductive wire, one end of the cylindrical shape being fixed to the diaphragm105. The voice coil106is disposed in the magnetic gap while the outer circumference of the sub-dome in the diaphragm105is fixed as described above. The sub-dome in the diaphragm105covers the through holes137of the cylindrical member135from above.

Adjacent to the reverse of the diaphragm105(below the diaphragm105inFIG. 4), a protector107is fixed to the top surface of the pole piece104. A constant gap is provided between the domal top surface of the protector107and the center dome of the diaphragm105. The protector107has a center hole171communicating with center holes141,131, and121of the pole piece104, the magnet103, and the yoke102, respectively.

Adjacent to the obverse of the diaphragm105, a resonator108has an outer circumference fixed to the upper outer circumference of the cylindrical member135. A constant gap is provided between the central domal ceiling surface of the resonator108and the center dome of the diaphragm105. The resonator108has a center hole181for introducing external sound waves to the diaphragm105. The resonator108additionally has multiple holes182around the center hole181. A lid110is fit to the lower end of the unit frame101. A lower opening of the unit frame101is closed by the lid110to define a relatively large air chamber111.

The diaphragm105vibrates in the anteroposterior direction in response to a variation in the sound pressure from received sound waves. The voice coil106also vibrates in the anteroposterior direction together with the diaphragm105. The voice coil106vibrates in the magnetic flux passing through the magnetic gap and outputs audio signals in response to the variation in the sound pressure. The dynamic microphone illustrated inFIG. 4electro-acoustically converts the signals as described above to output the audio signals from both ends of the voice coil6to an external device.

The dynamic microphone illustrated inFIG. 4further includes a noise canceling coil151for canceling hum noise flowing thereinto from the exterior. The noise canceling coil151is an air-cored coil. The noise canceling coil151is fixed so as to be wound on the outer circumferential surface of the resonator108fixed to the front end of the cylindrical member135.

Another example dynamic microphone illustrated inFIG. 5includes a noise canceling coil152fixed to the obverse of a resonator108. This dynamic microphone has the same configuration as that of the microphone unit illustrated inFIG. 4except for the arrangement and size of the noise canceling coil152. The noise canceling coil152composed of an air-cored coil has a central axis coaxial with that of the voice coil106. The noise canceling coil152is also provided in order to cancel hum noise flowing from the exterior into the dynamic microphone unit.

The noise canceling coils151and152illustrated inFIGS. 4 and 5are connected in series to the voice coil106and wound in the direction of canceling hum noise in the voice coil106induced by an alternating magnetic field flowing into the microphone.

Next, explanations will be given on undesirable vibratory noise in a dynamic microphone and an example dynamic microphone having a mechanism for eliminating the vibratory noise. In a dynamic microphone, a voice coil has large inertia force due to its large mass and therefore generates vibratory noise in response to mechanical vibration applied from the exterior. In particular, a first-order pressure-gradient microphone employs a mass control for controlling a bidirectional component. A diaphragm is therefore designed so as to have a resonant frequency lower than a main sound acquisition band. As a result, large vibratory noise is generated at the resonant frequency of the diaphragm.

Such vibratory noise can be cancelled by, for example, a dynamic microphone described in Japanese Examined Patent Application No. 61-30800. This patent discloses a vibration detecting device in a microphone case. According to the disclosure, the vibration detecting device detects noise signals outputted from a microphone unit due to mechanical vibration applied from the exterior, converts the detected signal into the opposite phase, and outputs the converted signals. The detected signals with the opposite phase are then added to the output signals of the microphone unit to eliminate the noise signals. The vibration detecting device can be referred to as a vibration pickup. The vibration detecting device includes, for example, a canceling coil in, for example, a microphone unit case and a magnet fixed to the inner surface of a microphone case so as to face the canceling coil, the magnet generating a magnetic field around the canceling coil. The microphone case receives unnecessary external vibration to cause vibration of the microphone unit due to its inertia force relative to the microphone case. This generates electromotive force in the canceling coil. The invention utilizes this electromotive force as signals for canceling the noise signals.

SUMMARY OF INVENTION

Technical Problem

As illustrated in examples ofFIGS. 4 and 5, dynamic microphones can reduce hum noise to a certain level by further including noise canceling coils for canceling hum noise flowing thereinto from the exterior. The hum noise cannot however be reduced sufficiently. This is because hum noise cannot be canceled which corresponds to a phase difference between alternating magnetic fields applied to the voice coil106and the noise canceling coils151and152.

Additionally, the dynamic microphone described in Japanese Examined Patent Application No. 61-30800 needs a magnet and a detecting coil functioning as a vibration pickup in order to eliminate noise caused by mechanical vibration.

It is an object of the present invention to solve the above problem in conventional techniques, i.e., to provide a dynamic microphone unit and a dynamic microphone that can sufficiently reduce noise.

Solution to Problem

A dynamic microphone unit according to the present invention includes:

a diaphragm that vibrates in response to received sound waves;

a voice coil that is fixed to the diaphragm and vibrates together with the diaphragm;

a magnetic circuit that generates a magnetic field in a magnetic gap, the voice coil being disposed in the magnetic gap;

a resonator that is disposed adjacent to the obverse of the diaphragm; and

a noise canceling coil that is fixed to a surface of the resonator so as to face a position of fixing the voice coil, the surface facing the diaphragm.

DESCRIPTION OF EMBODIMENTS

Dynamic microphone units and dynamic microphones according to embodiments of the present invention will now be described with reference toFIGS. 1 to 3.

FIG. 1illustrates a unit frame1serving as a base of a microphone unit. The unit frame1also functions as a part of a magnetic circuit, i.e., an outer yoke. The cylindrical or substantially cylindrical unit frame1is composed of magnetic material. As illustrated inFIG. 1, the unit frame1has a center hole having a smaller diameter in its substantially lower half and a step portion in its middle in the vertical direction.

In the center hole of the unit frame1, a disk inner yoke2is fixed to the step portion. A disk magnet3is fixed above the inner yoke2. A disk pole piece4is fixed above the magnet3. The inner yoke2, the magnet3, and the pole piece4have center holes21,31, and41, respectively, having the same diameter. The unit frame1, the inner yoke2, the magnet3, and the pole piece4are bonded to each other. In this configuration, the outer circumferential surface of the inner yoke2is in close contact with the inner circumferential surface of the unit frame1. In contrast, a round gap is provided between the outer circumferential surface of the pole piece4and the inner circumferential surface of the unit frame1. A magnetic flux from the magnet3returns to the magnet3through a magnetic circuit including the yoke2, the unit frame1also functioning as an outer yoke, the gap, and the pole piece4. In this configuration, the gap functions as a magnetic gap.

A cylindrical member35is fixed to the upper outer circumference of the unit frame1. The cylindrical member35has an inward flange36at the upper inner circumference. The flange36is fixed to the upper outer circumference of the unit frame1, for example, with an adhesive. The cylindrical member35substantially functions as a part of the unit frame1. The inward flange36has multiple through holes37in the vertical direction. A cylindrical space between the inner circumferential surface of the cylindrical member35and the outer circumferential surface of the unit frame1communicates with a space above the cylindrical member35through the through holes37. The through holes37have upper ends covered with an acoustic resistor18.

The circumference of a diaphragm5is fixed to the upper end of the cylindrical member35. The diaphragm5is formed by shaping a thin film material such as synthetic resin or metal. The diaphragm5includes a center dome51and a sub-dome52surrounding the center dome51. The center dome51is a partial spherical shell. The sub-dome52has an arc-shaped cross section. The sub-dome52extends along the circumference of the center dome51and is integrated with the center dome51. The outer circumference of the sub-dome52is fixed to the outer circumference of the cylindrical member35. The diaphragm5has the sub-dome52having an outer circumference fixed to the cylindrical member35as described above and can therefore vibrate in response to the sound pressure from received sound waves, in the anteroposterior direction (the vertical direction inFIG. 1) around the outer circumference of the sub-dome52as a supporting node.

A voice coil6is fixed along the boundary between the center dome51and the sub-dome52in the diaphragm5. The voice coil6is formed by winding a thin conductive wire and by fixing it into a cylindrical shape. One end of the cylindrical voice coil6is fixed to the diaphragm5. The voice coil6is disposed in the magnetic gap while the outer circumference of the sub-dome52in the diaphragm5is fixed as described above. The voice coil6is separated from both the unit frame1and the pole piece4. The sub-dome52of the diaphragm5covers the through holes37of the cylindrical member35and the acoustic resistor18from above.

Adjacent to the reverse of the diaphragm5(below the diaphragm5inFIG. 1), a protector7is fixed to the top surface of the pole piece4. A constant gap is provided between the domal top surface of the protector7and the center dome51of the diaphragm5. The protector7has a center hole71communicating with center holes41,31, and21of the pole piece4, the magnet3, and the yoke2, respectively.

Adjacent to the obverse of the diaphragm5(above the diaphragm5inFIG. 1, i.e., on a side for introducing sound waves), a resonator8functioning also as a protector for the diaphragm5has an outer circumference fixed to the upper outer circumference of the cylindrical member35. The resonator8has a central domal ceiling surface. A constant gap is provided between the resonator8and the center dome51of the diaphragm5. The resonator8has a center hole81for introducing external sound waves to the diaphragm5. The resonator8additionally has multiple holes82around the center hole81. The resonator8has the center hole81and the multiple holes82to thereby define a front acoustic terminal. The resonator8enhances the frequency response in a high sound range due to resonance in a front air chamber defined by the inner surface of the resonator8and the diaphragm5.

A lid10is airtightly fit to a lower opening of the unit frame1. This defines a relatively large air chamber11in the unit frame1. The air chamber11may contain an acoustic resistor.

The dynamic microphone unit is configured as described above. The dynamic microphone unit is implemented into a microphone case to complete the dynamic microphone. The microphone case has a front side covered with, for example, a front mesh. A connector connected to a microphone cable is implemented on the rear side of the microphone case.

The above-described dynamic microphone unit includes a noise canceling coil15. The present embodiment is characterized by a manner for implementing the noise canceling coil15into the dynamic microphone.

InFIG. 1, the noise canceling coil15is an air-cored coil formed by winding a conductive wire into a flattened cylindrical shape. The noise canceling coil15is fixed to a surface of the resonator8so as to face a position of fixing the voice coil6, the surface facing the diaphragm5. The resonator8has a convex surface83on its surface facing the diaphragm5, the convex surface83being formed by a flattened cylindrical surface along a circle concentric with the voice coil6. The inner circumferential surface of the noise canceling coil15is fit along the convex surface83to align and fix the noise canceling coil15.

The noise canceling coil15has the same wound diameter as that of the voice coil6. In this embodiment, a definition of the noise canceling coil15having the same wound diameter as that of the voice coil6includes cases of not only completely the same diameter but also substantially the same diameter. The noise canceling coil15and the voice coil6are overlapped with and away from each other in the central axial direction of the microphone unit. The noise canceling coil15is disposed near the voice coil6so as not to contact with the voice coil6even when the diaphragm5vibrates at the maximum amplitude.

The noise canceling coil15has the same number of turns as that of the voice coil6. In this embodiment, a definition of the noise canceling coil15having the same number of turns as that of the voice coil6includes cases of not only completely the same number of turns but also substantially the same number of turns. In order to mutually cancel noise signals generated in the noise canceling coil15and the voice coil6by an alternating magnetic field flowing into the microphone unit, the noise canceling coil15and the voice coil6are configured as described below. The noise canceling coil15is connected in series with the voice coil6. The noise canceling coil15has a winding direction opposite to that of the voice coil6.

An alternating magnetic field flowing into the microphone unit affects the voice coil6and the noise canceling coil15under substantially the same condition to generate substantially the same noise signals in the coils6and15. According to this configuration, noise signals generated in the voice coil6and the noise canceling coil15are mutually cancelled to output audio signals having extremely low hum noise, one type of noise signal, from the voice coil6.

A dynamic microphone unit according to a second embodiment of the present invention will now be described. This embodiment can cancel not only hum noise but also noise caused by external mechanical vibration. The description of the second embodiment will focus on different points from the first embodiment.

InFIG. 2, the inner circumferential surface of the noise canceling coil15is fit along the outer circumferential surface of the convex surface83defined by the cylindrical surface of the resonator8. This configuration is the same as that of the first embodiment. The resonator8has a thin plate connecting portion85between its outer circumference and the convex surface83. According to this configuration, the connecting portion85can have resilience so as to vibrate due to vibration applied from the exterior. The noise canceling coil15is attached at a position similar to the first embodiment, i.e., at a position so as to face the voice coil6across the diaphragm5. The protector7and the resonator8are composed of magnetic material. A leakage magnetic flux from a magnetic circuit including the unit frame1, the inner yoke2, the magnet3, and the pole piece4passes through the protector7, the resonator8, and then the noise canceling coil15.

Mechanical vibration applied to the microphone unit other than acoustic aerial vibration causes a central portion of the resonator8and the noise canceling coil15to vibrate due to resilience of the connecting portion85in the resonator8. A resilient coefficient and other design conditions of the connecting portion85are determined so as to equalize a resonant frequency of the noise canceling coil15due to mechanical external vibration to a resonant frequency of the diaphragm5. This enables the noise canceling coil15to vibrate in the same phase with the diaphragm5due to the external vibration.

The noise canceling coil15vibrates in the above leakage magnetic flux and thereby generates electricity, i.e., cancel signals in response to the external vibration. As a result, the noise canceling coil15functions as a vibration pickup without a dedicated magnetic flux source for picking up vibration. The diaphragm5vibrates in response to external vibration to generate vibratory noise from the voice coil6. The noise canceling coil15and the voice coil6are electrically connected so as to cancel the vibratory noise with cancel signals outputted from the noise canceling coil15. The noise canceling coil15cancels hum noise similar to the first embodiment. A configuration other than the above is the same as the first embodiment. Descriptions thereon will therefore be omitted.

According to the second embodiment illustrated inFIG. 2as described above, the noise canceling coil15cancels not only hum noise but also noise due to external vibration. In other words, the noise canceling coil15substantially constitutes a vibration pickup. As described in Japanese Examined Patent Application No. 61-30800, a vibration pickup inevitably includes components at a microphone case and a microphone unit, for example, a magnet at the microphone case and a coil at the microphone unit. In contrast, the second embodiment of the present invention employs the magnet3originally included in the dynamic microphone, as a magnetic flux source necessary for a complete function for the noise canceling coil15to cancel vibratory noise. As a result, the second embodiment of the present invention can advantageously not only simplify the configuration for detecting vibration but also provide the vibration pickup within the microphone unit.

Reductions in vibratory noise in the dynamic microphone according to the second embodiment will be described with reference toFIG. 3. InFIG. 3, a horizontal axis represents a frequency [Hz], and a vertical axis represents an output level [dB re 1V] from the voice coil due to mechanical vibration. Graphs were obtained when mechanical vibration was applied from the exterior. A thick solid line “a” indicates an output level after canceling of vibratory noise with output signals from the noise canceling coil15. A thin solid line “b” indicates an output level before canceling of vibratory noise. A dotted line “c” indicates signals outputted from the noise canceling coil15as a vibration pickup due to the mechanical vibration.

As is apparent from comparison between the lines “a” and “b” in the graph, the second embodiment of the present invention reduces vibratory noise by approximately 11 db at a maximum. InFIG. 3, a symbol E indicates a frequency range with a reduction in vibratory noise equal to or more than 3 dB. As is apparent fromFIG. 3, the second embodiment extremely reduces vibratory noise.

The dynamic microphone unit according to each embodiment described above can be contained in the microphone case having a microphone connector and other necessary components implemented therein to complete the dynamic microphone.

Any modification can be applied to the dynamic microphone unit and the dynamic microphone according to the present invention without departing from the scope and spirit described in the accompanying claims. For example, the basic configuration of the dynamic microphone unit is not limited to the configurations illustrated inFIGS. 1 and 2. A basic configuration of a conventional dynamic microphone unit may be employed to add thereto the noise canceling coil in the unique configuration according to the present invention.

Advantageous Effects of Invention

In the dynamic microphone described above, an alternating magnetic field flowing into the microphone unit affects the voice coil and the noise canceling coil under substantially the same condition to generate substantially the same noise signals. Additionally, in the dynamic microphone described above, noise signals generated in the voice coil and the noise canceling coil are mutually cancelled to output audio signals having extremely low noise from the voice coil.