An electro-acoustic transducer includes a needle electrode, an opposite electrode, a discharge region between the needle electrode and the opposite electrode, a high-frequency oscillating circuit in the discharge region causing a high-frequency discharge and modulating and extracting an audio signal in accordance with a sound wave introduced to the discharge region, or converting the discharge in the discharge region into a sound wave, the discharge being performed in accordance with a high-frequency signal modulated by an audio signal, and an inert gas supply channel that supplies inert gas toward the peripheral surface of the needle electrode. The electro-acoustic transducer includes a needle-electrode cover as a part of the inert gas supply channel. The needle-electrode cover extends beyond the tip of the needle electrode toward the opposite electrode and has a gas flow outlet disposed beyond the tip of the needle electrode toward the opposite electrode.

TECHNICAL FIELD

The present invention relates to an electro-acoustic transducer that eliminates the use of a diaphragm by performing electro-acoustic transduction that involves high-frequency discharge. More specifically, the present invention can effectively prevent wear of the tip of a needle electrode and reduce wind noise.

BACKGROUND ART

A typical electro-acoustic transducer such as a microphone or speaker includes a diaphragm. A typical microphone receives vibrations of a diaphragm in response to sound waves as electromagnetic changes, changes in capacitance, or optical changes, which are then converted into electrical signals. A typical speaker electromagnetically converts audio signals into vibrations of a diaphragm to output sound waves.

The diaphragms in these electro-acoustic transducers are used for mutual conversion between aerial vibrations and electrical signals. In other words, the electro-acoustic transducer has a diaphragm that connects audio, mechanical vibration, and electrical circuit systems. Such a diaphragm for the mutual conversion in any type of conventional and typical electro-acoustic transducers may cause poor frequency responses.

Thus, the inertial force due to the mass of the diaphragm, even of an extremely minimized one, in the conventional electro-acoustic transducer may result in a poor frequency-based sound collection.

An example electro-acoustic transducer having no diaphragm and detecting a particle speed by utilizing electric discharge to perform electro-acoustic transduction is disclosed in Japanese Patent Application Laid-Open No. 55-140400 (Patent Literature 1). The transducer described in Patent Literature 1 includes a needle discharge electrode and an opposite electrode surrounding the discharge electrode, the opposite electrode being apart from the discharge electrode. The opposite electrode is composed of a spherical conductive material having punched pores to propagate sound waves therethrough. The discharge electrode extends towards the center of the spherical opposite electrode. A high-frequency voltage signal is applied from a high-frequency voltage generating circuit to the discharge electrode, the high-frequency voltage signal being modulated with a low-frequency signal which is to be converted into sound waves. The high-frequency voltage signal causes a corona discharge between the discharge electrode and the opposite electrode, which emits the low-frequency signal, i.e., a sound wave.

The transducer described in Patent Literature 1 is an ion speaker, which converts an electrical audio signal into a sound wave using electrical discharge. The ion speaker described in Patent Literature 1 cannot be used as a microphone, and such a use is not suggested.

The present inventor has invented a microphone that includes a needle electrode; an opposite electrode facing the needle electrode; a discharge region defined between the needle electrode and the opposite electrode; a high-frequency oscillating circuit that includes the discharge region, the high-frequency oscillating circuit causing high-frequency discharge in the discharge region; a sound wave introducer that introduces a sound wave to the discharge region; and a modulated signal extractor that extracts a signal modulated in accordance with the sound wave which is generated in the high-frequency oscillating circuit and introduced to the discharge region.

The electro-acoustic transducer or microphone, which involves high-frequency discharge, generates a non-uniform electric field between the needle electrode and the plate electrode and applies a high-frequency high voltage to the non-uniform electric field to form plasma. The plasma is generated in the vicinity of the needle electrode in a high electric field and extends towards the plate electrode. The needle electrode in contact with the plasma is in a high temperature, which wears the tip of the needle electrode of the electro-acoustic transducer and leaves corona products at or near the tip. This results in deterioration of discharging characteristics, for example, a plasma length or shape. The deteriorated discharging characteristics cause an abnormal discharge, which results in a defect, such as an abnormal sound wave from the discharging region.

The present inventor has presumed that the substance in the air causes corona products at or near the tip of the needle electrode, and filed a patent application for an electro-acoustic transducer involving high-frequency discharge, the electro-acoustic transducer including a channel that supplies inert gas toward the perimeter of the needle electrode to prevent wear of the tip of the needle electrode and adhesion of corona products, and abnormal discharge during a continuous high-frequency discharge (See Japanese Patent Application Laid-Open No. 2011-188037).

An example configuration of the electro-acoustic transducer described in Patent Literature 2 is shown inFIG. 2.FIG. 2illustrates a microphone unit20including a needle electrode23and an opposite electrode24to cause discharge therebetween. The needle electrode23has a cylindrical base which is covered by an insulating cylinder26. The insulating cylinder26is further fit to an insulating cylinder25. The insulating cylinder25is fit to a base21while penetrating therethrough in the width direction thereof. In other words, the base of the needle electrode23is fixed to the base21through the insulating cylinders25,26, while penetrating through the base21in the width direction thereof.

The base21corresponds to the bottom of a cylindrical case22. The case22includes a flange at the perimeter of the open end. The tip of the needle electrode23is formed into a cone shape with a sharp end. The tip of the needle electrode23extends substantially coaxially with the case22and is located within the space defined by the case22. The needle electrode23may be composed of tungsten having a tip curvature of 50 μm.

An opposite electrode24covers the open end of the case22, the open end being opposite to the base21, which corresponds to the bottom of the case22. The opposite electrode24is a plain electrode. The opposite electrode24is composed of, for example, a punched metal sheet having numerous pores or a conductive wire net so as to propagate sound waves therethrough. The opposite electrode24is covered with an insulating material. The opposite electrode24may be composed of, for example, a stainless steel sheet having a lot of pores to propagate sound waves therethrough, the stainless steel sheet being covered with ceramic (silica) having a thickness of 0.1 mm.

The opposite electrode24faces the tip of the needle electrode23at a certain distance therebetween, and the opposite electrode24and the needle electrode23define a discharging region. The discharging region, in which high-frequency discharge is generated, is included in a high-frequency oscillating circuit. The discharge or the torch discharge generates a torch flame27. The opposite electrode24defines a sound wave introducing region that introduces sound waves into the discharging region in the case22as described above. The peripheral wall of the case22may have pores to introduce sound waves to the discharging region in the case22.

To cause a high-frequency discharge between the needle electrode23and the opposite electrode24, a high-frequency high voltage needs to be applied thereto. The high-frequency oscillating circuit thus has a vacuum tube as an active oscillating element, the vacuum tube being capable of withstanding a high voltage. The high-frequency oscillating circuit is configured such that the discharged current running through an electrical discharge channel between the needle electrode23and the opposite electrode24returns to the circuit. In other words, a high-frequency self-oscillating circuit is formed.

The needle electrode23and the opposite electrode24define a high-frequency discharge region. The particle speed in the high-frequency discharge region depends on the particle speed of a sound wave, and thereby equivalent impedance changes. The equivalent impedance in the high-frequency discharge region depending on a sound wave allows a signal from the high-frequency oscillating circuit to be modulated by the sound wave. The modulated signal includes frequency modulated (FM) components and amplitude modulated (AM) components, the number of the FM components being larger than that of the AM components. The FM signal is extracted and input to a frequency demodulating circuit to be converted into an audio signal in response to the sound wave from the sound wave introducing region.

The needle electrode23is surrounded by an inert gas guide channel251up to the vicinity of the tip thereof. The inert gas guide channel251is provided to carry the inert gas along the peripheral surface of the needle electrode in the electro-acoustic transducer. The inert gas guide251is composed of a cylindrical insulating material, the251being integrated with the insulating cylinder25that holds the peripheral surface of the insulating cylinder26, the insulating cylinder26supporting the needle electrode23. The inert gas guide251surrounds the needle electrode23with a space therebetween, and includes a gas flow outlet252through which the inert gas flows along the peripheral surface of the tip of the needle electrode23. The tip of the needle electrode23extends through the gas flow outlet252, which achieves the discharge between the needle electrode23and the opposite electrode24.

The inert gas guide channel251is in communication with a pipe42. The pipe42is fixed to the cylindrical case22while extending therethrough in the radial direction. A first end of the pipe42is fit to the inert gas guide251. This configuration allows the internal space of the pipe42to be in communication with the inert gas guide channel251.

A second end of the pipe42resides outside the case22and is connected through a coupling41to a pipe40. In addition to the above-mentioned inert gas guide channel251, the pipe40, coupling41, and pipe42define an inert gas supply channel from a gas cylinder (not shown) to the needle electrode23of the microphone unit20or the electro-acoustic transducer. Example inert gases used include helium or nitrogen gas.

The technique in Patent Literature 2 causes a high-frequency discharge between the needle electrode23and the opposite electrode24to achieve electro-acoustic transduction, while inert gas is supplied from the inert gas supply channel. The inert gas introduced through a supply channel including the pipe40, the coupling41, and the pipe42into the inert gas guide channel251flows through the inert gas guide channel251to the gas flow outlet252. The inert gas focused through the gas flow outlet252flows along the perimeter of the tip of the needle electrode23, which prevents the perimeter of the needle electrode23from contacting with air.

The technique in Patent Literature 2 can prevent corona products resulting from substance in air from adhering to the needle electrode23. The technique in Patent Literature 2 can prevent an abnormal discharge caused by the corona product adhered to the needle electrode23, thereby obtaining an electro-acoustic transducer involving a stable high-frequency discharge. The technique in Patent Literature 2 can also prevent wear of the tip of the needle electrode due to an air flow, which has not been achieved by the conventional technique.

SUMMARY OF INVENTION

Technical Problem

The electro-acoustic transducer involving high-frequency discharge, which is shown inFIG. 2, supplies inert gas from the inert gas supply channel to the peripheral surface of the needle electrode during the discharge between the needle electrode23and the opposite electrode24. Such an electro-acoustic transducer can prevent the surface of the needle electrode23from contacting with air and corona products resulting from substance in air from adhering to the needle electrode. Thus, the electro-acoustic transducer shown inFIG. 2can prevent an abnormal discharge caused by the adhesion of the corona products.

The technical development for enhancement of the performance of the above-mentioned electro-acoustic transducer involving high-frequency discharge suggests the need of a further improvement in the performance characteristics of the electro-acoustic transducer. One of the performance characteristics to be improved is sensitivity. One countermeasure to enhance the sensitivity of the electro-acoustic transducer involving high-frequency discharge is an increased power of discharge.

Unfortunately, the increased power of discharge in the electro-acoustic transducer involving high-frequency discharge readily causes a spark discharge. To prevent such a spark discharge, the flow rate of the inert gas may be increased; however, the increased flow of the inert gas blows the plasma, which causes noise similar to the wind noise that is commonly observed in microphones, thereby deteriorating a signal-to-noise ratio.

Accordingly, an object of the present invention is to solve the problem of the conventional electro-acoustic transducer involving high-frequency discharge, that is, to provide an electro-acoustic transducer involving high-frequency discharge that can enhance the sensitivity while preventing wind noise.

Solution to Problem

An electro-acoustic transducer according to the present invention includes a needle electrode, an opposite electrode facing the needle electrode, a discharging region between the needle electrode and the opposite electrode, a high-frequency oscillating circuit that includes the discharging region, the high-frequency oscillating circuit causing a high-frequency discharge in the discharging region and modulating and extracting an audio signal in accordance with a sound wave introduced to the discharging region, or converting discharge in the discharging region into a sound wave, the discharge being performed in accordance with a high-frequency signal modulated by an audio signal, and an inert gas supply channel that supplies inert gas toward the peripheral surface of the needle electrode, wherein the inert gas supply channel includes a needle-electrode cover that covers the peripheral surface of the needle electrode, and the needle-electrode cover extends beyond the tip of the needle electrode toward the opposite electrode and has a gas flow outlet disposed beyond the tip of the needle electrode toward the opposite electrode.

Advantageous Effect of Invention

The electro-acoustic transducer according to the present invention can effectively prevent a spark discharge even at a low flow rate of inert gas and with an increased power of discharge, and thus can reduce wind noise while enhancing the sensitivity and improving a signal-to-noise ratio.

DESCRIPTION OF EMBODIMENTS

Embodiment

With reference toFIG. 1, an embodiment of the electro-acoustic transducer in accordance with the present invention will now be described. Many components common between the electro-acoustic transducer shown inFIG. 1and the conventional electro-acoustic transducer shown inFIG. 2are denoted by the same reference numerals.

InFIG. 1, a microphone unit20includes a needle electrode23and an opposite electrode24to cause discharge therebetween. The needle electrode23has a cylindrical base. The peripheral surface of the base is covered with an insulating cylinder26. The insulating cylinder26is further fit to an insulating cylinder25. The insulating cylinder25is fixed to a base21while extending therethrough in the thick direction. In other words, the base of the needle electrode23is fixed to the base21via the insulating cylinders25,26while extending through the base21in the thickness direction. A cylindrical case22extends from the outer perimeter of the base21while being integrated therewith. The base21corresponds to the bottom of the cylindrical case22. The needle electrode23is located substantially coaxially with the case22and within the space defined by the case22. The tip of the needle electrode23is formed in a cone shape and extends into the case22. The tip of the needle electrode23has a sharp end. The needle electrode23may be composed of tungsten having a tip curvature of 50 μm.

An opposite electrode24covers the open end of the case22, the open end being opposite to the base21. The opposite electrode24is a plain electrode. The opposite electrode24is composed of, for example, punched metal sheet having numerous pores or a conductive wire net so as to propagate sound waves therethrough. The opposite electrode24is covered with an insulating material. The opposite electrode24may be composed of, for example, a stainless steel sheet having a lot of pores to propagate sound waves therethrough, the stainless steel sheet being covered with ceramic (silica) having a thickness of 0.1 mm.

The opposite electrode24faces the tip of the needle electrode23at a certain distance therebetween, and the opposite electrode24and the needle electrode23define a discharging region. The discharging region, which is included in a high-frequency oscillating circuit, causes high-frequency discharge. The discharge, i.e. the torch discharge, generates a torch flame27in the discharging region between the needle electrode23and the opposite electrode24. The opposite electrode24defines a sound wave introducing region that introduces sound waves into the discharging region in the case22as described above. The peripheral wall of the case22may have pores to introduce sound waves into the discharging region in the case22.

To cause high-frequency discharge between the needle electrode23and the opposite electrode24, a high-frequency high voltage needs to be applied thereto. The high-frequency oscillating circuit thus has a vacuum tube as an active oscillating element, the vacuum tube being capable of withstanding a high voltage. The high-frequency oscillating circuit is configured such that the discharged current through an electrical discharge channel between the needle electrode23and the opposite electrode24returns to the circuit. In other words, a high-frequency self-oscillating circuit is formed.

The needle electrode23and the opposite electrode24define a high-frequency discharge region. The particle speed in the high-frequency discharge region depends on the particle speed of a sound wave, and thereby equivalent impedance changes. The equivalent impedance in the high-frequency discharge region depending on a sound wave allows a signal from the high-frequency oscillating circuit to be modulated by the sound wave. The modulated signal includes frequency modulated (FM) components and amplitude modulated (AM) components, the number of the FM components being larger than that of the AM components. The FM signal is extracted and input to a frequency demodulating circuit to be converted into an audio signal in response to the sound wave from the sound wave introducing region.

The frequency demodulating circuit functions as a modulating circuit if the electro-acoustic transducer in accordance with the present invention is a microphone. The microphone converts an audio signal into a sound wave by causing discharge in the discharge region in accordance with a high-frequency signal, the high-frequency signal being modulated by an audio signal. Thus, the modulating circuit performs conversion between a modulated signal (generated signal) and a sound wave.

The top of the insulating cylinder25resides inside the case22and is fit to the base of a needle-electrode cover50. The needle-electrode cover50, which extends beyond the tip of the needle electrode23toward the opposite electrode24, has a gas flow outlet51disposed between the opposite electrode24and the tip of the needle electrode23. Since the needle electrode23has a tapered tip, the gas flow outlet51has a small radius. The needle-electrode cover50is a cylindrical member, the internal space thereof defining an inert gas guide channel52. Thus, the entire needle electrode23is surrounded by the inert gas guide channel52in the length direction.

The inert gas guide channel52is provided in the electro-acoustic transducer to carry inert gas along the peripheral surface of the needle electrode23. The insulating cylinder25is integrated with the needle-electrode cover50, the insulating cylinder25holding the peripheral surface of the insulating cylinder26to supports the needle electrode23. The inert gas guide channel52extends along the needle electrode23to surround the needle electrode23. A gas flow outlet51is disposed at the top of the needle-electrode cover50. The inert gas along the perimeter surface of the tip portion of the needle electrode23flows through the gas flow outlet51. The gas flow outlet51allows the needle electrode23to achieve discharge with the opposite electrode24.

The inert gas guide channel52is in communication with a pipe42. The pipe42is fixed to the cylindrical case22while extending through the cylindrical case22in the radial direction. A first end of the pipe42is fit to the insulating cylinder25. This configuration allows the internal space of the pipe42to be in communication with the inert gas guide channel52. A second end of the pipe42resides outside the case22is connected through a coupling41to a pipe40. The pipe40, coupling41and pipe42define an inert gas supply channel, which extends from a gas cylinder (not shown) to the microphone unit20or electro-acoustic transducer. In addition to the inert gas guide channel52, the pipe40, coupling41, and pipe42define the inert gas supply channel Example inert gases used include helium or nitrogen gas.

The electro-acoustic transducer in accordance with the embodiment shown inFIG. 1causes a high-frequency discharge between the needle electrode23and the opposite electrode24to achieve electro-acoustic transduction, while inert gas is supplied from the inert gas supply channel. The inert gas is supplied through the supply channel including the pipe40, the coupling41, and the pipe42into the inert gas guide channel52and then flows through the inert gas guide channel52defined by the needle-electrode cover50toward a gas flow outlet51. The inert gas focused through the gas flow outlet51flows along the perimeter of the tip portion of the needle electrode23, which prevents the perimeter of the tip of the needle electrode23from contacting with air.

The electro-acoustic transducer in accordance with the embodiment shown inFIG. 1can prevent corona products resulting from substance in air from adhering to the needle electrode23and inhibits an abnormal discharge caused by the adhesion of the corona products, thereby providing a stable high-frequency discharge. The electro-acoustic transducer in accordance with the embodiment shown inFIG. 1can also prevent wear of the tip portion of the needle electrode due to an air flow, which has not been achieved by the conventional technique.

During the operation of the exemplary electro-acoustic transducer shown inFIG. 1, the needle electrode23covered with the needle-electrode cover50is surrounded by the inert gas, thus does not contact with air. The electro-acoustic transducer in accordance with the embodiment shown inFIG. 1can prevent the tip of the needle electrode23from defects, such as wear caused by discharge and adhesion of corona products. The electro-acoustic transducer in accordance with the embodiment shown inFIG. 1thus can prevent an abnormal discharge caused by the corona products.

The needle-electrode cover50, which defines the inert gas guide channel, extends beyond the tip of the needle electrode23toward the opposite electrode24and has the gas flow outlet51disposed between the opposite electrode24and the tip of the needle electrode23. The exemplary electro-acoustic transducer having such a structure shown inFIG. 1can effectively prevent a spark discharge even at a low flow rate of inert gas.

Thus, the electro-acoustic transducer in accordance with the embodiment shown inFIG. 1can increase power of discharge to enhance the sensitivity while preventing a spark discharge by regulating an inert gas at a low flow rate, thereby reducing wind noise due to the inert gas flow to improve a signal-to-noise ratio.

The embodiment described above is a microphone including the electro-acoustic transducer of the present invention. The electro-acoustic transducer of the present invention can also be applied to an ionic loudspeaker. The electro-acoustic transducer in accordance with the present invention modulates a high-frequency signal, which is generated in the high-frequency oscillating circuit, into an audio signal in a modulating circuit, and causes discharge between the needle electrode23and the opposite electrode24using the modulated signal.

Thus, the electro-acoustic transducer in accordance with the present invention can be used as a microphone and a speaker.