Shielding housing for a condenser microphone
In a condenser microphone having a microphone case formed by a casting
, the resistivity of the surface of the microphone case is decreased surely, and the shielding function of the microphone case is made stable.
In a condenser microphone in which a substrate including an audio output circuit connected to a condenser microphone unit is housed therein, and a cylindrical microphone case 10 mounted with an output connector is provided on one end side, the microphone case 10 being formed by casting, a conductive layer 10b having a lower resistivity than that of a raw material of the microphone case 10 is formed integrally on a casting surface 10a on the outer surface side of the microphone case.
A condenser microphone includes a microphone unit in which a diaphragm and a backplate are arranged so as to be opposed to each other. The microphone unit incorporates an impedance converter because of its very high impedance. As the impedanceconverter, a field effect transistor (FET) is usually used, and on rare occasions, a vacuum tube is used.
An electronic circuit for audio output of the condenser microphone is housed in a metallic microphone case in a state of being mounted on a substrate. Usually, at one end of the microphone case, a three-pin type output connector specified inEIAJ RC5236 (Audio latch lock round type connector) is mounted. The output connector is connected to a phantom power source via an output cable (balanced shielded cable).
If strong electromagnetic waves radiated from a cellular phone or the like are applied to the microphone or the output cable, the electromagnetic waves pass through the output cable and intrude into the microphone via the output connector. Inthe microphone, the electromagnetic waves are sometimes demodulated by the impedance converter and delivered from the microphone as noise having an audio frequency.
To prevent this phenomenon, No. 1 pin for grounding of the three pins that the output connector has is connected to the microphone case, and an outer ring of a male plug on the output cable side, which connects with a shield coating, is broughtinto contact with the inner surface of the microphone case to provide electrical connection, by which a shielding function is given to the microphone case.
A high frequency current flowing in the microphone case due to the electromagnetic waves flows on the surface only due to the skin effect. Usually, the microphone case is manufactured by casting (die casting) of zinc, aluminum, etc. from theviewpoint of workability of microphone case and cost constraints, and the resistivity of casting surface is increased by an oxide film as compared with the base material, so that a sufficient shielding effect cannot be achieved.
The resistivity of the surface can be decreased to some extent by removing the oxide film on the casting surface by cutting. However, usually, since a casting has blowholes therein due to a gas at the time of casting operation, even if thesurface is cut, there is no guarantee that the resistivity of the surface necessarily decrease. Also, in the case where painting is performed, the resistivity of the surface is increased by the surface treatment.
Accordingly, an object of the present invention is to provide a condenser microphone having a microphone case formed by a casting, the microphone case having a surely decreased surface resistivity and hence having a stable shielding function.
In the case where the microphone case is formed by a casing of zinc or aluminum, the conductive layer is preferably formed of silver or copper, which has a lower resistivity than that of the microphone case. Also, as a preferable forming methodfor the conductive layer, a plating technique or an ion plating technique can be cited.
According to the above-described configuration, since the conductive layer having a low resistivity is formed integrally on the casting surface on the outer surface side of the microphone case, a high frequency current due to electromagneticwaves flows easily on the outer surface of the microphone case due to the skin effect. As a result, the shielding function of the microphone case becomes stable, and noise can be effectively prevented from being generated by the intrusion ofelectromagnetic waves into the microphone. Also, the surface strength of the microphone case can be increased.
To achieve the above object, the present invention provides a condenser microphone in which a substrate including an audio output circuit connected to a condenser microphone unit is housed therein, and a cylindrical microphone case mounted withan output connector is provided on one end side, the microphone case being formed by casting, wherein a conductive layer having a lower resistivity than that of a raw material of the microphone case is formed integrally on a casting surface on the outersurface side of the microphone case.
Shielding housing for a condenser microphone
By: Saphia
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