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Structure and working principle of microphone


Structure and working principle of common microphone


1. Moving coil microphone


Moving coil microphone is also called electrodynamic microphone, which is similar to electrodynamic loudspeaker in structure and is composed of magnet, sound coil and sound film, as shown in figure 12-11. The voice coil of a moving coil microphone is located in the magnetic field of a magnet. When sound waves act on the sound membrane to make it vibrate, the sound membrane drives the voice coil to vibrate correspondingly, so that the voice coil cuts the magnetic field lines and generates induced voltage, thus completing the acousto-electric conversion. The impedance of the voice coil is very low. The function of the impedance matching transformer is to change the impedance of the microphone to match the input impedance of the amplifier. Moving coil microphone output impedance of the high resistance and low resistance, high impedance output impedance is 1000-2000 Ω commonly, low impedance Ω output impedance is 200-600. The frequency response of the moving coil microphone is generally 200 5000Hz, and the high quality microphone can reach 30-18000hz. Moving coil microphone has the characteristics of durability, stability, one-way directivity, low price, suitable for language, music amplification and recording.


2. Capacitor microphone


Capacitive microphone is a microphone that USES the change of capacitance to cause the acoustic and electric conversion. Its structure is shown in figure 12-12. It is a variable capacitor with a small spacing composed of a vibrating diaphragm and a fixed electrode. When the diaphragm vibrates under the action of sound wave, the distance between the vibrating diaphragm and the fixed electrode changes, which leads to the change of capacitance. If there is a load resistance R and a dc polarization voltage E at both ends of the capacitor, then when the capacitance varies with the sound wave, alternating audio voltage will be generated at both ends of R. The output impedance of capacitive microphone is capacitive, because the capacitance is small, but the capacitive reactance is large at low frequency. To ensure low frequency sensitivity, an impedance converter whose input impedance is greater than or equal to the output impedance of the microphone should be connected to it. This impedance converter usually USES a field effect tube. Capacitive microphone has high sensitivity, large output power, simple structure and good sound quality, but it is not very convenient to use the power source, so it is mostly used in theater and language with higher requirements and music broadcasting occasions.




3. Electret microphone


Electret microphone is composed of two parts: acousto-electric conversion and impedance conversion, as shown in FIG. 12-13. The key element of the acousto-electric conversion part is the electret vibration membrane, which is a very static plastic membrane, on which a layer of pure gold film is evaporated, and then after the electret is conducted by the high-voltage electric field, the two sides are respectively occupied with opposite charges. The evaporation of the membrane is externally connected with the metal shell, and the other side of the membrane is separated by a thin insulating washer. Thus, a capacitor is formed between the evaporation of the membrane and the metal plate. The impedance conversion part is performed by the field effect tube, whose main function is to transform the impedance of tens of megohm into the impedance matching with the amplifier. The G electrode of the fet is connected to the metal plate, and the D and S electrodes are connected to the external circuit. The output form is shown in figure 12-14. Figure 12-14 (a) as the source output connection, the connection of the output impedance is less than 2 k Ω, circuit is stable, wide dynamic range, but the output signal is weak. FIG. 12-14 (b) shows the drain output connection method, which has a higher gain but a smaller dynamic range than the drain output connection method.



The working principle of electret microphone is that when sound waves vibrate the electret diaphragm, the electric field of the capacitor formed between the electroless metal film and the metal plate changes correspondingly, generating audio frequency signals that change with sound waves, which are output through the field effect tube. Electret microphone has the advantages of small size, simple structure, good electro-acoustic performance, low price and so on.

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