An electret microphone is a condenser microphone that replaces a polarized power source with an electret that has been previously charged with charge. There are two types of electret microphones. One is to use the diaphragm film material as the diaphragm (vibration mode). At this time, the diaphragm is responsible for both the acoustic wave receiving and polarization voltages. The other is to use the electret material. The rear plate (back pole type) is made, at which time it only acts as a polarization voltage. Since the microphone does not require a polarization voltage, the structure is simplified. In addition, due to its good electroacoustic characteristics, microphones with additional polarization voltages have been gradually replaced in recording and sound reinforcement and outdoor noise measurement.
The electret microphone has two metal plates, one of which is coated with an electret film (mostly polytetrafluoroethylene) and grounded, and the other plate is connected to the gate of the field effect transistor. A diode is connected between the sources. When the electret film itself has a charge, the surface charge is Q, and the interlayer capacitance is C, the ground voltage U=Q/C is generated on the pole, when subjected to vibration or friction by the airflow. Because the vibration changes the distance between the two plates, that is, the capacitance C changes, and the electric quantity Q does not change, it will cause a voltage change. The magnitude of the voltage change reflects the intensity of the external sound pressure. This voltage change frequency reflects the frequency of the external sound; this is the working principle of the electret microphone.
The diaphragm of the electret microphone is mostly made of perfluoroethylene propylene, which has good humidity performance, generates a large surface charge, and is less affected by humidity. Since the microphone is also a capacitive structure, the internal resistance of the signal is large. In order to extract and amplify the voltage signal generated by the sound, the output terminal must also use a field effect transistor.
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