Electret microphones consist of two parts: acoustic electric conversion and impedance conversion. The key component of acoustic electric conversion is the electret vibration film. It is an extremely thin plastic film, with a layer of metal film evaporated on one side. Then, after being polarized by a high-voltage electric field, opposite charges are placed on both sides. The vapor surface of the membrane is facing outward and connected to the metal shell. In a polarizing microphone, there is a field-effect transistor for pre amplification, so a certain bias voltage is required for normal operation of the polarizing microphone, which is generally not greater than 10V.
