The structure of an electret condenser microphone (capsule) is shown in the diagram below. From top to bottom, it includes: PCB, FET, gate conductive ring, gate insulating ring, electret diaphragm, insulating sheet, diaphragm, and the outer shell, making a total of 8 parts. The functions of each part are described as follows:
Electret Condenser Microphone (Capsule) Structure Diagram
1. PCB and FET Assembly
On the surface of the PCB, components such as FET, capacitors, and resistors are mounted. The main role of the FET is to convert the impedance matching, provide appropriate DC bias with the resistors, output AC signals, filter, and shield against interference. It also helps to fix the components and prevent foreign objects from affecting them.
2. Gate Conductive Ring
The gate conductive ring connects the electret diaphragm to the gate (G) of the FET. It transmits the weak electrical signal converted from the small vibrations detected by the diaphragm to the gate and also provides mechanical support.
3. Gate Insulating Ring
The gate insulating ring fixes the electret diaphragm and the gate conductive ring, preventing them from short-circuiting with the outer shell (the source (S) and gate (G) of the FET).
4. Electret Diaphragm
The electret diaphragm is coated with a layer of Teflon plastic film, which stores electrostatic charges. It forms one of the electrodes of the capacitor with the diaphragm, and the non-film side is connected to the gate (G) of the FET through the gate conductive ring.
5. Insulating Sheet
The insulating sheet supports the distance between the two electret diaphragms, leaving a gap to allow diaphragm movement, which alters the capacitance.
6. Diaphragm
The diaphragm is a stretched PET/PPS plastic film adhered to a metal ring. The side of the film that contacts the metal ring is coated with a thin metal layer, forming a variable capacitor with the electret diaphragm. It is also a movable diaphragm.
7. Outer Shell
The outer shell serves as the supporting part of the entire microphone. All other components are encapsulated inside, and the outer shell acts as the ground for the microphone. It also provides electromagnetic and optical shielding, as well as dust and foreign object protection.
Electret Material
Electret materials are insulating materials that act as permanent magnets. At high temperatures, they undergo strong electric field treatment, such as ionizing air under DC negative high voltage (-10Kv to -15Kv). The negative ions that are generated are permanently stored in the Teflon material under the influence of the high voltage electric field. The Teflon material with stored charges is a very excellent industrial electret material. A thin layer of Teflon is applied to the back electret diaphragm of the condenser microphone capsule, and after polarization, it permanently stores a specific level of static charge. This eliminates the need for an external DC biasing polarization voltage for the microphone, making this type of microphone capsule known as an electret condenser microphone capsule.
