Electret Condenser Microphone, as the name suggests is a parallel plate capacitor and works on the principle of a variable capacitance. It consists of two plates, one fixed (called the back plate) and the other moveable (called Diaphragm) with a small gap between them. An electric potential charges the plate. When sound strikes the diaphragm it starts moving, thereby changing the capacitance between the plates which in turn results in a variable electric current to flow.
These microphones are widely used in electronic circuits to detect minor sounds or air vibrations which in turn are converted to electrical signals for further use. The two legs as shown in the image above are used to make electrical connection with the circuit.
The two plates of the capacitor, which is the heart of the condenser microphone is shown in the image above. The diaphragm (inside the metal body) is a very thin film of conducting metal. It is deformable and elastic in nature. The diaphragm is pasted on a solid hollow ring which provides the necessary support to the diaphragm. The back plate is a thick solid perforated metallic plate. It is coated with a permanently charged electret material to eliminate the need of polarized power supply. These electrets are made by melting a dielectric material (like wax, plastic, etc) that contains polar molecules and solidifying them back in a strong electrostatic field. The polar molecules of the dielectric material are inclined towards the direction of electrostatic field and create a permanent electrostatic bias in them. In the modern microphones, PTFE (Poly Tetra Fluoro Ethylene) plastic is used to form the electric material.
The design of the holes plays a very vital role in reducing the air damping and mechanical thermal noise. The two plates have a small gap between them to allow the movements of diaphragm. A very fine thin film of plastic is placed between the two plates to provide electrical insulation. The diaphragm moves in the air gap between the plates with the slightest disturbance caused in the surrounding air pressure by the sound signal.