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An electret is a material that possesses a static electric field. This word is a combination of “electricity” and “magnet.” The material used in electrets is a special dielectric, an insulator with a special ability to retain a significant amount of strong electric field where it was immersed at the time of its manufacture.
Electrets exhibit a strong electrostatic polarization. This means the retained potential, in the form of electrostatic polarization, is similar to the way that the magnetic polarization of the common permanent magnets used in school laboratories is retained. The difference between an electret and a permanent magnet is that a permanent magnet has a near-permanent magnetic field, while electrets have a near-permanent electric field.
One use of electrets is to be the dielectric in a condenser microphone. In electrical circuit theory, a condenser, usually referred to as a capacitor, is comprised of two conductive plates separated by a dielectric material. A capacitor can be energized by charging it with a direct voltage source, such as a penlight battery, to produce a positive electrical voltage on one plate and a negative voltage on the other. The capacitor usually holds the voltage for a long time, only limited by the characteristics of the insulator or dielectric in between. It is now electrostatically polarized by the charging voltage.
When an electret replaces the dielectric in the capacitor or condenser microphone, there is no need to supply an external voltage source to produce the electrostatic field because the electret already has an electrostatic charge. When the air compression and expansion of the voice "move" the plates of the condenser microphone, there is a small but significant voltage generated that is proportional to the voice or sound upon the microphone. Electret microphones are used in many audio devices, such as portable media players and voice recorders.
The piezoelectric transducer also uses an electret in the form of a ceramic with a strong electrostatic polarization. There are two plates, one of which is fixed and one that is plated on the dielectric. When electrical signal in the audio spectrum of a sufficient level is applied to both the fixed and movable plate, the movable plate vibrates at the same rate as that of the electrical signal. This type of electret transducer is able to transform electrical signals into audible sound and at rates beyond audibility.