A form of electrostatic or capacitor microphone. Instead of creating an electrostatic charge within the capacitor capsule with an external DC voltage, an electret microphone employs a special dielectric material which stores a static-electric charge introduced during its manufacture. A PTFE film is normally used, and where this is attached to the back plate of the capsule the device is called a ‘back electret’. Some very early electret microphones used the dielectric film as the diaphragm itself, but these sounded very poor which is why designs which used the back-electret configuration were specifically denoted as such.
Designs which attach the PTFE film to the diaphragm are known as Front Electrets. Modern electret capsules compare directly in quality with traditional DC-biased capacitor capsules, and are available in the same range of configurations — large, medium and small diaphragm sizes, single and dual-membrane, fixed or multi-pattern, and so on.