Electret materials have been known since the 1920s and were proposed as condenser microphone elements several times. However, people considered them impractical before the invention of the foil electret tat Bell Laboratories in 1961 by James West and Gerhard Sessler, using a thin metalized Teflon foil. This became the most common type, used in many applications from high-quality recording and lavalier use to built-in microphones in small sound recording devices and telephones.
There are three main types of electret microphones, with a difference in way of using the electret material:
Foil-type or diaphragm-type
The diaphragm itself uses a film of electret material. This is the most common type. Nevertheless, it also has the lowest quality since the electret material does not make a particularly good diaphragm.
An electret film is applied to the back plate of the microphone capsule. The diaphragm consists of an uncharged material, which may be mechanically more suitable for the transducer design being realized.
In this newer type, the back plate does not exist within the design. Also, the diaphragm and the inside surface of the capsule form the capacitor. Therefore, the electret film adheres to the inside front cover. Also, there is a connection between the metalized diaphragm and the input of the FET. It is equivalent to the back electret in that any conductive film may be used for the diaphragm.
Condenser microphones similarity and difference
Electret microphones are very similar to the condenser microphones. The main difference is that the usage of the polarized material (an electret) eliminates the externally applied bias voltage. Typical electret microphones use a thin polymer film coated on one side with metal as the moving diaphragm. The film is permanently polarized at a level comparable to that used in biasing a conventional condenser microphone. An alternative that is increasing in popularity is to place the electret film onto the stationary plate and use a thin metal foil as the moving diaphragm. This configuration has a name: backplate electret. It offers the advantage or permitting separate optimization of the diaphragm performance and electret performance.
Because electret microphones have similar levels of biases as condenser microphones, their sensitivities are roughly the same. Similarly, the linearity, frequency response, and transient response of an electret microphone are close to those of a condenser microphone of similar size. However, due to the use of different amplification strategies, the electret transducer tends to have a broader dynamic range and lower self-noise.
The advantage of electret microphones, when compared to conventional condenser microphones, is that they operate without the need for an external power supply. They are also quite affordable. Electret microphones are now the standard microphone type in consumer products such as telephones and tape recorders. People bought over 600 million electret microphone units worldwide. To conclude, they are available in a wide variety of sizes and configurations. Some of which inhibit strong directivity.
Additional Resources & Source Texts