Electret Microphones
Last Edited: Dec 25, 2023
Electret materials have been known since the 1920s and were proposed as condenser microphone elements several times. However, people considered them impractical before James West and Gerhard Sessler invented the foil electret at Bell Laboratories in 1961, 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 an electret material film. This is the most common type but has the lowest quality, as the electret material does not make a perfect diaphragm.
Back Electret
The back plate of the microphone capsule has an electret film applied to it. The diaphragm consists of an uncharged material, which may be mechanically more suitable for the transducer design being realized.
Front Electret
The design of this newer type does not include a back plate. The diaphragm and the inside surface of the capsule form the capacitor. Therefore, the electret film adheres to the inside front cover. A connection exists between the metalized diaphragm and the input of the FET. The diaphragm is equivalent to the back electret in that any conductive film may be used.
Condenser Microphones Similarity and Difference
Electret microphones are very similar to the condenser microphones. The main difference is that 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 of permitting separate optimization of the diaphragm 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 different amplification strategies, the electret transducer tends to have a broader dynamic range and lower self-noise. The advantage of electret microphones, compared to conventional condenser microphones, is that they operate without needing 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 various sizes and configurations. Some of which inhibit strong directivity.
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