To talk about electret microphones we first need to talk about electret materials.
Electret materials have been known for a long time. Some people suggested they were an important component for condenser microphones. Nevertheless, these seemed impractical before the creation of foil electret. And what is a foil electret, you ask? Well, this material involves a thin metalized Teflon foil. After its appearance, it quickly became the most used material for applications with a high-quality recording and a lavalier for built-in mics. A diversity of tiny recording sound devices and phones use foil electret too.
The three most common electret microphones are the following: the foil-type—also known as diaphragm-type—, the back-electret, and front-electret. Here you will find a brief description of each.
Types of Electret Microphones
Foil-type or Diaphragm-type Microphones
Firstly, the foil-type or diaphragm-type is the most common type of electret microphone. It involves a layer of electret material, but of very poor quality. This is due to the fact that the electret material does not make a good diaphragm.
Second, the back-electret microphones involve a layer of electret material applied to the back of the microphone container. The diaphragm contains uncharged material which can be mechanically more suitable for the transducer design.
Thirdly, the front-electret microphone is the latest type of these microphones in the market. This type does not involve a backplate within its design. Both the diaphragm and the inner part of the container form the capacitor, so the electret layer adheres to the inside front cover. Notably, there is a connection between the metalized diaphragm and the input of the FET that is similar to the back electret. The front-electret microphone can use any sort of conductive layer within its diaphragm.
Electret Mics vs Condenser Mics
Electret microphones and condenser ones are alike. Namely, the most notable difference is that the electret removes the externally applied bias voltage. Within this scenario, the electret is a polarized material. The most common electret microphones contain a thin polymer layer adhered on one side of the container with metal material as the moving diaphragm. This layer is permanently polarized.
Electret mics and condenser mics have similar levels of biases, so they tend to have comparable sensitivities as well. Their frequency response, linearity and transient response are similar as well. Because of the difference in their amplification strategies, the electret transducer has a lower self-noise, as well as a broader dynamic range. Electret mics are pretty inexpensive and can be found in a variety of sizes and configurations. Also, they can operate without external power. They are popular among common devices such as tape recorders and phones.