The principle of operation of electret microphones is similar to the principle of operation of condenser microphones. The difference is that they do not require an external power supply. The membrane of these microphones receives an electrical charge during operation. To power them, only a small voltage (about 1.5 Volts) is needed, which is created using a battery installed in the microphone.
An electret microphone is a type of condenser microphone. It is used as part of a set of electronic equipment for professional and household purposes, in professional studios and amateur conditions, in sound recording and other devices. They are often used by shortwave radio amateurs. The electret microphone is very reliable, lightweight and has a flat frequency response.
Electret microphone arrangement
These microphones are made in the form of capacitors, a certain number of plates of which are made of a very thin plastic film located on a ring. An electron beam is applied to the film. It penetrates to a shallow depth, creates a space charge, which has the ability to persist for a long time. These materials are called "electret", which is why the microphone is called electret.
Then a very thin layer of metal is applied to the film, used as one of the electrodes. Another electrode is a metal cylinder, the flat surface of which is located next to the film. Its vibrations, created by acoustic waves, are capable of creating an electric current between the electrodes. Due to the fact that the current strength in this case is very small, and the output resistance reaches a large value (gigaohms), the transmission of the signal generated by the microphone is rather difficult.
To match the low impedance of the amplifier and the high impedance of the microphone, it is necessary to use a special stage, which is created on a field-effect (unipolar) transistor. It is located in the body of the microphone capsule (this is the name of the device where not only the microphone itself is located, but also the matching stage). The housing should be metal, capable of shielding the microphone and matching the cascade, protecting it from external fields of electricity.
In order to understand the suitability of a particular amplifier for connecting to a microphone, it is enough to connect to the input jack of the device (multimeter). If, as a result, it shows a voltage of 2-3 Volts, this means that the amplifier is suitable for working together with an electret microphone.
Working principle and design
According to the principle of operation, electret microphones are the same as condenser microphones, but the constant voltage in them is provided by the charge of the electret applied to the membrane in the form of a thin layer. This charge can be maintained for a long time (up to 30 years or more)
The work of electret microphones is based on the ability of some materials, which have a high dielectric constant, to change their surface charge due to the effect of a sound wave. These microphones have a very high impedance, which makes it possible to connect them to amplifiers with high input impedance. By their design, microphones are divided into several types, when the electret material in them is located in the frontal position, is located on a flexible membrane and mounted on the back plate.