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The electret microphone has gained widespread usage as a cheap but effective microphone for many applications.
Although some high quality versions are available, it is at the low end of the market where it has really gained its main usage.
Electret microphone construction
The key to the electret microphone is the dielectric that is used between the two plates of the capacitor that form the microphone.. This dielectric retains a permanent charge equivalent to around 100 volts.
The electret microphone dielectric is typically manufactured by heating a form of plastic whilst held within an electric field. As a result it takes on the charge which it retains. Although there is some leakage it can take a hundred years of more to fall appreciably.
Electret microphone performance
Within an electret microphone the diaphragm is thicker than in other microphones – this means that these microphones generally have a poor high frequency response, and a resonance at 5 kHz or a little more.
This peak in response can typically be reduced by cutting the treble if required and this can generally be achieved without affecting the speech frequencies unduly.
It is also found that most electret microphones have a falling bass response.
Electret microphone use
Electret microphones need a high input impedance buffer and preamplifier. This is normally contained within the electret assembly and is often powered by a 1.5 volt source. For handheld versions this is typically contained within the overall casing of the microphone and not supplied as a form of phantom power which is normally reserved for higher quality microphones.
It is also worth remembering that electret microphones should not be subject to moisture which can be an issue for closely ‘mic’ed’ vocals.
These drawbacks can be overcome relatively easily and often electret microphones can make an attractive proposition in view of their low cost.
They are also used in many items of electronic equipment where their performance is quite adequate and their low cost very attractive.