Electret Condensor Microphone: Works under the same principles as normal condenser microphones, but the electret variety are made with permanently charged material to read sound waves, thus eliminating the need for polarizing voltage. Most of these microphones do require a battery, but the power drain is so minimal that they may have an integrated one or only call for a single AA. [ Audio Glossary ]
Image Source: http://www.mwit.ac.th/~physicslab/hbase/audio/mic2.html
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