What is an Electret and What are Electret Microphones?

 Electrets are the electrostatic equivalents of permanent magnets. While a permanent magnet has two magnetically charged poles, an electret has two electrically charged ends. It has internal and external electric fields just like the magnet has magnetic fields. In short, electret is a dielectric material which has electrical polarisation. Electrets can be produced by heating a material and cooling in the presence of strong electric fields. They can also be manufactured by embedding excess negative charge in a dielectric using a particle accelerator. Alternate method is to embed charges on the surface using high voltage corona discharges. Embedded charges decay as a function of dielectric constant and resistivity. Material like polytetrafluoroethylene (PTFE) with extremely high resistivity can retain the charges for extremely long periods. Just as there are naturally occurring permanent magnets, quartz and and other forms of silicon dioxide can behave as natural electrets.

An electret microphone has a diaphragm which forms a capacitor that incorporates an electret. The constant charge present in the electret eliminates the need for the polarizing power supply needed for a non-electret condenser mic. But a preamplifier is incorporated to boost the audio signal. Sound waves move the diaphragm, changing the capacitance (ΔC) and produces a corresponding voltage change (ΔV) across the capacitor which is directly proportional to the charge in the capacitor (Q) and inversely proportional to the change in capacitance with movement (ΔV = ⁠Q/ΔC). As seen in the diagram, the integrated preamplifier requires small amount of power. In some electret mics, a 1.5 V battery is incorporated which runs for a long time as the current drain is usually very small.