An electret microphone is a type of condenser microphone that uses a permanently charged material to convert sound into an electrical signal. It’s the most common microphone design in the world, found in everything from smartphones and laptops to hearing aids and security systems. What makes it special is that thin layer of permanently charged material, called an electret, which eliminates the need for an external power supply to charge the microphone’s sensing element.
How an Electret Microphone Works
At its core, an electret microphone is a tiny capacitor: two conductive plates separated by a gap. One of those plates is a thin, flexible diaphragm. The other is a rigid backplate. Sandwiched in or coated onto this structure is the electret, a special plastic material that holds a permanent electric charge, similar to how a magnet holds a permanent magnetic field. That charge can persist for decades.
When sound waves hit the diaphragm, it vibrates. Those vibrations change the distance between the two plates, which changes the capacitance. Since the charge stays constant, the voltage across the capacitor shifts in direct proportion to the sound. That fluctuating voltage is your audio signal.
A traditional condenser microphone needs an external voltage (sometimes 48 volts) to charge its capacitor plates. The electret’s built-in charge skips that step entirely, which is why electret microphones can be so small and cheap to manufacture.
The Built-In Amplifier
The voltage that comes off the electret element is real, but it’s extremely weak and has very high impedance, meaning it can’t drive a cable or input without losing most of the signal. To solve this, nearly every electret microphone capsule has a tiny transistor (called a FET, or field-effect transistor) built right inside the housing. This transistor acts as an impedance converter: it takes the fragile signal from the electret element and outputs a stronger, more robust version that can travel through a wire to your recorder, computer, or mixer.
That internal transistor does need a small amount of power to operate. Consumer devices like laptops and phone jacks typically supply this as “bias voltage” or “plug-in power,” usually between 1.5 and 9 volts delivered through the audio cable itself. This is not the same as the 48-volt phantom power used with professional studio condensers, and plugging an electret mic into a phantom power source (or vice versa) without checking compatibility can cause problems.
Who Invented It
The foil electret microphone was invented in the 1960s by James West and Gerhard Sessler, researchers at Bell Labs. Earlier electret materials lost their charge too quickly to be practical, but West and Sessler identified modern polymer films that could hold a charge essentially indefinitely. Their design made high-quality condenser microphone technology available at a fraction of the previous cost, and it quickly became the dominant microphone type worldwide.
How Long the Charge Lasts
The electret’s permanent charge is remarkably stable. Modern electret materials are typically made from fluoropolymers (the same family of plastics as Teflon). Studies on Teflon-based electrets have measured half-lives ranging from about 14 years for thicker materials to 68 years for thinner films, meaning it takes that long for the charge to drop by half. In practical terms, the charge will outlast the microphone itself. You don’t need to “recharge” an electret, and the gradual decay over decades is too slow to noticeably affect audio quality during the microphone’s useful life.
Sound Quality and Performance
Electret microphones cover a wide frequency range, typically capturing sound from about 50 Hz up to 15,000 Hz or higher. That comfortably spans the full range of human speech and most musical content. Low-cost electret capsules tend to be remarkably flat through the midrange but often show a bump in the 5,000 to 10,000 Hz region, which adds a slight brightness or “presence” to recordings. They also tend to roll off below 50 Hz, meaning very deep bass is attenuated.
Sensitivity varies by model. Inexpensive electret headset microphones tested in a 2022 study published in the American Journal of Speech-Language Pathology showed sensitivities between roughly -58 dB and -44 dB (re 1V/Pa). Lower numbers mean the mic needs more amplification, which can introduce noise. Higher-end electret capsules perform better on this front, and some measurement-grade electret microphones rival traditional studio condensers in accuracy.
Most small electret capsules are omnidirectional, picking up sound equally from all directions. Some are designed with cardioid patterns (favoring sound from the front), though testing has shown that cheap cardioid electrets don’t always reject off-axis sound as effectively as their specs suggest.
Where Electret Microphones Are Used
The combination of tiny size, low cost, and decent sound quality makes electret microphones the default choice for an enormous range of applications. Your phone’s microphone is almost certainly an electret (or its close cousin, the MEMS microphone, which uses similar principles). The same goes for laptop microphones, Bluetooth headsets, hearing aids, intercoms, baby monitors, and voice recorders.
In professional audio, electret capsules appear in lavalier (lapel) microphones, boundary microphones for conference rooms, and some shotgun microphones used in film production. Measurement microphones used for acoustics testing and sound-level meters also frequently use electret elements because of their stability and flat response. By some estimates, over 90% of all microphones manufactured today use electret technology in some form.
Electret vs. True Condenser Microphones
Both electret and “true” condenser microphones work on the same principle: a vibrating diaphragm changes the capacitance of a charged element, producing a voltage. The only fundamental difference is where the charge comes from. A true condenser requires an external polarizing voltage, delivered through phantom power (typically 48 volts). An electret supplies its own charge from the permanently polarized material.
In practice, true condensers tend to appear in high-end studio and live-sound applications where larger diaphragms and ultra-low noise are priorities. Electrets dominate everywhere else, especially where size, cost, and simplicity matter. The performance gap has narrowed considerably over the decades, and many modern electret microphones deliver audio quality that’s difficult to distinguish from externally polarized designs at several times the price.