Electret Microphone

An electret microphone is a type of condenser (or capacitive) microphone that uses an electret material with a permanent electrostatic charge. Unlike traditional condenser microphones that require an external bias voltage, electret microphones have this charge built-in, making them simpler, smaller, and often less expensive. They are extremely widespread in a vast range of applications, from mobile phones to professional recording systems.

Here is a detailed explanation of electret microphones:

1. Operating Principle

The electret microphone operates on the principle of capacitance variation in a capacitor caused by sound waves. It consists of two main parts forming a capacitor:

• Diaphragm: A thin membrane (typically made of Mylar or Teflon plastic) coated with a conductive material (often a thin layer of gold or aluminum). This diaphragm is one plate of the capacitor and vibrates in response to incoming sound waves.
• Backplate: A fixed metal plate that constitutes the other plate of the capacitor. One of these two plates (usually the backplate or sometimes the diaphragm) is made of electret material.

What is an electret?

An electret is a dielectric (electrically insulating) material that has undergone permanent electrical polarization. This polarization is analogous to the permanent magnetization of a magnet. Once charged, an electret retains an internal electrical charge for a very long time (decades or even centuries) without requiring an external power source. Common electret materials include certain polymers like Teflon FEP, polycarbonate, and polypropylene, which are charged by applying an intense electric field during manufacturing or by electron bombardment.

How sound is converted into an electrical signal:

1. Sound Waves: Air pressure variations caused by sound waves make the microphone's diaphragm vibrate.
2. Capacitance Variation: The distance between the vibrating diaphragm and the fixed backplate (the electret) changes with these vibrations. Since a capacitor's capacitance is inversely proportional to the distance between its plates (C = εA/d, where C is capacitance, ε is the permittivity of the material between the plates, A is the plate area, and d is the distance between them), the capacitance of the capacitor formed by the diaphragm and backplate varies with sound waves.
3. Charge (or Potential) Variation: The electret, having a permanent electrical charge, maintains a constant charge on the capacitor. When the capacitance changes due to diaphragm movement, the voltage (or potential) between the capacitor plates must vary (Q = CV, where Q is charge, C is capacitance, and V is voltage). Since Q is constant (thanks to the electret), a change in C causes a change in V.
4. Amplification by an Integrated FET: The voltage variation produced by the electret capacitor is very small and has a very high output impedance. To make this signal usable, most electret microphones integrate a field-effect transistor (FET) directly into the microphone housing. This FET acts as an impedance converter (or buffer), transforming the capacitor's high impedance into a much lower output impedance, allowing the signal to be transmitted over longer distances without significant loss. The FET requires a small external power supply (typically between 1.5 V and 10 V) to operate. This power is often called "bias voltage" even though it is not used to bias the electret itself.
5. Output Signal: The FET's output is an alternating voltage signal representing the sound pressure variations captured by the diaphragm.

2. Key Components of an Electret Microphone

• Electret Capsule: This is the heart of the microphone and contains the diaphragm, backplate (with the electret), and often the impedance-matching FET. It is typically small (a few millimeters in diameter).
• Diaphragm: Vibrating membrane coated with a conductive material.
• Backplate (Electret): Fixed plate made of electret material or a metal plate with electret material attached to it.
• FET (Field-Effect Transistor): Impedance amplifier integrated into the capsule or very close to it. It requires an external power supply to operate.
• Housing: Protects the internal components and provides a mechanical interface for mounting.
• Output Connector: Allows the microphone to be connected to a preamplifier or other recording device. Common connectors include TRS (jack), XLR, and proprietary connectors for certain devices.

3. Types of Electret Microphones

Electret microphones can be classified based on several criteria:

Diaphragm Type:

• Foil Electret Microphones: The electret is a thin foil separate from the backplate.
• Back Electret Microphones: The electret is integrated into the backplate. This is the most common type.
• Front Electret Microphones: The electret is integrated into the diaphragm itself.

Directivity (Polar Pattern): Like all microphones, electrets are available with different directivities, determining their sensitivity to sounds from different directions:

• Omnidirectional: Sensitive to sound from all directions.
• Cardioid: More sensitive to sound from the front and rejects sound from the rear.
• Supercardioid and Hypercardioid: Narrower directivity than cardioid, with some rear sensitivity.
• Bidirectional (Figure-8): Sensitive to sound from the front and rear, and rejects sound from the sides.

Output Type:

• Two-wire microphones: Have two connection wires: one for the output signal and FET power, and the other for ground.
• Three-wire microphones: Have three wires: one for the output signal, one for FET power, and a separate ground.

Applications:

• Mobile phone and portable device microphones: Small, lightweight, and low-power.
• Headset and earphone microphones: Often miniaturized with cardioid directivity to isolate voice from ambient noise.
• Lavalier (lapel) microphones: Small omnidirectional or cardioid microphones used for interviews and presentations.
• Recording microphones (amateur and professional): Available with various directivities and audio qualities.
• Surveillance and security system microphones.

4. Advantages of Electret Microphones

• Simplicity and Compact Size: The lack of need for complex external bias voltage allows for very compact designs.
• *Lower Cost: They are generally cheaper to manufacture than externally biased condenser microphones.
• *Low Power Consumption: The integrated FET amplifier consumes very little current, making them ideal for battery-powered portable devices.
• *Good Sensitivity and Frequency Response: They can offer sensitivity and frequency response comparable to traditional condenser microphones for many applications.
• *Relative Robustness: While sensitive to significant mechanical shocks, they are generally more robust than some other microphone types.

5. Disadvantages of Electret Microphones

• *Electret Lifespan: Although the electret charge is designed to last a very long time, it can slowly degrade over time, especially under extreme temperature and humidity conditions, which may lead to reduced sensitivity.

• *Variable Audio Quality: Audio quality can vary significantly depending on design and component quality. Low-end models may have more noise and less linear frequency response than high-quality studio condenser microphones.

• *Power Requirement (for FET): While they don't need biasing for the capsule itself, they require a small power supply for the integrated FET.

• *Sensitivity to Electromagnetic Interference (EMI): The integrated amplifier circuit can be susceptible to interference if the microphone is not properly shielded.

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