Blog  /  How Does an Electret Microphone Work? A Comprehensive Guide

How Does an Electret Microphone Work? A Comprehensive Guide

Have you ever wondered about mechanisms microphones employ to bring out various sounds? Microphones convert sound energy into electrical signals. However, different types of microphones use varied methods in the conversion process.

In our case today, we'll discuss electret condenser microphones. The electret microphone has been in existence since 1961, and it works on an electrostatic principle, and our scope of interest gives details on applications, types, among others.  

What is an Electret Microphone?

 

An electret microphone or electret condenser microphone (ECM) is an electrostatically working capacitor-based microphone.  

electret condenser microphone

(electret condenser microphone)

 

Its function is to eradicate the need for a polarizing power supply since it uses a permanently charging material.  

Electret Microphone Type

 

You can determine the type of electret microphone by considering its electret charging material. As such, there are three electret types.  

  1. Front electret microphone 

 

It has no backplate. Instead, the inside surface of the mic capsule and diaphragm forms the capacitor. Then, manufacturers fix the electret film to the front cover inside the mic. Finally, they connect the metalized diaphragm to the FET input.

An electret microphone capsule/element

Source: https://en.wikipedia.org/wiki/File:Electret_condenser_microphone_capsules.jpg   

  1. Back electret microphone 

 

It has a permanently charged capsule because manufacturers fix the stationary backplate (not the diaphragm) to its electret material. In that way, the diaphragm accuracy and the electret's durability increase as it is now immobile.  

  1. Foil electret microphone/ Diaphragm-type microphone

 

Instead of having a different diaphragm/front-plate coated with electret film, foil electret mic has its electret material as its diaphragm. Despite a foil electret being the most common type, it has the lowest quality, and it is because electret films as diaphragms perform poorly.

Note:

Electret microphones lack polarizing voltage, and instead, they have integrated preamplifiers that need power (assumed as bias or polarizing force). Also, the preamplifier receives phantom powering in the studio and sound reinforcement applications.

Further, we can classify project-grade and consumer electronic ECMs based on their connector types. For example, we have terminal-type, wire-type, and pin-type microphones.  

Electret Microphone Working Principle

 

Electret MIC internal structure

 

The diagram below shows an ECM capsule structure. Even though not indicated, it also has a JFET in the middle section that acts as the impedance converter.

 

  1. Sound wave
  2. Diaphragm
  3. Backplate
  4. Battery
  5. Resistor
  6. Audio signal
  7. Electret (between the diaphragm and backplate).

Structure of a condenser microphone 

Source: Wikipedia

The Electrostatic Principles Behind the ECM Transducer

The electrostatic principle of an ECM gives the basis of its functionality.  

Explanation as follows;

 

First, the capsule is generally a parallel-plate capacitor with a stationary backplate and movable front plate (membrane/capsule diaphragm).

Moreover, the capacitor needs to maintain a constant charge between the backplate and diaphragm to function correctly.

Since the capsule has a fixed charge, we can use the formula below to understand its working;

Q = C.V

Q = Fixed electrical charge

C = Capacitance

V = Potential difference or Voltage level across the electrodes/plates

From the formula, a change in Capacitance is inversely proportional to any change in voltage. In other words, the Capacitance of the capacitor should oscillate within its setpoint to produce an AC voltage mic signal.  

MIC capacitance calculation

 

The electret has a fixed charge C. To alter the Capacitance within a capsule, the formula below will apply;

C = Ɛ0 (A/d)

Whereby,

A = surface areas of plates.

d = distance between plates.

C = Capacitance of parallel-plate capacitor.

Ɛ0 =dielectric constant

From the formula, the area of plates and dielectric constant (stationary diaphragm) are two constants. Moving the diaphragm changes the distance of the container while also causing a variation in levels of sound pressure.

All in all, sound waves at the diaphragm produce a coinciding mic signal.  

The Impedance Converting Transistor

 

Unfortunately, ECM capsules have an issue of extremely high impedance at their output. They should, thus, be in a position to maintain high impedance to prevent leaking of stored charge crossway the plates.

An impedance converter (a JFET) is effective in taking away audio signals from an ECM capsule.  

Junction-gate field-effect transistor (JFET) symbol

Source: Wikipedia  

Gate (G) is the high-impedance input, and it receives an output signal from the ECM capsule without degrading it.

The power supply of JFET is from an external power supply such as DC biasing or phantom power. It subsequently causes electric current flow through the source (S) and drain (D) terminals. The current has low impedance and hence, can be the final mic output.

In summary, input signals can control the output signals. Therefore, a high-impedance call through the gate-source JFET terminals can control the low-impedance motion at the source-drain terminals.  

Microphone output calculation

 

A signal-to-voice ratio (SNR) determines the desired electrical output signal level instead of the unwanted calls.  

Power supply principle

 

Despite ECMs operating as permanently charged capsules, they still are active microphones. That's to say, ECM impedance converters may still need some power to work efficiently. The power supply can either be DC biasing, phantom power, or a device's same battery.

An ECM circuit 

Source: Wikipedia  

Application of Electret Microphone

 

An electret condenser microphone so far has a wide variety of applications. But first, let us consider the examples before listing the areas of use.  

Typical ECMs include;

 

  • Consumer electronics like mobile phones,
  • Measurement microphones,
  • Professional medical devices such as hearing aids,
  • Student condenser microphones, and
  • Film microphones like lavalier mics or shotgun mics.

A microphone 

Source: Wikipedia  

Areas of application 

  • Recording sessions for instrumentals and voice since it's an omnidirectional microphone,
  • Live performances like band performances or stand-up comedy,
  • TV and radio broadcasting,
  • Mobile devices, and
  • Hearing impairments.

 

a child using hearing aids

(a child using hearing aids).

 

Summary

 

To conclude, electret condenser microphones are sensitive and compact. Additionally, they can capture then respond to sound vibrations from a 360° angle. Hence, that makes them the most common type of microphones in current use in electronic circuits.

Till this point, we hope that you've gained much from the post.

Do you still have any questions? Please do contact us.