In terms of audio technology, the Electret Capacitive Microphone (ECM) is a powerful and classic sound acquisition solution with reliable performance, strong adaptability, and a wide range of applications. MEMS microphone design is easy to integrate, compact in appearance, and highly favored by users, while ECM plays an important role in electronic design due to its rich installation configuration, wide directional modes, and acoustic multifunctionality.

This article will study the working principle of ECM, explore its internal circuits and directional response capabilities, from omnidirectional mode for capturing wide environmental sounds to unidirectional configuration for centralized sound isolation. In addition, key performance indicators such as sensitivity, signal-to-noise ratio (SNR), and frequency response will be explored to guide correct component selection.

Basic knowledge of electret condenser microphone
Electret condenser microphone (ECM, also known as condenser microphone) works through an electret. Electret is a ferroelectric material that is permanently charged or polarized. The high resistance and chemical stability of an electret ensure that the embedded charge remains intact for hundreds of years without significant decay. In English, the term 'Electret' comes from the combination of 'electrostatic' and 'magnet', reflecting the process of embedding electrostatic charges into materials. This is achieved by arranging the static charges in the polarizer, similar to the way magnets are generated through magnetic domain arrangement.

This inherent characteristic has significant advantages in microphone design. Traditional condenser microphones require an external polarization voltage to function, while the built-in electrostatic charge of the polarizer does not have this requirement. This simplification reduces the complexity of circuit design, enabling ECM to work efficiently in compact low-power applications, making it an ideal choice for various audio systems.

The working principle of an electret capacitor microphone is to use the diaphragm as one plate of the capacitor and the back plate as the other fast plate. When sound waves cause diaphragm vibration, the distance between the diaphragm and the backplate changes, resulting in a change in capacitance. These changes are determined by the capacitance equation:

C = Q / V

among which

Q=Charge, measured in Coulombs (since the charge is embedded in the polarizer, the amount of charge remains constant)
C=capacitance, measured in Farads
V=potential difference, measured in volts
Due to the movement of the diaphragm, the capacitance (C) changes, and the voltage (V) on the capacitor will undergo an inverse proportional change, generating an electrical signal corresponding to the sound vibration.

Then, this changing voltage is fed into the field-effect transistor (FET) inside the microphone for amplification, in order to facilitate better transmission. The DC blocking capacitor on the output stage can eliminate any unnecessary DC offset, ensuring the output of pure audio signals. This simple and effective design makes ECM a reliable choice for capturing sound in various electronic applications.

Working principle diagram of ECM
Figure 1: Working principle of ECM. (Image source: Same Sky)

The typical structure of a capacitive electret microphone includes the following key components:

Non woven fabric: prevents dust while allowing sound to pass through
Shell: Encapsulate and protect internal components
Polar ring: Strengthening the polarizing material used for the diaphragm
Diaphragm: vibrates in response to sound, thereby changing capacitance
Gasket: used to maintain the distance between the diaphragm and the backplate
Backboard: The fixed electrode that constitutes the capacitor
Base: Provides structural support
Copper ring: Ensure conductivity and electrical connection
PCB: Used to accommodate FET and other signal amplification circuits
The disassembly and assembly structure of ECM is shown in the following figure.