In electrostatics, a parallel plate capacitor has the following relationship:
C = ε · S / d
- C is the capacitance
- ε is the dielectric constant of the material between the plates
- S is the surface area of the plates
- d is the distance between the plates
Additionally, when a capacitor is charged with Q, the voltage across its plates is given by:
V = Q / C
Internal Structure of Electret Microphones
An electret condenser microphone contains a capacitor formed by a diaphragm, a spacer, and a backplate. The diaphragm is a plastic film permanently charged (electret). When exposed to sound pressure, the diaphragm vibrates, changing the distance d between the diaphragm and the backplate, thereby changing the capacitance C.
As C changes and Q remains constant, V must also change. This variation in voltage represents the initial conversion of sound into an electrical signal.
Impedance Matching and Amplification
The electrical signal generated is very weak and comes from a high-impedance source, so it cannot be used directly. Thus, impedance conversion and amplification are required.
This is achieved using a Field Effect Transistor (FET), a voltage-controlled device. The drain current of the FET is influenced by the voltage between the gate and source. In the ECM, the capacitor's electrodes are connected to the gate and source terminals of the FET. When the diaphragm vibrates, the voltage changes across the capacitor, causing variations in the FET drain current.
This changing current causes a voltage drop across the load resistor R1. The output voltage variation can then be extracted through a coupling capacitor C0. Since this voltage is driven by the original sound pressure, the entire process completes the conversion from acoustic to electrical signal.
Electret microphones convert sound into electrical signals by using a charged diaphragm and a FET amplifier to deliver usable output. The changing capacitance due to diaphragm vibration is key to this conversion process.
