Question

With the help of diagram, explain the working of capacitance microphone.

Hint:

Because capacitance microphones have extremely low-mass diaphragms, they offer excellent high-frequency transient response and a flat frequency curve, making them the standard choice for professional studio recording.

Answer 1

Step 1: The Electrostatic Principle of Operation:
A capacitance (condenser) microphone operates on the electrostatic principle. The electrical capacitance ($C$) of a parallel-plate capacitor is given by the formula: $$C = \frac{\epsilon_0 \epsilon_r A}{d}$$ Where $\epsilon_0$ is the permittivity of free space, $\epsilon_r$ is the relative permittivity of the medium, $A$ is the active plate area, and $d$ is the distance between the plates. By changing the distance ($d$) via sound waves, we vary the capacitance ($C$), which is converted into an electrical signal.

Step 2: Detailed Structural Breakdown of the Microphone Circuit:
A capacitance microphone circuit consists of the following components:

• The Diaphragm (Moving Plate): A very thin, highly flexible, conductive membrane (usually gold-sputtered Mylar) stretched tightly, acting as the moving front plate of the capacitor.
• The Backplate (Fixed Plate): A thick, rigid metal plate positioned parallel to the diaphragm, acting as the fixed back plate. The backplate is perforated with tiny holes to allow air to pass through, reducing air damping.
• The DC Biasing Source (Phantom Power): A stable external DC voltage source (usually $48\text{ V}$) connected across the plates through a high-value series resistor ($R$). This places a constant electric charge ($Q$) on the capacitor.
• The Series Resistor ($R$): A very high value resistor ($10\text{ M}\Omega\text{ to }1\text{ G}\Omega$). Because of its high value, the charge $Q$ on the plates cannot change rapidly during audio-frequency oscillations. This maintains a constant charge ($Q \approx \text{constant}$).
• Output Terminals: Connected across the high-value resistor, feeding into a high-input-impedance preamplifier (usually a Field-Effect Transistor or FET).

Step 3: Step-by-Step Working Mechanism:
The microphone converts sound waves into electrical signals through these steps:

• When sound waves (acoustic pressure) hit the flexible diaphragm, they cause it to vibrate.
• This vibration changes the distance ($d$) between the diaphragm and the fixed backplate.
• Changing the distance ($d$) alters the capacitance ($C$) of the capacitor.
• Because the series resistor ($R$) keeps the electrical charge ($Q$) constant ($Q = C \cdot V$), any change in capacitance ($\Delta C$) forces a corresponding inverse change in the voltage across the capacitor plates ($\Delta V$): $$V = \frac{Q}{C}$$
• When the diaphragm moves inward, $d$ decreases, $C$ increases, and the voltage $V$ decreases. When the diaphragm moves outward, $d$ increases, $C$ decreases, and the voltage $V$ increases.
• This fluctuating voltage matches the incoming sound wave frequency and amplitude, producing an electrical audio signal.