Condenser Microphones: Precision Through Capacitive Transduction

Condenser microphones (also called capacitor microphones) achieve superior sensitivity and frequency response by using electrostatic rather than electromagnetic transduction principles. Instead of a moving coil in a magnetic field, condenser microphones employ a lightweight diaphragm that forms one plate of a variable capacitor, with changes in capacitance converted to electrical signals through appropriate circuit design.

The basic construction consists of an extremely thin diaphragm (typically 2-10 micrometers thick) made from materials like gold-sputtered polyester or metal foil, positioned very close (10-50 micrometers) to a rigid backplate that serves as the fixed capacitor electrode. The narrow air gap between diaphragm and backplate creates a capacitor whose capacitance varies as the diaphragm moves in response to sound pressure:

C = ε₀A/d

Where ε₀ is the permittivity of free space, A is the electrode area, and d is the gap spacing. Small changes in gap spacing due to diaphragm motion create proportional capacitance changes that can be converted to voltage signals.

The conversion from capacitance changes to output voltage requires external circuitry because the capacitive element itself generates no power. Two main approaches are used: constant charge systems and constant voltage systems. In constant charge operation, the capacitor maintains constant charge Q, so voltage changes follow:

ΔV = ΔQ/C ≈ Q(Δd/εA)

The constant voltage approach maintains constant voltage across the capacitor through a high-impedance bias circuit, creating current flow when capacitance changes:

ΔI = V(ΔC/Δt) = V(εAv/d²)

Where v is the diaphragm velocity. This approach provides output proportional to diaphragm velocity, which corresponds to acoustic pressure for frequencies well above the diaphragm resonance.

Polarization voltage (bias voltage) is essential for condenser microphone operation, typically ranging from 48V to 200V depending on the microphone design. This voltage can be supplied externally (phantom power from mixing consoles or preamps) or internally (using permanently charged electret materials). The polarization voltage affects both sensitivity and maximum signal handling capability:

Sensitivity ∝ V_bias Maximum SPL ∝ V_bias²

Higher bias voltages increase sensitivity but also increase the risk of diaphragm collapse if the voltage exceeds the mechanical limits of the diaphragm suspension system.

Electret condenser microphones use permanently polarized dielectric materials to eliminate the need for external bias voltage. The electret material (typically fluoropolymer like Teflon) maintains a permanent electrical charge that provides the necessary electric field for capacitive transduction. While electret microphones are more convenient and economical than externally biased condensers, they may exhibit some performance limitations including temperature sensitivity and gradual loss of polarization over very long periods.

The impedance converter circuit in condenser microphones addresses the extremely high output impedance of the capacitive element (typically many megohms). Field-effect transistor (FET) amplifiers provide the high input impedance needed to avoid loading the capacitive source while converting to lower impedance outputs suitable for connection to external equipment. Modern impedance converter circuits achieve noise levels below 10 dB-A equivalent input noise, enabling condenser microphones to capture very quiet sounds with excellent signal-to-noise ratios.

Frequency response of condenser microphones can be exceptionally flat and extended because the diaphragm mass is much lower than in dynamic microphones. The absence of a voice coil eliminates the mass loading that limits high-frequency response in dynamic designs. High-quality condenser microphones can achieve flat response from 20 Hz to 20 kHz or beyond, with some specialized designs extending to 100 kHz for ultrasonic applications.