How Microphones Actually Work — Capsule, Diaphragm, and Signal

After fifteen years of placing microphones in studios and on stages, I have learned that understanding how these devices actually convert sound waves into audio signals changes how you approach recording decisions. The difference between a great take and a mediocre one often comes down to matching the right transduction method to your source material.

This guide examines the three fundamental components that make every microphone work: the capsule design, diaphragm construction, and signal conversion process. We will explore how dynamic, condenser, and ribbon microphones handle these elements differently and why those differences matter for your recordings.

The Capsule: Where Sound Becomes Motion

The capsule houses the diaphragm and the mechanism that converts its movement into an electrical signal. In condenser microphones like the Neumann U87, the capsule contains a thin diaphragm suspended just micrometers away from a solid backplate. This creates a variable capacitor where sound pressure changes alter the distance between the two surfaces, generating voltage fluctuations that mirror the original sound waves.

Dynamic microphone capsules work differently. The Shure SM57 uses a moving coil capsule where the diaphragm connects to a small coil of wire suspended in a magnetic field. When sound waves move the diaphragm, the coil cuts through magnetic field lines and generates electrical current through electromagnetic induction. This robust design explains why dynamic mics handle high sound pressure levels without distortion.

Ribbon microphones like the Royer R-121 feature the most delicate capsule design. An ultra-thin aluminium ribbon suspended between magnetic poles serves as both diaphragm and signal generator. The ribbon responds to air particle velocity rather than pressure, creating the smooth frequency response that ribbon mics are known for. However, this design makes them vulnerable to phantom power and wind damage.

Diaphragm Construction and Frequency Response

The diaphragm material and tensioning directly influence how a microphone responds to different frequencies. Large diaphragm condensers typically use gold-sputtered plastic membranes that provide excellent sensitivity across the frequency spectrum. The AKG C414 uses a one-inch diaphragm that captures subtle detail in vocals and acoustic instruments, though its mass can limit high-frequency transient response compared to smaller diaphragms.

Small diaphragm condensers like the Audio-Technica AT4041 employ lighter membranes that respond more quickly to high-frequency content. This makes them excellent for capturing cymbals, acoustic guitar picking, and other sources with fast transients. The trade-off comes in reduced low-frequency sensitivity and slightly higher self-noise due to the smaller surface area.

Dynamic microphone diaphragms must be robust enough to handle high sound pressure levels while remaining responsive to sound waves. The Electro-Voice RE20 uses a Variable-D diaphragm design that maintains consistent frequency response regardless of distance from the source, making it ideal for broadcast applications where mic technique varies.

Signal Conversion and Output Characteristics

Once the diaphragm moves, different microphone types convert that motion into electrical signals through distinct methods. Condenser microphones require phantom power to charge the capacitor formed by the diaphragm and backplate. When sound pressure changes the capacitance, it creates voltage fluctuations that an internal amplifier buffers and sends to your preamp. This active circuitry allows condensers to produce strong output signals with excellent frequency response.

Dynamic microphones generate their own electrical signal without external power. The moving coil design produces relatively low output voltages, which is why dynamics often sound quieter than condensers when plugged into the same preamp. However, this passive design makes them virtually indestructible and immune to phantom power issues. The Shure SM58 exemplifies this reliability, working consistently across decades of live performance abuse.

Ribbon microphones produce the weakest electrical signals of all microphone types. Modern ribbons like the sE Electronics VR1 include active electronics to boost output levels, while passive ribbons such as the Coles 4038 require preamps with substantial clean gain. The delicate ribbon element responds symmetrically to positive and negative sound waves, creating the natural harmonic distortion characteristics that many engineers prize for certain sources.

Polar Patterns and Capsule Design

The capsule construction determines not only frequency response but also directional characteristics. Single-diaphragm capsules create cardioid patterns by using acoustic ports that delay sound arriving from the rear. The delayed rear signal cancels with the front signal at the diaphragm, creating directional pickup. Microphones like the Rode PodMic use this single-capsule approach for consistent cardioid response.

Dual-diaphragm capsules enable multiple polar patterns by electrically combining signals from front and rear diaphragms. The Neumann TLM 103 uses this design to offer cardioid, omnidirectional, and figure-eight patterns. When both diaphragms receive equal electrical weighting, the result is omnidirectional pickup. When only the front diaphragm is active, you get cardioid response. Figure-eight patterns emerge when the diaphragms are wired in opposite polarity.

True omnidirectional microphones like the DPA 4006 use single-diaphragm capsules without rear ports, allowing equal sensitivity to sound from all directions. This design provides the most natural frequency response since there are no acoustic phase cancellations, but it also means no rejection of ambient noise or feedback in live situations.

Impedance and Loading Effects

Microphone output impedance affects how the signal behaves when connected to different preamp input impedances. Most modern microphones specify output impedances between 50 and 200 ohms, designed to work optimally with preamp inputs rated at 1000 ohms or higher. This impedance relationship ensures maximum signal transfer and prevents frequency response alterations.

Ribbon microphones traditionally featured much higher output impedances, sometimes exceeding 300 ohms. Vintage ribbons like the RCA 44 require careful impedance matching to avoid high-frequency rolloff. Modern ribbon designs address this with built-in impedance conversion circuits or specify appropriate preamp loading requirements.

Some engineers deliberately mismatch impedances for sonic effect, but this practice can introduce unpredictable frequency response changes. The safe approach involves using preamps with input impedances at least five times higher than the microphone output impedance, ensuring consistent performance across different signal chains.

Conclusion

Understanding how microphones convert sound waves into electrical signals through capsule design, diaphragm construction, and signal conversion helps you make informed recording decisions. Each transduction method offers distinct advantages: dynamics for durability, condensers for sensitivity, and ribbons for character. Choose based on your source material, recording environment, and desired sonic outcome rather than following generic recommendations.