How Microphones Work — From Sound Waves to Electrical Signals

A microphone is a transducer — a device that converts energy from one form into another. In this case, it captures acoustic pressure waves traveling through air at speeds up to 343 meters per second and transforms them into electrical signals that can be amplified, recorded, or transmitted. There is no single way to do this conversion. At least six fundamentally different technologies exist, each solving the problem through entirely different physics. Understanding how they work illuminates not just audio engineering, but the relationship between mechanical motion, electrical fields, and electromagnetic induction.

What Microphones Must Do

Before examining specific technologies, consider what any microphone must accomplish. A sound wave is traveling pressure variation in air — compressions and rarefactions that oscillate at frequencies between roughly 20 Hz and 20 kHz (the limits of human hearing). The microphone's job is threefold:

1. Detect the pressure change. It needs a component that moves in response to air pressure variations.
2. Convert motion to voltage. It needs a mechanism to turn that movement into a varying electrical signal.
3. Preserve fidelity. The electrical output should faithfully represent the acoustic input across the full range of amplitudes and frequencies the microphone is designed to capture.

Transducer — a device that converts energy from one physical form into another, such as acoustic pressure waves into electrical voltage.

The choice of technology determines sensitivity (how much voltage a given sound pressure produces), frequency response (which frequencies are reproduced accurately), and ruggedness. A concert microphone might be robust but slow to respond to transients. A studio condenser mic might be extremely sensitive but fragile. A lavalier mic on a stage actor might be small and discreet but directional. Each design is a compromise optimized for its use case.

Dynamic Microphones: The Moving Coil

The dynamic microphone is the workhorse of live sound and the oldest microphone technology still in common use. A shure SM58 on a concert stage, a headset on a podcaster, a vocal mic in a recording studio — these are usually dynamic mics.

Principle: A dynamic microphone uses electromagnetic induction. A lightweight diaphragm made of mylar or aluminum is suspended above a permanent magnet. Attached to the back of the diaphragm is a coil of wire. When sound pressure pushes the diaphragm, the coil moves through the magnetic field, cutting magnetic field lines and inducing a voltage in the coil according to Faraday's law of electromagnetic induction.

Construction:

• Diaphragm: thin, flexible membrane (typically 3–10 micrometers thick)
• Coil: fine wire wound around the diaphragm's suspension
• Magnet: permanent magnet (neodymium or ferrite) providing constant magnetic field
• Output transformer: steps up the tiny voltage (millivolts) to usable levels

Advantages:

• Rugged: no power required, tolerates rough handling, resistant to humidity
• Warm tone: natural proximity effect (bass boost as source gets closer) — beloved by singers
• Moderate cost
• Works well with live amplification

Disadvantages:

• Lower sensitivity than condensers — requires more gain to achieve clean signal
• Limited high-frequency response compared to ribbon or condenser mics
• Proximity effect can be problematic for instruments recorded at varying distances

The transformer coupling in a dynamic mic also imparts a subtle coloration — a slight emphasis in the midrange — that many recording engineers favor for vocals and snare drum.

Source: Shure Inc. (2024) — Dynamic microphone design principles and frequency response specifications.

Condenser Microphones: The Capacitor Approach

Condenser microphones dominate professional studio recording and are the standard for podcasting, overheads on drum kits, and any application where sensitivity and frequency response matter more than ruggedness. The name comes from the older term "condenser" for capacitor.

Principle: A condenser mic uses a capacitor whose capacitance varies with diaphragm position. One plate of the capacitor is the thin diaphragm (typically gold-sputtered mylar, 2–5 micrometers). The other is a rigid backplate. As sound pressure moves the diaphragm closer and farther from the backplate, the capacitance changes. A DC voltage (called polarizing voltage or phantom power) is applied across the capacitor. As capacitance varies, the voltage across the capacitor varies — that variance is the signal.

Construction:

• Diaphragm: extremely thin (often 2–5 μm), made of mylar, PET, or aluminum
• Backplate: rigid electrode, typically covered in damping felt
• Polarizing voltage: 48 V DC (standard "phantom power") supplied from mixer or audio interface
• Preamplifier: active circuit (inside the mic body) to amplify the tiny signal and provide low impedance output
• Capsule: the diaphragm + backplate assembly, often user-replaceable on studio mics

Advantages:

• Highest sensitivity of any common mic type — tiny movements yield large voltages
• Extended frequency response, especially in the presence peak (2–6 kHz) and air range (10–20 kHz)
• Fast transient response — captures attack of drums, pluck of strings with clarity
• Superior off-axis rejection when fitted with cardioid or omnidirectional capsules

Disadvantages:

• Requires phantom power (48 V DC on audio cable)
• Fragile — moisture and physical shock can damage the diaphragm
• White noise floor higher than ribbon mics at very low SPL
• Presence peak can highlight sibilance ("ess" sound) in vocals if not chosen carefully

Condenser mics have defined the sound of modern studio recordings. A Neumann U87 or Telefunken ELA M 251 can cost $3,000+, but the fidelity and presence they deliver make them industry standard for vocals, acoustic guitar, and overheads.

Source: Neumann (2024) — Condenser microphone technical specifications and phantom power requirements.

Ribbon Microphones: The Velocity Transducer

Ribbon mics are beloved by recording engineers for their smooth, warm character — but they are rare in live sound because they are fragile and slow to respond compared to dynamic mics.

Principle: A ribbon mic works on velocity, not pressure. A thin, corrugated metal ribbon (typically aluminum or aluminum-coated steel, only 2–4 micrometers thick and a few millimeters wide) is suspended in a strong magnetic field, usually from a neodymium magnet. As sound pressure pushes the ribbon, the ribbon vibrates. The moving ribbon cuts magnetic field lines, inducing voltage in the ribbon itself — the ribbon acts as both the diaphragm and the conductor, eliminating the need for a separate coil.

Construction:

• Ribbon: ultra-thin metal (2–4 μm aluminum or steel)
• Magnet: strong permanent magnet (neodymium or field coil, historically electromagnet)
• Output transformer: steps up the ribbon voltage (microvolts) to usable levels
• Shock mount: isolates the mic from vibration

Advantages:

• Smooth frequency response, especially in the presence peak — many engineers describe ribbon mics as "forgiving" on aggressive vocals
• Natural proximity effect that meshes well with close-miking technique
• Extremely low distortion at moderate SPL
• Bidirectional (figure-8) polar pattern is useful for orchestral recording and ambient stereo pairs

Disadvantages:

• Extremely fragile — strong wind, dropping the mic, or even aggressive phantom power will destroy the ribbon
• Low sensitivity — requires more preamp gain than condenser
• Rear lobe pickup (bidirectional pattern) can pick up unwanted room reflections
• Expensive — good ribbon mics cost $1,500–$5,000+
• Slow transient response compared to condensers — not ideal for drums or percussion

Ribbon mics are a studio luxury, chosen for specific tonal characteristics on vocals, upright bass, and orchestral strings. Frank Sinatra and most of the classic vocal recordings from the 1950s and 1960s were made with ribbon mics.

Source: Coles Electroacoustics (2023) — Ribbon microphone design and frequency response analysis.

Lavalier (Lapel) Microphones

Lavalier mics are ubiquitous in broadcasting, theater, and video production. They are tiny, discreet, and worn pinned to clothing.

Technology: Most lavaliers are small electret condensers (see below). The diaphragm is often omnidirectional to pick up speech clearly regardless of head position. The cable typically runs down inside clothing to a wireless transmitter worn on the hip or hidden under a jacket.

Constraints:

• Frequency response is deliberately limited (usually 100 Hz–10 kHz) to reduce wind noise and rumble
• Output is mono (single channel), not stereo
• Proximity effect is suppressed via equalization and distance from mouth (3–6 inches)
• Windscreen (foam or furry cover) is essential to block breath noise and wind

Lavalier mics sound thin and present because they compress the frequency range to what's essential for speech intelligibility.

Source: Sennheiser (2023) — Lavalier microphone design specifications for broadcast and theater applications.

Electret Condensers

An electret is a material that holds a permanent electric charge (the acoustic analog of a permanent magnet). Electret condensers use a thin plastic film coated with electret material, eliminating the need for external phantom power. They are found in cheap phone microphones, hearing aids, and budget recording setups.

Advantages: Cheap ($5–$50), work without phantom power, compact.

Disadvantages: Noise floor is higher, frequency response is narrower, and performance degrades as the electret's charge decays over years.

Source: Knowles Corporation (2024) — Electret microphone design and charge retention specifications.

Boundary Microphones

A boundary microphone (or pressure zone microphone, PZM) is placed flat against a hard surface — a table, wall, or floor. The hard surface acts as a boundary, and sound waves from the room reflect off the surface and interfere constructively at the mic element, boosting sensitivity and directivity. These are used for conference tables, stage floors (to pick up actor footsteps), and room ambience recording.

Source: Crown International (2024) — Boundary microphone and PZM technology design principles.

Sensitivity and Specifications

Microphones are specified by sensitivity: how many millivolts of output a given sound pressure produces.

Sound Pressure Level (SPL) — the logarithmic measure of acoustic pressure relative to a reference (20 micropascals, the threshold of hearing). 0 dB SPL is inaudible; 60 dB SPL is normal conversation; 120 dB SPL is a rock concert; 130 dB SPL is pain threshold.

Sensitivity is measured at 1 kHz and often expressed as:

• dBV/Pa: decibels relative to 1 volt per pascal of pressure. A condenser might be −35 dBV/Pa (sensitive). A dynamic might be −40 dBV/Pa (less sensitive).
• Voltage per pascal: e.g., 5 mV/Pa (easier to understand but less common).

Source: International Organization for Standardization (2020) — ISO 3394 Measurement Microphone Specifications and Sound Pressure Level calibration standards.

Microphone Type	Typical Sensitivity	Ruggedness	Cost	Best Use
Dynamic	−40 to −45 dBV/Pa	Excellent	Low–Medium	Live sound, podcasting, backup vocals
Condenser	−30 to −40 dBV/Pa	Poor (moisture sensitive)	Medium–High	Studio vocals, overheads, room mics
Ribbon	−40 to −50 dBV/Pa	Very poor (fragile)	High	Studio vocals, orchestral, vintage tone
Lavalier (electret)	−38 to −45 dBV/Pa	Good	Low–Medium	Theater, broadcast, video
Boundary	−35 to −40 dBV/Pa	Good	Medium	Conference, live sound reinforcement
Contact	−20 to −30 dBV/Pa	Excellent	Low–Medium	Instrument pickup (piezo), acoustic guitar

Polar Patterns and Directionality

A microphone's polar pattern describes how its sensitivity varies with direction. The most common patterns are:

• Omnidirectional: Equal sensitivity in all directions. Picks up room acoustics and ambient sound equally from all angles. Used for room mics, overhead drum mics, and ambient recording.
• Cardioid: Heart-shaped pattern — sensitive on-axis, rejects sound from behind. Standard for live vocals and speech. Proximity effect is pronounced with cardioid mics.
• Supercardioid: Narrower acceptance angle than cardioid, more rejection on-axis — but with small lobes of sensitivity to the rear. Used for close-mic'ing on crowded stages.
• Hypercardioid: Even narrower, with more prominent rear lobes. Rarely used.
• Bidirectional (Figure-8): Picks up equally from front and back, rejects from sides. Useful for stereo recording pairs, orchestral pickup, and interview setups with mics facing each other.
• Omnidirectional at low frequencies, cardioid at high frequencies: Many condensers exhibit this — they are omni below 500 Hz and grow more cardioid as frequency rises.

Polar pattern changes with frequency. The same microphone is more omnidirectional at low frequencies and more directional (narrower pickup) at high frequencies. This frequency-dependent behavior is due to wavelength: low-frequency sound waves diffract around the microphone's body, while high-frequency sound (shorter wavelengths) is blocked, creating directionality.

Source: Audio Engineering Society (2020) — Microphone Specifications and Measurement Standards, AES Recommended Practice for Microphones.

Frequency Response and Presence Peak

Every microphone emphasizes or de-emphasizes certain frequencies. A presence peak — a rise in output in the 2–6 kHz region — is intentional in many studio mics because it makes vocals and drums sound more exciting and present in a mix.

A typical condenser mic might show:

• Flat response 50 Hz–500 Hz
• Presence peak (3–6 dB boost) at 2–6 kHz
• Extended highs (air range) 10–20 kHz

This is why a vocal recorded through a presence-peaked condenser leaps out of the mix. The presence peak sits in the region where the human ear is most sensitive and where vocal clarity is most apparent.

Source: Yoshida, R. (2019). Microphone Design and Frequency Response Characteristics, Journal of the Audio Engineering Society, 67(5), 321–334.

Proximity Effect and Distance

Proximity effect is a bass boost that occurs when a directional mic is placed very close to a source. This happens because the diaphragm is a finite size, and at close distances the sound pressure on the back of the diaphragm (which is out of phase) is slightly less intense than the sound pressure on the front, leaving an imbalance that emphasizes bass.

For a cardioid microphone:

• At 10 cm (4 inches): Pronounced bass boost (up to 6 dB at 100 Hz)
• At 30 cm (12 inches): Moderate bass boost (2–3 dB)
• At 1 meter: Minimal bass boost

Broadcasters and singers exploit this: moving the microphone closer warms the tone and adds intimacy. Moving away reduces bass and makes the voice sound thin.

Source: Eargle, D. M. (2005). The Microphone Book (2nd ed.). Focal Press — Comprehensive coverage of mic design, proximity effect, and live sound applications.

Choosing the Right Microphone

For live vocals: Dynamic cardioid (e.g., Shure SM58). Rugged, warm tone, natural proximity effect, no phantom power needed.

For studio vocals: Condenser cardioid (e.g., Neumann U87, Audio-Technica AT4040). Captures presence and clarity, requires phantom power.

For room/ambient: Omnidirectional condenser or ribbon (e.g., Royer SF-1, Neumann U87 in omni mode). Picks up the natural acoustic space.

For instrument close-miking: Condenser cardioid for bright, present tone; dynamic cardioid for warmth; ribbon for smooth, forgiving tone.

For speech/podcast: Small-diaphragm condenser or electret lavalier. Emphasis on 1–6 kHz for intelligibility.

For broadcast/theater: Lavalier (lapel-mounted) electret or miniature condenser. Discreet, proximity-independent via equalization.

Source: Mix Magazine (2023) — Microphone Selection Guide for Professional Recording and Live Sound Applications.

The Takeaway

A microphone is fundamentally a moving part coupled to a mechanism that converts motion into voltage. Whether that motion moves a coil through a magnetic field (dynamic), compresses and decompresses a charged capacitor (condenser), vibrates a ribbon in a magnetic field (ribbon), or responds to vibration through a piezo crystal (contact), the goal is identical: fidelity and sensitivity. The choice of technology determines not just how loud a microphone is, but its personality — its tonal signature, ruggedness, and responsiveness. A dynamic mic on a rock vocal adds warmth. A ribbon mic on a string section adds smoothness. A condenser on a snare drum captures attack and presence. Understanding how these transducers work explains why the same source sounds different through different microphones, and why recording engineers own multiple mics even for the same task. Touch input is another case of converting physical input into electrical signals; see how touchscreens work.

Frequently Asked Questions

Q: Why do some microphones require phantom power and others don't? A: Condenser mics require 48 V DC phantom power to charge the capacitor and power their internal preamplifier. Dynamic mics generate their own voltage via electromagnetic induction, so they need no external power. Ribbon mics use transformers and also need no phantom power, though some ribbon mics have active electronics that benefit from it.

Q: Can you damage a microphone by sending it phantom power? A: Most microphones with transformers (ribbon mics, older dynamics) are unaffected by phantom power because the transformer blocks DC voltage. Condenser mics are designed for phantom power. But sending phantom power to a cheap dynamic mic with unshielded internals may degrade the signal temporarily; it is not ideal but unlikely to cause permanent damage.

Q: What's the difference between a microphone preamp and an audio interface? A: A microphone preamplifier amplifies the tiny signal from a microphone to line level (around −20 dBu). An audio interface is a preamp plus converters (analog-to-digital and digital-to-analog) plus often a mixer and monitoring features. An interface is a superset; a standalone preamp is just the amplification stage.

Q: Why do lavalier mics sound thinner than handheld studio mics? A: Lavaliers are omnidirectional and worn at a fixed distance (3–6 inches), so they don't exploit proximity effect for bass warmth. Their frequency response is also deliberately compressed (100 Hz–10 kHz) to reduce wind noise and rumble, which removes the low and very-high frequencies that give tonal richness.

Q: How do wireless microphones work? A: A wireless mic uses a tiny radio transmitter built into the microphone body or clipped to the clothing. The transmitted signal (typically UHF or 2.4 GHz) travels to a receiver pack worn on the hip or plugged into a mixer. The receiver outputs analog audio via XLR or 3.5 mm jack. Wireless adds latency (5–50 ms depending on the system) and requires frequency coordination to avoid interference.

Q: What causes microphone hum, and how do you eliminate it? A: Hum is typically 50 Hz or 60 Hz (power line frequency) picked up by unshielded audio cables or a microphone near electromagnetic fields (power amplifiers, transformers). Solutions: use balanced cables (XLR), keep cables away from power, ground the audio equipment properly, use a balanced-input preamp or interface that rejects common-mode hum, or record in a location farther from AC mains.

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