A microphone is a transducer, a device that changes one form of energy into another. It begins with sound, which is a pattern of pressure changes traveling through air. Inside the microphone, these pressure changes move a very light diaphragm.
That motion is converted into a changing electrical signal that can be recorded, amplified, or analyzed.
Different microphone designs use different physical effects, but they all preserve the timing and shape of the sound wave as closely as possible. In a dynamic microphone, a coil moves in a magnetic field and generates voltage by electromagnetic induction. In a condenser microphone, sound changes the spacing between charged plates, which changes capacitance and creates a voltage signal.
Engineers choose microphone designs by balancing sensitivity, frequency response, durability, noise, and the sound pressure levels they need to capture.
Understanding How Microphones Capture Sound
The electrical signal from a microphone is tiny, so it usually needs a preamplifier before it can travel through cables or enter a recorder. A preamplifier raises the signal level while trying not to add hiss, hum, or distortion. This is why a quiet room can still produce noise in a recording.
Some noise comes from air movement and electronics. Some comes from the microphone itself.
Condenser microphones need power for their internal electronics and charged capsule. This may come from a battery or from phantom power supplied through a balanced audio cable.
Microphones do not hear every direction equally. Their directional pattern is called polar response. An omnidirectional microphone responds to sound from nearly all directions.
It can capture a natural sense of a room, though it may pick up unwanted background noise. A cardioid microphone is most sensitive at the front and less sensitive at the rear. Students meet this design in handheld stage microphones because it helps reduce feedback from loudspeakers behind the performer.
Shotgun microphones use an interference tube to narrow their pickup at higher frequencies. They are common on film sets, where a microphone must stay outside the camera view.
Distance changes more than loudness. When a person speaks very close to a directional microphone, low frequency sounds can become stronger. This is called the proximity effect.
It gives some radio voices a deep, heavy sound. Moving farther away reduces this effect, but the microphone then captures more room reflections. Reflections arrive slightly later than the direct sound.
Hard walls, windows, and desks reflect strongly, while curtains and soft furniture absorb more sound energy. In a classroom recording, microphone placement often matters more than buying a more expensive model. Keeping the microphone close to the speaker and away from noisy fans usually improves clarity.
A useful microphone should reproduce important frequencies without changing them too much. This behavior is its frequency response. Human speech contains low frequencies that give voices fullness, plus higher frequencies that carry consonants such as s, f, and t.
A microphone with weak high frequency response may make speech sound dull. One with an exaggerated high range may make it sound sharp. Very loud sounds create another limit.
If the diaphragm, electronics, or recorder cannot handle the signal, the waveform clips. Clipping removes the rounded shape of peaks and produces harsh distortion that cannot be fully repaired later.
When learning about microphones, pay attention to the whole signal path. The source, room, microphone position, cable, preamplifier, and recording settings each affect the final sound.
Key Facts
- Sound pressure moves the diaphragm: larger pressure variations usually produce larger diaphragm motion.
- Dynamic microphone principle: a moving coil in a magnetic field produces voltage by electromagnetic induction.
- Faraday's law: V = -N dΦ/dt, where V is induced voltage, N is coil turns, and Φ is magnetic flux.
- Condenser microphone capacitance: C = εA/d, where A is plate area and d is plate separation.
- Frequency and period are related by f = 1/T.
- Sound pressure level: SPL = 20 log10(p/p0), where p0 = 20 µPa in air.
Vocabulary
- Transducer
- A device that converts energy from one form into another, such as sound energy into electrical energy.
- Diaphragm
- A thin flexible membrane in a microphone that vibrates in response to sound pressure changes.
- Dynamic microphone
- A microphone that uses a moving coil and magnet to generate an electrical signal from diaphragm motion.
- Condenser microphone
- A microphone that uses a charged capacitor whose spacing changes as the diaphragm vibrates.
- Frequency response
- A description of how strongly a microphone responds to different sound frequencies.
Common Mistakes to Avoid
- Thinking a microphone records sound directly, which is wrong because it first converts air pressure variations into an electrical signal.
- Confusing frequency with amplitude, which is wrong because frequency sets pitch while amplitude is related to loudness and signal size.
- Assuming all microphones work the same way, which is wrong because dynamic, condenser, ribbon, and MEMS microphones use different conversion mechanisms.
- Ignoring the direction of sound pickup, which is wrong because microphone placement and polar pattern strongly affect the captured signal.
Practice Questions
- 1 A sound wave has a period of 0.0025 s. What is its frequency in hertz?
- 2 A condenser microphone has plate area A = 1.0 x 10^-4 m^2, plate spacing d = 2.0 x 10^-5 m, and ε = 8.85 x 10^-12 F/m. What is its capacitance using C = εA/d?
- 3 Explain why a microphone diaphragm must be lightweight but still strong enough to resist damage from large sound pressure levels.