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.
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.