A microphone turns changing air pressure into a changing electrical signal that can be recorded, amplified, or sent to speakers. When a singer sings or a guitar string vibrates, it creates compressions and rarefactions in the air called sound waves. These pressure changes push and pull on a thin surface inside the microphone called the diaphragm.
Recording music depends on capturing those tiny motions accurately without adding too much noise or distortion.
In a dynamic microphone, the diaphragm moves a coil of wire near a magnet, producing voltage by electromagnetic induction. In a condenser microphone, the diaphragm acts like one plate of a capacitor, so its motion changes capacitance and creates a changing signal when powered. The microphone's output is an analog waveform that matches the pattern of the incoming sound wave.
Engineers choose different microphones and polar patterns to control tone, loudness, background noise, and feedback.
Key Facts
- Sound is a pressure wave that travels through air at about v = 343 m/s at room temperature.
- Wave speed, frequency, and wavelength are related by v = fλ.
- A microphone diaphragm vibrates with the same frequency pattern as the incoming sound wave.
- Dynamic microphones use electromagnetic induction: moving coil plus magnet produces a voltage.
- Condenser microphones use a charged capacitor: changing plate spacing changes capacitance and signal voltage.
- Higher sound amplitude makes the diaphragm move farther, which usually produces a larger electrical signal.
Vocabulary
- Diaphragm
- A thin flexible surface in a microphone that vibrates when sound waves hit it.
- Transducer
- A device that converts one form of energy into another, such as sound energy into electrical energy.
- Dynamic microphone
- A microphone that creates an electrical signal when a diaphragm moves a coil of wire in a magnetic field.
- Condenser microphone
- A microphone that uses a charged capacitor whose electrical properties change as the diaphragm moves.
- Polar pattern
- A map of how sensitive a microphone is to sound coming from different directions.
Common Mistakes to Avoid
- Thinking a microphone records sound as air inside a wire, which is wrong because the microphone converts air pressure changes into an electrical signal.
- Confusing loudness with pitch, which is wrong because loudness depends mostly on amplitude while pitch depends on frequency.
- Assuming all microphones pick up sound equally from every direction, which is wrong because different polar patterns reject or accept sound from different angles.
- Placing a microphone too close or too far without considering the source, which is wrong because distance changes signal level, tone, room sound, and the risk of distortion.
Practice Questions
- 1 A singer produces a note with frequency 440 Hz. If the speed of sound is 343 m/s, what is the wavelength of the sound wave?
- 2 A microphone signal has a frequency of 250 Hz. How many complete pressure cycles reach the diaphragm in 4.0 seconds?
- 3 A band is recording vocals in a noisy room. Explain why a cardioid microphone pointed at the singer can reduce background sound better than an omnidirectional microphone.