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.
Understanding How Microphones Record Music
The diaphragm cannot copy every sound perfectly. It has mass, stiffness, and a natural tendency to vibrate most strongly at certain frequencies. These properties create a frequency response, meaning that a microphone may react more to some pitches than to others.
A vocal microphone is often shaped to make speech and singing sound clear. A studio microphone may aim for a flatter response, so it changes the original sound less. The diaphragm must be light enough to follow fast vibrations, yet strong enough not to bend or rattle during loud sounds.
Direction matters because sound reaches different parts of a microphone from different places. A cardioid microphone is most sensitive at the front and less sensitive at the back. This helps a singer stand near the microphone while reducing sound from monitors, drums, or a noisy room behind it.
An omnidirectional microphone receives sound from nearly every direction. It can make a more natural room recording, though it picks up more unwanted sound.
Directional patterns come from the way air pressure acts on one or both sides of the diaphragm. Small openings and air paths inside the microphone delay sound slightly, changing how the diaphragm responds to sound arriving from each direction.
The electrical signal leaving a microphone is very small. A preamplifier raises it to a useful level before mixing, recording, or sending it to speakers. Raising a signal raises unwanted electrical noise too, so engineers try to get a healthy signal at the microphone itself.
This is one reason performers work close to a microphone. Moving closer changes more than loudness. With many directional microphones, bass frequencies become stronger at close range.
This is called the proximity effect. It can make a voice sound warm and deep, though too much can make words sound muddy.
Digital recording adds another stage. An audio interface measures the changing microphone voltage many thousands of times each second and stores those measurements as numbers. The sampling rate must be high enough to represent the highest frequencies humans hear.
Bit depth controls how finely each voltage level is measured. More bits allow a wider range between the quietest useful sound and the loudest sound before clipping. Clipping happens when a signal exceeds the system limit and the wave peaks are cut off.
It creates harsh distortion that usually cannot be fixed later. When learning recording, pay attention to microphone distance, angle, room reflections, input level, and handling noise. A good recording often begins with careful placement rather than editing.
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.