The Science and Engineering of Microphones

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Notes

Audio Signal – An electronic representation of the actual sound wave

Dynamic Microphone – In a dynamic microphone a thin diaphragm is connected to a coil of wire called a voice coil which is precisely suspended over a powerful magnet.

  • As the sound waves strike the diaphragm it cause it to vibrate, moving the voice coil through the magnetic field generated by the magnet which generates a small bit of electricity which is sent down the output leads.
  • This is the electromagnetic principle.
  • ADVANTAGE: They are simply constructed and can handle loud sources without much distortion.
  • DISADVANTAGE: They are weak when trying to capture soft distant sources because the diaphragm needs lot of sound energy to move.
  • DISADVANTAGE: dynamic microphones have a heavy diaphragm along with the additional weight of the coil of wire.
    • It therefore takes longer for the diaphragm to react to a sound wave due to the effects of inertia hence causing a less accurate recording.

Ribbon Microphone – Instead of using a coil, ribbon microphones use a small strand of very thin 2 microns thick aluminum ribbon.

  • Variation on the dynamic microphone.
  • Ribbon microphones are almost exclusively used in the studio, not for location audio.
  • ADVANTAGE: It is more responsive to high frequencies
  • DISADVANTAGE: It is fragile and prone to tearing

Condenser Microphone – Condensers use two charged plates, one fixed and one which can move acting like a diaphragm.

  • There’s no coil.
  • The two charged electric plates create what’s called a capacitor. As sound waves strike the electrically charged diaphragm, it moves in relation to the fixed plate changing its capacitance and generating a very small electric charge which is amplified inside the microphone.
  • This is the electrostatic principle.
  • ADVANTAGE: Because you’re not moving a coil, condensers can be more responsive in the high frequencies.
  • ADVANTAGE: Because you don’t have any magnets, condenser microphones can be made very small.
  • Because condensers work with electrically charged plates, that means they require some sort of outside power.
  • Some microphones have the option of an onboard battery while all condensers can utilitize something called Phantom Power.

Phantom Power – +48v of energy sent down the microphone cable to a condenser microphone from the audio recording or mixing board.

  • This power enables the electrically charged diaphragm to move in response to sound waves.

Directional Response – Directional response is represented by something called a polar pattern.

Polar Pattern – Polar pattern is how well the microphone “hears“ sound from different directions.

“On Axis” and “Off Axis” – On axis is directly in front of the sound source. Off axis is not directly in front of the sound source.

Omnidirectional Mics – This mic polar pattern is responsive to sound from all directions, you don’t have to be “on axis” to be picked up.

  • Lavalier and lapel mics are small condenser microphones with an omnidirectional pickup pattern that can be placed on a person.
  • Boundary mics are omnidirectional condenser mics. They are positioned flush with a surface that capture sound as it rolls off the flat surface. Boundary mics are used in stage production and conference tables.
  • ADVANTAGE: These mics are useful for picking up sound in a general area.
  • ADVANTAGE: Lavalier / lapel mics are small and can be placed just about anywhere.
  • ADVANTAGE: Boundary mics do not draw attention to themselves because they lay flat on the floor or wall.
  • DISADVANTAGE: They will pick up all the unwanted sound in the area.
  • DISADVANTAGE: Lavalier, lapel, and boundary mics won’t have the same richness of sound as a shotgun or studio condenser mic.

Directional Mics: Cardioid Pattern – Most basic pattern.

  • Heart-shaped pick up pattern.
  • ADVANTAGE: Picks up what’s in front but not behind.
  • ADVANTAGE: It is suited for live performance as it picks up the sound on axis but won’t pick up what’s behind it, like crowd noise or feedback from a speaker.

Directional Mics: Hypercardioid and Supercardioid Patterns – More directional than cardioid.

  • Skinnier heart-shaped pick up pattern.
  • Picks up the front and sides and rejects 150 degrees to the rear.
  • Shotgun mics are supercardioid.
  • ADVANTAGE: Great for recording location audio while trying to filter out some of the unwanted ambient sound.
  • DISADVANTAGE: Can exhibit strange phasing sound effects when used in small spaces.

Directional Mics: Figure 8 Pickup Pattern / Bi-directional – The polar pattern looks like a figure 8.

  • ADVANTAGE: Useful for certain musical applications or interviews with a person on each side of the mic.