Microphones

Microphones are transducers which detect sound signals and produce an electrical image of the sound, i.e., they produce a voltage or a current which is proportional to the sound signal. The most common microphones for musical use are dynamic, ribbon, or condenser microphones. Besides the variety of basic mechanisms, microphones can be designed with different directional patterns and different impedances.


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Dynamic Microphones


Principle: sound moves the cone and the attached coil of wire moves in the field of a magnet. The generator effect produces a voltage which "images" the sound pressure variation - characterized as a pressure microphone.

Advantages:
  • Relatively cheap and rugged.
  • Can be easily miniaturized.
Disadvantages:
  • The uniformity of response to different frequencies does not match that of the ribbon or condenser microphones.

The geometry of a dynamic microphone is like that of a tiny loudspeaker, and that is not just a coincidence. A dynamic microphone is essentially the inverse of a dynamic loudspeaker. In a dynamic microphone, the sound pressure variations move the cone, which moves the attached coil of wire in a magnetic field, which generates a voltage. In the loudspeaker, the inverse happens: the electric current associated with the electrical image of the sound is driven through the coil in the magnetic field, generating a force on that coil. The coil moves in response to the audio signal, moving the cone and producing sound in the air.

A small loudspeaker can be used as a dynamic microphone, and this fact is exploited in the construction of small intercom systems. Depending upon the position of the Talk-Listen switch, the device on either end of the intercom system can be used as a microphone or a loudspeaker. Of course, this is not a high fidelity process, and for commercial dynamic microphones, the device is optimized for use as a microphone, not a loudspeaker.

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Ribbon Microphones


Principle: the air movement associated with the sound moves the metallic ribbon in the magnetic field, generating an imaging voltage between the ends of the ribbon which is proportional to the velocity of the ribbon - characterized as a "velocity" microphone.

Advantages:
  • Adds "warmth" to the tone by accenting lows when close-miked.
  • Can be used to discriminate against distant low frequency noise in its most common gradient form.
Disadvantages:
  • Accenting lows sometimes produces "boomy" bass.
  • Very susceptible to wind noise. Not suitable for outside use unless very well shielded.
Example of use
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Condenser Microphones


Principle: sound pressure changes the spacing between a thin metallic membrane and the stationary back plate. The plates are charged to a total charge

where C is the capacitance, V the voltage of the biasing battery, A the area of each plate and d the separation of the plates.

 Advantages:
  • Best overall frequency response makes this the microphone of choice for many recording applications.
Disadvantages:
  • Expensive
  • May pop and crack when close miked
  • Requires a battery or external power supply to bias the plates.

A change in plate spacing will cause a change in charge Q and force a current through resistance R. This current "images" the sound pressure, making this a "pressure" microphone.

Further discussion
Pressure Zone Microphone Application
Microphone discussion
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