SIDEBAR 1: Microphone Topology

copyright 1997 by Eddie Ciletti

There are many different types of microphones — dynamic, ribbon and condenser — each with unique  characteristics.  These names refer to the technology used to convert sound waves into electrical waves, some being more suitable than others for specific applications.

A dynamic microphone capsule is essentially a headphone driver in reverse.  Picture a loudspeaker, its cone (diaphragm) is attached to a "voice" coil which is then suspended in a magnetically-charged air space.  Moving the cone generates a voltage in the coil OR impressing a voltage on the coil will move the cone.


A speaker in free air radiates sound from both front and back (a figure-of-eight pattern) until placed in a cabinet so that sound radiation can be made directional.  This is the same principal used in reverse to make a uni-directional or Cardioid microphone.  A Ribbon microphone is a variation on the dynamic theme.  It suspends a metallic foil in a magnetic field.  The surface area of the foil also serves as a diaphragm, but with less mass and inertia than a dynamic mic, hence improved transient response.   Ribbon mics are also inherently bi-directional and, like dynamics, are naturally low- to medium-impedance devices.  Transformers are used to match coil or foil impedance to the 200 ohm standard.

A "Condenser" capsule consists of a metallized plastic diaphragm suspended like a drum head over a metal back plate.  These two conductive surfaces don’t electrically touch.  The surface area and the air space between them determines the capacitance, which is typically less than 100 pico farads (pF).  This highly vulnerable sound source requires a buffer amplifier whose input impedance can be as high as one gig-ohm,  about one thousand times that of a guitar amplifier!  The built-in amplifier can be a vacuum tube or Field Effect Transistor (FET) followed by a matching transformer or an electronic impedance matching circuit.

A single diaphragm condenser capsule can be mechanically designed for omni-, uni- or bi-directional characteristics although the latter is not common.  More common are dual-diaphragm capsules with electronically variable patterns.  A fixed polarizing voltage is always sent to the front capsule.  Making the  voltage on the "rear" capsule more or less positive changes the patterns.  This can be accomplished with a switch or made continuously variable with a pot.  Electret microphone capsules are designed to retain their  electric charge.  External or "phantom" power is only required for the preamp.


As you can imagine, the sonic "signature" of a microphone primarily originates from its capsule design.  The mechanical and electrical methods of achieving the directional characteristics also happen to be great sonic contributors.  Omni mics are generally flatter, though some have a rising top end.  Both Cardioid and Bi-directional (Figure-of Eight) characteristics exhibit Proximity Effect, a substantial low-frequency "warmth" when these mics are used close-up for vocals.

The grill and the microphone body also contribute to the sonic character.  So do transformers or their  electronic equivalent.  Like most things analog, most "flaws" are often perceived as sonic assets at best or as limiting the microphone’s application to specific instruments or situations, at worst.

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