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.
HEART AND SOUL
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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