Condenser Microphones
Condenser (or
capacitor) microphones use a lightweight membrane and a fixed plate that
act as opposite sides of a capacitor. Sound pressure against this thin
polymer film causes it to move. This movement changes the capacitance of
the circuit, creating a changing electrical output. In many respects a
condenser microphone functions in the same manner as an electrostatic
tweeter, although on a much smaller scale and "in reverse."
Condenser microphones are preferred for their very uniform frequency
response and ability to respond with clarity to transient sounds. The low
mass of the membrane diaphragm permits extended high-frequency response,
while the nature of the design also ensures outstanding low-frequency
pickup. The resulting sound is natural, clean and clear, with excellent
transparency and detail.
Two basic types of condenser microphones are currently available. One
uses an external power supply to provide the polarizing voltage needed for
the capacitive circuit. These externally-polarized microphones are
intended primarily for professional studio use or other extremely critical
applications.
A more recent development is the electret condenser microphone (Fig.2).
In these models, the polarizing voltage is impressed on either the
diaphragm or the back plate during manufacture, and this charge remains
for the life of the microphone.
Figure 2
Electret Condenser Element
The best electret condenser microphones are capable of very
high-quality performance, and are used extensively in broadcast, recording
and sound reinforcement.
Due in part to their low-mass diaphragms, condenser microphones are
inherently lower in handling or mechanical noise than dynamic microphones.
For all of its electret condenser designs, Audio-Technica has elected to
apply the polarizing voltage, or fixed-charge, to the back plate rather
than the diaphragm. By doing this, a thinner material may be used for the
diaphragm, providing a considerable performance advantage over electret
microphones of conventional design. Many Audio-Technica microphone
diaphragms, for example, are only 2 microns thick (less than 1/10,000th of
an inch)!
Condenser elements have two other design advantages that make them the
ideal (or the only) choice for many applications: they weigh much less
than dynamic elements, and they can be much smaller. These characteristics
make them the logical choice for line - or "shotgun" - microphones,
lavaliers and miniature microphones of all types.
Attempts at miniaturizing dynamic microphones result in greatly reduced
low-frequency response, overall loss in acoustic sensitivity, and higher
mechanical or handling noise.
Phantom Power for Condenser Microphones
While
the electret condenser microphone doesn't need a power supply to provide
polarizing voltage, an FET impedance matching circuit inside the
microphone does require some power. This may be supplied by a small
low-voltage internal battery or by an external "phantom" supply.
Phantom powering is a technique which delivers a DC voltage to the
microphone through the same shielded two-conductor cable that carries the
audio from the mic. The phantom power may be supplied either by the mic
mixer or from an external supply that is "inserted" into the line between
the microphone and mixer input. For phantom power to function, the line
between the power supply and the microphone must be balanced to ground,
and uninterrupted by such devices as filters or transformers which might
pass the audio signal but block DC. Phantom power also requires a
continuous ground connection (Pin 1 in the XLR-type connector) from the
power supply to the microphone. The supply delivers positive DC voltage
equally to both signal-conducting leads, and uses the shield as a return
path, or negative. Balanced-output dynamic microphones are not affected by
the presence of phantom power, since there is no connection between the
shield and either signal lead and, therefore, no circuit for the DC
voltage.
Phantom power supplies are available in various output voltages ranging
from as low as 9 volts up to 48 volts. They may be designed to operate
from AC line voltages or from internal batteries.
Externally polarized or "discrete" condenser microphones seldom have
internal battery power. Instead, a phantom power source is used to provide
both the polarizing voltage for the element and to power the impedance
converter. This type is sometimes called a "pure condenser."
Other Types of Microphones
There a number of
ways to translate sound into electrical energy. Carbon granules are used
as elements in telephones and communications microphones. And some
low-cost microphones use crystal or ceramic elements that are generally OK
for speech, but are not seriously considered for music or critical sound
reproduction.
One other type sometimes found in recording studios is the ribbon
microphone. It is a form of dynamic mic, with a thin metallic ribbon
(which serves as both voice coil and diaphragm) suspended between the
poles of a magnetic circuit. While it is capable of excellent performance,
the ribbon element must be protected against high acoustic pressures or
wind, since it is relatively fragile. For this reason, ribbon microphones
are rarely seen in sound reinforcement applications or non-studio
recording.
Ribbon microphones are often designed to respond to sound from both the
front and back, and are sometimes used when a bidirectional pickup pattern
is required - which brings us to the next major microphone classification.
Continue
to the next section (What's the Pattern?)
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