Basic Microphone Information
So what is a microphone,
and what does it do?
A microphone is basically a speaker
in reverse. While a speaker is designed to turn electrical
current into the movement of air waves, a microphone is
designed to do the opposite. They are both referred to as
tranducers, which just means that they change one form of
energy into another. The technology a microphone uses to
create this change differs, depending on the type of microphone
used. A microphone is designed to capture sound waves in
a form that can be recorded or transmitted into electrical
energy, so the sound waves can be repeated later. Vibrating
air waves hit the microphone membrane, causing it to vibrate
at the same frequency, which is then turned into an electrical
signal. This seems a bit abstract, but it's not as bad if
you can think of electricity as a movement of electrons.
The membrane moves, causing the electrons to move. A speaker
is eventually sent the signal captured by the microphone,
essentially repeating the microphone's action in reverse,
making the air waves move again at the same frequency the
speaker is vibrating.
What are the main types of microphones, and how
do they work?
The main types of microphones used
in live and recorded audio are Dynamic, and Condenser.
Also, very expensive ribbon mics have been used
in some studios over the years.
The Dynamic mic uses a thin membrane
connected to a coil of wire, which is electromagnetically
charged and suspended around a magnet or something charged
with magnetic polarity. When the air waves hit the membrane
it vibrates at the same frequencies, causing the electromagnetic
field to vary at a certain speed, which moves an electrical
signal through whatever you plug the mic into.
The Condenser mic is really just
an electronic capacitor. It works on electrostatic principles,
rather than electromagnetic principles like the dynamic
and ribbon mics. It has one moveable plate and one fixed
plate. When air waves hit the membrane it vibrates at the
same frequencies, causing the two plates to move closer
together. As the distance between the plates decreases,
the capacitance increases. When the distance increases,
the capacitance decreases. According to the electronics
equation Charge=Capacitance x Voltage: if the charge is
constant (as it is with condenser mics) and the capacitance
changes (as it will when the membrane starts vibrating with
the sound waves), then the voltage changes in proportion
to the sound waves. This is the basic principal behind a
condenser mic. Because the membrane doesn't have to move
through a magnetic field a lighter membrane can be used,
resulting in a more accurate response.
A ribbon microphone works on similar
principals as the dynamic mic. It uses a very thin aluminum
ribbon to pick up the sound waves, though. The ribbon is
suspended in a strong magnetic field of flux. As the air
waves cause the ribbon to vibrate, it moves across the magnetic
lines of flux. This creates current in the ribbon proportional
to the sound waves. Because the ribbon is both the diaphragm
and the wire it can be made lighter, which creates a more
accurate microphone. However, ribbon microphones are notorious
for their susceptability to temperature changes and their
tendency to break with rough handling or in high volume
miking situations. They also tend to be unpractical in price,
so if you do own one you'll probably treat it like a deity.
Which is why only some studios use this type of microphone:
they're very expensive, and they're easy to break.
What are the different types of pickup patterns
microphones use?
There are five main patterns used: Cardioid, Supercardioid,
omni-directional, figure eight (bi-directional),
and lobar (shotgun).
A cardioid pickup pattern picks up
sound from in front of the mic capsule while rejecting sounds
from behind. It picks up sound from directly in front the
strongest, while gradually weakening as you move you at
an angle from the front. Its pattern looks a bit like a
heart. It's good for situations where you don't want a lot
of ambient noise or stage noise.
A supercardioid pickup pattern is
basically a tighter version of a cardioid pattern. It's
pickup strength drops more sharply as you move away from
the center. It's good for live situations where you want
an extreme degree of seperation between different mic signals.
An omni-directional pickup pattern
is one that picks up sound equally from all sides. A large
bodied mic tends to prevent sound waves from reaching the
capsules at certain angles, so omni-directional mics are
often made with smaller bodies. Its pattern looks like a
sphere. It's particularly useful for recording applications
where you want a lot of ambience.
A figure eight pickup pattern, or
bi-directional, picks up sound from the front and back equally,
but rejects sound directly from the sides. As you move away
from the front or back towards the sides the pickup strength
weakens. Its pattern looks like a three dimensional figure
eight. It's good for different stereo miking techniques,
such as the bloomlein and mid-side techniques. It's also
sometimes used for cymbal overheads on drums, when just
a single mic is desired.
A lobar pickup pattern is what is
found in shotgun microphones. It has an incredibly tight
pickup that picks up sound almost only from things directly
in front of it. This is why it is generally pointed directly
at the sound source. Its pattern looks like a long balloon
used for tying balloon animals. It's used for movie and
television recording, as well as surveillance or distance
field recording.
