Ribbon microphones are a popular choice among
engineers for recording
bass. To understand a bit more about how they work, we turned
to our friends at Electronic Musician.
A RIBBON MICROPHONE IS A TYPE OF DYNAMIC
mic in which a thin, corrugated strip of aluminum suspended
between two poles of a strong magnet serves as the diaphragm
and voice coil. The ribbon reacts to velocity of air particles (rather
than the pressure, as with moving-coil dynamic mics), and as it
moves within the magnetic flux field, it generates a small AC voltage
proportional to this velocity. Clamps attached to either end of
the ribbon also serve as contact terminals: Wires carry the signal
to a step-up transformer, which then raises the output voltage
and boosts the output impedance to a usable level for a preamp,
typically around 150 to 300 ohms.
Because the ribbon element responds to sound waves arriving
from the front or back, but is insensitive to sound coming from the
sides, most ribbon mics have a natural bidirectional polar pattern
[see diagram], which makes them ideal for eliminating unwanted
noise between two sources or for use in M/S and Blumlein stereo
recording configurations. Classic ribbon designs do not contain
internal electronics—just the ribbon, magnets, transformer and
occasionally a passive highpass filter network.
THE BIRTH OF THE RIBBON
Ribbon or “velocity” microphones were developed in the 1920s
by engineers at RCA, with the first commercial model, the 44,
appearing in 1931. The 77A ribbon microphone, introduced a few
years later, combined two ribbons (one velocity and one pressuretype)
for a unidirectional (cardioid) pattern. The ribbon element design, which offered
superior directional characteristics
for reducing noise and improving speech
intelligibility, was instantly embraced by the broadcast
and recording industries. In addition, the
“talkies” movie boom created a huge demand for
directional microphones, and many manufacturers
in both America and Europe, including IBM,
Shure, Electro-Voice, Bang & Olufsen, Western
Electric and Marconi, began developing ribbon
mics. Some notable examples include the STC
4033A, which was produced to meet the needs
of talkies and contained ribbon and moving-coil
elements, with variable pickup patterns; and Beyer’s
first durable “short-diaphragm” ribbon mic.
In addition, the BBC designed many successful
ribbon mics for its own use, such as the legendary
PGS, which was later manufactured by STC
as the 4038, taken over by Coles in the 1950s
and is still in production today. Ribbon microphone
development peaked in the ‘40s and ‘50s,
but these fragile, heavy designs fell out of favor
during the next decade, eclipsed by the development
of new smaller and lighter condenser microphones. With the exception of a
of models, ribbon mic production stopped until
the late 1990s, which ushered in the era of reissues,
vintage reproduction and new high-tech
designs that continue today.
CARE & FEEDING
Recording engineers often swap stories about
tough lessons we’ve learned: We remember when
we didn’t record a rehearsal that was the best performance.
We learn that media is cheap and performance
magic is unpredictable. Here are important
lessons we’ve learned about using ribbons, ranked
roughly in order of importance.
Strong wind is an enemy. Don’t blow into a
ribbon microphone. A ribbon element is as thin
as any condenser diaphragm and it’s only clamped
at the ends. This is good for bass and smooth, fast
transients, but large air movements can stretch a
ribbon to the point where the sound quality changes.
Air movement can be caused by theater curtains
closing, slamming a mic case shut or spring breezes
when the cartage company opens studio doors.
Don’t leave your amp turned up when changing
an instrument. Air blasts are a ribbon’s worst
enemy. If you don’t turn the amp down when you
plug an instrument in, the speaker cones try to
launch themselves out of the cabinet.
Cover a ribbon microphone when not in use.
This protects it from wind and “tramp iron,” the
fuzz that covers a magnet after dragging it through
a sandbox or across a workbench. If enough gets
collected, it will obstruct the ribbon.
Phantom microphone power can snap a ribbon
if you have a shorted mic cable or bad power
supply. Usually, a ribbon mic ignores phantom
power. Rarely, however, is not never, as we know
well from repairing mics. Turn off the phantom
power a few minutes before plugging in and let
the voltage bleed off .
Store long ribbon mics vertically. The middle
of the pleated ribbon can droop from the pull of
gravity. This lowers the tension, which can dull
the response or cause the output sensitivity to
change, depending on where the ribbon fl ops in
relation to the magnetic gap.