All About Ribbon Mics

November 1, 2012
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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 handful 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.

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