09/05/2024
#2 Ribbon mics
“Dynamic” in mic terminology refers to the mic converting mechanical energy (sound waves in the air) to electrical energy by a specific process: moving an electrical conductor through a magnetic field.
A conventional dynamic mic does this by moving a coil of wire which is attached to the mic diaphragm through a magnetic field. A ribbon mic is different in that the ribbon itself, which is the part of the mic that moves in reaction to incoming sound waves, moves within the magnetic field; the transducer and the “diaphragm” are one and the same.
The ribbon in a ribbon mic is a narrow, pleated strip of metal, usually aluminum, and is very thin. How thin? A human hair is maybe 50 or more times thicker. If you’ve ever been taught never to blow into a mic, the reason is that if it’s a ribbon mic, you’re likely to destroy the ribbon. Yes, it’s that thin. Most modern ribbons are actually somewhat tougher than the older ones, but still, don’t blow into the mic!
Ribbon mics have a very low output, and for most sound sources they require a preamp with a lot of gain. To get around this, some ribbon mics are active; that is, they have a small preamp onboard to boost the output of the mic to a more normal mic level, so you have a much wider choice of mic preamps.
Ribbon mics are known for their natural and “warm” sound, which is due, in part at least, to their typical high frequency rolloff.
Historically, ribbon mics were widely used in the U.S. in the early days of broadcasting, recording, and film making (once talkies came in). RCA, which built much of the radio equipment back then, also made microphones, and was known for their ribbon mics, the BK-5b, the 44b, and the 77DX, among others.
Ribbons gradually fell out of favor in the late 40s and 50s, but experienced a resurgence in the late 90s when Royer introduced their R-121. Since then, there have been many new ribbon mics put on the market.
One of the coolest things about ribbon mics is their pick-up pattern, or polar response, which is a direct result of the ribbon itself. Almost all ribbon mics exhibit a “figure-8” or bi-directional polar pattern. Sound that comes at the ribbon from directly in front of or behind the ribbon is picked up as it “hits” the flat part of the ribbon, while sound that hits the ribbon at an angle of 90 degrees, since it’s affecting both sides of the ribbon equally, is not picked up. It’s this deep null of the figure-8 pattern that can be very useful.
A few words (ok, more than a few) about mic pick-up patterns. There are three main patterns: omni-directional, cardioid or uni-directional, and figure-eight or bi-directional.
Omni mics pick up sound from all directions. Cardioid (heart-shaped) mics pick up sound mainly from the front, and figure-8 mics pick up primarily from the front and the back.
Two charts can tell you a lot about how a mic sounds and functions. A frequency response chart is a graph with frequency on the horizontal scale and level (in db) on the vertical scale that measures a mic’s frequency response on axis, or directly in front. The frequency is usually noted as 20 to 20,000 Hz. If the plotted response of a mic is say, +10 db at 100 Hz falling off to -10 db at 10,000 Hz, you can expect that mic to sound pretty dark, without a lot of high-end. Not a good mic for cymbals.
The polar response chart consists of several concentric rings (representing level in db) with radial lines indicating 0 degrees through 180 degrees and back to 0. 0 degrees is the front of the mic, and 180 degrees is the back side. A cardioid mic will have a graphed response showing it picks up sound better (with more level) from the front than from the side or back. But wait! It gets better! Most polar response charts are drawn with different lines (solid, dashed, dotted, etc) representing different frequencies. Some mics have a pretty flat response from the front, but when you look at the polar chart, you may see that from the back or sides, the mic’s frequency response isn’t flat at all from that angle. On a practical level, this means that the “bleed” into the mic, from nearby instruments or sound sources hitting the mic from any angle other than the front, can sound unnatural. Maybe that works for your application, and maybe it doesn’t. The Neumann KM 84 is a cardioid mic that has a very flat on-axis response and a pretty darn good off-axis response as well, so sounds entering the mic from directions other than right in front still sound fairly accurate, just quieter. As you might expect, that kind of engineering comes with a cost. But that’s Neumann, great engineering but expensive.
Ribbon mics aren’t the only mics that have a figure-8 pattern, many large-diaphragm condensers (and a few small-diaphragm ones) can be used in omni, cardioid, and figure-8 modes. They can do this because they have two mic capsules (the actual working part of a mic) back-to-back inside the grill. For cardioid, they just use one capsule, for omni and figure-8, they combine the output of both capsules in different ways to produce the desired pattern.
Anyway, back to the deep null at 90 degrees that a figure-8 mic exhibits. I’ve recorded acoustic guitar player/singers with a figure-8 mic on the guitar, and another for the vocal. If you carefully aim each mic so the front of the mic is pointed at what you want and the null is pointed at what you don’t want, it’s amazing that the two mics can be so close to one another, and each mic is pretty close to both sound sources (voice and guitar), and yet the guitar mic picks up lots of guitar and very little vocal, while the vocal mic picks up plenty of vocal and hardly any guitar.
Now, I would rarely use a ribbon mic on either an acoustic guitar or a vocal, but I have used a ribbon mic on an electric guitar amp many times, and that null can come in very handy. Many electric guitar players use stomp boxes or foot switches. Sometimes, they’re quiet and sometimes they’re not, plus the players themselves, being actual humans, may make noises that you don’t want to pick up. So what you do is set up your ribbon mic on the amp to get the sound you want, and then put the player 90 degrees off to the side of the mic, in the null. They can hear their amp just fine, and the mic won’t “hear” much of any switching or other noise they may make.
Besides guitar amps, I like to use ribbons on violins (and fiddles!) and horns, sometimes on percussion.
Next time: small-diaphragm condensers.