Figure-of-8 ribbon mics have the best rejection of any of the mics out there in the world. At first glance, most audio engineers would think that this statement is an oxymoron. How can a mic that picks up sound on both the front and the back have better rejection than a cardioid microphone, one that picks up sound only on the front?
This is because ribbons have a dead space between both sides of the ribbon, where absolutely no sound called be captured by the microphone. This is called the null. In fact, the side, top, and buttom rejection nulls in ribbon microphones are one of their biggest pluses. Whether in live performance or recording in a studio, savvy sound engineers use a ribbon’s figure-of-eight pattern and side nulls in creative ways to tactically to block out specific sounds and to emphasize targeted sources.
What Are Nulls?
While a figure-of-eight microphone picks up sound on both sides of the ribbon, any sound entering from the top, bottom and sides of the mic is muted — or nulled. Nulls are dead zones on a microphone where little to no sound is captured. Nulls make ribbon microphones among the best available for rejecting unwanted sounds.
Why Do Ribbon Mics Have Nulls?
Ribbon microphones have nulls because of the fundamental nature of the ribbon element and the way it captures sound. The front of the ribbon is the positive side and the back is the negative side. Any sound that enters the top, bottom and sides of a figure-of-eight ribbon mic is rejected and not picked up.
This is because the positive and negative sides of the ribbon cancel each other out when the sound passes equally over the null. The null allows ribbons to be very useful when recording multiple musicians in the same room.
In the figure-of-8 polar pattern image below, you can see that between the front and back lobe, the sound disappears.
Using the Nulls
To effectively use the nulls, an engineer must determine which sounds are to be avoided by the mic. For example, when recording on a small live stage, a figure-of-eight ribbon can offer far greater isolation than a cardioid condenser mic when positioned correctly.
One of the most popular applications for a pair of figure-of-8 ribbons is to use the nulls when recording singers who play acoustic guitar. By using two ribbon mics, one on vocals and the other on the guitar, each mic can be used to null out the opposing sound.
Start by placing a mic in front of the singer’s mouth with the bottom or side null of the mic pointing directly at the acoustic guitar. Place another mic facing the acoustic guitar with the top or side null of the mic pointing at the singer’s mouth.
This technique can create a large amount of isolation with depth that can’t be achieved even with directional cardioid microphones. It also sounds better, offering that smooth, natural ribbon sound.
Another way the nulls can be used is with drums. One way is to close mic a kick drum with a ribbon mic while avoiding cymbal bleed. Position the top of the mic so that it points towards the cymbals at the top of the drum set. The cymbal sound should evenly pass over both sides of the mic leaving a minimal bleed and a thumping kick.
Watch an example of the R92 on kick
Then there’s the opposite scenario. Ribbons make for some of the best tom and snare mics because the null allows their rejection of the low end of the kick drum. Just the reverse of the previous method.
Ribbon mics can be used either way on drums, both to great effect.
When recording live with multiple loud instruments, bleed is undoubtedly going to happen. While the back side of the ribbon is likely to pick up some of this bleed, it will mostly be from room reflections — much like using a room mic.
Sounds entering the back of microphone will be softer based on their relative amplitude and distance from the microphone. With the typical gain structure required for a mic that is in front of a guitar cabinet, the incoming signal from behind the mic is going to be considerably softer by comparison.
Using the nulls, avoid direct bleed from instruments and encourage ambient bleed. This minimal bleed can offer a major benefit to your sonic signature. It is often the glue that holds the tracks together.