Using multiple figure 8 pattern microphones to record in stereo

Posted: 17th January 2012 by Mezzanine Floor Studios in Recording Techniques

Polar patterns indicate the “directionality” of a microphone. They are relatively straightforward but are often misunderstood. This is especially true with the figure-8 pattern that is natural to ribbons and available on some switchable large diaphragm condensers.

Think in 3D

When first studying the polar pattern chart on a microphone’s documentation, people tend to think in only two dimensions. This is sufficient for understanding how Omni or Cardiod mics work, but an engineer with only a two dimensional understanding of polar patterns cannot fully understand and utilize microphones with a figure-8 pickup pattern (or indeed with hyper-cardioid or super-cardioid microphones either.)

Thus, the figure-8 pattern is commonly described in words as a pattern where sound is “heard” by the microphone from the front and the back, but not from the sides. This is true but incomplete. A mic with a true figure-8 pattern also does not “hear” sound from the top or bottom.

Practically speaking, this means that the engineer will want to point the front and/or back of the figure-8 microphone at what they want the mic to “hear”, and the null points at the sides, top, and/or bottom of the microphone at what they don’t want the mic to “hear.”

Recording separate sources with 2 figure-8 mics

When using two figure-8 microphones to record two different sources, such as a voice and an acoustic guitar at the same time, it can be very helpful to keep the top/bottom null points in mind, as with some mic stands it would be easier to point the top of one mic at the vocalist and the top of the other at the guitar than to try to point the sides of each microphone at what they don’t want to hear.

Recording in stereo

Generally speaking, one of two techniques will be used when recording in stereo with figure-8 microphones- Blumlein or M/S (Mid/Side). If focused on the microphones themselves these two techniques appear identical. In both cases the microphones are set up so the front of one mic is rotated 90 degrees from the front of the other. In this way, the “null” point where one mic doesn’t hear anything is covered almost completely by the other microphone, and vice versa. The similarities between the techniques end there, however.

Blumlein

Using the Blumlein technique the front of each microphone would be pointed at a 45 degree angle to the sound source being recorded. In this way, one microphone clearly captures sound from the left side of the source and the other clearly captures sound from the right side of the source.

  • This could be left and right overhead microphones for drums, recording an acoustic guitar in stereo to capture both the sound-hole and the 12th fret, stereo room microphones, etc.)
  • The back of the figure-8 pattern also picks up sound. Thus, one mic really picks up the left side of the sound source and the right side of reflections from the room behind the microphones, and the other microphone picks up the right side of the sound source and the left side of reflections from the room behind the microphone. This means that generally the backside of a Blumlein pair should not point at something you don’t want to hear. If using a Blumlein pair for room mics, for instance, it is best not to put the pair right up against the back wall of the studio.

The result of Blumlein is a “normal” stereo pair of tracks where one would be panned hard left, the other hard right.

Mid-Side

There are two differences between the Blumlein technique and the Mid-Side technique.

First, the microphone pair will be effectively rotated together 45 degrees. The front of one microphone* will be pointed directly at the source of sound that is being recorded. The other mic would have it’s side (null point) pointed at the sound source being recorded, and would capture only sound that comes in from the left and right sides of the room. In general this means the front facing microphone will pick up mostly direct sound from the source, and the side microphone will capture sound that is reflected off the side walls in the room.

Second, the signal from each of these microphones then must be encoded properly to obtain the proper results (see the Wikipedia article above for more detail.) The end result of the Mid/Side technique is a recording where the engineer can control the center of the mix and the left/right level of the mix separately, and where the left and right signals collapse into Mono perfectly, making it an excellent technique for recording in stereo when some listeners will only be able to hear in mono (which makes it very popular for radio engineers.)

* The front facing “mid” mic in a mid/side pair can actually be a microphone with any directional (non-Omni) pattern, although Cardioid and Figure-8 are the most common.

Capturing tone

Some figure-8 patterned microphones, including many ribbon microphone designs sound different from the front than they do from the back. Generally the back of these microphones will have a darker tone. Figure-8 mics with different tone in front and back are best used for close-micing, passable as room mics when solo or in Blumlein configuration if the tone differences don’t adversely affect the desired tone, and are generally avoided when choosing side mic for a Mid/Side configuration.

Other figure-8 patterned microphones, including some ribbons and most switchable large diaphragm condensers sound virtually identically in front or in back. These don’t give as many tonal options, but are more consistent and open sounding and tend to work well for close micing, room micing solo, or in pairs in Blumlein or Mid/Side configuration.

Think 3D, part 2

Remember when we talked about thinking in three dimensions with the figure-8 pattern? When recording in Blumlein or Mid-Side remember that the “top” and “bottom” are null points for both microphones. Usually Blumlein or Mid-Side mic positions are defined by what you want each mic to hear, but there are a few cases where you might also point the top or bottom of the configuration at what you don’t want to hear so that sound is reduced or cancelled entirely.

  • Think about using the Blumlein pair to record drum overheads in stereo in your basement. You just might have an AC vent in the ceiling you don’t want the microphones to hear. Rotating the Blumlein pattern so the top of each mic points at the vent may accomplish this very well as long as it doesn’t significantly change the microphone’s relationships to what you want them to hear
  • Think about using a Blumlein pair to record acoustic guitar. One mic could be pointed at the sound-hole and the other at the 12th fret. If you do this with the top of the mics pointed at the ceiling, though, you might miss an important opportunity. If your figure-8 mics have strong null points at the top and bottom, try raising the configuration a little so it looks down on the guitar slightly and rotation it forward so the top of each mic points at the guitar player’s face. Provided they don’t move much while playing this may help cancel out the guitar player’s breaths.

 

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