Recording - November 2011 - 41

Fig. 3

directly in front of the mic. And if the sound comes from 90 degrees the output of the mic will be 0, or silence. Keep in mind that these are theoretical numbers in an imaginary space where there are no wall reflections. If you sing into the side of a figure-8 microphone you will hear something, but it will be much quieter than if you sang into the front. I have colored the top (front-facing) lobe of the figure-8 in blue, and the bottom (rearfacing) lobe in purple, to indicate polarity. The first figure-8 mics were ribbon mics, which were made by stretching a vertical strip of metallized ribbon across the field of a magnet. If positive pressure came from the front of the mic, the ribbon would be pushed backward and the mic would produce a positive voltage. If positive pressure came from the back of the mic, the ribbon would be pushed forward and the mic would produce a negative voltage. So we say that a figure-8 mic reverses the polarity of sounds arriving from the rear. (See Figure 2.) This was not an intentional design feature, it’s just what happens when you build a mic that way. Any figure-8 mic works this way, even if it’s not a ribbon. That’s just the nature of the beast. If you duplicate an audio track in your DAW, reverse its polarity, and mix it back with the original, you will hear silence, as long as the faders and pan knobs on the two tracks are set the same. Now keep this concept in mind as we continue down the rabbit hole. For now—an omni and a figure-8 Mid-side stereo recording is usually done with a figure-8 and a cardioid mic, but it is easier to explain if we start off with a figure-8 and an omnidirectional, and it works that way too. Omnidirectional mics pick up sound from all directions, and do not alter polarity at all. Cardioid mics pick up sound from the front and sides and reject sound from the rear. See Figure 3, where the polar patterns of omni, cardioid, and figure-8 mics are compared. Let’s say we are recording a blue-eyed singer from Hoboken, New Jersey. You would point the omnidirectional mic right at him, as usual. (While an omni technically has no preferred direction, many real-world omni

mics tend to have better high-frequency response from the front of the capsule, so pointing the omni does make a difference.) Then you would place a figure-8 mic right above or below the omni, and as close as possible to the omni. But you would point the figure-8 mic sideways with respect to the singer. For the purposes of our discussion, let’s say you point the mic’s front-facing lobe to the singer’s left and the rear-facing lobe to the right, although again with a perfectly symmetric mic this won’t matter. In Figure 4, the upper mic is set up as a figure-8 and pointing sideways with respect to the singer; the lower mic is set up facing the singer (and could be an omni or cardioid in this position).

Fig. 4

Record the two mics to separate mono tracks on your DAW. Once you have recorded them, make a duplicate of the figure-8 (Side) track. You now have three tracks. Pan the omni (Mid) mic down the center. Pan the Side track all the way to the left. Then pan the duplicate of the Side track all the way to the right, and reverse its polarity. (In any DAW you can easily reverse or “flip” the polarity of a channel with a button on the channel itself or via a plug-in. Some DAWs refer to this as inverting the audio. If you have a pro analog console you might also find a polarity flip button on each input channel.) Set all three faders the same for now. What you have just done in your DAW is the matrixing. That wasn’t that scary, was it? In Pro Tools, using the Trim plug-in to invert the polarity of the duplicate Side track, your mixer channels would look like they do in Figure 5.
RECORDING NOVEMBER 2011

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Recording - November 2011

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