Mid-Side Mic Technique
Mid-side or M-S mic technique is typically done with one figure-eight pattern mic and one cardioid pattern mic, or omni in some cases. But what would you say if I told you that it could be done with three cardioid mics? I'll show you, but first let me explain how standard mid-side mic'ing works.
The concept is pretty simple. A cardioid mic is pointed at the sound source. A figure-eight mic is arranged so that it is aligned in the cardioid mic's vertical axis and as close to the cardioid mic as possible. The cardioid mic is pointed at the center of the sound source. The figure-eight mic is pointed 90 degrees away from the cardioid. Typically the figure-eight will be facing in the direction of stage left. This leaves the opposite side facing stage right. On a typical figure-eight mic, the front will be the ' + ' or normal phase and the back will be the ' - ' or reverse phase side. It's important to note that getting the elements of the two mics aligned on the same vertical axis is important to this setup since this technique depends upon the signals being phase aligned.
The cardioid mic (top) in this case is an Audio-Technica AT4050 set to cardioid. The figure-eight mic (bottom) in this case is a Blue Woodpecker. You will note that the Woodpecker is arranged so that it's aligned in the AT4050's vertical axis and is extremely close to the AT4050.
From this arrangement you derive three feeds, the center feed is from the cardioid mic facing center. The left feed is from the figure-eight mic, the right feed is also from the figure-eight mic but it is inverted. So you have a left feed which you pan hard left, a right feed which you pan hard right, and a center.
Now for the magic part: Record at normal levels, and later during mix down you can adjust the width of the stereo image. Or mix to mono without artifacts from phase cancelation.
The stereo effect comes from phase addition and cancelation. Sounds coming from stage left on the figure-eight left feed will be in phase with the front cardioid feed but out of phase with the sounds coming from stage right. Sounds coming from stage right on the inverted figure-eight right feed will be in phase with the front cardioid feed but out of phase with the sounds coming from stage left. The width of the image is adjusted by increasing or decreasing the level of the center feed as compared to the left and right feeds. The levels don't need to be close together to get phase cancelation, but you do hit a point of diminishing returns. Generally more center equals a narrow stereo image and less center equals a wider stereo image with respect to side levels. It's similar to having an coincident pair setup, but having the ability to change the agle of the pair at mix down. I should mention that side levels (left and right) should stay even for best effect.
In my example photo I use different mics with different responce curves. Since this technique relies on phase cancelation, you get a little better stereo imaging using matched multi-pattern mics. I used non-matched mics to illustrate that this works with any option you have lying around. I typically will use a pair of AT4050s, but have used the arrangement shown with excellent results.
In some cases an omni mic is used in place of the cardioid. This is normally done in a closed environment so that the omni mic is pointed towards the sound source and there is no sound source at the rear of the mic. This changes the stereo image a bit. Instead of being like a coincident pair that you can dial in the angle of the pair, it's more like have two mics set 180 degrees apart but you have control of the left and right mic patterns. More center (with respect to the side channels) moves towards a wide-cardioid pattern and less center moves towards a hyper-cardioid pattern.
Using an omni center mic for mid-side recording in a live setting is also possible. This has the effect of capturing a whole room as opposed to just the side the center mic is facing. If you wanted to capture a performance and the audience in the same feed, you may opt for using an omni mic and placing the mid-side arrangement at the front of a stage. This will give you a stereo image of both the performance and the audience's reaction to the performance.
Now, as I hinted earlier, you can do this with three cardioid mics as well. As you see I have arranged three SM57's as a proof of concept. With three mics you have three feeds from three mics and no need to invert anything. Center mic is center feed. Right facing mic is panned hard right. Left facing mic is panned hard left.
This brings me to another fun thing to play with. One of the things that makes ribbon mics sound magical is that they are figure eight (most of them) mics. As such, when you use one, the mic not only picks up whatever you put in front of it, but it also picks up sounds from directly behind it (the room) in reverse phase. If you want to play with this effect but don't have a figure eight pattern mic, use two cardioid mics set up as the side SM57's are set up above. The mic pointed at the source goes to one channel. The mic pointed to the rear gets inverted and goes to a second channel. Set the levels equal so that the mics sum in your mixer or DAW. This offers you a way to play with this effect without buying a new mic. Even if you have figure eight mics, I find it interesting to experiment with this setup using different mic patterns.
Try any combination you have lying around, you may be surprised at the results you get.