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This article, Microphone Basics - Part 3, covers the M/S Pair (Mid-Side) microphone placement and decoding techniques. Recording engineers can gain a significant improvement in their results, through a solid understanding of the principles surrounding microphone selection criteria, and microphone placement, relative to a sound source, when they possess a firm knowledge of the design of each type of mic, and intended applications for them.

continued from Part 2

Microphone Basics - Part 3

Finally, let's review the Mid-Side pair, or M/S pair. Here, we will use both a bipolar mic, and a single cardioid condenser. Place these so that the bipolar is facing “across” the source – you want each “hot” side of the mic to face 90 degrees away from the direction of the source. Then, place the cardioid mic, so that it “fills” the centre space between the “sides” of the bipolar, and aim it straight at the source, right through the middle of the bipolar's cross-axial orientation. The objective, is to blend both mic's inputs, so that the “mid” mic (the cardioid) will pick up the “centre” information from the placement, while the bipolar will pick up each of the “sides”, or the room sound, due to its double-faced design and positioning. The issue that presents itself here, is that, because the bipolar's capsules are faced in reversed directions to each other, they are also exactly 180 degrees out of phase with each other (described in other articles on this site), which is the cause of their excellent null-point rejection, however, as such, audio from one side of the bipolar, which mixes well with the centre mic, will most definitely not mix properly, if it arrives at the capsule on the opposite side, because the two sides will be out of phase with each other. So, to fix this, we would have to find a way to “split” the bipolar mic, and reverse the phase of one side of it, so we could get the same in-phase mix from both sides, with the centre mic added in.

The solution? First, take the bipolar mic's signal, and split it into two separate signals, so that each “side” can be connected to its own input channel. Then, reverse the phase of one of the sides, either with the phase reverse switch on one of your input channels, or, if you don't have these, by using a specially-wired cable to switch the “hot” and “cold” components of the circuit. Either way, once this is done, the two sides need to be panned left and right, and the mid mic can then be mixed into the centre, creating a roomy stereo image, plus a strong, 100% mono-compatible centre spot. Balancing the mix of the mid and side signals will “widen” or “narrow” the stereo image presented. This is probably the best configuration for doing solo acoustic guitars, or any soft and dynamic instrument, especially in a nice-sounding room.

By choosing the right mic or mics for the job, and then positioning it/them correctly, the vast bulk of possible problems in recording will be immediately eliminated, allowing you to concentrate on the performance your are capturing, rather than on problems with the sounds you are getting.

Go to Part 1 or Part 2 of this article.

In the following article, Mic Placement Techniques - Part 3, the concept of spot listening to a source is discussed, with the technique applied to microphone placement for recording. Engineers wishing to refine their results, particularly in acoustic instrumental recording, will find the ideas covered here to be useful in helping them to position their microphones for the best effect.

Mic Placement Techniques - Part 3

continued from Part 2

Commonly-arranged mic pairs are all named by design. In the coincident group, we have the X-Y pair, the Blumlein pair, and the M-S pair. The spaced grouping includes the A-B pair, the NOS pair, the ORTF pair, the sometimes-called Nashville pair, the 3:1 ratio pair, and a couple of interestingly tricky pairs used for isolating different aspects of a recorded performance, such as a guitar and a vocal.

Probably the best known coincident stereo mic pair is the X-Y pair, because the mics are arranged roughly along two axes, and roughly perpendicular to each other, with the mic's capsules (the business end) placed as close together as possible, without continuous or sporadic contact with each other. Using two cardioid mics, the result is a mic pair that provides two different spots in the stereo field, both captured from exactly the same point in space.... so, the image has essentially zero phase anomalies, and no smearing at all. The image can be widened or narrowed by altering the angle of the mics to each other, as long as the capsules remain in the same spot. The range of angles runs between about 70 degrees apart, to about 110 degrees, at which point the result is either two near-identical signals (loss of stereo width, as the angle is reduced), or two largely out-of-phase ones (possibly correctable by switching the polarity of one side).

The Blumlein pair, one of the true original mic pairs, developed by Allan Blumlein through a series of early experiments with stereo micing, is a very characteristic mic pair with some intriguing features, including ultra-discrete stereo separation, and pinpoint panoramic location of a source within the stereo field. To build it, we need two bipolar (figure-8) microphones. Place the mics one above the other, capsule-to-capsule (the top mic upside down), with the label-side hot spots each facing the same "side", but at 90 degrees to each other. In other words, the mics are aimed "across" each other, in a centred crosshair layout, so that each mic is directly above/below the other, and so that the two "hot" sides are pointed at the source, at a right angle to each other. The source should be centred on the mics, so that it plays "into" the midst of the 90 degree "hot zone" created between the two mics. Now, because each bipolar mic is very sensitive in the centre, but almost completely rejects sound entering from the sides, and because the side of one mic is placed directly where the centre of the other one is, and vice-versa (90 degrees to each other), the amount of separation between the channels is amazing, making this pair extremely satisfying for things like vocal acapella groups and small string groups, where the "performance image" can be recorded, with each source's position in the image accurately portrayed, and again, because the Blumlein is a coincident pair, with no transient smearing. An additional advantage of the Blumlein pair, is that most all bipolar mics are large-diaphragm mics, which will collect the lower, warmer frequencies better than small-diaphragm mics will, creating a more realistic final recorded image.

Moving up to the M-S pair, we enter a whole new world, involving the use of an unmatched mic pair (or, potentially so) to create the image. Unlike most all other mic pairs, that use two identical mics to create two identical "sides" to a stereo image, the M-S pair uses two different mic patterns to create two distinct components in a signal - one "mid" signal, and one "side" signal, and these are then decoded electrically, to create the stereo image we want.

To begin, we will need one bipolar "side"mic, and one "mid" mic, usually a cardioid, though this can be changed, for different effects. The setup, involves the coincident placement of the capsules, so that the mid mic is aimed directly at the source, while the side mic (the bipolar) is aimed so its two poles (the hot spots) are pointed out to the sides, at 90 degrees to the mid mic, and the source. The result, after tracking, is a direct, centred signal from the mid mic, and a separate, roomy and indirect signal from the side mic. Now, the issue is, that the side mic, having two opposite poles, back-to-back, when mixed with the mid mic, will have one side in phase with it, and one side exactly opposite in its phase, creating severe problems. The solution, is to create a copy of the side mic channel (in analogue terms, "mult" it), and then, flip it 180 degrees in polarity, and finally, combine the original with the mid mic, in one channel (left or right), and the "flipped" copy with the mid mic in the other channel.... and voila... we have a stereo image, with a perfectly in-phase centre, no matter what we do, and a nice, controllable room that can be both panned in and out, and changed in mix level, to alter the stereo characteristics of the result. In mono, the room will simply disappear, but the centre will remain strong, because it's already in mono.

As mentioned previously, most coincident micing applications, while very effective at capturing accurate transients and phase-aligned stereo information, tend to have a less-open, and somewhat more sterile sound to them, making spaced pairs a valuable alternative for great stereo image recording. As with all things, though, greatness comes at a cost, and spaced pairs require far more care in their setup, as a rule, to obtain truly stellar results.

Beginning with the simplest of ideas, the A-B pair is a spaced pair that shows just how great really simple can sound. The pair is, simply, two omnidirectional microphones, each positioned where the harmonic activity is most pleasant, and placed at about the same distance each from the source, to avoid any arrival-time smearing. The magic of this pair, is that the omni mics tend to produce an incredibly transparent and lifelike tone, and also exhibit almost no phase interreactions between themselves, nor any proximity effect to speak of, so their placement can be more freely decided, on the basis of a great tonal field, alone. Their disadvantage is that they are generally roomy, and need to be carefully-distanced from the source, and they can exhibit a phase combing effect, if their relative distances from the source don't match, and they are fairly close together.

To avoid some of the issues of arrival-time combing and smearing, a rule of thumb, called the 3:1 Rule, has been developed as a quick reference for mic spacing. Simply, mics should be placed at spots that are approximately 3 times the distance apart, as they are from the source. By following this rule, the probability is that the audio entering each mic will be differentiated enough that little or no phase combing will occur, and that the transient response of the further aspects of the source will not factor prevalently in the pickup of the closer stuff..... sound complex? Really, it's just a case of the Inverse Square Law, again, making the distant parts of the sound really quiet to the farther mic, and so, separating the signals from the two mics enough to allow the pickup of a stereo field, with few phase issues.

Click here for Part 4 of this article.

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Earthworks DK-50 Drum Kit

The Earthworks DrumKit mic combination kits represent a new level of simplicity and fidelity in the micing of drum kits. Using only three mics, the DrumKit system captures each transient virtually perfectly, revealing every tiny nuance of the drummer's performance.

Contact Rock Shop Pro Audio for more information on the Earthworks DrumKit system and other Earthworks products.

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