All About Mics – Part II

So last week I wrote about the common types of the different microphones that are used in recording and in live performance settings. Now I'm writing about the different capture or reception patterns of microphones and a few of the configurations commonly used for recording different sound sources in the studio and on the stage.

Microphones have been engineered in a way to be receptive to sounds coming from particular directions in three dimensional space over others. This is a deliberate method of capturing sound sources that can vary widely, depending on the scenario that presents. The need to avoid sound bleeding over from other sources in a physical space, or to capture sound from different areas in a physical space are both valid needs when recording or performing.

So as to how these patterns are created, they are the result (explained in a very simplistic manner...we're not in an Engineering class here) of combining or selectively cancelling the signal(s) from different reception patterns, e.g., the figure 8 pattern of a mic receiving sound from both the front and back and mathematically/electrically cancelling the sound from the behind the mic due to the phase difference of the sound moving the diaphragm 180 degrees out of phase, front compared to back.

Other systems utilize a variety of internal traps to acoustically isolate the mic from all but one preferred direction. As a picture indeed is worth 1,000 words, I'm going to use a nifty set of diagrams I discovered that illustrate the patterns along with the common name by which the pattern is known. Keep in mind that while the diagram may show that a pattern is able to receive sound from certain directions, the extreme edges of those directions are not nearly as well received as more to the center of the pattern. Also, the larger the diaphragm a microphone utilizes, the more the high end tends to be mitigated at the edges of those patterns , while a smaller diaphragm mic is less affected in this manner.

One area that I'm not going to delve into too deeply is recommending a wide range of specific mics for certain applications. There is such a wide variety of mics out there and often the types can be used interchangeably for certain applications. I'm referring to Dynamic vs. Condenser vs. Ribbon. Much of this depends on ones budget and how serious one is about their home studio or mic'ing up their band or whatever the scenario may be.

Truthfully, a handful of dynamic mics can fit the bill if money is tight. Yet, if one is not a brand or country-of-origin snob and is ok with "Made in China", there are some decent condenser and ribbon mics out there that cost no more than some of the more expensive dynamic mics. Are they the quality of an AKG or a Neumann? Of course not. But they provide the essential qualities of their given class and do a pretty good job.

The Patterns

Cardioid


This is the quintessential live performance mic. Because it rejects sound from the rear particularly well, it's not as susceptible to wedge monitor feedback. They're often the first for close mic'ing a speaker cab or acoustic instrument in a noisy environment, or mic'ing an acoustic instrument in the studio while the player is also singing.

Supercardioid


Like the Cardioid, but theoretically better at high gain before feedback so long as one points the wedges at it from the slight side-rear to avoid the reception area that it has directly from the rear. It has the same uses where more rejection is required, so long as there are no sources in the narrow, beam-like reception area that it has from behind the long axis.

Omnidirectional


These are best when mic'ing a group of singers on the cheap, or a speaker with a wearable mic, like a lapel mic. It picks up sound from pretty much everywhere.

Figure Eight


These kinds of patterns are the domain mainly of large condenser mics and ribbons. They are advantageous when recording two or more people singing at the same time as the singers can be positioned to see each other. Other uses are for mic'ing a cab or an acoustic instrument as both the instrument and the ambient room acoustic representation of the source can be recorded. Care must be taken with positioning so as to avoid combing and cancellation effects when utilizing this technique.

Frequency Response

Consideration too must be taken in the frequency response that a microphone provides. Some have a relatively flat frequency response over the auditory spectrum (take to be 20 Hz to 20,000 Hz for the human ear) and some have a response which colors the source signal ever so slightly.

Below are a couple of graphs that illustrate what this concept looks like graphically. Why would one want to color the source? If engineered in correctly, it actually produces a more pleasing sound from the PA or to the recording media. An example would be the famous "presence peak" that the workhorse Shure SM58 is famous for when used for capturing vocals. The second graph illustrates something akin to this peak.


Mic Placement

Again, I have some graphics that illustrate potential placements for mics in capturing sound for live or recorded music. Where I have practical experience, I'll interject such that I have found useful in my recording work and as the FOH person that sets up the mics for a show and runs the board.

Guitar


Usually I'll point a mic at the bridge area and another one to just off to the side at one of the upper bouts where the fingerboard ends. I like to use small diameter condenser or dynamic cardioids/supercardioids for this application. A similar technique works with resonator guitars, mandolins and banjos.

One might want to avoid pointing the mic at the sound hole. I've found one may get a strange pulsating, almost woofing effect, especially if the guitar is a Jumbo body size like the Gibson J-200. It was explained to me by a retired audio engineer that it has to do with the top compressing as it vibrates and "forcing" air out of the sound hole; the top is moving away from the mic at that moment. The opposite occurs when the top's vibration is 180 degrees opposite.

The sound is difficult to explain. The closest thing that I can relate it too is that it’s kind of analogous to the sound a compressor makes when set with the ratio way too high. It's not a consistent thing that has happened every time I have tried to mic the sound hole of an acoustic guitar directly with a traditional mic, but it does occur, so just be aware.

Piano


As one can see, there are a myriad of positions that one can utilize with a grand piano. In principle, a spinet or upright is similar. In practice, I park one mic beneath the soundboard, pointing straight up about a third of the way back from the middle of the keyboard and I park another one inside and have someone move it around while another plays a fairly full range of the keys until it's parked in a sweet spot where the highs and mids are balanced.

The one underneath seems to do a good job at accentuating the lows and it's just a matter of balancing the two channels in the mix. This is one area where a ribbon mic can be a good choice for the one underneath, but...I've gotten away with using a pair of the Shure SM57 Dynamic Cardioid mics this way. In fact, I've gotten away with using the SM57 in most applications as the work I've done generally entailed facilities on a low budget.

Drums


This set-up illustrates the use of about ten mics to mic up a kit and while I've done this in a professional studio, for most applications, be it your home studio or on the stage, ten mics is overkill...not unlike swatting a fly with a framing hammer. I find seven adequate in most cases, and if one is imaginative, five will get the job done.

For seven, I park two small diaphragm condensers to capture the cymbals (my condensers for this app have a low end roll off switch of about 10 dB so that the mics effectively ignore the drums themselves). I park a mic to capture the two center mounted toms, one on the floor tom, and one each on the snare, the high hat and one inside the kick drum.

If one has only five mics, I disable the bass roll off on the two overheads to capture the cymbals and the toms and then one each as before for the snare, the high hats and the kick. I have an inexpensive drum mic'ing kit that has five mics and the mounting hardware. The overheads I purchased separately.

I've also used in situations where the mics were supplied, the two overheads as described, a dynamic mic that looked something like a mortar shell ( a Sennheiser low frequency dynamic mic as I recall) for the kick and you'll never guess what the other four mics were I bet. If you guessed Shure SM57's, you've been paying attention. Those buggers are good for just about anything.

Guitar Cab


In this application, if I'm using one mic, I will park it right in front of the grill, slightly offset from the center of the cone of one of the speakers. Yes, of course an SM57 works here too, but I like the Sennheiser e906 side address dynamic mic. I also use an Audix i5 where I might use an SM57 at times. One can either put it on a short stand as pictured or one can hang it over the side in the same offset, on the grill position.

It has a frequency adjustment switch if there is a need to accentuate/attenuate certain frequencies, which can be a bonus. It only picks up from the side marked front. If you’re using one and can't really hear what's coming out of the cab, turn it around. If I use a second mic, it's really what sounds good in the given room. Usually the second mic is a studio only kind of situation. If the cab is open-backed, parking a mic behind it instead can yield some pretty cool results.

As always, when using a second mic, take care to listen first to ensure that there are no cancellation of phasing effects going on. Usually repositioning the second mic will eliminate such oddities.

I was considering, due to a request, to add some ideas about mic'ing brass, woodwind and traditional stringed instruments. As I thought about this, I realized that I have very limited experience with these. In most cases, the instrument has had some sort of clip on mic (clips onto the bell or horn end) or piezo pickup of some sort that is simply run DI into the board.

I can say that the few times that I have used a traditional mic on brass and wood winds, the mic is Cardioid and set out about five to eight inches from the business end of the instrument, excepting the flute, where a second mic is pointed at the aperture where the player would introduce the airstream by blowing.

Stereo Mic'ing

I located a handy chart that describes some of the conventions developed for stereo mic’ing using one of the standard XY, near coincident or side-by-side positioning patterns. Note that the arrows or rays are pointing to some point that would include a decent portion of the source sound. Experimentation with distance is generally the first consideration. I use these techniques where I might be mic'ing a vocal trio or string quartet, in addition to some sort of pickup on the stringed instruments themselves.

These are the three patterns that are used and there are variations of these, but the topic of stereo mic'ing is one of those things that can be the subject matter for an entire article. I'm accustomed to taking basic information like I've presented here and experimenting, as that is an excellent way to learn how to employ such techniques. This is true especially in one's home studio, as the variable of your room will require adaptation to any of the "standard" techniques, if one wants the best results.



Conclusion

Well, there is the long and short of it. For the reader that has little to no experience with microphones or mic’ing techniques, my hope is that my words have provided some useful information.

For the one with a modicum of experience, I am hopeful that I’ve provided at least one insight of value that such a one did not possess before. After all, we only have each other and if we share what we know, then as a community of musicians and musician-support folks, we’re that much better off for the collaboration of information and ideas.

-Kirk Bolas - MU Columnist
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