How to Mic Six-Strings Like a Pro
Three stereo miking secrets guaranteed to make your acoustic guitar tracks shine
Ask five engineers how they approach recording acoustic guitar, and you may very well get five different answers, if you get any at all, that is. While many factors influence an engineer's choice of recording techniques -- the character of the instrument, the style of music, the player's personal tonal and dynamic signature, and the recording environment -- some engineers prefer to keep their approaches a trade secret. But despite all the possible permutations of instrument, style, player, room, and ego, for most seasoned engineers the real secret to recording an acoustic guitar is stereo miking -- plain and simple.
Sure, if it's just "spaciousness" you're after, you could record an acoustic guitar in mono and fold in some enthusiastic stereo processing. Or if you're hankering for a dramatically textured sound, you could try combining a microphone with an acoustic guitar pickup. If depth and accuracy are what matter most, however, you'll find that stereo recording with two microphones is an exceptionally reliable way to record full-bodied, realistic-sounding acoustic guitar tracks.
Let's take a look at three of the most popular techniques, and also examine the questions of microphone choice and mono-compatibility. As you'll find, stereo recording can be a complex art, since the interaction between the two mics will determine many aspects of the sound -- including tone, image, and mono-compatibility. But like any art, you'll also find that practice makes perfect. So whip out those mics, pull out that guitar, and give these tips a try: You'll hear the difference.
Three Surefire Techniques
In most cases, you'll want to use a pair of cardioid (unidirectional) mics placed close to the instrument. Close-miking -- approximately six to 12 inches from the guitar -- is used in most pop and other contemporary recordings that feature acoustic guitar.
Cardioid mics are generally best used for close-miking guitar because they exhibit less bass proximity effect (or bass boost) than other directional types when placed close to the sound source. And we all know that acoustic guitars can sound boomy if miked incorrectly.
Let's explore three common approaches to stereo miking and acoustic guitar. Each of these techniques has been used on countless hit records. Be sure to check out the corresponding audio links, and of course, consider these as starting points for your own creativity. After you've mastered each one, feel free to experiment with your own variations on each method.
Spaced Pair, Version A 
Two mics are placed apart from each other at the same approximate height, one pointing at the 12th fret of the guitar and the other at the bridge.
With this approach -- as with any miking technique that uses two or more mics that are spaced apart from one another -- always be sure to follow the "3-to-1 rule." According to this rule, the distance between two mics should be at least three times the distance between each mic and the sound source. This keeps phase cancellations to a minimum, resulting in a smoother sound that also translates well to mono. So, for instance, if you've got each mic seven inches from the guitar, the 3-to-1 rule mandates that you spread the two mics at least 21 inches apart from each other. (One of a few exceptions to the 3-to-1 rule is with the X-Y technique, as described below.)
Spaced Pair, Version B 
Our second technique is a variation on the spaced pair. As in the setup above, one mic points to the 12th fret. The second mic, however, is hung from a mic stand at the performer's ear level, pointing down at either the bridge or at the strings just behind the soundhole. For example, if the performer is right-handed, this second mic would be placed over her right shoulder. (Once again, be sure to follow the 3-to-1 rule.)
You can also move this ear-level mic slightly out in front of the performer and angle it back towards the guitar (versus pointing straight down at the floor), for a brighter sound. This technique usually yields a more open -- but thinner -- sound than the simple spaced pair on a horizontal plane. Check out the links to hear the difference.
If you're not getting the sound you want with a spaced pair placement, try moving one or both microphones slightly to improve the timbre. Because spaced pair placement is subject to phase interference, moving one mic only an inch or two can dramatically change the sound. To learn more about how phase affects the timbre of stereo guitar tracks, go to the mono-compatibility link below.
The X-Y Technique 
X-Y, or coincident-pair, is the no-brainer approach to stereo miking. If you follow these steps precisely, and are willing to move the mics around a bit to find the sweet spot, you'll find it's hard to make a bad recording. (That's assuming, of course, that your room, your mics, and the instrument -- and, while we're making a checklist, the guitarist -- are half-way decent.)
Place the two mics close together so that their capsules are almost touching. The rear ends of each mic are spread apart at an angle of a roughly 90 to 120 degrees. The result looks like a wide V shape, with one mic's capsule positioned directly above the other.
(The 3-to-1 rule doesn't apply to the X-Y technique because the two capsules are so close that sound waves arrive at both at essentially the same time, minimizing objectionable phase cancellations.)
To start, try placing the two mics opposite the 12th fret of the guitar. If you have a really nice sounding room to record in, try backing the mics up to a distance of one to two feet from the guitar. This will capture more room tone and yield a more natural sound. While the best-sounding position can depend upon the guitar, the room, or the mics, typically, placing the mics around seven inches in front of the guitar's 12th fret will tend to de-emphasize midrange frequencies. That's because one mic will be pointing in the direction of the bassy soundhole, and the other towards the top of the neck, an area rich in high frequencies.
As you experiment, you'll find that the X-Y miking produces a much narrower stereo image than the spaced-pair techniques. But you'll also hear how X-Y lends a smoother, warmer, and more natural sound to acoustic guitar.
Choosing the Right Mics
By definition, stereo miking requires a pair of microphones. Certain types of recording situations -- such as chamber orchestra, or an acoustic jazz trio -- demand extreme accuracy, and in these cases, it's highly desirable to have a matched pair of microphones. This means more than just two of the same model; it also calls for two mics that have been factory-certified to produce virtually identical frequency response. (Some, though not all, manufacturers sell matched-pair mics.)
Fortunately -- unless you're, say, capturing an audiophile-quality classical guitar recording -- you won't need a matched pair to record acoustic guitar tracks. In fact, you might not even need to use two of the same model of microphone (though if you do you'll get a more consistent timbre in each channel than if you had used two totally different mics). That said, rules are meant to be broken, so feel free to experiment by mixing and matching mic models. And like any musical instrument, every mic has its own sonic character, so try to get your hands on as many different models as possible and experiment.
Generally speaking, condenser mics are the right choice for acoustic guitar. As a group, they offer a far more detailed and realistic sound than dynamic mics. But before you choose a specific condenser, first decide what kind of sound you want. Small-diaphragm condensers (those with a diaphragm smaller than one-inch in diameter) generally offer a better transient response than their large-diaphragm cousins, producing a less colored, more detailed sound. For pop and country productions where guitar tracks will be tucked into dense arrangements with drums and bass guitar, small-diaphragm mics are often the best choice.
Many engineers consider the AKG C480B ($987 with the CK61-ULS capsule), Neumann KM184 ($729) and DPA 4011 ($2,190) to be among the best small-diaphragm condensers on the planet. (All list prices are in US$.) All three sport cardioid (unidirectional) patterns -- meaning they tend to reject any sound that isn't directly in front of them -- and sound awesome on acoustic guitar. (Cardioid response is also required for most stereo miking techniques, in order for the resulting recording to have a "left-to-right" soundstage.) AKG's C480B is a modular mic, meaning that you can interchange various capsules -- each offering a different polar pattern -- with the mic body that holds the internal preamp. This mic features a 70Hz high-pass (low-cut) filter, useful for rolling off unneeded low frequencies when recording acoustic guitar.
The Neumann KM184 exhibits an inherent low frequency roll-off at 200Hz, delivering guitar sounds free of low-end "boominess." DPA's 4011 mic -- known prior to 1998 as the Brüel & Kjær (B&K) 4011 mic -- features a 1dB roll-off in the midrange frequencies along with a 1dB rise between 10 and 15kHz. The result is a crisp, though not overly bright sound. This 4011 has treasured place in many mic lockers (including my own). Check out these links to hear the DPA 4011 on a Guild M20 acoustic guitar, with various mic placements.
There are plenty of other small-diaphragm, cardioid condensers on the market, many of which offer decent performance for a lot less scratch. Some better-known alternatives that other enginneers report good results with include Shure's SM81 ($530), AKG's C1000S ($297), and Audio-Technica's AT3528 ($259).
Large-diaphragm mics -- those featuring diaphragms at least one inch in diameter -- can also provide outstanding results when recording acoustic guitar. All other things being equal, these mics tend to offer a slower transient response than their small-diaphragm counterparts. This causes a slight de-emphasis in high-frequency detail and tends to give them a rounder, warmer sound -- just the ticket for traditional jazz recordings and lean guitar/vocal arrangements. (It's this warmth that makes large-diaphragm mics so popular with vocals.) The Lawson L47MP Tube Condenser ($1,995) sounds great on acoustic guitar for these applications. I've also used the Manley Reference Gold Tube Condenser ($5,500) with excellent results. On a budget but craving that large-diaphragm condenser sound? Some candidates that I didn't have the chance to work with would include AKG's C414B/ULS ($1048), Neumann's TLM103 ($995), the Alesis AM51 ($549), and the AKG C3000B ($478).
And what about a dedicated stereo microphone? These mics -- such as Shure's VP88 ($1194) and Audio-Technica's AT825 ($525) -- typically have a pair of cardioid capsules mounted in one housing. While they may be useful in certain applications, they're actually less flexible than a pair of independent mics -- since their diaphragms are physically fixed relative to one another. In other words, if you want to try some of that mic-above-the-bridge, mic-above-the-fingerboard stuff, or any other interesting variations, you'll want a pair of mono mics.
Mono-Compatibility, EQ & Compression
Stereo-Miked Acoustic Guitar Tracks and Mono-Compatibility
When stereo tracks are collapsed to mono, the result can sound dramatically different from the original tracks. This is not only true of the listener's perception of the width of the stereo image and the discrete placement of different elements in a mix: Conversion to mono can also significantly change the timbre of individual instruments, especially if certain stereo miking techniques were used to record the original. (In rare cases, the instrument could even disappear from a mix, should the left and right signals be far enough out-of-phase to cancel each other out when combined to mono.)
True, in the last couple of decades -- with AM radio's diminishing role as a music medium -- mono-compatibility has become less of an issue. And in fact, some engineers prefer not to compromise their stereo tracks or limit their recording techniques to cater to the lowest common denominator. Still, many engineers choose to play it safe -- ensuring that their miking (or other processing) techniques won't sound terrible if played back in mono. However you feel about this issue, you'll be able to make more informed choices on how to record if you know what the sonic repercussions will be for mono playback. So what exactly happens to stereo acoustic guitar tracks when they are collapsed to mono? The answer depends on what miking technique you use to record.
Spaced pair techniques generally pick up a high degree of uncorrelated signal for each track. That is, many frequency components on the left-panned track are more or less out-of-phase with corresponding frequencies on the right-panned track. (This still applies even if you use the 3-to-1 rule for mic placement.)
This is because each of the sound waves emanating from the guitar will arrive at each mic at a different time. Due to that time difference the mic will pick up the wave at a different phase of its cycle. When the stereo signal is collapsed to mono, the constructive and destructive interference of these out-of-phase components combine to emphasize and de-emphasize their corresponding frequencies. The resulting timbre can be markedly different from your carefully crafted stereo tracks and can play havoc with your mix. You might be able to compensate with EQ, but this might exacerbate the problem.
Tracks recorded with an X-Y technique are far less prone to phase problems. Since the capsules are placed so close together, the sound reaches both mics at roughly the same time. As a result, tracks recorded in X-Y stereo are much more mono-compatible than those recorded with a spaced pair.
A Word on EQ and Compression
Aside from the occasional use of a microphone's passive high-pass (bass roll-off) filter, I prefer not to add equalization to the signal I'm recording. You can never tell in advance exactly how yet-to-be-recorded tracks will interact with the guitar you're recording, so any processing you add during recording is just a guessing game. Since you'll probably need to make subsequent adjustments in timbre (and possibly dynamics) at mixdown, you should avoid processing the signal twice. Your tracks will sound more pristine if you hold off adding processing until you have a complete picture of how the tracks will fit together. Instead, if you're not getting the sound you want during soundcheck, move the mics around until the timbre sounds right.
If you want to experiment with EQ as you're tracking, you can record the music dry and add EQ on the monitor returns. This way, you can hear the results of the EQ without committing to it.
Though many engineers will compress an acoustic guitar during tracking, I usually don't like to do so. As with other broadband, percussive instruments, guitar can easily cause a compressor to pump (cause audible changes in level) if it's not set up exactly right. Once these amplitude modulation artifacts are on tape, they are all but impossible to remove. For this reason, I compress acoustic guitar tracks at mixdown -- when I have multiple opportunities to get it right.
