How to Configure this Classic Stereo Mic Arrangement, and the Theory Behind It
By Phil O'Keefe
Human hearing is a wonderful thing. Our brain, in conjunction with our two relatively widely spaced (~17-20cm) ears, has an amazing ability to localize sounds and pinpoint the direction that they are coming from. This is primarily accomplished in two ways: through arrival time differences, where sound arrives at one ear a tiny fraction of a second before it arrives at the other ear, and amplitude or sound pressure level differences between the two ears. Amplitude differences are due in part to the baffle effect of the human head, which slightly attenuates the level and changes the frequency spectrum of the sound arriving at whichever ear is "behind" the head and furthest away from the sound source. The ability of the brain to perceive very small differences in sound pressure levels as well as phase also gives us the ability to tell which ear is closer and getting the "hotter signal", while even very small differences in when the sound arrives at each ear also provide localization information that can be unconsciously deciphered by the brain.
Stereo microphone techniques use time of arrival differences between the microphones, sound pressure level differences between them, or a combination of the two to achieve the stereo effect. For example, A-B stereo or a "spaced pair" of omni microphones relies primarily on arrival time differences, while the XY stereo technique under consideration here is intensity based, and depends on the level differences alone.
THE GEAR YOU'LL NEED, AND SETTING IT UP
XY stereo mic technique requires a pair of closely matched cardioid condenser microphones, or a suitable XY stereo microphone. You will also need two identical mic preamps, with identical gain and other settings dialed up on each. XY is a coincident mic technique, where the two microphone capsules are positioned as closely together as possible and adjusted so that the center point of both capsules is aligned vertically, but with the microphones angled at 90 degrees relative to each other, as shown in Figure 1 and Figure 2.
Figure 1: A pair of DPA 2011C small disphragm cardioid condenser mikes arranged as an XY stereo pair
Figure 2: XY from the sound source perspective. Note the stereo bar and single mic stand
As shown in the images above, a stereo bar is commonly used to help facilitate the placement of the two microphones. It is possible to use two separate stands and position the microphones correctly, but if you need to adjust or reposition them, you'll need to move two separate stands. It's much easier to adjust the positioning when both are mounted to a single stereo bar that is attached to a single mic stand.
THEORY AND CAVEATS
Since much of the sound that they pick up arrives off-axis to either one or both of the microphones, XY stereo pairs tend to sound best when using microphones with minimal off-axis coloration. Additionally, because directional microphones are used, proximity effect also comes into play. You will get a greater amount of low frequency information in the recording when the microphones are placed in closer to the source, and less low frequency boost when the microphones are placed at greater distances from the sound source. When setting up your XY mikes, experiment with the distance to the sound source until you achieve the desired amount of both low frequencies and room ambiance.
Because the microphone capsules are in nearly the exact same physical location with an XY setup, there is no significant difference in the arrival time of the sounds reaching them. Left, center or right - regardless of what direction it is coming from, sound arrives at both capsules virtually simultaneously. This means that XY doesn't provide any arrival time cues to help our ears determine the directionality of the sound. If you were to substitute two omni microphones in an XY arrangement instead of two cardioid microphones, the stereo effect would disappear. However, because of the attenuation of the cardioid polar pattern for sounds arriving off-axis - from the less sensitive sides and rear of the microphone - an XY stereo pair of microphones, or a purpose-built XY stereo mic such as the ones shown in Figures 3 and 4 does provide level differences that our ears can use to determine the direction the sound came from. It is also possible to use other directional polar patterns, such as hypercardioid. In fact, the only real difference between Blumlein stereo and XY stereo setups is the types of microphone used; Blumlein is basically an XY configuration with bi-directional microphones used instead of cardioid mikes.
Figure 3: Single unit XY stereo microphones, such as this Rode NT4, are also available
Figure 4: An MXL Revelation Stereo XY large diaphragm tube microphone. Note the dual capsules inside the head grille
In my earlier article on Blumlein stereo pairs, we discussed crossed bi-directional or "figure-8" microphones. While both XY and Blumlein stereo techniques use coincident pairs of microphones, due to their greater attenuation in the bi-directional pattern's null points, the stereo imaging of Blumlein tends to be wider than with XY stereo, but at the expense of greater ambient or "room sound" pickup. Because both XY and Blumlein stereo techniques use coincident pairs of microphones, their mono compatibility is equally excellent, and is exceeded only by Mid-Side stereo, where the side microphone is completely cancelled out, leaving only the signal from the Mid microphone when it is summed to mono. If mono compatibility is critical, Mid-Side mic technique is often the preferred choice, but it also provides a more ambient sound than XY stereo does. In cases where the room acoustics are less than ideal, or when you wish to capture less ambient sound and more direct sound from the source while still maintaining a stereo image, XY is an excellent choice. Give it a try on such sources as acoustic guitars, small vocal ensembles, drum overheads and percussion.
* A quick word about the microphones used in the illustrations: The DPA 2011C microphones that are used in some of the pictures in this article are indeed small diaphragm condenser mikes, but their use of "interference tubes" means that the capsules are not within a few millimeters of the front end of the body as with most small diaphragm condenser microphones; the diaphragms are actually located further back down the length of the tube, under the rearmost side grilles. When using these exact mikes, the ideal configuration would require "crossing" them further down the tubes so that the diaphragms are aligned on the vertical plane. However, this is contrary to the arrangement that is typical for the vast majority of small diaphragm condenser microphones (Neumann KM184, Rode NT5, MXL 604, Oktava MC012, Audio Technica AT4051, AKG C-451B, etc.), so for the sake of clarity, I have stuck with the more typical configuration when setting up the microphones for the illustrations.
Phil O'Keefe is a multi-instrumentalist, recording engineer / producer and the Associate Editor of Harmony Central. He has engineered, produced and performed on countless recording sessions in a diverse range of styles, with artists such as Alien Ant Farm, Jules Day, Voodoo Glow Skulls, John McGill, Michael Knott and Alexa's Wish. He is a former featured monthly columnist for EQ magazine, and his articles and product reviews have also appeared in Keyboard, Electronic Musician and Guitar Player magazines.