Fig. 1. A pair of Neumann KM84 cardioid microphones in an XY array.
A good stereo recording can be a thing of beauty. In addition to delivering a realistic sense of left-to-right position, a stereo recording can convey impressions of size, detail, and depth that cannot be obtained through other means of recording. We’re not talking here about manufacturing an image by recording mono tracks and panning them across the stereo field during mixdown. We’re talking about using two microphones to capture an audio event, including the space in which it happened. This month, we examine basic stereo-mic techniques that can be used alone or in conjunction with close-miking (on drum kits, for example); in a future issue, we’ll explore more advanced applications.
Stereo-mic techniques can roughly be separated into three categories: coincident, near-coincident, and spaced pairs. Each has variations with strengths and weaknesses. The best results will be obtained using two mics of the same brand and model; taking the results to a higher level usually requires a matched pair.
Fig. 2. A pair of Cascade Fathead II mics in a Blumlein array. These mics are figure-eight.
Fig. 3. Schoeps CMC64 cardioid mics in ORTF.
Fig. 4. Schoeps CMC64 cardioid mics in NOS configuration.
This Is No Coincidence A coincident pair comprises two directional microphones arranged so that their diaphragms are placed as closely together as possible (usually one atop the other), typically angled between 90 and 135 degrees. Figure 1 shows what is known as an XY array: two cardioid microphones crossed at a 90-degree angle. Regardless of how the mics are aimed, sounds coming from any direction reach the two diaphragms at the same time. This means that one, there is little or no phase cancellation between the mics because arrival times are the same, and two, the setup provides only what we call intensity or amplitude cues. In other words, your brain’s impression of the stereo field is based solely on how loud an instrument may be in one channel or the other, or in the middle. You’ll notice in the photo that both mics are pointed 45 degrees off-axis from the source but the cardioid patterns overlap in the middle, providing a solid, if somewhat small, stereo image. Along with the lack of phase issues, the solid center is one of the strengths of the XY array. When using an XY pair, you’ll never have to worry about mono compatibility (and you’d be surprised at just how much mono is still in use). For a wider stereo image, you can open up the angle as far as 135 degrees.
Variations A configuration of two figure-eight microphones in an XY configuration is called a Blumlein array, named for the man who pioneered stereo, Alan Blumlein. The strengths and weaknesses of a Blumlein pair are similar to those of an XY pair of cardioids, the biggest difference being that a Blumlein pair captures more room sound due to the rear lobes of the figure-eight patterns (Figure 2). A bit of experimentation may be needed to find the optimum distance from the source so that the stereo image doesn’t get wishy-washy. (That’s a technical term.) The Blumlein array is at its best in a good-sounding room, whereas an XY array of cardioids can more effectively be used to control the amount of room sound. If you’re seeking somewhat less-ambient pickup than with the Blumlein array, but more than you’d get using XY with cardioids, experiment with hypercardioid or supercardioid patterns, both of which have slight pickup lobes at the rear. Another variation of a coincident pair is called M/S (Mid-Side) stereo, which we will discuss in depth next time.
Fig. 5. Sony C48 multipattern mics set to omnidirectional and arrayed as a spaced pair.
Near-Coincident Pairs In order to create a more realistic stereo image, we need to add a time cue to our mic array. Imagine that your eyes are closed and a friend is speaking to you while he walks around the room. Even though you can’t see him, you know exactly where he is. That’s because (dependant on his location) his voice reaches one of your ears first and then the other slightly later—unless he is directly in front of you, in which case his voice reaches both of your ears at the same time. A coincident pair does not take this timing difference into consideration, but a near-coincident pair does. A near-coincident pair is an arrangement of two directional mics splayed at an angle between 90 and 120 degrees but separated by a distance akin to the space between one’s ears. The two most popular near-coincident arrays are NOS (Nederlandsch Omroep Stichting) and ORTF (Office de Radio-Television Diffusion Française), developed by Dutch and French broadcast organizations, respectively.
Figure 3 shows ORTF configuration: two cardioid mics placed approximately 6.5 inches apart, each angled 55 degrees off-axis. NOS pushes the mics slightly farther apart (11.8 inches), but tightens the angle to 45 degrees off-axis (see Figure 4). These two techniques provide an increased sense of spaciousness over a coincident pair because they give your brain intensity and time cues, while still providing a solid center image. They may not be 100 percent mono-compatible, so you’ll need to check your recording in mono. (Hint: If you use a near-coincident pair for drum overheads, listen in mono for ‘swishy’ sounds on the cymbals. That’s an indication of phase cancellation.) Adjusting the distance between the mics, or between the mics and the source may help alleviate phase problems.
Spaced Out Sometimes known as an A-B stereo array, a spaced pair of microphones delivers a less precise, more diffuse picture of the sound source (Figure 5). This makes A-B stereo a good choice for orchestral, choir, or chamber music applications, in which blend is important. Traditional spaced pairs employ omnidirectional microphones, though you certainly can try any pattern. Spacing between the two mics is usually determined by the width of the sound stage: Generally the distance between mics is a third to half the width of the sound stage. When mics in a spaced pair are separated by more than three to four feet, the image may develop a hole in the middle, so some engineers add a third mic which should be panned precisely center (when recording an orchestra, for example).
Using a spaced pair requires you to be aware of critical distance—the location in a room where direct and reflected sound are of equal strength. Generally the more lively the room, the closer the mics should be to the source. Omnis can be placed closer to a small group (or small instrument such as a drum kit) while maintaining coverage and reducing room tone. Don’t be sloppy about aiming (or not aiming) omnis at the source. Most omnidirectional mics become directional as frequency increases, and you may as well take advantage of this characteristic. Since phase issues are possible with a spaced pair, be sure to check the results in mono.
Stay tuned! In the future, we’ll discuss M/S (Mid-Side) and other advanced stereo techniques.
Steve La Cerra is an independent audio engineer based in New York. In addition to being an Electronic Musician contributor, he mixes front-of-house for Blue Öyster Cult and teaches audio at Mercy College, Dobbs Ferry campus.