Most of us take the stereo listening experience for granted. After all, we have two ears and stereo has two speakers! For decades our recorded music has been delivered and played back in stereo, but that doesn’t mean that capturing music effectively in stereo is a trivial operation. It’s taken many years for engineers to develop consistent and reliable methods for stereo capture. We’re going to explain the top 5 stereo microphone techniques to make great recordings. They are:
- Spaced Pair (AB)
- Coincident (XY and Blumlein)
- Mid/Side (written MS, M/S, or M-S)
- Near-coincident (ORTF and NOS)
- Sphere 180: coincident recording with the Sphere L22 microphone
Choosing the right one isn’t always obvious, even though we do offer suggestions on where to start. Making these choices is easier when we take a moment to study the world’s first stereo miking setup: our ears.
How we hear and how mics hear
Our ears and brain form a complex interactive system for judging music localization. They have evolved to use seemingly irrelevant information to great effect. For example, the pinna (the fleshy part of the outer ear) affects how audio travels down the ear canal, its particular shape dictating how we place sounds not only from left to right, but also above or below, before or behind.
The head itself is vital to stereo perception: the same sound comes to one ear directly but must pass around the head to reach the other, and the brain correlates these different timbres to precisely localize a sound source. The fancy term for this effect, which has been studied in great detail, is the head-related transfer function or HRTF.
The three most obvious factors used by our ears are differences in loudness, precedence, and phase. Loudness is pretty apparent—if a sound source is to our left, the left ear will hear it more loudly than the right. Similarly, precedence is easy to understand; a sound coming from the left will reach the left ear before it reaches the right. These time differences will also cause differences in the phase of the sound waves reaching our ears; these become very important when we discuss stereo miking.
A microphone is not an ear. It possesses none of the important physical attributes of the human auditory system; it has a polar pattern that dictates which direction it “hears” most sensitively, but that doesn’t inherently provide localization. To obtain stereo, we have to set up mics in such a way as to mimic parts of the human hearing experience.
The Top 5 microphone techniques to make great recordings
Here are some of the most common stereo miking techniques out there, with advice on where to use them and what to expect from them. Check out the diagrams below and follow along. Note that for all of the figures, we are looking straight down at the mics from above. The left channel is marked in green and the right channel in yellow.
Spaced Pair (AB): Just put up two mics
The most obvious stereo mic setup is the spaced pair (sometimes called AB, see Figure 1). Two mics (often omni) are set some distance apart, delivering stereo information by differences in loudness and precedence. This is often used for relatively large sound sources, like an entire drum kit or a grand piano. For really huge sources like an orchestra or a choir captured by distant mics, AB is pretty much the only setup that works. Compared to the other stereo techniques we describe below, a spaced pair typically provides the widest and most dramatic stereo image.
This method comes with two potential problems. One is that the distance between the mics means that sound waves arrive at different times and therefore will have phase differences, which can lead to drastically compromised sound when mixed down to a narrower soundstage or to mono. This can be mitigated by using the “3-to-1 Rule,” which says that you can minimize phase issues by making sure that the distance between the two mics is at least three times the distance from either mic to the source. That’s not always practical or possible, but it’s a good thing to aim for.
The other issue with AB is that if the mics’ distance apart is very large, the stereo image will seem artificially overstretched, with a dead zone in the center. Imagine hearing a choir as two chunks on either side of the stage with nobody in the middle! The only option here is to either move the mics closer together or to position them further away from the source.
Here's a short piano performance captured with a spaced pair of microphones. Notice how the wide stereo image creates a lush sound that complements the piece:
Coincident: XY and Blumlein
The “opposite” of AB miking is coincident miking (or XY, shown in Figure 2). Two cardioid microphones are set up as close to one another as possible, one stacked atop the other so their diaphragms are effectively at the same point in space with respect to the sound source. They’re typically angled at 90º and aimed so that the mic on the left captures the right channel and vice versa.
XY relies almost exclusively on differences in loudness; precedence times are equal and phase differences are minimal. This produces a signal that is clear and often excellent at specific localization, but with a slightly more “narrow” overall feel. (That’s not a bad thing—very wide stereo soundfields can sound unrealistic and are a common problem with AB arrays.)
The Blumlein Pair (see Figure 3) uses this same positioning but with figure-8 mics rather than cardioids; this captures out-of-phase information from behind the array and adds to the sense of spaciousness and realism.
We recommend XY and other coincident techniques as a great choice for beginners. They work well for many applications and have some built-in “failsafes” for the novice. The one thing that catches even experienced recording engineers when setting up stereo mic arrays is phase cancellation; what should be a massive soundstage ends up sounding thin, weak, and hollow in mono. A coincident array completely avoids this. They’re also more compact, which can be a consideration in tight studio spaces as they require only a single mic stand. Many portable field recorders come with XY mic arrays built-in.
Our Sphere L22 mic is particularly well suited to coincident recording. Not only can you capture your audio and then adjust characteristics like the polar pattern well after the recording session, but Townsend Labs’ Sphere 180 plug-in gives you options no other mic can match. We’ll talk more about that below.
One technique that’s become very popular in recent years is Mid/Side (M/S). It uses two coincident mics: the Mid is a cardioid mic pointed at the sound source, and the Side is a figure-8 mic pointed to the sides, as shown in Figure 4.
Why do this? If we split the Side mic signal and send it to two mixer channels, then invert one of those two, we can balance those two signals to adjust the stereo width in the mix—from an ultra-wide stereo soundstage down to perfect mono with no phase problems. Recently, several plug-ins have started to include built-in M/S processing, making it easy to play with the technique in your DAW. One excellent option is the free Midside Matrix plug-in from the awesome folks at Goodhertz.
In our experience, Mid/Side works well with close drum rooms, acoustic guitars, and other stringed instruments.
Near-coincident: ORTF and NOS
Another technique is near-coincident miking, usually named after ORTF, the acronym in French for Radio France (Figure 5a), or NOS, the Dutch Broadcasting Organization (Figure 5b). They vary slightly in mic distance and angle; in ORTF, the cardioid mics are angled outward at 110º and separated by 17 cm (6.7”), whereas in NOS, the mics are 30 cm (12”) apart and angled at 90º. Some stereo field recorders mimic these arrays in miniature.
Radio broadcasters developed this setup because it gives both loudness and precedence stereo. Plus, it’s possible (if not always easy) to tweak the mics’ relative position slightly for the best phase response. The result is a realistic stereo soundstage, but at the cost of some initial fiddling to get things exactly right. Stereo bars and positioners that hold two mics at once on a single stand are often marked with angles and distances to get you into the ballpark, and the rest is done by ear. The ORTF technique is a fabulous starting point for pianos, small ensembles, and percussion instruments.
Sphere 180: one mic becomes two
With the advent of the dual-capsule Sphere L22 and its flexible software, a unique and straightforward form of coincident stereo miking becomes available. The Sphere 180 plug-in allows the signal from the L22 to be processed as a pair of coincident mics with separate outputs. Just rotate the mic by 90º, so it’s at right angles to your sound source, and it captures phase-accurate audio from left and right. This technique is like XY but with a 180º angle between the mics, which creates a soundstage that’s wider than what you get with XY while still sounding very natural. It’s fast and easy, producing stunning results that easily stand up to any traditional stereo miking approach.
Thanks to the Sphere 180 software, there’s a lot of room to play with this technique. You can tweak the polar patterns to produce more or less stereo centering and width, and if you want, you can even run the two sides with different mic models—for example, to balance the stereo image of an acoustic guitar.
Check out these videos that show Sphere 180 in action, comparing it with conventional two-mic recording setups:
With the Sphere system, a single mic becomes a mic pair, with flexible and musical-sounding results that you can adjust to taste long after the tracking session is over and the musicians have gone home. You can download the free, fully-functional Sphere plug-in and our pre-recorded tracks to start exploring the unique possibilities in Sphere 180.
Using one instance of Sphere 180 with one L22 is the simplest application, but feeding one mic’s signal to two parallel instances of the conventional Sphere plug-in and pressing the Reverse button on one of them can produce similar effects with more tweakability. And why stop there? If you use these techniques with two L22 mics, each with its own instance of Sphere 180, you can create everything from Mid/Side and Blumlein arrays to multichannel surround setups.
The Top 5 stereo microphone techniques: a great start!
A basic understanding of stereo miking is essential no matter which mics you own. The top 5 stereo microphone techniques are a great starting point to make great recordings, and will serve as a solid foundation for growing your practical miking skills.
With Sphere you now can:
- Record with the sound of microphones many have only dreamt about
- Change mic type, polar pattern, and other microphone characteristics, even after tracking!
- Audition the sound of different microphones without tiring the vocalist
- Reduce bleed, undesirable room coloration, and other common issues using Off-Axis Correction™
- Record in stereo from a single microphone
We invite you to install the free, fully-featured Sphere plug-in for all major platforms and DAWs. Try it now and reimagine our library of pre-recorded tracks.