XO Wave: Stereo Microphone Techniques

XY Microphone setup using a Schoeps UMS-20 microphone bar.

ORTF Microphone setup using a Schoeps UMS-20 microphone bar.

Making great recordings with two microphones is not only an invaluable skill in and of itself, but also a great technique to use in conjunction with other techniques. Stereo recording techniques can be used to record orchestras and small ensembles as well as adding a little life to recordings of rock bands. With a few simple tricks and techniques under your belt, along with a little practice, stereo recordings with two microphones can be a powerful asset in your recordings. You should find this article a good jumping-off point for understanding the basic stereo techniques, and it will help you if you are short on time or don't know where to begin, but there is no substitute for experimentation and using your ears!

Two Ears to Listen with...

People have two ears and recordings are generally made in stereo, so it stands to reason that mikes should be used in pairs. In fact, a great many recordings have been made with only two mikes, and even more have been augmented by using microphones in stereo pairs. Even in this modern day of surround sound, a "stereo pair" of mikes is the fundamental part of many recordings.

The basic of idea of stereo recording is to place a pair of mikes in such a way that the listener can tell where the sounds are coming from. This gives the listener a sense of being there, because they can tell not only where the instruments are but also where the echos are coming from. This reverberation helps them feel like they are in the room the recording was made in. Traditionally, you might record a track in mono and pan it to one side or another in the mix, but with an orchestra, you can use mike placement to achieve the same effect for all instruments simultaneously (and automatically) if you use stereo miking.

If you are unfamiliar with basic microphone types and uses, you may want to read our Tour of Microphone Types before continuing.

Basic Stereo Techniques

There are several methods of placing microphones to achieve this effect. One of the most popular configurations is called "XY", or "coincident pair". In this configuration, you take two cardioid microphones and place them one on top of the other (or side-by-side). Instead of pointing the mikes directly at the source, one mike is pointed 45 degrees to the left, and the other is pointed 45 degrees to the right. The idea is that sounds on one side of the pair will be picked up more in one mike, and sounds on the other side will be picked up more in the other mike. Sounds in the middle, of course, will sound about the same in both mikes. This technique is relatively easy to start with and works well in a variety of situations.

Another configuration is called "A-B" or "spaced pair" or "spaced omnis". The idea here is to space a pair of omnis about 18" apart and point them at your source. In this case, the volume of the sound hitting the mikes is about the same regardless of where the sound is coming from, but the time at which the sound arrives is different. For example, a sound to the right of the mikes will hit the right mike first and then the left mike. When this is played back, the listener can perceive this delay and use it to determine where the sound is coming from. This technique is commonly used to record classical music and small ensembles, and to add ambiance to virtually any recording. It is a bit tricky to master, but the results can be astounding -- especially with high quality microphones.

Another common technique is called ORTF, which is a French acronym for Office de Radiodiffusion Television Française, the broadcasting network that first developed the technique. The idea is to place two directional mikes about ears-width apart (17 cm) and at a 110 degree angle. This technique reproduces both timing and volume discrepancies, making it, in some sense, a hybrid of the spaced pair and XY techniques. While it might seem to be the ideal configuration, because it preserves both timing and volume information, ORTF can be difficult to master, and can sometimes produce unpleasant results when played back in mono.

Microphone Placement

When placing a stereo pair, it's important to realize that the best place for the mikes may not be the best seat in the house. There are several reasons for this, but the most important is that, even in the front row, there are often things in between the sound source and the microphones that get in the way of the sound. Also, in many halls, low frequencies tend to "hug" the ground. Try moving the mikes a few feet above the ground, where they will be free of low-frequency build-up and on-stage obstructions. Another problem with placing the mikes at the best seat is that microphones are simply not as good at separating out the various instruments and echoes as human ears are. Try placing the mikes closer to the source than you would normally want to sit to improve the clarity of the instruments.

Many recording engineers like to listen with headphones while they adjust the mike placement until they find the right balance. Another technique it to plug one ear and move your head around until you find the spot with just the right balance of instruments, tones and reverberation and put the mike pair in that spot. Using your ears, you can often find a "sweet spot" where everything sounds just right, and even moving an inch or two might ruin the sound.

The Soloist Effect

Recordings made with a stereo pair often exhibit a strange phenomenon I call the "Soloist Effect". When we are watching a performance live, we often use a number of visual and auditory cues to separate the various sounds. Many of these cues are simply not available when recording, and so even if a recording is otherwise well-made, it may sound like someone has turned the volume down on some instruments -- usually the all-important soloists. This is especially the case when working with spaced omnis because spaced omnis often have an ill-defined or "weak" quality near the middle. There are a few ways of combatting this problem, such as moving the soloist closer to the mikes, or changing the musical arrangement; however, these are often out of the question.

The very best solution is to use your ears to search around for the "sweet spot", but sometimes you just can't -- either because you can't find it, or you can't get to it, or maybe you just ran out of time. In this case, all is not lost. The simplest solution is to add another mike, or even pair of mikes, just for the soloists. Be careful when doing this, though, because timing difference between the mikes can cause bizarre artifacts, usually described as "hollow", "phazy", or "thin" sound. Often what works is to put a mike very close to the soloist, and just mix a little of that in with the stereo pair. This is not ideal, because it can reduce the richness of the recording, so I would never do this for a commercial recording, but it can get you out of a pinch, such as when someone is planning to use your recording as an audition or practice tape.

Mono Compatibility

An important concern when working with stereo mike pairs is how the recording will sound when played back in mono. This is not just for the thousands of television sets out there with mono sound, but also for making sure the recoding will sound good when played back on sound systems with less than ideal design in rooms with far from ideal acoustics -- which is how most people listen to music. A quick check for this is to listen to your two mikes mixed to mono and make sure it still sounds rich, full, and detailed. You'll know you did something wrong when switching to mono sounds like you put the orchestra is a small cave, or like you suddenly cupped your hands over your ears.

Mid-Side and Decca Tree

Both spaced omnis and ORTF are notoriously difficult to get good mono sound out of, and XY can be tricky as well. For this reason, two other techniques have been developed for stereo recording: Mid-Side (MS) and Decca tree. The Decca tree technique starts with spaced omnis, usually spaced wider apart than normal for omnis, and adds another omni in the middle and to the front. The front omni is mixed equally in both channels, while the other omnis are only mixed to their respective left or right channels. Though useful in a variety of settings, the Decca tree is the standard for film scoring because it is known to sound good in mono, stereo, and across the wide variety of processes used for film sound. Many recording engineers also like the flexibility of the Decca tree. There is usually no need to break out a tape measure to measure the distance between the mikes, and the center mike can be moved closer or further from the main pair to vary the sound. Variations on the Decca tree have also been used for surround-sound recordings.

The Mid-Side technique is theoretically equivalent to XY, but the realities of mike design often make Mid-Side more stable and reliable, and easier to set up. The idea is to take one mike (usually a cardioid condenser, though any mike will work) and point it at the sound source. This is your "Mid" mike. Then take a figure-8 mike and place it behind the mid mike, pointing 90 degrees away from the "Mid" mike. This is the "Side" mike. Now, instead of plugging one mike into your left channel and the other into your right channel, you'll need to "decode" the signal by finding the sum and difference of the signals in the two mikes. That is, the left channel on the final recoding is created by summing the signals from the two mikes and the right channel by taking the mid signal and subtracting the side signal. When combined to mono, you are left with the Mid signal, making mid-side inherently mono-compatible.

Conclusion

Which technique do I prefer? Well, of course it depends on the mikes available to me and the situation at hand, but if I had only one pair of mikes I would pick a pair of omnis and use the spaced omnis technique. The results of using this technique can be truly amazing. The accuracy, transparency and naturalness of recordings made with this technique is hard to beat. Armed with a pair of omnis, a small mixer, stereo recorder, and perhaps a few extra mikes to augment certain instruments, you can tackle virtually any live recording situation. Spaced omnis can come in handy in the studio, too, especially if you have a good sounding room.

Microphone placement is a tricky beast. Hopefully this article will help you no matter what kind of music you are recording. Just remember to use your ears!

-- Bjorn Roche