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The Ultimate Guide to Stereo Microphone Techniques

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Just when you thought you had gotten to grips with using a microphone to record with you find out that you will be needing more. For that stereo sound you know? You don't? Well, that's just all right since there is nothing to be nervous about stereo recording techniques. If anything, they are even more pleasurable than normal recordings, giving you more sonic information from the instrument at hand. Tell me, which would you rather have; a one microphone acoustic guitar that only has a little body and attack to it, or would you rather have a stereo recording of that same acoustic complete with the fullness and richness of the body and the brilliance of the strings? You don't even need to answer that, who doesn't want an amazing sounding guitar track?

In the following Premium tutorial we'll be going through all the usual stereo microphone techniques that are famous out here in the audio engineering world. Read on through this tutorial and get to grips with how to squeeze a wide stereo image into your recordings! Whether you want to close mike an instrument in stereo with coincident pairs or put up spaced pairs as room mics, the following explanations will teach you which to use.


Coincident Pair Techniques

Let's get the ball rolling by talking about coincident pair techniques. By using coincident pair techniques we have a greater degree of control over phase, which is one of the biggest perks of using said techniques. Because both microphones are almost at the same spot, sound waves arrive at the exact same time more or less to both microphones. Since stereo microphone techniques are used to capture an acoustic, live or full sound the use of condensers is encouraged. The dynamic microphone just doesn't have the full frequency range nor the transient response needed for an accurate reproduction in most of these cases.


The X/Y Technique

Not to be confused with the Coldplay record of the same name, the X/Y Technique uses two condenser microphones at an angle of 90-110°. Like before, we don't have to worry about phase differences since we are positioning them so close that they touch each other. The X/Y technique give you a very good stereo image, perfect for a variety of recording applications. I recommend staying at an angle of 90°, with a wider angle there is greater risk of losing the stereo image since you do not get accurate reproduction of the center.

See the image below for a good explanation on how the X/Y technique works. Both microphones pick up the center sound source, resulting in a stronger sound in the center at the same time the mics pick up the full spectrum of the instrument. If we are close miking an instrument we would try to have the sweet spot in the center, where it gets picked up by both microphones, but still capturing the full spectrum of the instrument. If we were ambient miking with this technique we would point the instrument at the center, where the polar patterns overlap. Then the rest of the polar patterns would capture the ambience of the instrument, or the sound of the room.


http://en.wikipedia.org/wiki/File:XY_stereo.svg

The Middle Side Technique

The Middle Side Technique is a little tricky. It requires a few recording tricks for it to work. It is my favorite recording technique for acoustic guitar as I get both the sweet spot of the instrument as well as the ambience and body. We need two different microphones for this technique to work, a normal cardioid condenser and a bi-directional or figure eight condenser microphone. If you are confused about polar patterns I recommend checking out The Beginner's Guide to Microphones.

We place both microphones close together so that they are touching, or almost touching. The cardioid microphone is aimed at the instrument while the bi-directional mic is perpendicular to the other mic, with its sides facing out. See how the microphones are positioned in the picture below.


Photo by Pablo Albacete

Now that we have both microphones aligned correctly it's time for some studio tricks. We plug the cardioid microphone into whatever channel we like and leave it for now. We create an aux for the bi-directional mic and we route a copy of the second microphone to the aux track. Have the aux send set to post fader so you can control the aux track by only controlling the original bi-directional track. We flip the phase of the aux track so that the copy is 180° out of phase with the original.

By testing these two microphones in mono we align the faders so that they completely cancel each other out. So, you're doing good if you've stopped hearing a signal from the two bi-directional channels. By flipping your recording program or mixer back into stereo and panning both the bi-directional mic and its copy hard left and hard right we get a nice ambience. And by putting the cardioid condenser back into the mix we've got the full attack and center of the instrument you are recording. That's audio engineering for you, creating some great sounds with careful placement and ingenious mic techniques.

The middle side technique is definitely the most complicated but it has its benefits. Once you get the hang of it you won't ever like going back to doing just simple stereo techniques like X/Y.

To recap:

  • Cardioid mic is on channel one.
  • Bi-directional mic is on channel two.
  • A copy of the bi-directional mic is sent to the third track and flipped out of phase.
  • In mono, the bi-directional mics are aligned out of phase so that they are cancelled out.
  • Back in stereo we pan the bi-directional mics hard left and hard right.
  • Add the cardioid mic to the center.
  • Instant awesomeness!

The Blumlein Technique

Alan Blumlein was a British engineer known for creating and innovating many things in the audio world. He was the pioneer who created the X/Y Technique as well as this stereo microphone technique that bears his name. The Blumlein technique consists of two figure-8 condenser microphones aligned in a 90° angle. Very similar to the X/Y technique except for the difference in polar patterns.

Because of the nature of the figure-8 polar pattern, the Blumlein technique not only picks up a great stereo image of the sound source with minimal phase differences - like the X/Y – but it also picks up the ambience of the room since half of the microphone is pointing back from the sound source. Naturally, this technique has its perks when used in a great sounding room, but could sound horrible in a less than optimal recording space. I recommend checking out this site I stumbled upon when I was researching the details for this article. It is a visual representation of most of the microphone techniques I talk about here.


Near-Coincident Microphone Techniques

The following microphone techniques are the so-called non-coincident microphone techniques. They are not completely next to each other, but not far apart enough to be called a spaced pair, which I talk about later. These techniques have the microphones spaced apart, which can result in phase problems if you are not careful. Also, if you are not careful with the placement of the angles you might not accurately capture the center of the stereo spectrum, therefore losing a big chunk out the width of the signal.


ORTF

This technique was invented by the French Radio and Television Broadcasting (Office de Radiodiffusion Télévision Française). By using two condenser microphones with a cardioid polar pattern, spaced 17cm (6.6inch) apart and at a 110° angle we get a nice microphone technique that is suitable for applications such as recording classical music.


Taken from http://en.wikipedia.org/wiki/File:ORTF-Stereo.svg

Since we are dealing with two cardioid microphones we don't pick up as much ambience as we would with any other polar pattern, which is suitable when we only want to pick up the sound source itself with minimal ambient interference. Although, it must be said that the amount of ambience you will pick up is related to how close or far you are miking up the instrument.

I've never used the ORTF mic technique myself personally, but a friend of mine used it in conjunction with a close mic when he was recording cello and violin with great results. You would think that it won't reproduce the stereo field, or the center correctly but due to the fact of how they are positioned, in conjunction with them being cardioid pattern results in a great sounding stereo image. And it even works well in mono!


NOS

The NOS technique is by far my favorite name of a stereo microphone technique. It's short for Nederlandse Omroep Stichting, which basically just means Holland Radio. The NOS technique is similar to the ORTF except it has the microphones separated by 30cm (12inch) instead of 17cm (6.6inch) and are spaced at an angle of 90°. Just like all the other stereo microphone techniques, this one was designed to capture a rich stereo sound and does so without problem.


http://commons.wikimedia.org/wiki/File:NOS-Stereo.svg

I'm not a big fan of near-coincident pair techniques. Maybe it is because I fear that I'll screw something up and it'll all be out of phase. I'd rather place my microphones in a coincident pairing like the X/Y to avoid phase differences than getting out my ruler and measuring how many centimeters the microphones are supposed to be apart.

But these techniques weren't invented for nothing and they do work well for those who use them. So if you are anxious to get experimenting I encourage you to do so, although I'm perfectly content with my X/Y and M/S. I also don't do much classical recording where you would need techniques like this, but if you want to be the recording engineer of your choir, big band or classical quartet, please try these techniques out.


Binaural Techniques

Binaural techniques are created for headphone usage. The complete effect of binaural techniques can't be enjoyed to the fullest without headphones.

Binaural recording emulate the way our ears sound so that if you would be listening to a binaural recording on headphones with your eyes closed, you would feel like are actually there in the room with the music. Put on your headphones and listen to this Youtube video of the virtual barbershop. Here you can feel the complete immersion of binaural recording since it almost feels like you are getting a haircut yourself.

Binaural recordings are created with using a dummy head with two omnidirectional microphones placed in the ears. With the shape of the head, the polar pattern of the microphones interacting with each other as well as the separation between them we get the binaural effect of feeling like we are actually there in the room. Since it is developed for headphone use only the effect isn't reproduced when played back through normal speaker systems.


Quasi Binaural Techniques

It can be fun trying to experiment with creating binaural techniques. Although a scientifically made dummy head and molded microphones inside the ear is the most accurate reproduction of a binaural recording you can get by with using some other methods.

By using the technique known as Optimal Stereo Signal, or the Jecklin disk we can recreate a fairly effective binaural technique without any trouble. The OSS technique uses two omnidirectional microphones separated by 36 cm (14inch). In the middle we place a disk with a diameter of 35cm (13 3/4inch).

By using this sound absorbing disk we can recreate some of the characteristics of the human head. Although not as effective as the binaural techniques, it does a pretty good job of emulating our hearing experience.


Spaced Pairs

Spaced pair techniques have two microphones spaced apart at a certain length. Spaced pairs have the biggest risk towards phase, so placement of these pairs has to be done very carefully in order to avoid phase cancellations. There are a few things to consider when using spaced pair.


The A-B Technique

The A-B spaced pair is used with two microphones positioned apart from each other facing the same way towards the sound source. Normally, the distance between the two microphones is around 50-60cm (19-23inch) but we also have to consider the size of the sound source we are recording.

If the sound source is a large and wide sound source, such as a choir we might need to distance the microphones from each other in order to accurately pick up the sides of the choir. Rule of thumb is that the pair is spaced apart 1/3 of the width of the sound source. So if we have a 6m (19feet) wide choir, we would need to space the two microphones 2m (6feet) apart in the center of the choir for accurate reproduction of everyone.


The 3:1 Rule

This is an easy rule to remember and one that I use frequently to place overheads on drum-kits. The rule states that the distance between the microphones has to be three times the distance from the sound source. So if the distance from a particular sound source you want to record in stereo is 1m (3feet), the other microphone must be placed 3 meters (9feet) apart. It's a good rule when you are placing overheads, recording piano or recording a string quartet for example.

On a side note, it's also good practice to have the overheads equidistant from the snare drum, to reduce phase cancellations that might occur. We don't want phase problems with one of the most important pieces of the drum kit.


The Faulkner Array

The Faulkner Array microphone technique is a spaced pair technique that uses bi-directional, or figure-8 microphones instead of the more commonly used cardioid or omnidirectional. The two bi-directional microphones are spaced 20cm (approx. 8inch) apart.

The microphones in a Faulkner Array face the same way at a 0° angle. Because they are figure-8s instead of cardioids they also pick up the ambience of the room, which make them perfect when you are recording, you guessed it, in a great sounding room. If you are recording a small jazz quartet or a small classical string ensemble, the Faulkner Array wouldn't be a bad choice to try.


Decca Tree

Talking about classic recording techniques, the Decca Tree is a testament to the ingenuity of the sound engineers of yesteryear. The Decca Tree technique was developed by the engineers at Decca Records to be able to accurately record orchestral recordings. By using two microphones at the sides and one in the center we get a greater degree of control over all the facets of the recording, both the source and the acoustics of the room.


Depending on what we are recording, there are a few ways to use the Decca tree. For orchestral and classical recordings we would just put omnidirectional, small diaphragm condenser microphones everywhere in a great sounding room with an amazing orchestra, playing a great movie score. The omnis would then be picking up everything from the orchestra as well as being accented by the acoustics of the great hall used for this particular recording.

Now if we were recording something a little smaller, like a jazz concert, we can have the option of using other polar patterns and larger diaphragms for a different sound. We could mix and match polar patterns, using only cardioid patterns for a more direct sound. Also, putting an omnidirectional in the center with the two side microphone sporting a cardioid polar pattern can be a good choice. Of course, like any other recording trick, the sound is what matters and if you like the sound that two cardioids and an omni give you more than anything else, you should go with that. If it sounds good, it is good.


Which Technique to Choose?

In the stereo mic world it's safe to say there are quite a few options to choose from. Microphone techniques have been experimented with and tested ad infinitum since the beginning of stereo and make no mistake, the amount of experimentation gone into developing these said techniques has been substantial. But when faced with wanting to do a stereo recording, which technique should you use?

If you are just recording in your home studio, where the acoustics are sub-par you would be wise to choose a technique that doesn't involve omnidirectional nor bi-directional microphones. Focusing on cardioid microphones is the way to go if you want to isolate your source sound from the sound of your room. I always start off with coincident pairs, just because with them you completely bypass the need to worry about phase. Throwing up a X/Y pair or a middle side technique is a quick and easy way to capture the sound of your desired instrument without the need to measure anything.

But if you are recording in a nice sounding room you might like to try out some of the near-coincident techniques as they offer a very good stereo image that work well in mono. Since you can measure the techniques accurately you can be sure you are using the near-coincident techniques the right way, instead of trying to measure the source and/or microphone distance with the A-B technique or the 3:1 rule. The Decca Tree might be the most common space pair microphone technique, and in rooms with great acoustics it is a winning technique.

I like using the X/Y technique for drum overheads. If you want a close overhead sound you can place the X/Y pair above the drummer's head where it will be picking up all the drums from the drummer's perspective. This combined with a different stereo pair for room mics can accomplish a great overhead and room sound, whether that's using a spaced A-B pair or a near coincident pair like the NOS or ORTF technique.

If you are doing pre-production with a band and you want to grab some demos of themselves rehearsing in their rehearsal space you could use the Blumlein technique. Throw up two figure eights in the center of the room and capture all the band. Later on you listen back and feel like you are in the center of the room.

My favorite mic technique for acoustic guitar must be the M/S technique. Although it involves a little bit of work setting up I just like the added control you get with the extra cardioid microphone in the center. Some say you shouldn't use it in mono but I disagree. It completely works in mono because of that extra microphone. The figure-8 microphone will cancel out, which is a shame, but you don't have to worry about your acoustic guitar sound disappearing just because of someone listening back in mono.


Conclusion

Well, now that you've learned all the technicalities of these aforementioned stereo microphone techniques I think it's about time you go out and experiment. Whether you're going to use a close miked M/S or X/Y in your bedroom studio or throwing up spaced pairs for the church choir you now know which techniques are more viable than others. Assess your recording situations and elect the best mic technique for the task at hand. No nervousness any longer, just plain skill and knowledge.

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