I wanted to write this time about stereo miking techniques. Some of the best recordings use only two or three microphones. And as with all things audio, there are no hard and fast rules. Fortunately, there are some really good starting places. There are several "standard" techniques that are designed to capture an acurate stereo image of whatever you are recording. Some are simple. Some are not. One sounds very good in headphones. Regardless, a superb stereo recording is wonderful to listen to.
While stereo miking can be simple, the microphone selection and placement can be very tricky. Two microphones can be placed in so many combinations that the stereo image can change with just a centimeter difference. It's up to you to find what works best and what sounds most pleasing to your ear. These techniques can be used for acoustic guitars, pianos, string quartets, small chamber ensembles, horn sections, orchestras big and small, big bands and any other instrument or group of instruments you want to capture in stereo.
Complete flexibility is the name of the game with an A/B technique. Take a matched pair of cardioid microphones and place them wherever you want to get the stereo image you want. When recording, pan one left and the other right. A couple of guidelines to consider. When placing them apart (non-coincident), place the mics at least three times as far apart as they are from the source. This is known as the 3:1 rule. Following this rule helps minimize phase cancellation. Another technique to minimize phase cancellation is to position the mics so the capsules are as close together as possible. This is known as a coincident pair. These are also called X or Y pairs as the mic placement mimics those characters.
Coincident pairs sometimes aren't effective when trying to capture Mahler-size orchestras. A popular technique then is a variation of the A/B technique. Place a matched pair of omni-directional microphones far apart. Again, pan one left and the other right.
Nederlandse Omroep Stichting (Holland radio) developed a methodology to capture accurate stereo recordings. Using two matched cardioid microphones, angle them 90 degrees relative to each other and space the capsules 30cm apart. There are hardware devices with mic clips to help. Pan one left and one right. Accurate is the word that best describes this technique.
Similar to NOS is Office de Radiodiffusion Télévision Français (ORTF). The difference is the two cardioid mics are positioned at an angle of 110 degrees and 17cm apart. And, just like NOS, ORTF is very accurate.
Oh, boy. The Decca Tree. This technique was developed in the 1950's and used by Decca Records to create exceptional stereo recordings. It's bit of a slant and not a true "stereo" technique because it uses three microphones. Therefore, it requires three mic preamps and possibly a three-track recorder.
I have a friend who welded a Decca Tree frame out of old mic stands. It's possible to do it with three separate stands. As I already mentioned, to make a Decca Tree recording, you'll need three matched microphones. Three omni-directional mics to be exact. The left and right mics are spaced six feet apart. The third mic is postioned directly between those two and three feet out in front. This mic will be panned in the center. Decca recordings are accurate and pleasing to listen to.
Of all the stereo techniques, the Blumlein technique is my favorite. I did a recording once of a string quartet that sounds so wonderful and lush, I want to listen to it over and over. I like this technique when recording in a space with wonderful acoustics. I first used this technique at the suggestion of Fred Vogler who records the Los Angeles Philharmonic. He was visiting Beno May while I was at Bernie Grundman Mastering. I was telling him about an upcoming session I had. He told me that Blumlein was his main stereo pair for the LA Phil. I tried it and liked it. It worked so well, I didn't need to EQ or compress the recording at all.
To make a Blumlien pair, you'll need a matched pair of figure-8 or bi-directional microphones. Place one with the front facing left. Position the right upside down and directly on top of the other facing right. Pan them left and right respectively. What's different about the Blumlein is the rear of each mic is picking up the room reflections. It's not ideal when an audience is present because anyone coughing or sneezing will also get picked up. But, in a controlled environment, this technique is wonderful.
This is the trickiest technique to setup. It's not very accurate as it has some phasing inherent in it. But, the sound is very smooth. Almost ethereal. When recording a solo piano with a new age style or a solo acoustic guitar, this is extremely effective.
M/S or Middle-Side requires a figure-8 or bi-directional mic and a cardioid mic. Follow closely. This gets tricky quickly.
Place the figure-8 mic so the front points left and perpendicular to the source. The cardioid mic will go (as close as you can) on top and be pointed directly at the source. Now, split the figure-8 mic signal AFTER the preamp into two channels on your console or interface. Invert the polarity of one channel. Adjust the input gain until the signal cancels as much as possible. Pan one hard left and the other hard right. Now, bring up the third channel with the cardioid microphone. Notice as you bring this up, the center image tightens and the stereo width diminishes. You can adjust the center mic to taste. More center mic makes a tighter, narrower image. Less center creates a wider stereo image.
Logically, it would appear that the best stereo recordings would come from replicating our own heads and put mics where our ears ought to be. That's precisely what binaural records are. You can buy the kits with dummy heads and mics already placed inside. Or, place your own matched omni-directional mics in a dummy head. Simply put the head where it sounds the best to your ears. Binaural recordings sound pretty good on loudspeakers. They are fairly accurate and don't have a lot of phase issues.
Where binaural recordings exceed are through headphones. It stands to reason they would. Binaural recordings through a pair of headphones puts you right there in that sweet spot. You feel like you're in the room with the group.
As you can see, there are a plethora of techniques for stereo recordings. I leave it to you to experiment and find one that works for you. There isn't one that works for every application. As I tell many people, recording music is an art form. If it were easy and/or scientific, everyone would be doing it.
Rock. Roll. Repeat.