TRY THESE FOR YOURSELF
I’ll briefly introduce you to a few stereo miking techniques and why you would use them but you have to try them and experiment with them because your ears will prove that the smallest adjustments made to the microphones can have amazing results. In a later post, I’ll discuss the broad topic about moving the microphones before using EQ. The best engineers know when and how to move the microphones for tonal changes and perceived space. Actually, they know how to trust their ears when moving the microphones since there are no hard, fast rules for placement and only what is believed to sound the best.
Using 2 cardiod microphones of the same type and manufacturer, place the capsules as close as possible (coincident) or within 12 inches of each other (near-coincident) and facing each other at an angle of 90-130 degrees depending on the sound source.
The pair is placed with the center facing directly at the sound source and panned left and right. Because of the close proximity of the microphones to each other and the source, there is a reduced chance of phase problems more common with A-B techniques.
The Blumlein Pair is when the microphones are placed at 90 degrees to one another when using bi-directional microphones. It can look like the X-Y when using Figure 8 mics or it can look like the image below. It provides an exceptional stereo image but is very sensitive to the sound of the room since there is little off axis rejection. Notice in the illustration how these pick up the stereo image when placed correctly.
The M-S Technique or Mid-Side is a technique that’s shrouded with a little bit of controversy around it’s matrix encoded cinema compatibility. Place a figure 8 or omnidirectional microphone facing sideways and a cardiod microphone at a 90 degree angle to the omni, facing the sound source. This has excellent mono compatibility which makes it very popular in broadcast.
The idea behind this configuration is that the cardiod, facing the sound source, is picking up the middle. The figure 8 or omni is picking up the sides. You simply add the Left signal to the Center as well as the Right signal that is phase flipped. If you don’t use a matrix decoder, you can adjust the stereo width after the recording is done. It sounds easier than it sounds. Since most of us are using Digital Audio Workstations anyway, this is actually as simple as creating 3 tracks. Track 1 is the cardiod, middle signal. Track 2 is the figure 8 or omni. Track 3 is a copy of track 2, phase reversed. I’d suggest linking the faders between tracks 2 and 3 so that your side signals are even. Now, turn them all the way down and turn up track 1. That’s the middle, mono. Slowly turn up the sides and you’ll start to hear the stereo image start to appear.
There are MS Decoder plug ins available and some preamps actually have them built in. It eliminates the need for duplicating tracks which can be useful if you have a lot of M-S recording in your project. There’s some controversy around whether or not M-S can create translation issues in matrix encoded Dolby cinema. I have not personally run into any issues yet and have nothing to add to the discussion.
A-B TECHNIQUE (Time Difference Stereo)
This is a pretty straight forward technique. Using 2 cardiod or omnidirectional microphones spaced evenly apart and equidistant from the sound source in a parallel configuration. Typically these would be spaced anywhere from 3-10 feet depending on the source. If you’re recording an acoustic guitar, 10 feet would be a bit too wide. For miking an acoustic guitar you’d normally want some body area sound and some neck area sound. If your miking the overheads of a drum kit using this technique, you’d space them further apart. Therefore, if your miking an ensemble, the pair would be even further apart.
There are a couple ways to check the phase of the microphones. You can zoom in on the waveforms in your editor and simply look at when the sound starts. Using your ears is a better solution, personally. A mono reference source can be used to check phase. When the program is switched to mono you hear the frequencies jump out or fall out, then you can trust that they are out of phase. This is important because most supermarkets and broadcast playback are in mono. Your laptop and cell phone are likely to be mono. So you can’t afford to lose the frequency content with phase issues. For mono compatibility, the coincident pair above is the most accurate.
ORTF (Office de Radiodiffusion Télévision Française)
Using 2 cardiod microphones as similar as possible, place the microphones away from each other at an opening angle of 110 degrees with the capsules 17cm apart. The source should be central to the microphones. Since the cardiod patterns reject most of the room sound, since it’s off axis, you pick up reasonably realistic stereo field that has decent mono compatibility. This is a little bit like the way our ears are.
Move the mics! The angle and the distances can be adjusted as needed. Use your ears when setting this up and listen for the differences in just the smallest adjustments. You’ll notice that this configuration can be placed further away from the sound source. I believe you’re adjustments will typically prove that this angle and distance is most often the most useful. At great distances from the sound source, you’ll start to lose low frequency content since this is a very wide configuration.
I should probably mention the NOS Nederlandse Omroep Stichting since it is very close in design as the ORTF. It employs a 90 degree angle and a 30cm distance spacing. This would be used for smaller ensembles or even piano as the farther you are from the sound source the more you lose low frequency content. Therefore, this cannot be used as far from the source as ORTF.
Another similar configuration to both of these is the DIN (German System) which employs a 90 degree angle and 20cm distance spacing. This would be used for even smaller ensembles and piano. This would not work well for very large ensembles since you can’t get far enough to get a wide image without losing a lot of the low end information.
So, ORTF, NOS, and DIN are all very similar but used for different purposes. Fact is, if the ensemble is truly large, none of these are going to work best alone. If the ensemble is large enough, you might consider using ORTF in the center and flanking 2 A-B microphones for the wider part of the stereo image. Be very aware of phasing relationships between the microphones even though this is a very common way to record large ensembles.
This is essentially 3 Omnidirectionals in a triangle. The outside microphones are 60-120cm apart the the center mic can be tucked in a little bit from them. Use your ears and adjust as needed. The outside mics are so far apart that a hole will be created in the middle. This is true of the A-B technique as well. (Conversely, the ORTF, NOS, and DIN configurations will lack the far sides if too far from the sound source.) That’s why there’s a third omnidirectional microphone placed in the middle.
Most of the time the Decca Tree is positioned behind the conductor in some way and rigged so it can be raised or lowered to get the best sound. This configuration usually gets a very realistic image.
You can see that this configuration is really an A-B with a center microphone. It’s most effective for ensemble recordings. While they do make Decca Tree stands, you can use 3 boom mics and place them all yourself. It just makes the adjustments more tedious.
HOW DOES IT SOUND?
Why do these work and why don’t they work? Time delay and loudness. Since there is more than one single microphone at work, you now have control over how loud one is over the other as well as the time delay since you can move one and not the other, closer, farther, up, down, etc… This will create the sonic illusion that a sound source is either left, right, forward, or back. Move the mics! Make problems for yourself on purpose. See what all of this means in practice. Often times you’ll see a drum kit with a dozen or more microphones involved. Sure you can really capture a lot of what’s going on but you’re certainly asking for phase issues. But if you’re patient, careful, and know how to work out the issues, you can do whatever you want. Alan Sides and Steven Miller created the Ocean Way Drums product using more microphones on the kit than you could ever imagine. It CAN work. (But check out Glyn Johns’ method for tracking drums with only 3 microphones. Or, listen to Led Zepplin, Stones, Hendrix…)
I’ve merely introduced you to the concepts. There are a lot of articles on the web from many of the microphone manufactures as well as the individual societies that discuss these techniques and others in greater detail. Try Harmony Central for example. Set them up, move the mics, listen for the results. You will not believe what subtle adjustments will do to the characteristics of your sound. Try these in different spaces and see what large rooms, small rooms, high ceilings, hard floors, diffusion, and absorption can do to your sound. What worked best and for what? If you liked what you read here, check out this nice resource, Microphone Data. There are a few articles worth your time.
It’s really not feasible to squeeze all of this into one article. Each technique deserves it’s own article and explanation. However, I hope you found this a little bit helpful.