This post focuses on the various ‘standard’ ways to get a stereo image with microphones.
A class-A stereo image is one of the holy grails of audio engineering. It’s easy to get a decent image, but to achieve a stereo image that condenses down to mono, sounds rich, and evenly captures the entire performance is very difficult.
Most stereo techniques involve using two microphones but there are some that use more. Additionally, there are some very good stereo mics on the market such as the Studio Projects LSD-2 or the time-honored, appropriately expensive AKG C24.
Find the Spot
Use your ears. If you can afford it, have the performers do their thing while you wander around the room. Find the spot where you can hear as much of the performance as possible, where you hear the character you need, and where the stereo image is full. Place your mics there.
Techniques with two Mics
Spaced Pair (AB)
A good general-purpose placement, spaced pair is achieved by placing microphones evenly apart (usually around six feet apart) and sufficiently away from the performers so that there is a delay between the two microphones. The delay it takes for the sound to reach one or the other mics is what generates the stereo image. Cardiod microphones will produce a more focused image with less ambience. They will also not require quite as large a spacing. Omni microphones require larger spacing as they reject less of the sound – and will produce an image with more of the room. The downside of spaced pair is that it takes careful placement to avoid phase issues. Also, you should be aware of reflections from walls and the ceiling in achieving balance.
If the mics are spaced quite far you can end up with a ‘hole’ in the middle where the sound drops in volume a bit. Two ways to solve the ‘hole’ problem are to make a Decca Tree or use…
To set your mics up in X/Y, point them towards each other a bit and towards the performers. The angle you choose relative to each other will determine the width and focus of the image. I usually use 90 degrees – right angles. If you need a more focused center image, try 60 degrees. If you need a wider image, try 120 degrees. Also, to achieve better phase coherency in certain situations, it can work well to stack the mics vertically instead of horizontally. The mics should be as close as you can get them. Microphones build for stereo are most often build in the X/Y format. Generally cardiod or super-cardiod mics are used here. There won’t be enough of a delay difference to use omnis effectively.
ORTF is basically like X/Y, but the mics are pointed outwardly instead of towards each other. An angle of 110 degrees is usually used with cardiod microphones. The capsules should be spaced exactly 17cm apart. Lots of research was done to achieve these numbers… I don’t really know the specifics of why. ORTF condenses to mono fairly well and has good ambient rejection at the expense of the center image if placed close to the performance.
M/S is a funky little arrangement. It involves using one bidirectional microphone and one cardiod. The cardiod microphone should face the performance head-on while the bidirectional mic should be placed such that its poles are at right-angles to the cardiod facing left and right. I usually position the bi-directional mic right above the cardiod. Omni, as showing in this diagram, can also be used. To use this technique, you’ll need to use an M/S decoding plugin (often included in DAW packages such as Cubase) which translates the signals into a stereo image. The benefit is that you can raise or lower the volume of the cardiod mic to produce more or less center volume.
Decca Tree is essentially like spaced pair but with an additional mic to fill up the center. The spacing is in a T shape with the center mic behind. Omni mics work especially well in this arrangement such as the DPA 4003 series or the classic Neumann M-50 series. The benefit of this is that you can place the tree quite a bit further back with a larger spacing if you like.
For lots more information on stereo recording, check out Wes Dooley’s ‘The New Stereo Soundbook’. It’s a really good read.