Using different stereo microphone techniques, you can record a sound source using two microphones at the same time, producing a stereo image of the sound. This stereo image is created due to the tiny amplitude and timing differences that each microphone will pick up, helping you to capture realistic recordings.
I often come across new starters who think that some sound sources are only recorded with one microphone pointing directly at the source. But using more than one mic can help add an extra dimension to your productions that you would otherwise miss out on if you only stuck to the traditional single mic approach.
There are potential problems with these stereo microphone techniques that need to be looked out for, but they’re easily overcome by following a few tips and guidelines.
There are a few different aims behind the use of stereo recordings:
- Capture accurate instrument positions
- Record some room ambience
- Achieve an even tone and frequency balance across the stereo image
Below are some of the more common techniques you’ll find used in the recording studio, with an acoustic guitar as an example.
XY (Coincident pair)
- 2 cardioid mics of the same make and model, at 90-135 degrees to each other
- generates stereo image from amplitude difference, not timing differences
- narrow stereo spread
- good centre image
- no phase issues
- mono compatible
AB (Spaced pair)
- 2 omnidirectional mics of the same make and model, 3-10 feet apart
- time difference and level difference between the two mics generates stereo image
- brings out the sound of the room
- smooth, spacious sound
- can produce phase issues, which can cause problems in mono
- great pickup of stereo image
- strong central image
- mono compatible
- uses a cardioid mic facing forwards, and a bidirectional mic facing sideways
- increasing the level of the bidirectional mic increases the size of the stereo image
To create the stereo image, you need an M/S decoder, or you can create one yourself using certain settings on the mixing desk:
- the cardioid mic is panned centrally
- you duplicate the signal from the bidirectional mic, and then one signal is panned hard left, and one is panned hard right
- the signal panned hard right has it’s phase reversed
Microphone Phase Issues
There’s an important issue to think about when recording with stereo microphone techniques where the two microphones are at different distances to the source. Sound will enter the microphone that’s closest to the source before it enters the microphone that’s furthest away, causing the different recordings to move out of phase.
These phase issues have a negative effect and cause the combined signal to sound ‘hollow‘ – certain frequencies will cancel each other out, and it can especially affect the lower frequencies.
One of the ways to rectify this is by shifting the audio waveforms inside your DAW so they move back in phase.
Roughly speaking, sound travels at a distance of a foot every millisecond. So for example, if your first microphone is up close to a guitar amp, but the second mic is 3 feet away, then shifting the waveform of the second mic backwards 3ms will move the audio waveforms back in phase. But be careful with this – you may have to zoom in closely to look at the waveform itself, to check that they’re perfectly in sync and in phase.
The 3-to-1 Rule
Another method to combat potential phase issues is known as the 3-to-1 rule (also seen as 3:1). This basically says that for every unit of distance between the first mic and the sound source, the second mic should be three times that distance from the first mic.
Let’s look at some examples:
- If the first mic position is 1 inch from the source, then the next mic position should be 3 inches away from the 1st mic.
- If the first mic position is 4 feet from the source, the next mic position should be 12 feet away from the first mic.
- So, if you have a microphone pointing just behind your acoustic guitar’s bridge at a distance of a foot, and another mic pointing at the guitar’s neck at a distance of a foot, make sure there is a distance of 3 feet between the two mics.
It’s an important rule to live by when using any of the stereo microphone techniques, and it will stop most phase issues before they even begin.
As you’ve seen, there are many different stereo microphone techniques that you can use to record your vocals and instruments for use in your productions. I recommend that you try some out and listen to the different images you can create. Recording an instrument in stereo will often lead to more realistic and natural results compared to sticking to the usual single mic techniques.