The A/B stereo recording technique is also known as “Spaced Pair”. It involves two microphones positioned anywhere from 3 to 10 feet apart, meant to capture the same sound source. The distance between them will create time-of-arrival (phase) and level (amplitude) differences in the input they receive. These will be more evident as we increase proximity to the sound source.
Level and time-of-arrival differences can be used to recreate location cues our ears receive and brain use to locate sound sources across the horizontal spectrum. For example, at a distance of 60 cm from each other, microphones can capture a source with a time difference of 1.5 to 1.7 milliseconds. This is usually enough to match the location of a sound source to that of one of our stereo system’s speakers. For this, we need to pan the mic tracks hard left and right, respectively. When not panned like this, the same time or phase differences will produce phasing in the output, to varying degrees. Both signals mixed to mono produce the most extreme phasing effect.
A/B recording technique applications
Therefore, this technique works great in the studio for creating wide stereo images. It’s common when miking drums, full bands, and for capturing “room sound.” It’s best to locate the pair of microphones symmetrically within the acoustic space to prevent capturing uneven diffusion on each mic. For instance, a mic in the middle of a room will capture a completely different sound than a mic in the corner of the room.
While not strictly important, positioning the spaced pair equidistant from low-frequency sources can help focus the low frequencies in the mix. Good examples are kick drums and bass guitar cabinets.
A/B stereo recording technique can use different microphone types. The most common choice is either cardioid or omnidirectional.
When using spaced cardioids, aside from both mics being several feet apart, they should point directly at the sound source. Depending on the spacing between them, they will produce narrower or broader stereo images. One of the characteristics of this setup is it won’t capture too much of the room acoustics.
On the other hand, we have omnidirectional mics, which we can be position in the same way as the cardioids. However, omnidirectional mics can recreate a generally broader and more diffuse stereo image. They can also capture a more realistic acoustic environment.