Sidebar 2: A-B UNPLUGGED: CLASSICAL RECORDINGS

In true stereo recordings, listening in A-B can give you insights into the stereo miking techniques used. True stereo recordings using coincident microphone pairs will usually have an increase in reverberance in A-B, while recordings made with spaced-apart omnidirectional microphones will change little, if at all, when the A+B and A-B signals are compared. If the spaced pair is relatively close together, a loss of low-frequency information will be observed in A-B. The closer the pair are to each other, the less low-frequency information will be present. You can even calculate, approximately, how far apart the two mikes were by figuring out at what frequency the low frequency sounds begin to roll off. Frequencies with wavelengths much longer than the distance between the two mics (about twice the distance or more) will essentially be “in phase” at both mics and will therefore cancel each other out when summed for A-B listening. At the same time, signals coming from the side (which is the worst case for phase shift) will, at some frequency, be 180° out of phase. In A-B, that frequency will be in phase, and will consequently be boosted by 6 dB. To apply this insight: if the level is clearly down at 100 Hz., but slightly increased around 200 Hz., then the mikes are almost certainly about half a wavelength apart at 200 Hz. (2.5 feet) and approaching or less than a wavelength apart at 100 Hz. (greater than 5 feet), so you can reasonably infer a 5-foot spacing between the mics. Clever, eh?

Orchestral recordings that involve the use of numerous spot microphones (“highlight mics") will reveal the presence of those mics in two different ways: highlights that are panned to the center will disappear, while those that are panned Left or Right will be revealed more strongly. Gain-riding of highlight microphones panned Left or Right becomes quite apparent as well, sometimes embarrassingly so.

Often, in such multi-microphone concert-hall recordings, reverberance will be reduced when listening in A-B, because the reverberance is being primarily picked up by the main stereo pair, and that will be reduced in level in A-B, leaving the highlight mics to stand out with lower reverb levels.