Because running a single mono PA stack leaves us with some serious coverage limitations - and only selling seats for the exact middle of the venue will put a big dent in ticket sales - focusing on sending dissimilar signals to the left and right PA sides is the way to go.
The goal is to reduce the amount of audio occurring in the perceptual center without compromising the mix on one side or the other.
Put another way, we want both the left and right sides to reproduce as different a signal as possible while still having each side well mixed and tonally balanced.
Achieving that goal can result in a minimal of buildup in the center, a wide stereo image, and a great sounding mix regardless of listener position.
The most basic way to shift the perceptual center of a source is to slightly delay one side. The advantage is that left and right tonality remains matched while shifting the center to one side or the other.
The issue is that since the actual signal being reproduced by both sides is still identical, it does not help with comb-filtering, but rather just moves the comb-filtering issues horizontally off center to one side or the other.
Utilizing two different microphone types on a single instrument, equalizing them to sound similar, and then panning them hard left and right is a very effective way to introduce differentials in the signals being reproduced. By differing mic types, I’m referring to using a dynamic mic in combination with a condenser mic or a ribbon mic.
There are several interesting things that occur when pairing mismatched mics. In addition to the inherent tonal differences, there will also be differences in volume linearity that will occur. This means that as the instrument is played, the sound will have a volume dependant “pull” to one side or the other.
Also, dynamic mics tend to have relatively heavy diaphragms made of plastic which makes them a bit slow to start and stop moving. Condenser and ribbon mics tend to have very light weight, metalized ultra-thin diaphragms that are very quick.
This “speed” factor, due to differentials in the mass of the diaphragms, introduces a different type of “pull” where the lighter weight diaphragm tends to lead and the heavier dynamic diaphragm tends to lag. Keep in mind that both mic types chosen should sound great independently to the point where either one could easily be used alone.
Mic’ing distance is also a useful tool. When mic’ing a single mono instrument with two mics, using a mic positioned very close to the instrument for the signal sent to one side of the PA and a more distant position for the mic sent to the other side is a natural way to introduce some delay to one side.
Not only is there a slight shift in the perceptual center, the added ambience of the distant mic adds a “far away” feel that tilts the perceptional angle from which the instrument radiates.
If directional mics are used, the closer mic will have more low-end than the distant mic. The phase shifts introduced by the channel EQ used to match the sound of the two mics also introduces additional differentials between the left and right signals.