One thing I see frequently in live sound reinforcement is the classic configuration of two main loudspeakers per side, coupled as tightly as possible with a ratchet strap.
While structurally sound, this isn’t an optimum electroacoustic approach.
The cabinet edges of a loudspeaker usually have nothing to do with the horizontal dispersion angle, and for most models, coupling along the edges and driving both sources at the same gain results in a horrible, comb-filtered mess, with some summation (Figure 1).
To create a “unity” splay for coupled mains, simply splay them to the angle of their coverage relative to each other. This creates a line of minimum variance (if you walked along it, you would hear no change in level) in an arc around the front of the array, and the cancellations in the comb filtering are less severe (Figure 2).
The tradeoff is that sonic energy is being spread across a wider area. Sometimes this can be a problem when, for example, you are worried about leakage in a ballroom with air walls, or in a recent event that I worked, leakage through a tent.