I’ll try to describe some of the issues to consider when using multiple speakers in a live situation. We can see from the earlier discussion that you’re almost always going to need more than one speaker system on each side of the stage, unless you are in a very small room, and are mixing the very quietest of bands. One speaker system just can not generate the SPL needed to project a reasonable volume at any distance. (note that when I mention “multiple speaker systems, I mean multiple IDENTICAL speaker systems. To simplify the discussion, let’s assume that you are using all-in-one 3-way cabinets, plus subs. A “speaker system” can therefore be comprised of any number of these all-in-one cabinets, plus as many subs as is required. I will refer to “system” as the entire number of cabinets)
There are, as with most things, advantages and disadvantages of using multiple speaker cabinets aimed at one area.
The first advantage, obviously, is that you can achieve higher SPL at a particular listening position than with only one.
Secondly, putting several cabinets in close proximity decreases the dispersion angle and increases the ‘Q’ of the system over that of a single cabinet. This puts more sound where you need it, and decreases the amount that you send off into the reverberant field to bounce around and cause problems.
Thirdly, it will extend the low frequency response over that of a single cabinet by increasing the radiating area and horn mouth area. This is a HUGE advantage, especially in subwoofers.
Also, multiple cabinets covering every portion of the audience’s listening area provides a degree of redundancy in case an amp, or speaker fails (and it will eventually happen to you).
The disadvantages mainly relate to time/phase issues. Hearing the same sound from different sources produces phase cancellations, which produce a “comb filter” effect on the sound. The amount of this comb filtering depends on the number of cabinets you have aimed at a particular listening point, the distance between the drivers, and, to an extent, whether the drivers are side-by-side or vertical. If two speakers are side-by-side and you rock back and forth as you listen, the frequency of this comb filter will change as you move, which makes it much more noticeable. If the drivers are vertically aligned, the change will only happen as you move your head up and down. Since people move horizontally much more often than they bob up and down, this is a better choice if you can do it.
This is the problem that the new “line arrays” from JBL, EV, and others is trying to solve. The speakers are hung in a large vertical array and angled so that (as close as possible) only one cabinet is aimed at any particular point. Through massive digital processing, they attempt to correct any phase issues which remain. There are two problems I have with these: you are forced into a situation where you only have one cabinet aimed at any given point, which greatly limits SPL (and therefore headroom), and the digital processing makes it sound horribly (to my ears) processed. I’ve read the “white papers” from those companies, and I understand what they are doing, and why, but the resulting sound quality is, to me, not acceptable.