Minimum Distance Is About Stereo Coverage, Not Interference
The minimum width between a left and a right speaker is, in effect, the width between two ears — like headphones. The next point about minimum distance is about how many people can listen to both speakers. If the speakers are too close together, very few people will experience true stereo.
Comb Filtering and Stereo Pairs: Why It's Not the Issue You Think
Comb filtering does occur with stereo pairs but it's less noticeable because the brain isn't processing the two inputs trying to determine what frequencies are missing. Different frequencies are missing from the input to each ear, so the other ear fills in the blanks.
Comb filtering is worst when two adjacent sources are producing identical output. As long as the two speakers are on opposite sides of your head and playing different (stereo) signals, comb filtering is not going to be a noticeable issue.
When Comb Filtering Actually Becomes a Problem
Comb filtering becomes an issue when two speakers are playing the same signal and are at different distances from only one of your ears — on the same side of your head. The null occurs where one source is farther from the listener by half the wavelength of that frequency.
When Coverage Angles Do Matter: Speaker Arrays
The coverage angles and their effect on comb filtering are of greatest concern when arraying multiple speakers. Two speakers 5 meters apart have their sound fields intersecting 1.4 meters in front of the midpoint. Narrower coverage will move that point farther away — but is that desirable?
Use one left speaker and one right speaker and try to fit the audience between them. The coverage angles are largely irrelevant in a stereo pair scenario.
