++1 Russell. There was a long thread on PSW below which addressed this about halfway down in
PSW Recording Forums: Brad Blackwood => 2 subs
I wrote this about 3-way boxes in two posts, starting with a rant in #364327:
"...One sub can *never* allow time coherence, period. Put the subs and every other driver in correct time alignment with the ear position. That usually requires setting higher-freq drivers farther from the ears, depending on the crossover and the rise times of the drivers. That's a big reason many tweeters now are recessed a half-inch or so into flared mounting plates. Manufacturers should provide correct ear-to-driver distance for any 3-or-more-driver rigid loudspeaker, because *one and only one* distance is correct. What, yours don't?** Big surprise...
Unless you can adjust drivers singly or in pairs for time alignment, there will be *only one* sweet spot or, sadly, none at all. You can do it yourself most quickly by using single-miked percussion samples, listening to one speaker channel only. Adjust woofer-to-midrange first, then mid-to-tweeter. Do separate subs vs. woofers last. Pray that the design *has* a coherent point..."
From message #366103:
"... Let's say you're a speaker designer and you've chosen your drivers and crossovers and recorded those absolute offsets. You then draw a box with some slope of front baffle with three points on it to represent the infinite-distance acoustic center of those drivers. Now draw the ear-to-tweeter line and the other two. Is there any ear-to-tweeter distance and speaker angle at which the lines converge at the ear? If yes, you have found your *only* time-coherent listening position for that speaker. Now, if just one driver on the speaker can be adjusted to move forward or back, *and* the other two drivers can be tilted, you will have a large area in which your speakers can be placed wrt your ear yet still achieve time coherence..."
Regards