Thanks for the positive feedback :)
Yes centre to centre is correct. There was several test boxes to get both the Vb
for the woofers right as well as to get the spacing of the tweeters right. I started researching this many years ago when I was given a flyer with the original B&W THX certified system (CM8? I think the LCRs were called) - 2x 6.5"W + 3x 1"T. It was an all passive system (including the two subs that were wedge shaped that the LCRs sat on as stands. Discontinued now...
Anyway, their tweeter spacing was close, the small rectangle panels touched, but there was a gap between the top tweeter and the flanking woofer and the bottom tweeter and the woofer, and I knew it was calculated.
So I began researching D'Appolito's 3/2 geomertic arrays, and applied the same physics given that these THX speakers were 5/2 geomertic arrays. B&W listed all the specs including Vb
and Xc, so from there I could do the math, which I found to be one wave length from centre of the top woofer to centre of the top tweeter, then center of the top tweeter to center of the bottom tweeter, then center of the bottom tweeter to centre of the bottom woofer. So bacically three full wavelengths from top to bottom.
Given that, I then figured that it might be just how M&K did theirs, so I started building test boxes.
VIFA actually released (for a very limited time) three 12ohm 1" tweeters on a single plate, but from my calculations (and I sent these to VIFA), the product was not really suitable below 4.8K crossovers, and given that the tweeters themselves could be crossed over at 3K, that the plate should have been bigger.
Anyway thanks again or the positive feedback, and if there is anything you need to know to get your DIY happening, let me know...