Beyond what frequency range CTC distance becomes a non issue?

It looks like I have started the thread almost 10 years ago.

The question was really about supertweeters, and tweeters crossed very high to a wide range mid. It is usually a non-issue with subs and mid to woofer xo because the wl at xo point is usually large enough.

I have no new wisdom regarding the original question.

But I have been experimenting on the effect of CTC distancing recently (rather the effect of lowering xo point with fixed CTC). Using DSP of course.

Test bed was a two way with large-ish planar (neo8s) and a 15" midwoofer.

I tried xo points from 800 down to 300hz. And the image got better and better; the tightness and stability of the center image improved at every step without any collapse in the size of the total sound stage.

Putting a mid driver btw T and W does not solve the problem. Total height of the driver column is also important. Ie Your T to W distance also needs to be as close as possible (1/4 wl is probably a good guide)

Hence I purchased a coaxial driver for my next project 😊