green mountains

Bombaywalla, perhaps I'm the one that's mistaken, but I think you might be confusing electrical phase for mechanical phase?

Thiel's previous products before the use of concentric drivers had different plots, e.g.:

http://www.stereophile.com/content/thiel-cs2-2-loudspeaker-measurements

http://www.stereophile.com/content/thiel-cs36-loudspeaker-measurements

http://www.stereophile.com/content/thiel-cs5-loudspeaker-measurements-0

BTW, for those that might not have been aware of this (IMHO, perhaps the best thread to ever appear on Audiogon):

http://forum.audiogon.com/cgi-bin/fr.pl?cspkr&1032037028&openflup&151&4#151

Bombaywalla, I can only guess why. Perhaps for cost savings, improved time and dispersion coherence, and ease of loudspeaker placement? Since it appears as though the concentric drivers use a mechanical rather than an electrical cross-over, perhaps the sum at the listening position is corrected for?
Considering what was posted by Roy in that thread, I have to wonder why there isn't more use of sealed box enclosures by the loudspeaker designers that adhere to this philosophy.?
I think ultimately somehow digital technology will make these ideas more accurate and practical.

Bombbaywalla, I don't think Apogee's are time and phase coherent.