How high can an isobaric midwoofer pair reasonably be used?

[DaveFred]

Is there any inherent reason a pair of 6.5–7" midwoofers in a conventional isobaric arrangement can't be run cleanly to around 2 kHz?

Assume both drivers individually work well to 2 kHz. I am specifically wondering whether the isobaric arrangement itself — trapped air between the drivers, coupling, spacing, cavity resonances, etc. — creates problems as frequency rises, and what determines the practical upper limit.

It has obviously been done. The Totem Mani-2 reportedly ran its iso pair to around 3 kHz, and I just found the Dellichord FR6, a current 6.5" two-way using two identical SB Acoustics midwoofers isobarically, a passive radiator, and a 1.7 kHz crossover.

I am going to build a test box and compare a single driver directly against the iso pair, but I would be interested in any measurements, experience or technical explanation of what happens to an isobaric pair as frequency increases.

Please keep this to the question of using an isobaric pair into the midrange. I understand the usual iso tradeoffs and alternatives. “Use a different driver,” “use DSP,” “there's no reason to use isobaric anymore,” etc. may all be perfectly reasonable advice, but aren't the question I'm trying to answer.

https://www.soundstageaustralia.com/index.php/reviews/761-dellichord-loudspeakers-fr6-speakers

[5th element]

No you can use this up as high as you want. The isobaric loading basically only affects the drivers responses around the low frequency tuning and resonance. Otherwise it's as if the other driver isn't even there. Of course there could be some interference between the two drivers, at higher frequencies, but that's not because of the isobaric loading, as per say, but because of having the second radiating surface in close proximity to the first.

I would expect the effect to be minimal otherwise drivers in tiny cabinets would display horrendous response issues from the back reflections but they do not.