Yup, I believe the idea is to have an additional enclosed volume of air. Then tune it as a Helmholtz resonator to the relevant frequency, just like you would a regular port. It then acts like a kind of narrow band acoustic black hole. Obviously the volumes and port sizes are much smaller for the higher frequencies involved.
I should say that I've not tried this technique myself. I also believe that Elipson have a patent on this application, so not for commercial applications.
Neko88, remember that with the driver removed that you will effectively have made the driver cutout a port/helmoltz resonator too, this is likely to be the 185Hz resonance.
There could be coincidence going on here. A sphere has the least troublesome diffraction signature of any enclosure, but it still produces a step in response. That could be combining with the coincident internal resonance (its got to be a similar frequency as the physical dimensions between the outside and inside are similar in a sphere).
Unless you have some measuring kit (its really cheap these days: PC+ARTA+behringer mic) its going to be hard not to continue guessing. Try the divider, it might do the trick. Good luck!
You will need a bridging piece between two hemispheres as above, because two wooden hemispheres will not have the same shape perimeter. This bridge will mess with any standing waves. Other than that, build and listen. No audio device is perfect, the skill is to match its failings with your own prejudices.
Cheers Steve
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