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Wow, Jessie, thanks for the detail deals of your idea. I have spent last evening to contemplate many possible design configurations and I desired to discard all my former ides of metal fine-tuning, even my machinist in on standby expecting my order.

I just will not have space to put in there any maniacal devise as the volume that I need to be left will be too small.  So, I decided to do what I did with upperbass horn - to displace the volume with non-compressible foam. Here is a close to final design that I will order my carpenter to render.

The back chamber will be 3 layers of ¾ Baltic Birch, glued together. On a back it will be a door that will be hold by 12 studs, driven with epoxy into the plywood. The door will be sitting on a think cork gasket. The space in the back chamber will be filed with non-compressible foam. The driver will be installed, covered, the foam spayed. Then, what the foam expands and solidify the foe will be cut by knife in order to hit the target resonance frequency. I will be using one of those

http://greatstuff.dow.com/

I might also will glue piece of fabric on the basket of the driver, I do not know and I am still contemplating  to do it or not.

Now the controversy.  The method I described above is wonderful – it is super cheap but very effective and very accurate. However, all of those Polyurethane foam sealant let go with time, the foam juts is losing density after years.  It is valid argument and it is true. BUT I did check the back chamber tuning of my upperbass horn last week. It did not move for a single Hz after 9 years. So, I figured that the problem might be more theoretical in nature.

BTW, my carpenter pitched me an idea to fill the back chamber with sand. I do not hate the idea and kind of thinking about it.

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