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This is a complicated subject.

While building the horn I would need to know what the size of back chamber to make.  Usually the initial size of driver diameter plus two depth works very nice. It makes big chamber that eventually might be minimized to tune the resonance frequency properly.  I do not have the Jessie’s luxury to build metal, high-precision, calibrate back chamber. My back chamber will be simpler and I will be using acoustic foam to minimize the side tuning the resonance frequency to the horn rate.

http://www.romythecat.com/Forums/ShowPost.aspx?postID=2991#2991

There is however a kink in my situation.  I really would like do not open and close the Back Chamber and do not staff it with filler while the horn in attic. I would like to do it while the horn at ground level BEFORE the horn will be lifted up and slide into the wall. My horn is 1.4 size and after it will be installed it will “see” the ceiling of my cathedral room. It means that the volume of the air from my cathedral ceiling pitch will be added to the mass of the air in the bell of the horn mouth, changing the rate of throat reactance and consequentially driving the resonance frequency away.

So, there are 3 options to deal with it.

1)       Set the resonance frequency after the horn are lifted up and installed. I do not want to do it – too little room will be left in attic between the horns and large center air unit.

2)      Make a maquette (thanks coops) measure that deviation of load between floor and cathedral ceiling and to presume that the delta will be the same for large horn. I do not want to do it as well as I feel it will be very inaccurately. The large horn will behave very different.

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