Acoustic treatment for corners.

Consider a door or window the perfect bass trap and if your corner has a door or window then open it and you'll then have the "identical to an open area" which you seek.

For the rest of us, bass traps and usually lots of them if they are of the resistive kind are the necessity. Diaphragmatic traps will take up much less footprint and most people incorrectly set up their resistive bass traps so as not to work at peak efficiency. They need to be pulled away from the wall equal to one-quarter the wavelength of the transition zone (i.e. Schroeder frequency) which is usually 300-500Hz and extend into the room as far as you can to work below this transition zone frequency. Get a hold of Dr Floyd Toole's latest book and it'll set you straight.

Wow! What a room, Kevinzoe. Without doing the work, what would that mean in inches, for say an ASC tube trap in a corner. I gather that it would be pulled out less than 7.5 inches? Do Cathedral Sound Panels act as a Diaphragmatic bass trap? In my case, the problem corner(closed area) has about 3 feet of wall before it hits a sliding glass door. There is a little bit of improvement with the glass door open.

Thanks Wmakshak. If you plan on putting a resistive-type bass trap which I believe ASC are (but I could be wrong) then the maximize its effectiveness it would need to out from the wall 1/4 the wavelength of the transition zone frequency which could range from 500Hz for a smallish room to 200Hz for a larger domestic room. Below is some handy info:

Hz 1/4 wavelength (")
500 6.8
400 8.5
300 11.3
250 13.6
200 17

Note: a door (even a closed one) or wall of different construction type as the other walls could impact these theoretical distances. In my room I have a door at one end of the room so the "acoustical length" appears longer than the actual room length. A test tone disk and SPL meter will help detect which distance is best at reducing a peak bass mode.

Keep in mind that these are the starting points and that absorption should extend further INTO the room to affect lower frequencies. Clearly you need a good deal of real estate or a very tolerent significant-other.

Regarding Cathedral Sound Panels, I'm not familiar with them and their website is a bit sketchy on how they're made other than they're 2" thick. 2" may be sufficient for a true membrane trap but clearly not thick enough for a resistive bass trap. RPG makes membrane traps but they're costly.