I am a little surprised that your calculations didn't show any serious problems - given the fact that your room's measurements are very close to doubling each dimension, I would think some strong modes should exist.
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You can spend big bucks and experience a lot of frustration trying to eliminate dimensionally induceds nodes and still not succeed. It has been my experience that the only nodes that are important are the ones at the listening position. These can be controlled to some degree by speaker placement and listening position placement. A good CD with pink noise tones and a RS meter and graph paper will be of great help to you. There has been a lot of discussion on this subject on this site and on audioasylum - you might like to do a search and see what you can draw from the prior posts. As i recall they contained some excellent explanations and suggestions.
Zaikesman, I had some congestion between 77 Hz and 83 Hz, but not again till I got out to about 178 Hz. I wish I could show you the graph so you could make some suggestions or corrections if I'm wrong.
It was the first time I ever tried this.
Newbee, thanks for the suggestions. I'm willing to work at it.
I, like Zaikesman, am very surprised by your findings. However, every now and then some things serendipitously fall into place and what should be a huge problem is balanced out by something else that should also be a huge problem just by luck. That being said I would suggest that you do as Newbee suggested, but I would add a bit too it. Take the measurements at the listening position then pick 2 or 3 other positions that are various distances (say 1 to 3 feet) and various directions away from the primary listening positon. The purpose of this is to see if you are sitting in null points for the frequencies that should be accentuated. It's simple enough to calculate these and I would point you to our website tutorial the listening room . We also have a test CD that may be useful, as it has test tones calibrated for the Radio Shack analog SPL meter.
I am not quite as surprised by the results although in the perfect model doubling the measurement on the rectangle would cause problems. However what we don't know is the materials in the room,whether your measuring a blank room, ect. The one question concerning what's above the ceiling is the right one. Acoustical applications can make all the difference in the world. In my app. I paid a lot of attention to what went behind the wall,ceiling,and floor. Despite things I couldn't change about an existing configuration the result with regard to resonance and other issues was very satisfying.
Rives, I just went to the website, and found it to be most helpful. I wish I could have programed in the location of my stand and record racks. I moved the speakers around and the listening chair on the program. The funny thing was, I was somewhat close to where I should be. A little tweeking of lacations is required.
How do I find out about your room treatments? I'm interested in getting things into a better working order.
Gotta go and move my speakers!