Originally Posted by six172
here is a screen capture of my report.
Thanx, that helps. If you blow across the mouth of an empty Coke bottle, you can get that small chamber to resonate (make that boooooh sound). If you expand that small chamber to a 12x19x9 space, it will still resonate (of course at different frequencies than the Coke bottle, but you get the idea).
According to the chart you posted, the width dimension of your room will result in resonances (aka standing waves or room modes) at 47Hz, 94Hz, 141Hz, 188Hz, 235Hz, etc. Centering the woofers of your L/R speakers 2 feet in from the side walls should help minimize the peaks & dips associated with the first five of these width modes. If your speakers don't go down to 47Hz, then centering a subwoofer at the midpoint of room width (below the centre speaker) will help.
The chart shows that your room's length modes will be at 30Hz, 59Hz, 89Hz, 119Hz, etc. You can reduce the first and third modes by using a second sub, placed at the midpoint of the back wall. If you're willing to spring for four subs, you can get even smoother bass and better seat-to-seat consistency. If not, I would budget for 2 subs at a minimum.
Room modes create nulls (cancellations) at even divisions of room dimensions: halves, quarters, sixths, etc. So avoid those locations! Instead, place your main row of seating at an odd division of room length: one of the thirds or fifths. BTW, when I say seats, I mean the listeners' ears (if you watch movies reclined, take that into consideration).
That's a good starting point for reducing modal problems in your room; will certainly give the room correction system in your AV receiver a head start. Since you're going to be spending time, effort and money on a dedicated home theatre, I would recommend investing a relatively small amount of it in measuring gear. Finally, you mentioned that your room length could be changed. How much space do you have to work with?