Check the archives..... tons of info
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Around 100 Hz, the most effective and easy to integrate would be membrane panels. Even those have to be quite large to work below 50 Hz. Larger than can be shipped. More likely a DIY project. Do a few weeks of research first and contact me if you have questions. There is a way to build them into the walls invisbly but it's fairly serious surgery.
I have built bass panels and the best I could describe it would be a maniacal "Mwahahaha".
Have no idea how a box within a box would work but it doesn't seem like a good idea.
I found Ethan Winer's RealTraps, which uses solid fiberglass panels, to be very effective in cleaning up muddy bass in my listening room.
The open baffle subwoofer idea sounds like a nonstarter to me, unless the baffle is 8 ft. x 8 ft. You'll get almost no low bass because the front and rear radiation are going to cancel at long wavelengths. You'll end up with an inert, five-sided room divider with no extension below 100 Hz.
There is a way to use the unusual Machina Dynamica products to improve bass accuracy.
As I have to visit the Big Apple for a conference, if you would like to email me, then I can fill you in, and would be happy to let you use a few items to quickly hear if there is synergy with your own listening room.
These unobtrusive room treatments are often quite impressive. Even to the point of obviating need for subwoofer, with its own integration problems.
I believe you were asking for a simplistic answer and that answer is "YES". I provided a detailed response here on things we used to provide room treatments that were a part of the room and not necessarily the expensive items "true acoustic treatments" which can be recommended. Our room treatments were less than $400 and provided much better sonics than anything that could be attached to our wall. YMMV.
Realtraps! They provide the most absorption to prevent flabby bass in the smallest space... Check my system out as I use 4 Mondo Traps, 4 MiniTrap HF (1st order reflection points), 4 Minitraps for corners..
If you are looking to intergrate subs (especially building) consider a Tact 2.2x with 2 DAC cards and an upgraded powersupply then you can even time align the corner loaded subs and change everything about the crossover in real time. Ultimate in integration.. not the last in audiophile but WOW! You have to spend way more to beat that combo!
I do not think the 5-sided box will work.
The bass wavelengths are simply too long to "see" the box and be contained by it. For instance, at 40 Hz the wavelenghs are 28 feet long. Those wavelengths simply will not be significantly affected by features less than about 7 feet across.
For information on bass traps, you might find this article useful:
Aside from the bass-smoothing technique I described in another thread, there is another active technique that come to mind.: Use a monopole and a dipole subwoofer, and time-delay the signal to the dipole (by how much, I don't know). The combination of a monopole and time-delayed dipole produces a cardioid (heart-shaped) radiation pattern. The bass system used in the Martin Logan Prodigy and Odyssey was a passive cardioid-pattern type. Meyersound makes active cardioid-pattern prosound subwoofers.
Best of luck to you!
Yes treatment can improve room accuracy. But not much impact below 100 Hz ( unless you use specific tuned tube traps and you really need to know what you are doing ). I'd suggest treating all four corners with broadband absorbers. You need something at least a couple of feet wide and an average of at least 6 inches thick, IMHO (3 pounds per cubic foot or up to 8 pounds per cubic foot...foam, fibreglass, mineral wool...)..and as a minimum I'd shoot for 16 linear feet for treatment....that is what I have and it barely makes any difference in the ultra LF (although I am happy with what it does in the 100 to 600 Hz range...again a slight improvement in my room at the listening position).
For ultra LF - use a PEQ, something such as a Rives TACT - it seems to work well.
I'll just add this note I made on another thread before you think a TACT can do everything for you (in reality you need a TACT or PEQ as well as acoustic treatment)
You will find that a parametric equalizer is ineffective above about 80Hz for standing waves room mode equalization.
A typical room of 25 feet by 16 feet by 9 feet will have a total of six room modes below 60 Hz. At 120 Hz this jumps to thirty five room modes (many sharp peaks and troughs in the frequency response and spatially too).
In practice it becomes impossible to do much parametric equalization above 60 HZ to tame bass standing wave modes.
Concentrate on acoustic broadband absorption and speaker placement for modal issues above 60 Hz.
I find that four GIK corner Tri-Traps offer a modest improvement in room response above 100 Hz (subtle clarity improvement in lower mid range vocals was just audible and sweetspot became noticeably larger). See my virtual system to get an idea of the aesthetics. Any less size/surface area of treatments and you are unlikely to make an audible change. I should probably do twice as much treatment to get my RT60's even lower, but, for aesthetic considerations, I stopped at four of these thick broadband absorbers.