Originally Posted by Borderdog
What are your room dimensions?
There are ways to work around those nodes. It basically comes down to speaker position and listening position; it's more complicated than I'm implicating, but there are some simple work-arounds. It deals with quarter waves and other such things.
The other thing is bass traps. Bass trap is such a misnomer. Most commercially available bass traps are worthless.
Do you have any carpentry skills? Or know some carpenters? I can show you how to build a beautiful, aesthetically pleasing Bass Trap that works into the lower 30hz range. It's a sealed membrane design.
Room dimensions are approximately 13x16x8', with some variation (small closet in the rear corner, beam crossing the ceiling, sliding door, etc.). I do have carpentry skills and basic tools. And I already have some sealed membrane traps and some broadband bass absorbers.
I've tried making membrane traps that target the mid 30s and didn't have much success. The first ones I made were about 1' deep with 0.5" membranes of either OSB or plywood. They were ineffective because the thick membrane was too stiff. I also made a couple of 2x2x4' free standing traps with 1/4" low density particle board on the faces. They had some effect, but not as much as I had hoped. I'd have to pack all the corners of the room to meaningfully reduce the lowest modes, and that would take up a lot of space. I ended up using the corners instead for broader band bass absorption.
Now you've gone and given me a chance to show off
Here is a picture from behind the listening position. Room treatments are numbered. Legend is below the picture.
1 - Corners crossed with 2" OC705 fiberglass board with foil facing on the front. The cavity behind them is filled with fiberglass batting. The OC 705 boards are free to move in and out a little bit. The batting behind them acts like a spring to hold them against the front of the frame. This was an idea of mine that seemed to help extend their effectiveness a little lower.
2 - Shallow membrane traps tuned for wide bandwidth over the upper bass and lower midrange. There is an air gap between the membrane and the vertical wood slats in front of them. The slat spacing and air gap were chosen to resonate in an attempt to enhance the effectiveness of the membrane trap for a certain mode. It didn't really work, so the slats are basically decorative only.
3 - Membrane traps, mid-bass tuning
4 - Absorber, 4" mineral wool boards mounted on 1.5" standoffs and fiberglass batting behind.
5 - 2D Diffusor
6 - Membrane traps, tuned for the floor-ceiling axial mode (0,0,1), around 72 Hz IIRC
7 - Hybrid absorber/diffusor. 2" mineral wool boards with cardboard glued onto their face, mounted on 1.5" standoffs. A pattern of holes and slots was cut into the cardboard such that the mineral wool is more exposed at the first reflection points and less exposed away from them.
8 - 2" mineral wool boards mounted on 1.5" standoffs and fiberglass batting behind. The front half is a first reflection absorber, the rear half is covered by 1/8" fiberboard to minimize absorption above the bass.
9 - DIY acoustic door
I used Guilford of Maine acoustic fabric stretched over hobby board frames to hide everything. Unseen in the rear of the room are first reflection absorbers on the left and behind the listening position, more bass trapping, diffusion, and a small closet.
The concept I was going for was to make the plane containing the speakers and listener's head into a reflection free zone, by chasing down each reflection seen in the early impulse response and trying to knock it down with absorption and/or diffusion. But I wanted to minimize the mid and high frequency absorption everywhere outside of that plane because I wanted to keep a reasonably flat RT60 vs. frequency. I learned a lot and it worked out OK, although I'll do some things differently if I ever have the chance (unlikely with two young children now). RT60 ended up being flat, but short at 250ms. Waterfall response is relatively well controlled except at the already mentioned 35 Hz and 42 Hz.
The way I'm dealing with those modes is to use small speakers that don't excite them too much, and I have two subs placed at the midpoints of the side walls. Placing them symmetrically on opposing side walls cancels the odd-order axial width modes (42Hz, 126Hz, 210 Hz). And placing them at midpoints of the room length puts them in a null at the frequencies of the odd-order axial length modes (35 Hz, 105 Hz, 175 Hz). The harder problem to solve are nulls related to placement of the speakers and listening position. I don't have a ton of freedom with placement because of functional constraints. Another problem I've got is a heavyweight sliding glass door behind the blinds on the left, which sucks some midbass out of the left speaker only.