chimney + angled ceilings * 30 days = losing my mind ...help

I've been working on getting a new room into shape for a month now, and I've reached the end of my rope here. I'm grateful for all the information on this forum, and have been reading it voraciously throughout this process. I now humbly submit the details of my room's problems, hoping someone will have a fresh idea or two about fixing them. The pics: (that blue thing is my head in the listening position) Synopsis: This is a second story room in a rental house. The room will be used for both mixing and overdubbing. So far I've been concentrating on treating the mixing area, on the left side in the drawing. (The live/overdubbing area will be on the right side of the drawing.) I've been adding one type of treatment at a time, and running LOTS of frequency sweeps with Spectrafoo between each step. The treatment: All treatment so far is of the absorption type, rather than "panel" (resonator) type. Corner and sidewall traps made of 6" of rockwool. Back wall traps are made from 3" of OC 705. Those are all 2' x 4', and covered in burlap. Not shown in the drawings are the overhead treatments. On each side, flush along the angled ceiling, placed at the mix position reflection point, is a 32" wide x 48" tall area of 4" deep foam blade tiles (Markertek). These don't make it all the way up to the peak of the ceiling (since they're 48" tall, and that angled portion of the ceiling is over 6'). So, to augment overhead trapping, suspended directly over the mix position is a 2' x 4' panel made of 3" of OC 705, with the long dimension running front/back. I've been playing with the height and angle of this overhead panel; right now it's about 15" down from the peak, with the front end a bit lower than the back. The Problems: The chimney is the most asymetrical part of this layout, and I've tried treating it with panels of 705. This hasn't always helped, sometimes creating new peaks and dips. And it although it looks like it would create chaos for the left speaker, it doesn't seem to be messing things up too bad. At 25" from the chimney, you'd expect to see a big dip at 135 Hz, yes? (25" being the quarter-wavelength for that freq) There's a slight dip there, but only a couple dB's at most. The biggest low-end hole is seen around 350 Hz, where there's a fairly fast drop of about 9dB. If it helps, I can get a screen shot from Spectrafoo, but I'd say that, from 100 - 500 Hz, there are several significant peaks/dips (at least 6 dB) per octave. Ironing out these variances is what's driving me crazy. I ran the Harman room mode calculator, and overlaid it with my room. It didn't wind up telling me much, unfortunately. And the angled ceilings make it hard to calculate anything having to do with height. Questions: 1) Should I definitely put a trap up on that chimney? Would something less dense than 705 panel work better? And when the speaker is so close to a surface, how much of an air gap is best? 2) For the overhead panel of 705, should that be placed according to the mirror/reflection point? Or should that just get placed directly over the speakers? 3) For the overhead trap, is 705 the wrong material for a "reflection point" absorber? I've read here about how higher freqs will "graze" off denser panels. With the way it's hung now, with the front end a bit lower than the back, the sound (if you ray trace it from the front of the speaker) is moving almost parallel to the panel. 4) I had considered another room configuration: putting a speaker right on each side of the chimney (left and right, with the mixing position on the "bottom" part of that drawing, "below" the chimney looking up). Two things made me wary of this: having a solid mass between the two speakers, rather than air, AND having the speakers shoot towards the angled ceiling right behind me. Are these both valid concerns? 5) Any other suggestions? I can also post photos, and the aforementioned Spectrafoo screenshots as well. I tried to cover a lot of stuff here, so I probably skipped over a few important details... apologies in advance. Thank you kindly, Leigh

1) Should I definitely put a trap up on that chimney? Would something less dense than 705 panel work better? And when the speaker is so close to a surface, how much of an air gap is best? If most of your problems are in the lower, modal area, I wouldn't worry about going to a lighter absorbent; usually the best place for an absorbent behind speakers is IMMEDIATELY behind them; you could even try doing cutouts and "soffiting" your speakers with rockwool. Main thing is SBIR in this case, so absorbing as low as possible will help in most cases, because you'll have several peaks/dips as you move speakers closer or further from boundaries. Minimum air gap I'd use is about 4", with 4" or thicker material. 2) For the overhead panel of 705, should that be placed according to the mirror/reflection point? Or should that just get placed directly over the speakers? It should be centered at the ray trace points of speakers; with a peaked roof, this can be wierd. For your roof, I think I'd do some frames and "cloud" the entire roof peak. Otherwise you will likely see some compression artifacts coming off that ceiling and back to the mix position, out of the acute angled roof peak. 3) For the overhead trap, is 705 the wrong material for a "reflection point" absorber? I've read here about how higher freqs will "graze" off denser panels. With the way it's hung now, with the front end a bit lower than the back, the sound (if you ray trace it from the front of the speaker) is moving almost parallel to the panel. If you have lighter stuff, like 3 PCF, I'd use it; otherwise, another way is to put standard fluffy insulation in FRONT of the heavier stuff for a more broadband result. 4) I had considered another room configuration: putting a speaker right on each side of the chimney (left and right, with the mixing position on the "bottom" part of that drawing, "below" the chimney looking up). Two things made me wary of this: having a solid mass between the two speakers, rather than air, AND having the speakers shoot towards the angled ceiling right behind me. Are these both valid concerns? The solid mass thing will give a hard-to-predict apparent bass boost, and firing into that ceiling would require complete and deep absorption to keep it from totally screwing up phase coherence at the mix position. Alternatively, you could put your speakers against the side wall, absorb the chimney, and let the ceiling's upward slope give you RFZ characteristics for the ceiling, anyway. In your situation, I'd consider seriously building at least four portable absorbers of 4" or thicker absorbent; if you stay where you are, you'll want at least one of those behind you and in front of the window to avoid early reflections from the rear, and you could experiment with placement distance along the sides for the shallowest dips in response... Steve

Thank you Steve for this reply. A couple of followups...

knightfly wrote:

2) For the overhead panel of 705, should that be placed according to the mirror/reflection point? Or should that just get placed directly over the speakers? It should be centered at the ray trace points of speakers; with a peaked roof, this can be wierd. For your roof, I think I'd do some frames and "cloud" the entire roof peak. Otherwise you will likely see some compression artifacts coming off that ceiling and back to the mix position, out of the acute angled roof peak.

So this cloud would be 2' wide, and run the length of the room? Would it be best to place the edges of the trap right against the ceiling (like a corner trap), or hang it down further a bit?

knightfly wrote:

In your situation, I'd consider seriously building at least four portable absorbers of 4" or thicker absorbent; if you stay where you are, you'll want at least one of those behind you and in front of the window to avoid early reflections from the rear, and you could experiment with placement distance along the sides for the shallowest dips in response... Steve

That's a good idea, having a portable absorber to put across the window. Those 6" rockwool absorbers in my diagram are portable - they're built into frames with legs. Are you suggesting I build four more, to have more side wall coverage? Right now the two side ones are placed at the speaker/mix position reflection points. Cheers, Leigh

Today I tried putting a BIG trap in front of the chimney. This had the discouraging effect of causing a wide dip at about 350 Hz, where before there was just a small (smaller and narrower) notch there. The trap I used for this test was a whole package of 8lb rockwool, in total 12" deep. That's a lot of trap. And it didn't have a very positive effect. At least I didn't go and build a whole new trap for it - I was just using the bundle of rockwool, wrapped in plastic, as I bought it. On the other hand, I'm happy to say that putting some trapping across that back window did smooth out some of the bumpyness in the 100 - 500 Hz area. Leigh