31 Hz modal problem

Hello guys, I have built a control room in an odd shaped (octagon) room, 20.5 ft between parallel walls. These walls climb 10 ft vertically, after which they rise at a 10/12 pitch toward a point in the center. The center point is 18 ft above the floor. The walls are double framed, double layered drywall, but there is unfortunately about 100 sq ft of window surface. The floor is wood, over concrete floor. Using a calibrated mic and monitor pair, I have tracked pink noise in the room & found, out of 16 separate locations, a 31 Hz problem at every location. I believe it is obvious where it is coming from (1130 / 18ft = 62.8 Hz; 1/2 wavelength of ~31 Hz). The ceiling is the deepest corner in the room, converging at about 77 degrees. Because I believe the trouble is definitely forming in the ceiling, my plan of attack is to build a ceiling bass trap - probably a hemholtz type. I already have the formulas, etc. to design such a trap, but I've 2 questions I can't seem to answer: 1. How much surface area of this bass trap will I need to equalize the (average) modal peak of 8.5 dB at this frequency? 2. If I shift the design of the trap such that its physical dimensions are be deeper instead of wider (adjusting the perforation percentage to keep the tuning the same), will the increased depth allow me to get more absorbing power? [In other words, does a bass trap of higher volume have more absorbancy than a shallower bass trap of the same surface area, if both are tuned to the same frequency?] The room volume is 4900 cubic ft, if it helps.

I often cut my mixes @ 30 hz.. so.. :roll:

I have built a control room in an odd shaped (octagon) room, 20.5 ft between parallel walls. These walls climb 10 ft vertically, after which they rise at a 10/12 pitch toward a point in the center. The center point is 18 ft above the floor.

This might seem like a strange question, but why on earth did you build it like that? It doesn't make much sense, acoustically, as far as I can see. Why octagonal? That gives you FOUR sets of parallel walls, all at the same spacing! Kind of like two square rooms, superimposed on each other!!! I would expect that you'd have huge modal problems at 27.6 Hz, 55.1Hz and all multiples thereof from the first order axials, not to mention the related tangentials and obliques. Not to mention serious flutter echos at the same frequency, etc. Plus, your ceiling is going to focus reflections and do other strange things. It's also hard to see where you'd put your bass traps in an octagonal room! Even if you solve your 31 Hz dip (which at 8.5 dB isn't all that bad too stat with), you'll still have all the other issues to deal with. Anyway, that's just my 0.02...

Yes, I knew I'd be reprimanded for the room shape! It's the runner up to one of the worst shaped rooms of all - circular. The fact is, it's a den in a house overlooking the woods / pond on a hillside. Part of the idea was that the room would be living space - a sort of "music room" to those guests less privy to technical audio. If we ever decide to sell, it's much easier to sell a house with a "cool shaped den with lots of windows" than an even more odd-shaped (properly designed) "control room". From outside the house, it looks great. I know this doesn't impress you audio gurus, but the room is large enough that analysis suggests it is fairly free of most audio problems, as long as the source isn't stationed in the center of the room. I'm setting up the console near one wall, giving me about 26 ms of time delay in the room's natural delayed reflections (between my ears and the back wall). By the way, the bass frequency modal problems, for those who are curious are hereby listed in order of weight ... * 31 Hz (+8.5 dB) [showed up in 16 / 16 sample sites - 100% problem] * 52 Hz (+5 dB) [showed up 11 times out of 16 - 69%] *113.5 Hz (+6 dB) [50%] * 92 Hz (+7 dB) [44%] All other modes above these four seem to diminish significantly, and I'll probably stick to diffusion panels to take care of all that.

Any way, my plan is to attack the 31 Hz problem by building a bass trap for that frequency in the ceiling, where I believe it is originating. My trouble is understanding how much of a bass trap I need. Can anyone give me a "rule of thumb?" Is there a way to guesstimate how much coverage a trap needs to have, per decibel of peak, based on room volume?

ejb wrote:

Any way, my plan is to attack the 31 Hz problem by building a bass trap for that frequency in the ceiling, where I believe it is originating.

Accumulating might be the better word then originating. Folks talk about building overhead clouds in control rooms on a regular basis here. That is what you could use to soften that area up and reduce the focus, as was pointed out. But that room shape is going to keep getting in your way, acoustically. Have you done anything to treat the walls, stop the reflections, flutter echo, etc?

xSpace wrote:

Have you done anything to treat the walls, stop the reflections, flutter echo, etc?

No, sir. I was taught to treat acoustics in the following order: 1. Modal problems, first. Start w/ the lowest frequencies, then re-test to check effectiveness & see how the next frequency bands were affected. 2. Reverberation time. Treat specific problems first, then broadband absorption until reverberation level is up to par. 3. Keep some flat spaces in reserve for diffusers. This was the method I learned a few years ago. If things have changed, I'm open to learning the new techniques. But the opinion ingrained into me was that it was a mistake to treat other acoustic problems before attacking the modal problems first. I cannot post drawings due to computer equipment trouble. But I can post pics.....

then the control room above...

And here's the shot from outside - the last one taken just this past weekend. We got a lot of snow!

jbassino wrote:

I often cut my mixes @ 30 hz.. so.. :roll:

I have been contemplating this; however, we are looking into editing for video & film. Consequently, we may have to do some "sub" mixing at some point. And because the problem is so strong, I'd like to just take care of it before I trim it all out,if you know what I mean. once you get business up and rolling, it's hard to stop to band-aid problems. BTW,

Soundman2020 wrote:

I would expect that you'd have huge modal problems at 27.6 Hz, 55.1Hz and all multiples thereof from the first order axials, not to mention the related tangentials and obliques.

As you guessed it, 27 Hz is part of the problem. In fact, the 31 Hz mode is a wide one, about 10 Hz wide. For that low frequency, it's very wide.

ejb wrote:

xSpace wrote:

Have you done anything to treat the walls, stop the reflections, flutter echo, etc?

No, sir. I was taught to treat acoustics in the following order: 1. Modal problems, first. Start w/ the lowest frequencies, then re-test to check effectiveness & see how the next frequency bands were affected. 2. Reverberation time. Treat specific problems first, then broadband absorption until reverberation level is up to par. 3. Keep some flat spaces in reserve for diffusers. This was the method I learned a few years ago. If things have changed, I'm open to learning the new techniques. But the opinion ingrained into me was that it was a mistake to treat other acoustic problems before attacking the modal problems first. .

None of this matters in a poorly designed environment. Treating acoustical problems is a direct result of poorly defined rooms. What ever you learned...has changed.

Maybe its just me, but the more I look at that room, the less I see it as being a usable studio, without an awful lot of work being done on it, one way or another. Maybe the easiest would be to just take off all the inner drywall, consider the outer shell to be the outer leaf, and build another (decoupled) room inside it, with usable dimensions. But like I said, maybe that's just me...

I think the room is amazing. Would be great as a song writing room. Studio..... :idea: 1st I would hang a huge double sided octagon cloud in the ceiling. At leat 6inches thick. Then corner panels between each window bridging the "corners" Did you says the drum room was under this? [edit]- as in the drum room is an octagon as well?

I agree Lilith. As an open room it must be nice. But the OP said this: "I have built a control room in an odd shaped (octagon) room, 20.5 ft between parallel walls. " For that, we all know this is a very difficult issue to overcome without some understanding of what that kind of room is doing, first. Your suggestions seem like a good start.

But the OP said this: "I have built a control room in an odd shaped (octagon) room, 20.5 ft between parallel walls. "

Exactly. Plus the almost-parabolic-dome-shaped ceiling. The combination of those two looks to be rather complex, to me. I agree that as a room, it LOOKS great, and would no doubt be a pleasure to work in! But as a studio, it has serious issues. - Stuart -

Soundman2020 wrote:

I agree that as a room, it LOOKS great, and would no doubt be a pleasure to work in! But as a studio, it has serious issues. - Stuart -

Haha! Well, yes, it has serious issues, BUT... is the glass half full, or is it half empty? The room is already built. By the way, I recovered my old name ... just used "ejb" until I could figure out what happened to my old data after my move. I've actually been a member here for several years. My life has been in constant flux. Anyway, I'm back, and you'll see this avatar & user name from now on. Once this thread is finished (no more traffic for a while), I'll deactivate my old name. John says it will erase this thread, unfortunately. Anyway, my point is this: the room is built. And painted & trimmed. After the huge task of building, we are already scraping the bottom of the financial bucket. If I continue to make major changes now, I will not have the equipment I want. So, whether or not the room is "usable" is now a historic question. I HAVE to assume that it is. So the next question is, where do I go from here? Thank you all for the positive comments. It was a lot of work! [Yes, the drum room is ALSO octagonal in shape; however, that room will be much less a problem, because I'm setting it up as a semianechoic chamber. I am presently installing 10 inches (thick) of insulation on ALL wall surfaces. The ceiling will be outfitted with diffusers. I will also build some mobile reflective diffuser units to add to the drum room walls when I want something more reflective. Directly next to the drum sitting position, I have built a simple 1/4 wave, wide spectrum bass trap out of the underside of the stairs. No space wasted. According to calculations, it should be about 20% effective at its two extremes - 30 Hz & 350 Hz, and up to around 60% effective for the frequencies between 45 Hz & 280 Hz.] I like the floating cloud idea.

[quote="lilith_envy"]Then corner panels between each window bridging the "corners" Lilith, I forgot to address this comment. In fact, your suggestion mirrors what was on the back of my mind. Realizing that the amount of glass in the room is also a problem, it seemed plausible that diffusion spanned between the windows (and perhaps above & below, as well) might give me that edge this room needs. Two of the windows have glass (patio doors) opposing them, which isn't good. However, the other two have objects in their reflective path; a very large (8 ft x 8 ft) bookshelf, and a (8 ft x 4 ft) console on the perpendicular wall pair. There will also be a baby grand, couches & tables, and perhaps an array of hanging acoustical instruments (guitars, banjos, mandolins, etc.). All in all, I think there will be some diffusion anyway, but stereo balance is needed, and true diffusion will likely need special attention.