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Unexplained null

Discussion in 'Room Acoustics / Isolation / Treatment' started by Reverend Lucas, Oct 15, 2014.

  1. Reverend Lucas

    Reverend Lucas Active Member

    Let me start by apologizing for the length of this post.
    "I have made this longer than usual because I have not had time to make it shorter." Blaise Pascal

    I recently converted some of my 90-year-old basement into a one room project studio. It started by moving one four-foot section of wall to make a more convenient jam space, and moving one outlet on that wall. This led to moving the outlet on the wall. The scope of the project quickly escalated into replacing most of the electrical in the house and adding additional circuits (a lot of the house was still on knob and tube wiring), building walls, reading a dozen books, significant acoustic treatment, and a lot of time and money spent. Let's just say I've caught the bug in the last year. I can post details if anyone is interested.

    Anyway, I ran an analysis of the room using RoomEQ Wizard today, performed at the mix position about a third of the way into the room. I'm generally pleased with the results, the low end of which is posted below. They're a significant improvement over the bare concrete walls they once were. The room is now flat enough for my needs, except for a null of about 17 dB at 59 Hz that I can't explain. This corresponds with what I hear in mix translation. Things tend to translate well, except the kick, which tends to explode when listening to the mix elsewhere. The room is 15' x 25' x 7'3" to the unfinished floor joists stuffed with fiberglass (7'10" to the subfloor above). From my limited understanding, I would think that a null that deep would correspond to a prominent axial mode. Dimensionally, though it's nowhere close to an axial mode. Only a tangential mode gets close to 59 Hz.

    Is it possible that a tangential mode could create a null 17 dB deep?

    Does anyone have any ideas on what might be causing this?

    I'm grasping at straws. As far as treatment, I have triangular bass traps 2' thick straddling the front corners from floor to ceiling, and a dozen 2" 703 panels spaced 2" from the walls. The third corner is also straddled with 703 backed with fiberglass batts. Which leads to the last corner... The biggest irregularity in the room is the HVAC. The furnace with all its sends/returns are in one corner of the room, so I'm imagining that somehow acting as a Helmholtzish resonator and sucking out the 59 Hz. A stretch?

    I can post post pictures/make a drawing of the room, or anything else that helps.
    Thanks for reading my ramblings, and I appreciate any advice.

    REW Measuerements 10152014.jpg
     
  2. Space

    Space Well-Known Member

    "Is it possible that a tangential mode could create a null 17 dB deep?"

    Yes it is not only possible it seems to be happening to you. I wouldn't lose my mind over the 17 dB but I would start thinking about the placement of my monitoring desk. You could be sitting in the null, most likely are simply by where your desk is in relation to where your head is in relation to where that mic was.

    You say you are (your head is) at 8 feet 3 inches approximately is that correct? How did you define that position? Also this could be a direct interaction with the height of the room.
     
  3. Reverend Lucas

    Reverend Lucas Active Member

    Thanks for the input, Space.

    The
    The measurement was taken from the mix position, which is not quite 8' off the wall. It was basically defined by a compromise between getting distance between the walls and monitors with the acoustically ideal 1/3 into the room listening position(not remembering where I read that at the moment), and the practicality of getting the full band into the room for practice. There is a support in the middle of the room that breaks it up sort of awkwardly, and limits the options of the mix position.

    I think my next step is to take some more measurements, to figure out how spatially localized the null is. I don't know enough/lack the intuition to know this offhand. I don't hear massive changes in that frequency in moving my head. Ideally I'll be able to move the desk enough to get out of the null. Otherwise I guess I'll have to choose either learning to mix with it, or using a compensatory EQ bump while mixing. I'm not thrilled about either option. I suppose I shouldn't rule out more bass trapping. My other half loves it when I bring home fiberglass;) Gear in general, really.
     
  4. Space

    Space Well-Known Member

    "with the acoustically ideal 1/3 into the room listening position(not remembering where I read that at the moment)"

    I have never read it before in the past ten years I have been involved with this aspect of construction. There is, defined as a starting position, a rule of thumb developed by Wes Lachot that recommends 38% of the distance from front wall to back wall as the place your EARS should be.

    The following link is a discussion Eric Desart started, it is better information than this 1/3 assumption:
    http://forum.studiotips.com/viewtopic.php?f=1&t=3378
     
  5. Reverend Lucas

    Reverend Lucas Active Member

    I appreciate your expertise, Space. It's helping me fumble my way through waters that are probably over my head. Nonetheless, I'm having a good time attempting to not drown.

    Apparently I had since forgotten the percentage, too. In any case, I took measurements at 1' increments from the front wall and was surprised again. I wasn't in the worst of the null. The null was the worst in the middle of the room (28 dB!). The good news is I got lucky, and moving only a foot closer to the wall flattens the low end to within ±3 dB, at least until my monitors roll off. I'm comfortable with practicing my mixing skills in that environment, and now plan to take measurements of the rest of the space to locate any zones that would be dangerous while tracking.

    My new mystery is the frequency of the null shifts higher as it approaches the middle of the room. It moves steadily from 58 Hz at 5' from the front wall to 69 Hz in the middle of the room. Any idea why this might happen?
     
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