Hi Tony! Yup, 70 and 140 show up as room modes according to the calculators. Below 100 are tough to deal with. I thought my specialized ASC traps would make a bigger dent. Turns out subwoofer phase has a much greater impact! I learned from this Room Modes 101 article that room modes can be identified through peaks in the frequency response and slow decay in the time domain. It suggests that modal problems can be solved by:
Positional equalization (changing the location of the listening positions and speakers/subs)
But first.... I should be working on controlling early reflections. As an FYI to myself, I included a few related articles below. Please feel free to suggest more reading, and as always, your questions/comments are welcome!
The idea on reflections is to determine the kind you want and the kind you don't want, carefully work with the timing of the wanted ones, and absorb/diffuse the ones not wanted in a way that does not disrupt imaging. First reflections only, after that, the delay is generally enough that imaging is not affected. The first part involves careful measurements, first physically with a tape or laser distance measurer, then refining with REW and a mic. The second kind are eyeballed, because that is good enough. See where they appear most likely and use panels on those surfaces - ensuring that ALL diffused reflections are aimed AWAY from the LP.
Do you think this is something best learned through formal training and/or experience? The literature I've come across so far seems to lean in that direction. Care to comment on where I should head after getting good ETC measurements? I realize my questions are probably like a passenger expecting a pilot to teach them to fly by asking how to get on the plane!
I think I found the answer in your recommendations on early reflections in the thread HTS Two-Channel Speaker Setup Guide for a Deep Soundstage. It seems that I do need to generate an ETC from the IR graph. According to the REW101 Guide, these should be "... measurements that isolate one speaker, e.g. left front, right front, center, etc. Do not use measurements that combine multiple speakers, or measurements that include the subwoofer(s). .... Any peaks above -20dB represent reflections that should be researched."
Perfect room symmetry is not absolutely necessary. General symmetry plus a few treatments probably do the trick. A room with longer RT60 times might have "unbalanced spaciousness" effects where one side sounds bigger than the other. My own listening area is far from symmetrical, but RT60 is quite low and I do not notice anything like this. With longer RT60, absorptive treatment can help balance this effect out.
Early reflections can really throw off the imaging and soundstage. Treatments here are very beneficial. My own rules for early reflections:
Less than 5 mS (of additional delay relative to the delay of the direct wavefront from speaker to LP; on an impulse response plot, it will be easy to read that relative delay directly): disruptive to image clarity, must be absorbed.
Between 5 mS and 15 mS: must be VERY summetrical (direction and delay time) and must be perceived as coming from the vertical line of sight from speaker to LP; if the reflections are NOT coming along the vertical line of sight from speaker to LP, or if not symmetrical, then either absorb or scatter them. Scatter them with a reflective surface pattern that does not reflect ANY of the sound directly at the LP (I have gotten into long arguments about this last point, but experiments have proven to me that it is very important for the best imaging, and I do not care what expert says otherwise, they are WRONG).
Example 1: ceiling reflections (flat ceiling) would be on that vertical line of sight, and might be only delayed by 8 mS or so, and would probably be highly symmetrical. They are OK, though, will not smear the image L or R, and will add a small amount of spaciousness to the sound, as well as some height info to the soundstage.
Example 2: in a symmetrical room, reflections off the side walls, if only a few feet from the speakers, would be symmetrical, and would fall within the 5 to 15 mS window, but would NOT be on the vertical speaker to LP line of sight, so imaging would be smeared and softened, so they are not OK. Absorb or scatter (see above).
Example 3: Reflective panels on the front wall on the vertical line of sight from LP to speaker could be angled so the reflections are on or very close to that line of sight and are very summetrical and fall in the specified delay range. These are OK, can enhance the soundstage and imaging. However, the natural reflection points on the front wall just inside those panels would not fall on that line of sight and would mess up the imaging. Absorb or scatter (see above) at those natural reflection points.
More than 15 mS: less critical. In general, better scattered, and the direction of reflection is not important, although randomized is better.
In the name of all that is sane, what on earth is the meaning of this? Well, I'd like to think it's helping tame a floor-to-ceiling axial mode but truth be told, it's primarily acting as a diffuser for the back wall. It also doesn't hurt to hide the mess left on the back wall from dismounting the surrounds.
My attempts to advance into the realm of speaker timing and ETC-driven reflection remedies have dissolved. Well actually, they've morphed into iterative tuning cycles to find the best balance between theory and practice. I at first reached an acceptable configuration with all speakers crossed at 80Hz and sub phase adjusted for good SPL support through the crossover region. Measurements said that FR was not particularly good, but remembering that time domain behavior plays an important role told me that listening was important as well. I tried a 60Hz crossover configuration - which looked better on paper - but didn't sound nearly as good. Granted, the Dirac calibrations could have skewed results, but I strived for identical protocols. Moving back to the 80Hz setup with a modified house curve and slightly repositioned center channel went a long way toward imparting punch, clarity, and immersion. I'm very satisfied with the results and couldn't have done it without your help! Now it's time to kick back and become acclimated to the new sound. Do you think it'll take long to develop a re-tuning itch?
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