Insulation selection for inside out room

[norg]

The short version with more detail below: for an inside out mixing room, for the purpose of getting a room dead before tuning, is there any benefit to consider between 4" roxul safe at 4.5pcf vs 4" roxul afb at 2.5pcf? And is there another product I should consider for stuffing the cavities? I'm still referencing Bob Gold's coefficient page and know there's a likelihood I'm behind the times on products. But based on the information there, the coefficient ratings are pretty close with a bit below 1.0 in the 125hz range for AFB.

Longer version: for an inside out style mixing/mastering room, my plan is to go full dead then tune after. The walls are all 2x6 studs or girts so 4" thick stuffing is an opportunity and the ceiling joists are 2x12 which opens up a whole new world of deadification.

Apologies for being a bit light on details for the moment. The stage that I am currently at: I'm almost finished doing all of the removal of drywall from the sides and ceiling of an oddly shaped room.

As I've gone through each wall and exposed the innards, I've taken very careful measurements of each component and I'm building a complete model in SketchUp. And it's bugging me because I measured the room with drywalling still intact and made a thickness assumption based on the first wall I ripped apart, come to find later that different thicknesses were used in different places. Why even. So I'll tweak the layout after demo is done to correct the 1/8"-1/4" that I'm off in a few places.

Once ALL the demo is finished and the room is bare, which should be by the the of next week, I'll do empty room REW measurements for funsies, baseline reference, and later comparison. Then stuff the daylights out of the space until it's dead as a can of Spam and then run the REW tests again. All that before figuring out where I'll need hangers, super chunks, or both.

Which is why I'm here now asking ridiculous questions about insulation. The context is the room had to have all the drywall removed anyway in the aftermath of a natural disaster so I'm taking advantage of that need to make a better mixing space. And because there's an awesome contractor involved for the more destroyed parts of the building, he's very happy to order and have delivered any insulation products I need as part of the overall repair. So if I provide information to him now when I'm close to finished with demo prep, I'll be ready to stuff the room to get the new baseline in short order.

I could just pick one and get it ordered but one of many things I've learned over years of lurking here and I'm John Sayers forum is how many times someone did a thing only to read "if you had posted a question here first, you could have avoided <insert expensive regret here>."

End wall of text.

[Soundman2020]

I'm assuming this room is already fully isolated? In other words, you'll be adding insulation to the final, inner leaf of the room? No more isolation needed? So your intention is only to treat the completed room now, not isolate?

Just checking, because often there's confusion here.

OK, assuming this is only treatment: 4" of pure absorption isn't going to do a lot to deaden the room completely. Bass traps are often 24" deep, or at least 16", to really get down into the low end. If you do the entire room in 4" 'fluffy', you'll end up with a very dull but still "boomy" room. The mids and highs will be dead, but the low modes will still be present, to a greater or lesser extent. In fact, that would probably make it really hard to get the low mids back again at a decent level. My guess is that the room response would have a deep dip somewhere in the region 200 - 400 Hz. which isn't what you want..

My suggestion would be to empty out and strip down the room, as you say, so that you are left with just the final inner-leaf walls, floor, and ceiling, totally bare. Do a few REW tests like that, at key locations (mix position, client listening position, likely instrument locations, etc.), to see where your major bass issues are (room modes being the number one issue), then treat the room to deal with those issues: Probably rear wall (deep bass traps in the corners), ceiling (overall deep but varying thicknesses, as needed) and first reflection points on side walls. Your initial REW tests can also help you figure out the best location for the speakers and mix position. Once you have that figured out, mark them very carefully so you can always come back to those identical spots for future measurements. Very important! If your mic or speakers are off by just an inch or two for subsequent tests, then you can't really compare results validly. It's amazing how different a REW test looks from just moving the test mic a couple of inches up, down, left, right, forwards backwards. All your tests should be done with the mic in the exact same spot.

Then do more REW tests, with the mic and speakers in the exact same, identical locations, to see what worked and what didn't, then figure out your next steps from there.

You CAN go with the "very dead the liven it up" approach if you want, but do keep it balanced, and leave yourself with enough minimum life in the room so that it isn't to hard to get back to your goal. By "balanced", I mean try to get the decay times more or less the same across the entire spectrum, in accordance with the room size. Once you have them all in the same ball-park, you can tweak that to bring it all up, or all down, as needed. But getting to that "flat time domain response" isn't so easy...

- Stuart -

[norg]

I'm assuming this room is already fully isolated? In other words, you'll be adding insulation to the final, inner leaf of the room? No more isolation needed? So your intention is only to treat the completed room now, not isolate?

Just checking, because often there's confusion here.

Thanks so much for a very detailed response! Great question and important to clarify. This can be considered the inner leaf of a room. In the context of this space, no further isolation is necessary due to location, low volumes, and proximity to nearby spaces.

OK, assuming this is only treatment: 4" of pure absorption isn't going to do a lot to deaden the room completely. Bass traps are often 24" deep, or at least 16", to really get down into the low end. If you do the entire room in 4" 'fluffy', you'll end up with a very dull but still "boomy" room. The mids and highs will be dead, but the low modes will still be present, to a greater or lesser extent. In fact, that would probably make it really hard to get the low mids back again at a decent level. My guess is that the room response would have a deep dip somewhere in the region 200 - 400 Hz. which isn't what you want..

That's a fantastic point and I was wondering about how to proceed with that. I know that thick traps and superchunks and other corner trapping is going to be necessary but I wasn't sure exactly -what- would be necessary in terms of that level of tuning. My thought process was to first resolve the abundance of reflective surfaces and then figure out where the unique properties of the room show up and adjusting accordingly. I'm actually really curious about what the dips and peaks are going to be like in a room shaped this way. Which is something that will be answered soon and further in response to the next paragraph.

My suggestion would be to empty out and strip down the room, as you say, so that you are left with just the final inner-leaf walls, floor, and ceiling, totally bare. Do a few REW tests like that, at key locations (mix position, client listening position, likely instrument locations, etc.), to see where your major bass issues are (room modes being the number one issue), then treat the room to deal with those issues: Probably rear wall (deep bass traps in the corners), ceiling (overall deep but varying thicknesses, as needed) and first reflection points on side walls. Your initial REW tests can also help you figure out the best location for the speakers and mix position. Once you have that figured out, mark them very carefully so you can always come back to those identical spots for future measurements. Very important! If your mic or speakers are off by just an inch or two for subsequent tests, then you can't really compare results validly. It's amazing how different a REW test looks from just moving the test mic a couple of inches up, down, left, right, forwards backwards. All your tests should be done with the mic in the exact same spot.

Thank you for filling in a few gaps that I hadn't accounted for in my notes. Measuring the room with REW tests was and still is the game plan and your note includes reminders about making sure to mark in detail speaker locations, looking into some of the ways used before to make sure that the mic is in the exact same position and aimed the same for test locations. I have a mic stand that will be devoted to and locked in position for height to avoid altitude variance. I've read in here some folks who have had success with a string and a weight that marks the exact spot. Masking tape with numbers for different positions of all the things to keep track of what was in the back of my brain for that purpose. And re-reading your guide on REW measurements for the 17th time to be sure.

I already know that a sliding glass door to the right of one of the prospective listening positions is going to be a colorful reflection point. Even moreso given that I'm pretty sure it needs to move either further down the wall or to a different wall completely because its location makes what I think will end up being the better mixing position impossible by interfering with a soffit build for monitors. Or removed completely and replaced with a more appropriate door in a different spot. It was built really weird and some things just don't make sense but I'll make sure to include pictures of the layout I'm building in SketchUp when this all gets going. I've verified with the contractor that it can be moved if necessary without any structural concerns.

Then do more REW tests, with the mic and speakers in the exact same, identical locations, to see what worked and what didn't, then figure out your next steps from there.

You CAN go with the "very dead the liven it up" approach if you want, but do keep it balanced, and leave yourself with enough minimum life in the room so that it isn't to hard to get back to your goal. By "balanced", I mean try to get the decay times more or less the same across the entire spectrum, in accordance with the room size. Once you have them all in the same ball-park, you can tweak that to bring it all up, or all down, as needed. But getting to that "flat time domain response" isn't so easy...

So just to make sure I'm fully understanding: you're recommending hold off on installation of insulation to address reflections until after a complete battery of initial REW tests, evaluate the results, and then make a more educated guess at what and how much to install where. Which makes good sense.

Knowing that insulation is going to be a huge component of whatever plan is made for walls, monitor soffit, traps, and etc and having access to contractor discount pricing while he's still working on the other parts of the building, I'm trying to decide what to order initially to get that process going. I could also be jumping the gun by thinking about it prior to measuring anything.

I'll start a more proper topic when I am able to finish off the last few bits of the SketchUp model and have REW results. I'm just grateful that there are folks out there who understand and feel this same obsession.