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Post by ferrofluid on Dec 12, 2023 4:50:51 GMT
I plan to remove a rear wall in my room as recently advised in this forum. With it gone the room dimensions will be 122.5 W, 106.5H and 287 L.
The left and right walls (the longs walls) cannot be moved. Before describing the current construction of the front wall and ceiling and the space beyond each, I’d like to draw attention the to ratio between my W and H. 122.5/106.5 = 1 : 1.15. Although I cannot recall the name of the acoustician who the ratio tables are attributed to, I have repeatedly seen a table that indicates a ratio of 1.14 width to 1 height (14% wider that tall) is desirable. So….I’ve got that going for me, to paraphrase Bill Murray in Caddyshack. My length to height (287/106.5) = 2.69 or 1 : 2.69. Not sure what to make of that. But I love not having a wall close behind my seat.
Beyond the front wall and ceiling is attic space and neither wall has sheetrock on the attic-side (rafters and studs respectively). There is pink insulation between studs and rafters.
I think is is widely advised to put broadband absorption on the font wall and a cloud on the ceiling, so that is what I plan to do.
Question:
Instead of putting panels ON the wall and ceiling, would it be beneficial to create an aperture in the ceiling and front wall covered with acoustically transparent cloth? Beyond the front wall (cloth) there is 28” of space to layer rockwool or fluffy pink stuff. Or, if someone makes the case for such, I could move the entire wall about 24” which would make the room 311” or 25.9’ long. If I create the cloth pass-thru, how much roxul should I put (depth)? If I don’t make a cloth pass-thru, would it be beneficial to make the wall more rigid by adding thin plywood to the backside of the stud? Or sheetrock if that is somehow acoustically superior. Sheetrock is just heavier to haul into my upstairs attic. I imagine enclosing the attic side of the wall would lower the room ambient noise floor irrespective of stiffening considerations.
In the ceiling (attic space), there is room to make the rockwool as deep as needed. Alternatively, I can raise the ceiling by as much as 11” but doing so is by far the most laborious modification on the menu of options. Read: Costly. If I add the cloth pass-thru there will be rows of rockwool separated by 2 x 10 rafters running across the short dimension of the ceiling. My speakers fire down the length of the room, 90 degrees to the rafters. I mention this IN CASE it matters.
Thank you,
Rick
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Post by rock on Dec 12, 2023 14:27:25 GMT
Hi ferro,
You have a lot of questions here. I know I'm not addressing all of them. Maybe a diagram or drawing would help (I'm not sure what your plans are for the "pass thru"?). And just personally, feet/inches or decimal feet is a little easier for me to digest... but maybe that's just me.
#1. Your room modes are the distance between solid (massive) walls not the porous surface. So in cases where you can open up a ceiling or wall for insulation, you need to measure your "new" room size from the boundary behind the insulation. But yes, it's OK to do that (bigger rooms have lower modes which are generally less of a problem if they are low enough) but you need to recalculate your room for volume and modes etc. You should strongly consider fire retardant material especially for large areas.
#2. The "Magic" room ratios are nice. They are chosen for having room modes spaced apart and not piling up, but with tons of absorber thickness and area coverage, it reduces the reliance on room ratios alone because absorption will minimize room modes.
#3. If you do go with tons of built in absorbers, you may have a rather dead room. I think that can be good for a band room and recording but some people prefer a more "natural" room sound. After and in front all that insulation, you can add a little "life" with reflective materials like wood slats installed spaced along the walls or even diffusors ($$$).
I think you've got questions I didn't address so please feel free ask.
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Post by ferrofluid on Dec 12, 2023 19:42:07 GMT
Rock, I just happened to stumble upon this thread from earlier this year that asks what I am asking with respect to opening/removing the sheet rock to let the rooms insulation do the absorbing. the-audio-expert.freeforums.net/thread/858/pitched In case the link doesn’t work, the thread is called pitched. I will try to get around to posting a sketch if I can, but the open attic spaces beyond the front wall and the ceiling are huge. They are actually the same space. The attic that surrounds this room is large and open. I used the term cloth pass-thru but actually the execution of it would have to be sorted. I just know that if the drywall was removed (like is said in the linked thread) you must have something that supports the insulation but also is acoustically transparent. Although, I assume this has been done before and I’m loathe to reinvent the wheel, if left to my own devices I envision something like chicken wire on the studs and rafters to support the insulation and cloth stretched tightly across a rather think wooden frame to be attached to the drywall and covering the unsightly chicken-wired opening in the wall or ceiling. Think speaker grill. Cloth stretched across a frame attached to a baffle. Totally agree that I don’t wish to over damp the room. If I open the walls as described, I am challenged to know how large the area should be and, as I said before—-how deep the insulation should be. There are many moving parts here, for sure. |
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Post by rock on Dec 14, 2023 14:18:46 GMT
"but the open attic spaces beyond the front wall and the ceiling are huge. They are actually the same space. The attic that surrounds this room is large and open."
The modal response is determined by the solid, reflective walls so removing the inner walls will give you a different, larger room with lower frequency fundamental room modes (that's good).
"I just know that if the drywall was removed (like is said in the linked thread) you must have something that supports the insulation but also is acoustically transparent."
You don't need to make the entire thing transparent. You can use a 2x4 framework (on 24" centers is probably close enough) to support your insulation. It's true that the 2x4s will reflect some very high freqs, but not much. Use rigid insulation if possible but chicken wire or hardware cloth is ok too if you need it in between.
"Totally agree that I don’t wish to over damp the room."
I don't think I said you shouldn't completely damp reflections. I said if you do, you can add more life with reflective materials. The advantage of this is that the complete damping gives you a "blank canvas" to work from with the idea that wood slats or diffusors will not create low freq room modes.
"how deep the insulation should be."
As deep as you can afford. 8" min but up to 2 feet (or more?) if you have the space and deep pockets:)
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Post by ferrofluid on Dec 14, 2023 18:37:14 GMT
Great info, Rock.
“The modal response is determined by the solid, reflective walls so removing the inner walls will give you a different, larger room with lower frequency fundamental room modes (that's good).”
Q: In order to achieve what you describe, is the surface area of the opening critical beyond a certain minimum size? For instance, would removing the Sheetrock where a cloud would otherwise be hung, give a great result? I’ve not determined the size of a cloud for my room yet, but I can do whatever is needed. Shooting from the hip, I imagine the cloud would be around 24 sqft (6 by 4) and making it larger is fine, if beneficial. For perspective, a typical bedroom door is 80 by 32 inches or 17.8sqft. Of course, another question begs to be asked. If there is a minimum prudent opening size to target, wouldn’t the size of the openings in the front wall and ceiling combine? Reminder, both are open to the same cavernous attic because there is nothing separating my ceiling and front wall.
BTW, I would heavily augment or completely remove the old janky, poorly-positioned insulation above the room to improve thermal regulation even if I had no plans to open the space (remove sheetrock from my ceiling). Thus, my starting point will be 9.25” (the nominal width of a 2” by 10” rafter) of fluffy stuff—whether pink or rockwool. Adding more will improve acoustics and also temperature regulation.
I’ll keep rigid panels in mind.
Thank you,
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Post by rock on Dec 15, 2023 23:33:59 GMT
If I understand you correctly, I don't think it works that way.
Here's the way I think it works: For example, If you take down half the wall (to expose a deeper room behind), you'll still have a shorter length on one side and a longer length on the other side, each with it's own room mode frequency. So you still have some sound bouncing back and forth between the short wall space too. If you want a lower room mode in a given axis, you need to remove the entire wall. If you can't remove it all, you'll get some benefit as you'll lessen the total strength of the shorter mode (depending on where you stand or measure). This is one problem with complex rooms with more than 3 axes; that is calculating room modes.
It's not like water (in the room) that flows through a pipe (your opening in the wall) to fill a larger tank on the other side of the wall.
Regarding insulation, try searching threads here about it, there are differences but with thickness over 8", it seems that cheaper lower density "fluffy" will work as good as the higher density rigid. The physical problem with large volumes (vertical stacks) of fluffy is that it needs support to keep it from compressing itself under it's own weight. You can use wires or screen/chickenwire/hardware cloth etc.
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Post by rock on Dec 17, 2023 17:30:06 GMT
I re-read your last post and I didn't really answer your question about the cloud. (to be placed centered on the reflective paths between the speakers and the listening position). The size depends on the distance between the speakers and the LP. You really have two paths, one for each speaker so if you use one big cloud, it will cover both so that's how you determine the size (bigger is better).
Removing the DW to place the cloud flush with the existing ceiling is good. Again, the distance between the speakers and paths will help to determine the size. The main purpose for the cloud is to absorb the first reflection but it will also do double duty to null the room mode in the vertical axis as long as the thickness is enough (over 8", but thicker is better).
I don't think of openings in the front wall and the ceiling combining as they will each work for modal frequencies in their respective axes. (Technically they will combine when we consider the tangential and oblique modes but they are less dominant so I mostly ignore them and when you think about it, when you reduce the axials with bass traps, you're also reducing the other modes so yeah, I guess they do combine in a way. But then doesn't everything you do to the room combine to affect the whole?)
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Post by ferrofluid on Dec 17, 2023 21:33:13 GMT
Rock,
Thank you. I’m reading your posts, but have been and will be pressed for time to respond properly.
Beyond saying that I appreciate your taking the time..
absolutely agree that everything in acoustics combines. This morning while discussing how each move in acoustics affects more than one thing (speaker toe,-in was the topic) I said to a friend “if you look up multi-faceted in the dictionary, it says: Acoustics.
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