External Isolation vs. Internal Absorbtion, Same wall

[thecr4ne]

Digging into diaphragmatic absorbtion sections in the Master Handbook of Acoustics, there's specific mention of drywall's functional diaphragmatic low freq absorption. This got me wondering about using different depths of insulation and different thicknesses of drywall leaves to address room modes (from amcoustics calculator) rather than addressing them with additional apparatuses down the road. 2 walls of my room are exterior walls. If possible I'd love to better understand the interplay of external isolation and internal absorption by the same wall, or whether the two can be considered independently.

Can anyone offer clarification or point me in the right direction here? I'd like to maximize LF isolation while taking advantage of drywall as a diaphragm to plan ahead for internal treatment, so to speak.

[DanDan]

A friend Acoustician calls the absorption of studded partitions, Invisible Alpha Tm. For isolation, the void between Mass layers is best fully filled with light fibre.
For sure any floppiness will weaken modes. Newell calls this scenario A Limp Bag.

[thecr4ne]

Thanks DanDan,
I read through this post regarding modifications to the limp membrane idea for the "thin panel" version of a diaphragmatic trap.
https://www.acousticsciences.com/artblo ... ass-traps/

According to amcoustics room mode tool, the lowest axial modes in my 12ftx8ftx8ft room are about 46Hz and about 70.5Hz. Using the thin panel + insulation derived f=192/sqrt(mxd) for my walls I came to an ideal mass of about 3.2lbs/sqft for the front and rear walls (5.5in insulation) and 2.1lbs/sqft for the side walls and ceiling. The plan is to do 5/8 drywall all around (2.2lbs/sqft) and a layer of 3/16" hardboard (1.08 lb/sqft) added to the front and rear walls to achieve the goal mass.

I'm not going to get my hopes up that the ideal "floppiness" will be there, and also acknowledge that the "sealed box" definition of the diaphragmatic trap isn't necessarily true here (2 walls and ceiling decoupled with hat track/clips) I can at least aim for an ideal and hope for a positive outcome, and hope I don't accidentally excite something else undesirable.

Picking up drywall today and hope to have it hung this weekend once insulation is installed in the exterior walls. Ceiling and interior wall insulation will be added from the outside after the fact as those walls are open and accessible.

[DanDan]

Best of luck with that. Hardboard is often forgotten, but been in Studios since before there were Studios.....
This is Pegboard facing SoffiTraps.

Kiwi Bigger.png

[thecr4ne]

Just now wondering if the hardboard+drywall diaphragm will function properly as 2 leaves or if they would need to be glued together, and of course how that would change it's effectiveness as part of the MAM system isolating outside noise.

[DanDan]

I'm afraid I don't know. Is there a more certain way of achieving the Mass you want? e.g. Most manufacturers have an enhanced Plasterboard, such as Gyproc Soundbloc....

[thecr4ne]

Not without an exponential increase in cost. quietrock is available, but $65/sheet compared to the $10 a sheet of standard 5/8" I paid, other types like "High density" drywall aren't stocked by any of my local distributors and would likely be too costly for me anyway.

I've already purchased and picked up all the sheets, so I'll stick with this approach, 3/16" hardboard behind the drywall, unglued. gets me to about ~3.22 lbs/sqft, in 13/16" and if it ends up damping the 46Hz mode, I'll call that a bonus. The room will undoubtedly need other acoustic treatment anyway.

Maybe I'll get lucky and my logic here won't be totally flawed. The math, as derived from the modified limp membrane equation is 192/sqrt(3.22x5.5)=45.65Hz which is about as close as I can get to the actual 46Hz mode, based on listed weights for the drywall and hardboard.

May start hanging drywall today. Next thread will be about the 42" window on the ext wall.