Sound absorption: Damned if you do, Damned if you don't???

[amirm]

So when I was building my theater, I was getting conflicting advice. On one hand, the experts advising regarding sound proofing recommend the usual double drywall with viscoelastic (green) glue between the sheets. But the acoustics people worried that stiffening the walls that way, exacerbates the bass anomalies of the room. I take that as the more solid wall reflects more of the bass rather than absorbing it.

So what is the view on this? In my case, making a quiet room was extremely important so I did the double drywall anyway (we have bedrooms above the theater). But like to know where the right trade off is.

 

[DonH50]

I think those are two different things. I isolated my new media room using floating (Kinetics IsoClips) walls and ceilings (basement, so concrete floor), double drywall, 6" of mineral wool insulation, and sealed (acoustic-grade caulk) all joinings, outlets, etc. Isolation is not absorption, nor really has anything to do with treatment of the room. Nor can room treatment provide isolation, or not much, and virtually none at bass frequencies. Stiffening the walls may indeed increase the impact (amplitude and Q, bandwidth) of room modes, but you do that for isolation, the desired result, and I suspect (but have not looked it up) that any change in modal response in the room due to wall stiffening is minimal, particularly relative to everything else going on.

So, in my opinion, you did what you wanted to do in isolating the room from external noise while containing the sound you want within the room. The next step is treatment (usually absorption and diffusion) to do what you can to fix (perhaps "optimize" would be a better word) the sound within the room.

Hopefully that makes sense -- gotta' run. I expect Ethan will be along and clear up my mess. - Don

 

[Ethan Winer]

I expect Ethan will be along and clear up my mess. - Don

LOL, no real mess, but I can add a little anyway.

Stiff massive walls that isolate well also reflect bass more, so that type of construction helps isolation but hurts the bass response and ringing inside the room. However, I believe that isolation done using Green Glue or resilient channel etc can improve both. If a wall is free to flex, and the flexing is absorbed, you get some (frequency-selective) bass trapping due to diaphragmatic absorption.

--Ethan

 

[amirm]

OK, let me expand . The sound is absorbed by the viscoelastic glue between the two layers of drywall which converts acoustic energy into heat. The process of building that structure though, creates a higher mass, and more rigid wall structure. That means bass frequencies get reflected more than if the walls were more flexible. Result is that you then need to work even harder with acoustic treatment to tame the the low frequencies.

So yes, you are right that the two subjects normally are different and separate. But as I have explained, they are not as separate as it normally is talked about if the data I have gotten is correct. In other words, we may be creating more of a problem to solve in soundproofing the room.

And yes, other methods such as isolation doesn't have this side-effect so they can be deployed without concern. My question is related specifically to creating the wall structure from 2-3 layers of drywall.

Hope this is more clear .

 

[amirm]

Ah, our posts crossed . Ethan, the beneficial effect been characterized with Green glue?

 

[steve williams]

could someone explain how it dissipates sounfd energy into heat?

 

[Nyal Mellor]

The green glue acts as a damping medium; sound wave hits sheetrock, sheetrock flexes, green glue damps flexion by converting sound to heat.

BTW the green glue website is an awesome resource www.greengluecompany.com

 

[steve williams]

Thanks for the link Nyal

 

[amirm]

But surely a double drywall with glue in between is still a lot more rigid than a single sheet. In casual testing, exerting force on the structure, it seems to flex less than normal drywall but maybe that is my imagination . Hence the reason I wanted to read about acoustic properties (not sound isolation which they talk about).

 

[DonH50]

Thanks Ethan, I agree with your thoughts/facts (as usual).

Amir, shoot, I can't find the link, but a year or two ago when I was researching stuff for my new media room build I reviewed the properties and pros/cons of putting a viscous'ish layer between the drywall layers. I think it was on one of the pro sites -- Mason, Kinetics, something like that. There were measurements and stuff engineers could love... However, it was probably isolation data, which are acoustic properties after all (you know that, but ah! to catch Master in a slip! ) I don't recall any measurements such as you seek.

I was not looking specifically at Green Glue at the time (the stuff I was looking at was blue or grey, IIRC, and I had not heard of green Glue until now, unless I forgot -- probable). My conclusions regarding its properties and efficacy match what I tried to say and Ethan did, best I can recall (my notes are buried in the black hole I use for filing things of supreme importance so they are never lost). That is, a little better isolation, slightly better (lower) STC, perhaps a little more energy in the room. I did not care about the latter as it was minor and I planned extensive room treatment anyway, and when I reviewed all I was already doing adding the gummy isolation layer would not significantly change the STC (reduced isolation a little, but I was already doing about as much as practical so it would not have hurt much) so I decided it was not worth the cost (of buying and installing the material). It was a very minor addition to what I was already doing, probably deep in the mud in my case.

FWIWFM - Don