Drywall damping for room acoustics?

[the dreamer]

Hi all,
as Brian R recently posted a lot about damping and I just read a book from Philip Newel who uses a lot of constrained layer damping for soundproofing but also for in-room acoustics some things make me wonder.

Newell uses deadsheet in all of his designs. What exactly is deadsheet...MLV, bituminous stuff, massloaded felt??????

I define a case here:
I thought about us small room guys. Let's assume that costs are not the main factor for choosing material.
It is given, that one must highly soundproof the room but the space is limited. So big airgaps or 1m thick multilayer walls for both absorbtion and isolation are no option. So we end up with several layers of drywall for the inner shell. Now there's the problem that we have a really good isolation, but as no lower frequencies can leave the room and are reflected by the massive walls we have them IN the room.

Besides all the usual treatment is there a gain in in-room acoustics putting a damping layer between the 2 innermost drywall layers?
I mean will the inner layer of drywall absorb to a degree that it is worth to even think about it?
Treating every surface in this way (maybe with different drywall thicknesses), would it help to get better acoustics in a small room?

Thanks!
Florian

[Brian R]

Hi Florian,

at studiotips in a thread about panel traps i entered some relative absorption data for damped -vs- undamped walls. The absorption for two layers of drywall with GG in between appeared to be similar at low frequencies to a single layer of drywall with no damping.

i do not feel that it's reasonable to conclude based on this that Green Glue raises the absorption of the wall due to the fact that a twice-as-heavy wall had similar absorption to the lighter wall.

the reason i don't feel it's reasonble to conclude that Green Glue raised damping in that structure is that i am not at all convinced that adding a second layer of drywall without damping would lower absorption at low frequencies.

i have about 10 days of time coming up in a facility that can measure TL and absorption, and i hope to collect much more data for the learning.

Brian

[the dreamer]

Hi Brian, thanks a lot for responding! Very much appreciated:D

at studiotips in a thread about panel traps i entered some relative absorption data for damped -vs- undamped walls. The absorption for two layers of drywall with GG in between appeared to be similar at low frequencies to a single layer of drywall with no damping.

I will go for a search...

i have about 10 days of time coming up in a facility that can measure TL and absorption, and i hope to collect much more data for the learning.

I'm looking forward to that. (if I understand what you come up with :shock: :) )

Flaviu somewhere posted a link to a site today where I found this:
http://www.asc-soundproof.com/index-isowall.htm
What do you think about this!
Basically the same principle as GG?

They claim their visco-elastic stuff works exactly like the idea I had in mind...improve soundproofing and absorbing lower frequencies to inhance acoustics. Marketing hype???

Bituminous layers should have similar effects, right?

Thanks.
Florian

[Brian R]

Florian,

i just realized that most of the pictures i've ever posted that were hosted on www.audioalloy.com are gone. they changed the site and somehow they gost lost in the shuffle. i am hoping someone can figure out how to get them back up so people can see them.

i have seen the ASC commentaries. I offer everything i know about damping and low-frequency absorption here, as i won't have time to follow up for probably weeks


-Noral Stewart, at the google acoustics group, once entered in a thread discussing ASC Walldamp that the concept had merit. That damping could improve absorption


-it SEEMS to me that if you massively damped a panel trap it would cease to function, as the nature of the absorption of a panel trap is resonance. imagine you completely eliminated the resonance, it seems to me that you would also completely eliminate the absorption, but:


-in 3rd party tests for TL we took absorption in a previously-certified reverb room (the lab removed their own certification as they didn't want to work with the new process), the absorption of the wall at low frequencies was NOT reduced with the use of Green Glue. However, the TL dip at resonance was reduced from about 11dB to about 2dB. so i guess that's having your cake and eating it too.


-i STRONGLY believe that the absorption of walls is related to resonance and NOT related to this "increasing absorption with falling frequency" theory. The data we took - FOR EVERY SINGLE WALL THIS HELD WITHOUT EXCEPTION - in our last round of TL/absorption tests peaked at the resonance of the wall, and FELL with lowering frequency below that


-based upon the aformentioned belief, i feel that the best wall assembly for in-room acoustics - for low frequency absorption - will be a staggered stud wall, followed by a double stud wall. the reason is that these walls exhibit a series of resonances at low frequencies, and it MIGHT/SHOULD follow that they exhibit absorption over a broader frequency range.

A resilient channel wall will ... mostly... exhibit a strong mass-spring resonance. A single-wood-stud wall or single-steel-stud wall will exhibit (respectively) a very strong structural resonance and essentially no resonance below that, and a very strong airspring/structural resonance and essentially no resonance below that.

hence, i think that for in-room acoustics staggered studs make sense. that is based 100% on intuition, knowledge of the resonant behavior of walls, and what i know or seem to konw about walls and absorption


-green glue customers have, dozens of times, associated the application of the second layer of drywall + damping in their rooms with "better sound" or "reduced echo" or "less boomy".

i don't think any of them have supplied me with any type of measurement, like RT60 or whatever, so take that for what it's worth



i don't think i forgot anything.



-oh, the damped walls in our tests exhibited lower absorption at mid/high frequencies. However, noneo f the walls had alot of absorption at this frequencies

[the dreamer]

Thank you for the comprehensive response! :D
Florian

[Brian R]

you asked about bitumous layers and the like:

i don't know??? I can say that Green Glue will supply the most damping of any possible option available to you. maybe for this purpose (absorption) that's bad, maybe less damping is better, maybe some intermediate level is best, i don't know.

i simply don't know.



you MIGHT get good damping with a bitumous material, you might not. constrained layer damping is not kind to projects of hit-and-miss, it requires somewhat specific sets of properties. for example, put 5 gallons of Green Glue per 4' x 8' sheet, it will perform WORSE than with teh recommended amount, not better.


paint-on damping materaisl are always the-more-the-merrier. CLD and something untested and uncalibrated... is a crapshoot.

paint-on materials, OF ANY TYPE ANYWHERE, are massively, massively unlikely to yield any meaningful result.

[knightfly]

Also, bituminous materials are pretty temperature sensitive (damping-wise) and pretty NOT fire resistant... Steve

[the dreamer]

Thanks for the comment Brian!


Steve:
I'm talking about something like this: (sorry german only)
http://www.aixfoam.de/cgi-bin/shop/s...TkNfSUQ9ODAwNA
Looks like some bitumenous stuff and they state it's used for machinery, cars, isolation doors, walls, ducts and the like.
It's fire resistant.

Florian

[Brian R]

i have a huge pile of absorption data for walls, courtesy of Audio Alloy (1,000,000 thanks to those guys for allowing work outside of just Green Glue tests on their budget).

i will try to get it correlated somehow and see if

1) it all makes sense

2) if it offers an answer to your question

:)

[the dreamer]

Thanks Brian, that's fantastic!
Also thanks for the new interesting stuff on Studiotips, gotta read this a lot of times to grasp, if I ever do :? :D

Greetings
Florian