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Discussion Starter · #1 ·
I used the search function but haven't found anything yet.


My question is regarding a comparison of Acoustik-Mat and Green Glue for soundproofing the floor. Which is better?


My planned home theater is on second floor, right over master bedroom.


Thanks!


Regards,

Lawrence
 

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for impact noise or airborne sound, both?


you should plan to modify walls of the theater and ceiling below if very high isolation is required.
 

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Discussion Starter · #3 ·
Hi Brian,


I believe I need to reduce airborne sound, since there won't be much impact noise when everyone in the theater is sitting down and not walking around.


For the walls, I plan to use RISC clips and green glue and 2 layers of drywall: 5/8" + 1/2" (GG between).


For the ceiling of the theater, I plan to use green glue and 2 layers of drywall: 5/8" + 1/2" (GG between).


I won't have an opportunity to change the ceiling below. Tearing apart the master bedroom has very low WAF.


The floor is one thing where I don't know what to do yet. Before Green Glue, I noticed everyone was using Acoustik-Mat. Now, I have a choice but don't know which is better? Will the choice change if I want to reduce impact sound instead?


Thanks for the reply.


Regards,

Lawrence
 

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Discussion Starter · #4 ·
Sorry to drag this old thread up from the past... is there a definitive answer yet?


For 2nd floor with existing plywood subfloor, which option is better? By better, I mean for sound-proofing (keep the bass in) and impact-noise reduction (minimize sound of people walking around in theater).


option #1 - acoustik-mat then 3/4" plywood

option #2 - Green Glue then 3/4" plywood
 

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Just a real simple question, What is Green Glue?? Anyone got a manufacturer's site web address??


Thanks


SR1355
 

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There is no feasible way to significantly improve sound isolation through the floor with your restrictions. To increase impact isolation, you would use a resilient underlayment (Enkasonic, OwensCorning Silent Floor, Dow Ethafoam 222, Regupol 4010, etc) under two layers of plywood. This will slightly increase sound isolation, but not enough to isolate a bedroom from a movie playing. To significantly increase sound isolation will require redoing the ceiling above the master bedroom.
 

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ok, i was looking for this post a day or two, but the search feature on this site generally cleans my clock.


let's talk about impact noise first...


would i be wrong to presume that you will be using carpet/pad? that seems to be the trend for sound purposes?


under that presumption, a resilient underlayment will be of minimal value. But Green Glue (damping) will still be of value.


that's the short, simple answer.


Here's the technobabble (skip if you just don't care, read if interested or if my statement seems weird and you wanna know why):


the resilient surface (carpet + pad) reduces impact noise by reducing the ability of something hitting the floor to transfer energy to the wood/concrete - whatever the hard thing under the pad is. for example, if you had a small hammer, and tapped a concrete floor the hard surface to hard surface would make alot of racket.


cover it with carpet/pad and it makes alot less.


now the effectiveness of carpet/pad will depend on pad thickness, and the gains will fall to zero at some frequency. imagine you were 1) getting snapped quickly by a rubber band and 2) getting pushed by someone else, and in both cases you covered yourself with a carpet pad.


for the rubber band you get a quick slap, and the pad would probably totally absorb it. the push, on the other hand, would just compress the carpet pad wrapped around your chest and push you anyway. and so it is with carpet and pad - at lower frequencies it's effect diminishes rapidly, at higher frequencies it's effect improves rapidly. just where and how much is based on the properties of the pad, but thicker=lower frequency is a reasonable rule all else reasonably equal.


similar effect: imagine a hard floor above you, and two people taking turns walking around. one is barefoot, and one is wearing steel-soled work boots.


what do you hear for the barefoot walker? thud, thud, thud. what do you hear for the steel toed walker? clack clack clack. in this case the soft stuff under the bare foot acts as the carpet pad, see?


so higher frequencies are attenuated drastically (the effect is huge), but low frequencies not so much.



with a resilient underlayment (something hard over something soft over something hard, like plywood/fiberglass matt/plywood), you get a similar sort of behavior, but with a catch or two. They behave like all mass-spring systems. namely:


1. there is a resonance point

2. at the resonance point performance is made worse

3. below the resonance point you gain nothing

4. above the resonance point you gain rapidly, sometimes dramatically


and in general, you will get lower frequency results with carpet and pad on top than you will with an underlayment. so to add plywood over a resilient underlayment over plywood, but under carpet to make


carpet/pad

plywood

resilient something or other

plywood


will make a good thing a wee bit worse somewhere, not help at all where the carpet was failing, and add to the performance where it is already superb. all of which adds up to not much gain.


now damping is, i think, the solitary thing you can do to modify impact noise that is effective at all relevant frequencies (besides just ladling on more mass, but that usually doesn't pan out the way you hoped). in theory, the reduction of impact noise in some frequency band will be given by


10*log(new damping/old damping), or 10dB for every 10* increase in damping. Green Glue can raise the damping of something like plywood or OSB by 15-40 times (depending on construction).


so Green will improve the floor even where the carpet/pad fail, hence it still has value.


i'm not overly fond of promoting GG, lol, i'd be a horrid salesman, maybe someone should shrinkalyze that. i'm sure one of you folks is qualified. ;)


but that's the low-down, for impact noise under carpet, green glue by a mile.


now, onto airborne sound.


Brian
 

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ok, if you have no carpet/pad, the best option for impact noise would be this:


plywood/green/plywood with acoustikmat on top


then


plywood/acoustik mat/plywood/green/plywood


and i'd still take Green over just the underlayment given my druthers, and the reason again is that damping will affect impact noise across the frequency spectra, the resilient underlayment will make it a bit (none to 10dB, maybe more in an extreme case) worse over some range, and have no effect at low frequencies.


on top it will not make it worse over some range, and will work to a lower frequency.


for the most part, impact noise on wood floors is ALL low frequency noise.


if someone held a gun to my head and made me pick just one of these products to make a floor have low levels of impact noise i'd pick acoustik mat, and put 2 or 3 layers ON TOP with nothign hard over it.
 

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ok, here's a rough idea of how all of these things work

http://audioalloy.com/impactnoisetheory.gif


ok, let's hope that worked.


this is just FYI in case anybody cares, pay it no mind if you don't.


in reality the theories for resilient toppings (carpet pad) and resilient underlayments (something soft under something hard) are pretty good, and generally reflect just what is shown above with


1. not as smooth of course

2. widely varying frequency where the curves go up (or down)

3. different depth of dip (both much larger dips and lesser dips) for the underlayments


often mass doesn't give as much gains as expected


and generally damping makes choppy curves due to different damping at different frequencies, and the varying resonant behavior at varying frequencies. Generally, though, the expected gains materialize (better at some frequencies, less at others), and in some cases the damping material can act as an underlayment as well, causing much, much larger high frequency gains (tens of dB), but the big golden fruit is the capacity to improve low frequency performance. :)


k, that's how impact noise control works on the floor level, the ceiling below is obviously the other big factor.
 

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ok, folks, that got a bit windy, sorry about that, but it's a topic that I answered and i just wanted to explain why in case anybody cared.


more later and i'll keep it shorter
 

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Quote:
Originally posted by brianr820
ok, folks, that got a bit windy, sorry about that, but it's a topic that I answered and i just wanted to explain why in case anybody cared.


more later and i'll keep it shorter
Long is good. :) You are a great source for information.
 

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Quote:
Originally posted by japanesegeek
Long is good. :) You are a great source for information.
cool.


i should have added that over wood floors, the gains at higher frequencies from either carpet or underlayment are not nearly so drastic.


the reason is that wood is already fairly soft, so above when i said that the theory generally applies, it does, but over something hard (like concrete). over wood you get the same basic things, but much more of a limit to the high frequency gain.

http://audioalloy.com/impactnoisewoodfloors.gif


that's a better description for over wood.



now with respect to just making something heavier to improve impact noise, there are a few different factors, including stiffness lowering one particular resonance, that generally won't make this a facile of getting good results all by itself.


and finally, remember that results always vary. and "thicker" might mean "softer or more ideal in properties". so in those pics when i say "thicker" soft stuff on the top, it's probably better to think of it as "more effective" -vs- "less effective"


1/2" of someones super-engineered foamish carpet pad will go farther than 1/2" of cork, etc. (cork is harder)
 

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Brian,


Another question about flooring upgrade with GG...


In another thread you said:


" pick something of about the same thickness as the other layer (or a bit thicker), and that won't break the bank. Another layer of particleboard is a fine choice."


The question relates to the "same thickness" comment. Is this a critical issue in getting significant benefits? My theater is above my great room. The only thing that currently comes through is some deep bass when playing at relatively high levels in the theater. The construction (top down) is; carpet, pad, 1 1/8" T&G plywood glued and screwed, 11 1/2" cavity filled with fiber glass, RC, 5/8" sheetrock. Would there be any problem/benefit to adding a GG'ed layer of 1/2" OSB or particle board over this existing construction?
 

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ok, edit:


here's the short answer: that's some thick plywood (1 1/8"). you do not need to use that thick for the other layer. 1/2" will do a nice job. in a lab you could discern a difference between 1/2" and 3/4", how much it matters in the real world is another story.


if 3/4" is reasonably cheap, go for it, if 1/2" is what you need, so be it and don't worry about it at all.



a little technobabble with respect to this topic:


i remember that comment in the GG thread - a gent was upgrading his floor and thicker would be more mass (use 1/4" ply, add little mass, use 3/4" ply, add more).


and also, for sandwich, or constrained-layer, damping to work you have to create shear via motion between the two sides (which pulls and stretches on the stuff in the middle). and the risk with too thin of a layer in one of the sides of the sandwich is that you don't put any strain on the Green. imgaine


concrete block

green glue

aluminum foil


well aluminum is stiff, and concrete is stiff, so we're in good shape? no, the foil won't be able to exert any force on the GG, see? so the foil + GG would just sit there and accomplish a whole lotta nothing.


but, for example, 1/4" drywall works with 5/8" drywall and that's a stiffness ratio of ~20 times.


Brian
 

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Quote:
Originally posted by brianr820
... that's some thick plywood (1 1/8").

Brian
I had previously lived in houses with standard 3/4" plywood on the second floor. It always seemed to flex under load. When we had this house built, I told the contractor I wanted two layers of decking, 3/4" and 1/2" to stop the floor flexing. He came up with the 1 1/8 " T&G stuff as a less expensive alternative to the two layers. The second floor of our home is extremely rigid, you never feel any flex or "give" when moving around. Man is that stuff heavy! The framing crew had one strong dude that could toss a sheet up onto the second floor framing (11') by himself!


Thanks for the clarification on the flooring application.
 

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hey lawrence, for airborne sound:


is there any chance that you'll have access to the top of those joists?
 

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Discussion Starter · #18 ·
Brian,


I think I speak for many people when I say that I really appreciate your detailed explanation and graphs and all the effort that you put in to help us. My heartfelt thanks for that.


I am not sure what your question means "is there any chance that you'll have access to the top of those joists?" There is already a plywood subfloor there. Are you asking if I can remove the subfloor? I don't think I am going to go to that extreme. My wife will kill me. I think I am limited to adding things on top of the plywood.


Regards,

Lawrence
 

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Hey Lawrence,


thanks much - i'd be pleased to know that my various postings aided someones cause. It's an enjoyable topic for me, so it's hardly a chore.


rynberg commented above that if you can't modify the ceiling one way or another, that would limit to some extent the performance that you could attain. And that may be, but situations are what they are, and solutions still need to be found.


so, i don't know what your budget is, or height constraints, but i think it reasonable to offer that more would be more on the floor. if you have interest in combining the two products that you mention (Green Glue & Acoustikmat), you would not suffer (save on budget, and that sure isn't an unimportant consideration).


the biggest problem of a normal ceiling are these two: 1 the normal low frequency resonance that occurs in the 80-200hz range on this type of construction and 2 the direct connection between floor above and ceiling below allowing energy to transfer fairly easily. Green Glue floor and ceiling will essentially eliminate #1, just on floor will go a long long ways, and RSIC clips (or other well built sound clips like the kinetics things) will essentially eliminate #2... now you can't do RSIC but acoustikmat on the floor will help.


so, IMO, the more the merrier, within the the applicable limits.


Brian
 

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I know this is probably a stupid question but If I go with two 1/2" layers of dryawall will I have to get special screws for my light switch covers and socket face plates to screw on? Also will this mess up my door trim and window trim?
 
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