AVS Forum banner
Status
Not open for further replies.
1 - 12 of 12 Posts

·
Registered
Joined
·
158 Posts
Discussion Starter · #1 ·
So I keep reading "pack empty joist cavities," but I don't understand it. Based on the fact that fiberglass insulation absorbs little to no acoustic energy, what is it point of putting it in between joists? Is it just to keep things from having the space to rattle? Is the problem the same for ceilings, stages, risers and walls? What am I missing here?


Thanks,

stefan
 

·
Registered
Joined
·
158 Posts
Discussion Starter · #3 ·
Ok, so assuming that is is covered with sheet rock (in my case 2 layer with GG) on the walls and ceiling and plywood on the risers. What is the purpose? I guess I made a gross over generalization. The comment should have been that fiberglass insulation packed into walls between layers of sheet rock has little absorbing capability. This is why everyone uses GG or MLV instead of just fiber glass and 1 layer of sheetrock, right?


I understand that high density fiberglass is a good acoustic treatment for reflections, but isn't this more diffusion rather than actual absorption as you get with more mass? What I mean is doesn't the break up, delay and change the direction of the wave energy causing it to self cancel?


If not where does the energy go? With MLV this is pretty easy to understand because there is so much mass in there. Loose fiberglass insulation doesn't have that kind of mass so where does the energy go? I am picturing it like sparing with a pillow vs a punching bag.


Or is my memory of physics just way off? If so, then a direct question.


Here is what I have:

Walls: Concrete foundation wall - insulation nailed on - two sets of stud (there is another thread on this fiasco) - drywall - green glue - drywall.

Ceiling: Joists - drywall - green glue - drywall.


Question: What difference will it make if I put fiberglass insulation in between the studs/joists vs leaving it empty?


Picture: This actually show 3 layers of studs on the left because of a small jut out to match the other side. http://www.mystique.com/gallery/albu...2041.sized.jpg


--stefan
 

·
Registered
Joined
·
3,258 Posts
Hi Stefan,


As Ethan says, fiberglass is very absorptive. Its acoustical purpose here is to damp the mass-air-mass resonance that builds up between the wall or ceiling/floor surfaces. It is a very cheap way to gain some low-frequency sound isolation (or more precisely, not to lose additional low-frequency sound isolation). This resonance would otherwise directly "short out" existing sound isolation, or even amplify low-frequency sound transmission. Don't skip doing this!


Regards,

Terry
 

·
Registered
Joined
·
6,697 Posts
Quote:
Originally Posted by sbraab
Ok, so assuming that is is covered with sheet rock (in my case 2 layer with GG) on the walls and ceiling and plywood on the risers. What is the purpose? I guess I made a gross over generalization. The comment should have been that fiberglass insulation packed into walls between layers of sheet rock has little absorbing capability. This is why everyone uses GG or MLV instead of just fiber glass and 1 layer of sheetrock, right?


I understand that high density fiberglass is a good acoustic treatment for reflections, but isn't this more diffusion rather than actual absorption as you get with more mass? What I mean is doesn't the break up, delay and change the direction of the wave energy causing it to self cancel?


If not where does the energy go? With MLV this is pretty easy to understand because there is so much mass in there. Loose fiberglass insulation doesn't have that kind of mass so where does the energy go? I am picturing it like sparing with a pillow vs a punching bag.


Or is my memory of physics just way off? If so, then a direct question.
Hi Stefan,


Hopefully Terry will jump in here to correct anything I say that's not correct.


As he points out batt insulation between the joists can help in sound isolation, even with a couple of layers of drywall covering it. Correct me if I'm wrong, you're thinking that the insulation is rendered nearly useless by having a drywall covering because you are visualizing the sound reflecting off the walls? This is true for the higher frequencies, but for the lower frequencies it is partially absorbed by the drywall, but then passes through into the joist cavity.


Obviously, the air particles aren't actually forced through the wall. What happens is the wall is energized by the sound waves impinging on them and they start to vibrate. This vibration in turn is transmitted to the air inside the cavity. Without an absorber to fill the cavity, the cavity air column would start to resonate at it's tuned frequency dictated by it's dimensions. Think Pipe Organ. ;)


Once the sound wave is transmitted inside the cavity the fiberglass fibers slow the the velocity of the vibrating air particles, that is the fibers absorb the sound by converting kinetic energy of the air to a slight rise in temperature of the fiberglass.


As Terry points out there are two advantages to the batt insulation. Damping resonance and sound isolation. While it is true that Green Glue and Mass Loaded Vinyl, is more effective in sound isolation than fiberglass, they do not address the resonance of the air cavity.


Anyway that's my layman's take on answering your "How does this work?" and "Where does the energy go questions?"


Larry
 

·
Registered
Joined
·
220 Posts
Larry, did you stay at a Holiday Inn last night?


That was a great explanation, even a knucklehead like me can understand that.


Thanks,

Mark
 

·
Registered
Joined
·
6,697 Posts
Quote:
Originally Posted by Mark Ducati
Larry, did you stay at a Holiday Inn last night?


That was a great explanation, even a knucklehead like me can understand that.


Thanks,

Mark
Hi Mark,


Thanks. It did sound good, didn't it?


Now Terry can jump in and tell us how it really works. ;)


Larry
 

·
Registered
Joined
·
3,258 Posts
Good explanation, Larry!


- Terry
 

·
Registered
Joined
·
2,114 Posts
the most important function of insulation in a wall is to lower the frequency of mass-spring-mass resonance. This lowers the frequency at which decoupled walls actually decouple, and lowers the frequency at which the wall will perform badly.


some amount of damping can occur at low frequency resonance points, but generally not much, perhaps 1 or 2 dB.



the second most important function of insulation, and hte most famous, is to absorb sound passing through the cavity - sound trying to take an airborne path through the cavity to the other side. This is a middle/high frequency phenomenon, and insulation can be enormously effective.



an additional function, usually important only in deep walls like double-stud walls, is to eliminate "cavity resonances", which are basically room modes occuring within the wall cavity. in shallow walls these are generally very high in frequency, but as cavity depth increases, they move to lower bands.


none of this is entered in contrast to anything above, just as clarification, the attached graphs outline better than any amount of words could hope to.




basically, in most wall types, adding insulation will:



1. lower the frequency of worst wall performance

2. considerably increase mid/high frequency performance

3. because the freq-of-worst-performance is lowered, performance will be WORSE at some frequency


and there you have it




DE has associated empty wall cavities with in-room sound problems.


How expensive is insulation for material and labor relative to something like GG? about the same as 50% coverage with GG?


 

·
Registered
Joined
·
6,327 Posts
Quote:
Originally Posted by sbraab
Ok, so assuming that is is covered with sheet rock (in my case 2 layer with GG) on the walls and ceiling and plywood on the risers. What is the purpose? I guess I made a gross over generalization. The comment should have been that fiberglass insulation packed into walls between layers of sheet rock has little absorbing capability. This is why everyone uses GG or MLV instead of just fiber glass and 1 layer of sheetrock, right?


I understand that high density fiberglass is a good acoustic treatment for reflections, but isn't this more diffusion rather than actual absorption as you get with more mass? What I mean is doesn't the break up, delay and change the direction of the wave energy causing it to self cancel?


If not where does the energy go? With MLV this is pretty easy to understand because there is so much mass in there. Loose fiberglass insulation doesn't have that kind of mass so where does the energy go? I am picturing it like sparing with a pillow vs a punching bag.


Or is my memory of physics just way off? If so, then a direct question.


Here is what I have:

Walls: Concrete foundation wall - insulation nailed on - two sets of stud (there is another thread on this fiasco) - drywall - green glue - drywall.

Ceiling: Joists - drywall - green glue - drywall.


Question: What difference will it make if I put fiberglass insulation in between the studs/joists vs leaving it empty?


Picture: This actually show 3 layers of studs on the left because of a small jut out to match the other side. http://www.mystique.com/gallery/albu...2041.sized.jpg


--stefan
Looking at those pic's I gotta say I am hoping you are going to use GG.

The walls look like they are touching. :( This will transmit sound to the rest of the house. While your set-up is/will be better than an average 2X4 wall, you still will not see the results you could have. GG will eliminate and dampen lot of vibration that will transmit through your walls.

Another note: by looking at those pics, if your don't stuf those stud cavities with insulation, your walls will sound like the biggest base drum you have ever heard. ;)


Craig
 

·
Registered
Joined
·
2,114 Posts
the data in those graphs is all sourced from IR-761, National Research Council Canada, and copyright is held by the same.


you may find that marvelous document at www.nrc.ca
 

·
Registered
Joined
·
1,014 Posts
Stefan,


As Craig just posted, in one word, think "drum". Take an empty coffee can with the lid on and "play it" -- sounds like a drum. Now pack it full of batt insulation, and "play it", again. Sound any different? Shoud be "duller" -- not only will the insulation absorb some of the frequencies, but it will displace air, which is easier to move than the solid fibers.


Geordon
 
1 - 12 of 12 Posts
Status
Not open for further replies.
Top