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Discussion Starter · #1 ·
I have a 2x10 joist ceiling?:confused: How much insulation should be used purely for sound abatement? Is there a point of diminishing return or should I just stuff the cavity?


I was wondering since some sound will travel through the structure irregardless of the amount of insulation stuffed. :)
 

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AJ,


I suggest you stuff it. :D


Seriously, insulation between the joists above a sheetrock ceiling does damp the ceiling vibration, and it helps the ceiling to act as a bass trap a little, but it probably won't help with isolation as much as you'd hope.


Since fiberglass is inexpensive, go ahead and install as much as will fit.


--Ethan
 

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I sound proofed my ceiling and interior walls with a product called "sonopan." The pdf file shows how you can do it.
http://www.cascades.com/pdf/brochure_sonopan_en.pdf

It's a product made in Canada maybe you can find the equivalent in your area.

It works great because I can watch a film in my basement and not disturb anyone if they are sleeping.

The only problem is about 10% of the sound escapes through the hot air vents. It sounds like a system turned down to a quiet level if you're near a vent.

The other benefit is it cuts down the reverb in the room and improves the acoustics
 

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Discussion Starter · #4 ·
I was thinking Roxul mineral fiber with an RC-1 drywalled ceiling. Is the sound reduction linear with respect to the thickness ?


I've looked at Sonopn or similar but the large area I'm working with makes it cost prohibitive.


As for the heat vents any where near the theater I'm replacing sections with insulated flex duct. Metal duct is a very good transmitter of sound, too good! Even some returns I've changed to 8" flex duct.
 

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Quote:
I was thinking Roxul mineral fiber with an RC-1 drywalled ceiling. Is the sound reduction linear with respect to the thickness ?
Mineral fiber is a very good insulator, comparable to fiberglass batts, but slightly better above 250 Hz. Sound reduction isn't linear with thickness I'm afraid, but it does increase somewhat with thickness. The resilient channel will make a big difference, as will adding a double layer of drywall with staggered seams. Use acoustical caulk ABSOLUTELY EVERYWHERE at every layer, leaving no cracks or seams. If you don't seal all the leaks, your sound isolation will be ineffective.


Regards,

Terry
 

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Discussion Starter · #6 ·
Terry,


Sound reduction not being linear in my example. How much insulation is 1) practical 2) nice to have 3) over kill, in your opinion.


My media/game room will be less than ideal acoustically . I just want to maximize my return on investment.
 

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AJF,


I recommend 3 1/2 to 4 inches of soft insulation, such as mineral fiber. If you have more depth than this, you can fill the space with it. That may give you a couple more STC (Sound Transmission Class) points, depending on your construction.


This won't have a major effect unless you seal completely with acoustical (permanently flexible) caulk. I recommend resilient channel as well, which can add maybe 6-10 STC points. Resilient channel is not expensive.


Nice to: a double layer of drywall using a layer of 1/2-inch and a layer of 5/8 inch, all sealed well and with seams staggered. Overkill would be building an acoustically isolated room!


Regards,

Terry
 

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Terry hit it right on. Stuff the cavity with mineral wool or fiberglass. And whatever you do, no can lights or other holes in the ceiling. Actually, I would suggest no ceiling lights or other electrical boxes and put sconces on the walls because any hole is a bad thing. If your ceiling drops the sound by 20 dBA, but 1% of it is open - then twice as much sound energy will get through and it will only drop the sound 17 dBA.
 

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Actually, a 1% hole is even worse than that.


I just put a message in the "home theater general" section about a new sound isolation calculator I wrote as an Excel spreadsheet. I am making it available free for non-commercial use, and you can find it at www.componentacoustics.com . It uses the standard STC (Sound Transmission Class) rating, which is a frequency weighted dB-type number used for measuring interior sound isolation.


Here are some results from the calculator, which computes the power leaking through all materials, and then converts it back to the overall STC. Say you've got double stud construction with a double layer of 1/2" drywall on one side, and fiberglass batt insullation on both sides. This should give you a nice high STC of 60. Good stuff.


Here's what happens when you punch a hole in it:


0.001% open - down to STC 50

0.01% open - down to STC 40

0.1% open - down to STC 30

1% open - down to STC 20


In fact, the starting STC value turns out to be independent of the value you get after making a certain percentage hole, so long as that starting value is higher than the STC for that hole percentage (as given above)!


Example. A good-sized home theater might have 1000 square feet of surface. You've "soundproofed" it really well, to a whopping STC of 66, about the highest you can achieve with ordinary wood-frame construction. But you haven't caulked, leaving a total of one square foot open via all the seams in the drywall and the cracks around electrical boxes. It is 0.1% open. You will only get an STC of 30, which is about what you'd have with no sound isolation construction at all!


Ya got to download and play with this calculator. I've learned a lot from it already, and I thought I knew a thing or two about acoustics. ;)


Regards,

Terry
 

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Terry, what about your HVAC for the room (and some have separate supply and return for the projectors too) ie coming out of the ceiling or walls? How do you suggest getting around it so not to loose STC via these holes?
 

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Hello Frank,


The HVAC system is another kettle of fish - or can of worms? :)


The problem here is not so much transmission via the ducts to louder adjoining spaces, as with silencing the direct path to a noisy sound - the HVAC unit itself. This is not my specialty, but I know of duct silencers that act on the same principle as car mufflers. Duct liners are used to attenuate HVAC sound as well, as are reactive attenuators.


HVAC sound can actually decrease the need for a high STC, because it increases the steady-state background noise of the room. The rule used for sound isolating commercial cinemas from each other is this: the sum of the STC and the NC (Noise Criterion, the level of background noise) should be 95 or greater. So a quiet theater with an NC of 25 needs an STC of 70, while a theater with a loud HVAC system generating NC 40 only needs an STC of 55! Its kind of a raise the bridge/lower the river situation.


- Terry
 

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Terry - I'm sorry to say but that calculation does not fit with real world experience. Thus I would say that something in your calculation must be off. While I agree that open areas do affect sound transmission, it is not to the degree that you state.


Case in point: completely RC double drywall theater with 10" fiberglass above and double drywall. Basically close to ideal as is reasonably possible - that was my last theater. Sound upstairs was MUCH quieter afterwards. Now if I left the DOOR open (a HUGE opening) it still was much quieter upstairs than before. Sure closing the door made it less noisy, but the noise transmission was still less. Not caulking seams certainly would not be as bad as leaving a ~20 sq. ft opening.


Just my experience...
 

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Hi PAP,


Nothing wrong with the calculations - its just that the specifics of the real room need to be considered. The calculations (if you try to apply them for multiple wall/ceiling surfaces) assume that the sound transmission is the same through any of these surfaces from inside the room to a point on the outside.


This is clearly not the case for the different sound paths in the situation you describe. Sound transmission between your HT directly to the floor above through the ceiling is much greater than via the convoluted path out the door and up the stairs. Now, if that door were in your ceiling (!) you would indeed have invalidated your sound treatment.


I appreciate your comment, as it gives me a chance to clarify the proper usage of STC calculations. ;) They apply for single surfaces only - a ceiling, a wall, etc., or at best adjoining surfaces that communicate via a similar path to the same exterior space.


Regards,

Terry
 

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Quote:
what about your HVAC for the room (and some have separate supply and return for the projectors too) ie coming out of the ceiling or walls? How do you suggest getting around it so not to loose STC via these holes?
The key is to minimize penetrations of your barrier. For example:

Drywall and seal the room.

Install soffits/columns from MDF (don't frame them in with the walls.) This places the columns/soffits INSIDE the room barrier.

Have your electrical (romex) enter the room via a single hole through the barrier (very well caulked) and into a soffit area. Run your electrical through the soffit, down columns. Place your light switches/duplex outlets in the columns...no barrier penetration. Behind the stage, use surface mount electrical outlets. For wall mounted sconces, use surface mount 1/2" or 3/4" boxes..you have a small hole for the romex behind the box that can be well caulked.

Bring your HVAC supply and return into the soffit. Mechanically isolate the ducts from the wall, and seal any gaps. You supply/return diffusors then penetrate a column or soffit. You've thus reduced your barrier penetration to the two openings for the ducts, rather than the four, or so, for the diffusors.
 

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Terry have you given any thought to making your spreadsheet work with StarOffice or OpenOffice for us MicroSloth loathers out there. It is a free download and reads .xls - it just has a problem loading your file for some reason.
 

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Quote:
Originally posted by Terry Montlick
Use acoustical caulk ABSOLUTELY EVERYWHERE at every layer, leaving no cracks or seams. If you don't seal all the leaks, your sound isolation will be ineffective.
Hi Terry,


I understand your comment with regard to acoustic leakage, but the folks building my home have other practical concerns regarding this air tight approach.


I'm building a home in Florida and most of the various trade people have 'horror" stories relating to walls that "don't breath". In particular they state that the exterior walls suffer condensation even under normal conditions such that wall paper is not recommended on exterior walls. With an air tight wall they say the fiberglass batts would be water laden and moldy in no time. I realize that this is not your field, but if the insulation were to get wet I assume that it would have a more adverse affect on acoustics than a small crack at the bottom of my sheetrock.


Any thoughts?


Thanks.


Larry
 

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Larry - Just caulk the interior walls and ceilings, as these will be your sources of sound leakage from room to room.


Krag - send me your email address and I'll send you an xls file, which will hopefully work. If I get a lot of demand for this calculator, I'll rewrite it in Java.


Regards,

Terry
 

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Let's take a hung ceiling grid. 10 inch joists above. You want to fill the joists wit insulation BUT you do not want to stuff, don't compress this.


Now the ceiling grid. Say ceiling tiles with .65 to .75 NRC. On top of the tiles weight down with 1 inch compressed fiberglass backed by a piece of sheetrock. Got a few inches above the sheetrock left? Fill with more fiberglass batting.
 

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Discussion Starter · #20 ·
Mark,


It will be a drywall ceiling. So I get one shot. Set it and forget it.


But I have read posts that it's OK to compress because I'm led to believe density is what your after.


thanks real world feedback is good!
 
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