Just how crazy is this idea for ceiling sound reduction? - AVS Forum
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post #1 of 120 Old 09-30-2004, 03:32 PM - Thread Starter
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Background - my basement room that is becoming a HT was furred out & paneled on the walls w/ 12" acoustic tiles (stapled) in the ceiling. The type this is compressed like particle board but a little softer - on the face it had small holes. When I removed it, sound between the floors increased so it worked to some degree, though not great.

I have low ceilings in the basement (under 7') so minimizing loss is a consideration (I posted & got great feedback on low ceilings recently). So what I was planning on doing was filling the 8" joist cavity with fiberglass insulation, hanging RSIC truss clips (to minimize loss and level off the joists), hat channel & then 5/8" drywall.

Theres some office construction where I work - they hauling out old 24x48" acoustic tiles - about 1/2" thick. Since I can get these for free, what do you think about cutting them down and either gluing or stapling them to the bottom of the subfloor at the top of the joists? In other words, the top of the joist cavity would be covered by these guys. Then fiberglass insulation, etc. Sound crazy? Minimal effect? Since the original tiles did provide some benefit, it seems to me that this would have some benefit, but your feedback is welcome.

Thanks,
Bob

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post #2 of 120 Old 09-30-2004, 04:49 PM
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If the tiles are dense mineral fiber, virtually no effect -- not enough for the effort involved, that's for sure. If the tiles are fiberglass -- no effect.

Bottom line, not worth it.
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post #3 of 120 Old 09-30-2004, 07:06 PM
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hi bob,

not crazy at all, always good to think. generally in-room acoustic treatment products (like ceiling tiles) are porous. they need to be so sound can get in and be absorbed... good for absorption, bad for isolation.

increasing absorption in a room will decrease the amount of energy present in the reverberating soundfield... basically make less sound in the room, less sound to pass through. your comment may relate to this.

attached to the bottom of the floor the benefit that they could offer would be damping, absorption (cavity absorption) and mass. they will offer none of the former, and little of the latter. their absorption properties will be overridden by the fiberglass batting in this case. so, as usual, rynberg is on the money

Brian

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post #4 of 120 Old 09-30-2004, 07:24 PM - Thread Starter
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Thanks guys - plenty of other crazy ideas to come, for sure!

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post #5 of 120 Old 10-02-2004, 07:26 AM
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the last handful of times that i've logged into this fine site i've contemplated making this post. this thread seems like a good place to do it, so i'll throw it out there. i'll disclaim this at the posts end.

when contemplating the value of any material in a soundproofing (sound isolation) application, there are 5 and only 5 principles that you need to think of.

1. the first is mass. mass impedes the transmission of sound in a simple way - it's harder for the sound to shake a very heavy thing than a very light thing, no different than saying it's harder to push a shopping cart full of lead bricks than an empty cart.

2. the second is mechanical de-coupling, or mechanical isolation. this comes in the form of air cavities, resilient channel, resilient clips, spring hangers, etc., etc., etc.

all of these function by preventing vibration from moving through the mechanical parts of the wall to the other side of the wall, where it can/will produce sound. instead it has to pass through the air, where some of it will be lost, and through the insulation/absorbing material, where (at some frequencies) much of it will be lost.

100% of resilient isolators come with a resonant region in which their performance will fall below that which would be observed if no "isolator" were present. in resilient walls, the frequency range where this occurs is defined, i believe, essentially by the air spring (unless you have an immense air cavity). i cannot make that statement with absolute certainty as i haven't tested every resilient mounting options, and if a resilient mount was fairly stiff (like too-thick channel), it might push this frequency higher than the air spring frequency.

3. absorption. insulation in a cavity, increase the losses due to air cavity by eliminating/removing/destroying some sound. another effect of insulation in a cavity is to lower the resonant frequency of resiliently mounted walls.

4. resonance. resonance works AGAINST the good things done by 1 through 3 above by making it very easy for sound to vibrate a wall (and the vibrating wall vibrates air on the other side, making new sound, hence resonance increasing the ease with which sound is transmitted).

there are exactly two ways to deal with resonances

a. damp them - this reduces their magnitude,
b. move them. resilient channel, for example, moves the big, ugly low frequency resonance to below the 125hz STC cutoff. soundboard can do this as well by making walls more flexible. pros and cons to that, but if you can get it below 125hz, STC will soar, even if the wall isn't improved at other frequencies. RC, properly installed, does improve performance at other frequencies VIA #2 above - mechanical decoupling


5. conduction. this is related to flanking sound and also to #2 above (mechanical de-coupling).

imagine you had a floor that lost vibration intensity at 50db/foot. no flanking sound through that floor as the sound dissappears in transit at such a fast rate. imagine you had a concrete floor, sound could travel through that for hundreds (or more, i'm winging that figure) of feet before dropping by the same level. thus, important for flanking sound

if a panel had an infinite rate of energy decay, that would mitigate the direct-(non-resilient) connection between two layers of drywall. sound could not travel across the panel to the studs to travel to the other side.

conduction in materials varies with internal sound velocity and damping. in constructions it would also vary with things like continuous-vs- non-continuous subfloors/foundations, etc.



and that's it

so if we summarize these basic principles into a few things that a product or material or item can have that might allow it to improve sound transmission we get (~, roughly, approx) this list

1. heavy
2. mechanically decouples / resilient isolator including air space
3. improves damping
4. adds sound absorption between two panels

that's it, not in any particular order of importance

if something doesn't fall into one of those categories, forget it

i'm going to ponder this post when i get some time and perhaps come back to add or edit, but this is a really good/simple starting point, i believe

Brian

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post #6 of 120 Old 10-02-2004, 07:40 AM
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some general prudence is always in order, of course, some examples:

mass: gluing a piece of paper to your wall increases it's mass, but not by much, and so the effect would be trivial. as an EXTREMELY LOOSE general rule, doubling mass will buy you 6 more dB. these factors (mass, decoupling, resonance) interact, so you WILL NOT observe exactly 6db of change in a real wall with an air cavity. general rule only.

gotta add alot of mass to make a difference


damping: smearing some peanut butter on a wall will raise it's damping. if you had an infinitely precise test array you could test this, lol.

but not by much, and so the difference would be trivial. have to raise damping ALOT to make a big difference in resonant behavior or to reduce the ability of something to conduct vibration/sound.

absorption: throwing a little layer of straw from a farmers hay field, or some egg-crates, in your ceiling cavity will increase absorption. but not by much, and the effect would be trivial.

have to add alot of absorption - like mineral fiber, fiberglass, cellulose, other real, functional absorbing matrials - to make a big difference. regular fiberglass is well proven to be effective.


decoupling: harder to formulate a simple mental example here... but prudence applies here more than anywhere, because if you get it wrong you make things WORSE, not better (due to the resonant thing i mentioned above).

ex: have a normal wall, add resilient channel + more drywall in front and you will wind up with WORSE transmission loss over some frequency range. add studs and an air space in front of a concrete block wall and the same thing happens. make that air space too small and you will create severe problems in sensitive regions.

put your floor on rubber pads without calculating and you risk having severe vibration/resonance problems over ~1 octave. put a floating floor over a resilient underlayment and it will make MORE foot-step noise over some frequency range.

etc., etc., etc. so resilient decoupling is something that needs good prudence


a simple example of non-resilient decoupling that has been shown to positively affect sound-stopping (non-resilient, so it doesn't come with a resonant region drawback) is putting little rubber pucks between the studs and the drywall, under the screw positions.

happy hunting

Brian

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post #7 of 120 Old 10-02-2004, 07:48 AM
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couple quick add-ons and clarifications

-i lump the coincidence phenomenon in with resonance. i do this because it's damping controlled

-rubber pucks are not nearly as effective at mechanically decoupling as resilient mounting options, but they do something

-i excluded stiffness. stiffness can wickedly increase transmission loss at lower frequencies, and this is used to advantage at times in industrial noise control enclosures. it also affects the location of various resonances in various systems (see my lumping of coincidence with resonance above). it also affects the speed of sound in a panel or structure. stiffer=faster=greater distances before damping can have it's effect (it's effect=destroy the vibration/kinetic energy)

however, i do not think you can build a wall stiff enough to capitalize on the effect above, so i think that the exclusion of stiffness as a basic prinicple of sound-stopping is reasonable. feel free to argue

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post #8 of 120 Old 10-02-2004, 07:53 AM
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and for fun and then i promise to cease (lol)

you can actually soundPROOF a room in a few ways,

1. infinite decoupling. a vacuum. zero mechanical connections and the whole room encased in a vacuum.

2. infinite stiffness. sound could not bend the boundaries of an infinitely stiff room, and hence no sound could be re-created on the other side

3. infinite damping. damping results from forces which counter the motion of a panel (or other structure). the damping force is AGAINST the direction of motion. infinite damping = infinite force resisting motion = infinitely stiff, so perhaps this doesn't deserve it's own category.

4. infinite mass. infinite mass could not be moved with any force, hence could not vibrate, and sound could not be re-created on the other side


sound DOES NOT pass through walls. it vibrates the walls, which vibrate the air on the other side. hence it's new sound (if you want to look at it that way) on the other side of your wall, not sound that passed through.

sound passes through openings, but not sealed constructions.



ok, so for the heck of it, let's rank some materials by these 5 categories

concrete block:

mass = awesome
decoupling = zero, it's a single panel, not applicable
absorption = not appliable for the same reason
resonance/damping = poor, but the absence of an air cavity eliminates the most problematic resonance
conduction/flanking = horrid. stuff is a great conductor


concrete block with studs, air space, and drywall:

mass = still awesome
decoupling = good (presuming your studs/air are decoupled)
absoprtion = use it!
resonance/damping = lots worse. that air cavity is a spring, and so is the bending stiffness of the drywall between studs. as such you will have a spring resonance (same as above). published reports (NRC Canada, IR-586) have demonstrated 15+ dB of lost sound-stopping in the low frequency region
conduction/flanking = changed. drywall AND concrete conduct sound well, but if the vibration can't get from one to the other we've improved it (at higher frequencies). it follows that we would worsen it where we INCREASE the amount of sound that can make it through.




a 2x4 wall (normal, assume it's sealed)

mass = fair to partly cloudly
decoupling = poor (drywall directly connected to the studs)
absorption = add insulation!
resonance/damping = bad. crippled by resonant problems at lower frequencies
conduction/flanking = poor. drywall conducts sound very efficiently, as do studs/framework, etc.


a resilient channel wall
mass = fair to partly cloudy
decoupling= good
absorption - add insulation
resonance/damping = low frequency air spring resonance, other resonances in the drywall panels, but these are not as severe because the panels have no communication path between the studs
conduction/flanking = resilient mounts prevent higher frequencies from entering the studs, and hence isolate flanking sound at higher frequencies. where they fail (the resonant region) and below, they fail in this regard as well.

and so on and so on and so on

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post #9 of 120 Old 10-02-2004, 08:20 AM - Thread Starter
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Brian,

Fabulous post! Thanks so much. I'm especially interested in smearing peanut butter over the walls - something the kids can get into! Do you know the acoustic properties of peanut butter vs. almond butter? .

Again, thanks for the clear & consise summary.

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post #10 of 120 Old 10-02-2004, 08:30 PM
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thanks very much bob.

i often wonder what to post on various topics, and if what i chose to post did anyone any good. there really aren't any one-line answers to this topic.

it might be fun to follow sound through a construction, based on the principles above. this is a thought experiment on-the-fly, so we'll have to see how it goes

part 1 - sound hits a wall...

what happens is that the sound hitting the wall causes vibration in the wall (something called a bending wave is of the most interest here)

the two things that can resist motion due to sound pressure are mass and stiffnes. again, i think it reasonable to discount the latter for this discussion.

at any and all resonant points the amount of vibration that the sound can generate in the wall will be greatly increased

the first layer of the wall resists sound based purely on

[mass minus resonance]

for a single panel - like poured concrete wall or a large un-braced gypsum wall, that's it. mass minus resonance. one comment at the end of this post.

with an air cavity and two leaves in the wall, more happens

we have a normal wall with air cavity and two panels, one on each side.

sound got into the first panel via the above discussion. so what happens to it now?

if we have a normal 2x4 wall some of the sound will go through the air cavity, where absorbing material can eliminate some of it. but much will pass through the studs to the other side of the wall, where it is re-radiated as new sound.

if we have a resilient decoupling system in place, thi will prevent higher frequencies from passing through the studs to the other side, so higher frequencies have to go through the absorbing material. because much more of the sound can be absorbed, we can stop much more sound. :P


comment at posts end: stiffness is again discounted. perhaps i could be allowed to just say it's beyond the scope

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post #11 of 120 Old 10-02-2004, 09:07 PM
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Brian,

It's great when someone can clearly explain this as you have in your posts. It's also important since this topic is brought up again and again on this forum. I hope this post gets tagged so many others who will have the same question get to find and read it before posting Thanks!

-Jason
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post #12 of 120 Old 10-04-2004, 09:45 AM
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Great stuff, Brian.

Perhaps you could address LF specifically, especially related to mass and also decoupling. I am thinking especially of ceilings with decoupled joists (or drywall nailers to be accurate) with multiple drywall layers vs RC attached to the "real" joists, plus drywall.
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post #13 of 120 Old 10-05-2004, 03:50 AM
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hi guys, and thanks.

with respect to low frequencies...

boy. what is perhaps, and perhaps by far, the single most misunderstood aspect of sound isolation is the behavior of springs. to understand the behavior of a simple spring gives one a bit of ability to understand how things work in the low frequency region.

without the concept of a simple spring, discussion about low frequency sound isolation can't realistically occur.

here's a picture:



3 areas of interest

1. a resonant frequency. at and around the resonant point performance is grossly worsened
2. below the resonant frequency, you gain nothing.
3. at ~1.4 times the resonant frequency performance improves(so if the resonant frequency was 100hz, at about 140hz performance starts to improve)

air is a spring.

so the lesson here is simple: introduce a spring into a system to provide "isolation", and you will do so. BUT ONLY ABOVE SOME FREQUENCY. below that frequency it will be at best the same, and at worst FAR worse.

rubber pads are a spring, a spring is a spring, resilient channel is a spring, and an air cavity is a spring.

you CANNOT build a stud row in front of a concrete block wall and say "i've isolated the sound". you WILL have considerably worsened performance over some frequency range.

you CANNOT put a rubber pad under a floor and say "i've isolated it". you WILL worsen performance over some frequency range.

you CANNOT put a resilient mount on a wall and say "i've isolated it". below some frequency you will have isolated nothing.

etc.

and air is a spring. all springs come with a resonance.

where to take this post from here...? i could throw up some data or graphs to illustrate examples of spring-related resonances working their mischief... i'll think about it and come back.

but this reasonably simple concept - the behavior of a simple spring - is critical to understanding how walls behave with respect to sound-stopping at low frequencies.

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post #14 of 120 Old 10-05-2004, 05:25 AM
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brianr820:
Quote:


where to take this post from here...? i could throw up some data or graphs to illustrate examples of spring-related resonances working their mischief... i'll think about it and come back

My favourate example of resonance is the shared wall.

Two people move into a bran new subdivision of two-unit-townhouses. The basement has a shared 6" thick poured concrete wall.

Person 'A' builds a room-in-a-room with about 5" of decoupled airspace between him and his neighbour, and the isolation goes up, and neighbour 'B' is happy. (The room-in-a-room raises the STC by 30 points relative to the bare concrete wall, and the resonance frequency drops by 60hz to a new MSM near 30hz)

Then, neighbour 'B' decides to finish his basement too and sticks 1x1's on his side of the shared concrete wall with a single layer of 1/2" gypsum on that. Suddenly he can hear muffled conversations from 'A's basement. Why, because the 1" space is resonating (i.e. bad amplifier) and the MSM is in the low voice range (circa 135hz).

Person 'A' is now injuncted from enjoying his HT, until person 'A' pays for an acoustician and contractors to come in and remove neighbour 'B's wall, and replace it by screwing it directly to the concrete without the 1x1's and the problem goes away.

Alternatively, and this is one of those astoundingly rare examples, instead of removing the 1x1's and the 1/2" gypsum wall in person 'B's room, the walls could be filled with expanding foam (drill some little holes and inject it in the gap). This will couple the gypsum to the concrete, and should also get rid of the annoying resonance.

An amateur built the Ark. Titanic was built by professionals. Of course Noah took a little advice.
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post #15 of 120 Old 10-05-2004, 01:21 PM
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Quote:
Alternatively, and this is one of those astoundingly rare examples, instead of removing the 1x1's and the 1/2" gypsum wall in person 'B's room, the walls could be filled with expanding foam (drill some little holes and inject it in the gap). This will couple the gypsum to the concrete, and should also get rid of the annoying resonance.

I never thought of that. Thanks Bob!

Always learning
Andre
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post #16 of 120 Old 10-05-2004, 01:27 PM
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Quote:
Originally posted by BasementBob
My favourate example of resonance is the shared wall.

Ha, that's a great example, BB. I have experienced similar situations....
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post #17 of 120 Old 11-30-2004, 02:44 AM
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ok, i've long been meaning to add a bit more to this thread.

ok, so I offered 5 basic principles - mass, mechanical de-coupling, absorption, resonance, and conduction. Taking a look at those one at a time might make these posts a better reference for the DIY guy not seeking professional aid. Perhaps it would be useful to talk about them, and then talk about how application in the real world works in conjuction with the basic principles.

how reality changes/mutes/affects the application of these general principles.

Generally speaking, it's reasonable to think of sound transmission in terms of

1. sound shakes (forces vibration in) the wall inside the noisy room
2. sound/vibration travel by various paths (via air, through the structure) to the other side of the partition
3. new sound is created on the other side via vibration in the second leaf of the partition

for a single panel (like a one-leaf concrete block wall), the sound forces vibration in the panel, which creates new sound on the other side directly.

Mass was the first one that i offered, and a reasonable place to start.

Mass is as straight-forward as can be. The heavier that a partition (wall or floor/ceiling, etc.) is, the harder it is for sound to "shake" it. Just as it's harder for you or I to shake a shopping cart full of sand bags back and forth than it is to shake an empty cart.

In a single panel (no air cavity), mass is one of only two factors that substantially affect performance (the other being resonance).

To gain via mass you have to add alot of it. Something called "Mass Law" shows that if you double the amount of mass in a single-leaf (no air cavity) partition you can expect a 6dB improvement in performance. This table may be helpful:

% increase in mass / increase in performance (dB)
+33% / +2.5dB
+50% / +3.5 dB (add another layer of drywall on one side)
+100% / +6dB (add another layer of drywall both sides)
+150% / +8dB
+200% / +9.5 dB
+300% (4 times heavier) / +12dB
+500% (6 times heavier) / +15.5dB
+900% (10 times heavier) / +20 dB

These rules apply only loosely to normal air-cavity double leaf walls, at times the gains will be much less, and at other times much more. How much extra mass benefits a wall varies greatly with the amount of mechanical de-coupling that exists between the two sides.

so, next let's talk de-coupling.

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post #18 of 120 Old 11-30-2004, 03:35 AM
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de-coupling the two sides of a normal double-leaf (one air cavity) wall is probably the most familiar means of improving sound isolation to many, and the results can be over-whelming.

Resilient channel functions by de-coupling, as do sound clips, and room-within-a-room constructions. the ultimate level of decoupling would be a room floating on springs inside another room.

used in combination with absorbing materials (insulation in the cavity), de-coupling has a great effect. The sound can't go readily through the studs to the other side, so it has to go through the air/insulation, and much of it is absorbed/destroyed/converted to heat. like this



at higher frequencies, de-coupling can be wildly effective at improving sound isolation. the use of a clip (or spring hanger), room within a room, or properly installed channel can yield a greater improvement at many frequencies than almost any feasible amount of added mass.

Also, when de-coupled construction is utilized, the gains from adding mass become far greater than just throwing mass on a normal wall and calling it good. more on that in a minute.

the limitation of decoupling is the air in the cavity. Air acts like a spring, (as do other things, more on that later). At the resonance point the performance of the de-coupled design will fall below that of a single panel of same mass.

so, at the resonance point (and around it), performance will be worse for our trouble - less than coupled, and below it we won't gain anything - below the resonance point the air will simply couple the two sides.

as a general rule, it's a good idea to try to drive the resonance point as low as possible, for as we drive it down in frequency, we push it to frequencies that are less disturbing, and more importantly we drive the frequency where the wall de-couples and isolation goes up dramatically down.

to push the resonance frequency down these basic rules are helpful

1. use as much mass on both sides as you can
2. use as deep an air cavity as you can
3. use insulation in the air cavity

but, unless you have a very deep air cavity, and very heavy walls, you won't realistically be able to de-couple the sub-woofer region. for example, Terry Montlick comments on this general topic in this thread

http://www.avsforum.com/avs-vb/showt...&highlight=DMF

ok, this is a bit of fun, and i'll try to come back to finish later this week.

take care all,

Brian

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post #19 of 120 Old 11-30-2004, 07:55 AM
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Excellent stuff. This should be a sticky.

I am serious...and don't call me Shirley.
Bryan Pape - Lead Acoustician
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post #20 of 120 Old 12-01-2004, 02:50 AM
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hey, thanks bpape

next is absorption, along with de-coupling the most understood of the principles here, perhaps.

at midbass/mid/higher frequencies, absorbing materials function as exactly that - they absorb some or most of the sound that is traveling through the partition via airborne paths.

that's an important point - on single wood stud walls, the effect of insulation is less than on de-coupled walls, often drastically less.

in the subwoofer region, however, it is considerably difficult for absorbing material to actually "absorb" the sound. imagine placing a layer of R-19 fiberglass insulation in front of your main/center speakers... it would probably mute the sound considerably, wiping out alot of the higher frequencies. but run the same test by placing it in front of your subwoofer, would you expect that this would yield no bass in the room?

but insulation can have a different type of positive effect on low frequency performance, it can lower the frequency of resonance. And this is a good thing.

the most common question about absorbing materials is "which one is best". at mid through higher frequencies, lab tests have made clear that mineral fiber and cellulose type insulations tend to outperform regular fiberglass. but at lower frequencies, the advantages of these materials are not as clear and a very solid case could be made in favor of common fluffy fiberglass, i.e., the pink stuff.

some generally good guidelines are

1. the most important thing is to use some insulation or other
2. it is 110% ok to use normal fiberglass - in the real world the differences between types will be more modest than in labs
3. thicker beats denser or more exotic


and that's about it. use something, as thick as your budget will allow.

DE has correlated lack of insulation in wall cavities with in-room sound colorations, another consideration.

Understanding sound isolation
That link may be helpful
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post #21 of 120 Old 12-19-2004, 02:53 PM
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ok, on to resonance.

the short answers:

COINCIDENCEthere are two major resonance problems in any wall. The most "famous" is called "coincidence" and occurs at fairly high frequencies in drywall constructions. This has been associated with problems in various studies, but... by and large this isn't the biggest worry when it comes to walls.

To deal with coincidence: thinner materials of any given type will exhibit this problem higher in frequency. Bonding layers with something rigid like liquid nails will lower the frequency at which the coincidence dip occurs.

LOW FREQUENCY RESONANCE ISSUES the second resonance problem will occur at lower frequencies, and will usually represent the weakest point of any wall. this resonance you SHOULD try to deal with. There are two ways

step one for coping with low frequency resonance - drive it down in frequency the first is to move the resonance to a lower frequency. this can be accomplished by by

1. use a de-coupled design
2. use as much mass as possible on both sides of the wall - leaving one side very light while the other is heavy won't do as well.
3. use insulation in the cavity (a must in any case)

You cannot realistically drive the resonance below subwoofer territory. That would require perhaps 5-10 layers of drywall on both sides of a normal depth "room within a room" with RSIC clips to keep stiffness down.

common 2x4 wallon a single stud wall, you cannot drive resonance down with mass, only wider stud spacing can aid your cause here, and you can never drive it down very low. Only raising the damping of the panels and the rest of the structure can help at all with that problem.


the other way to cope with low frequency resonances is to damp them. insulation will have some damping effect, (and generally common fiberglass is fine in this regard as well).

the ability of damping materials to affect resonance behavior varies a bit from construction to construction, but that's the next installment here.

i'll add the tech babble later, but the key resonance problem in common walls is at low frequencies, and the following guidelines are the formula for success:

for de-coupled walls:

1. use as much mass as possible on both sides
2. use as deep of an air cavity as possible
3. use insulation, fluffy fiberglass is fine
4. use some means of raising the mechanical damping of the structure. only constrained-layer (sandwich) damping will have any meaningful effect in this regard

for common 2x4 walls:

1. raise the mechanical damping of the structure. only constrained-layer (sandwich) damping will have any effect at all in this regard

Understanding sound isolation
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Ceiling tiles are designed not just for absorption to improve acoustics inside the room, but for isolation to prevent voices from carrying through the ceiling from one cubicle or office to another. The intent is not to make the rooms silent, but to prevent voices from being heard intelligibly in the other room.

Ceiling tiles are rated for Ceiling Attenuation Class (CAC). The higher the number, the more isolation they provide. I'm not sure how CAC relates to STC, but ceiling tiles can range in CAC values from .1 to .4. There is also an additional coating for the backs of the tiles that is supposed to improve it a little more.

That isolation rating is no doubt determined with the tiles installed in typical fashion - suspended below the real ceiling with a large air gap in between. All bets are off if you just staple it to the bottom of the floor.
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This is an amazing thread. Brian, keep contributing, I'd love to read more. I'm going to re-read this thread a few times, to make sure I understand everything.

I think it's safe to say the following:

2x4 walls - standard
Staggered 2x4 walls (2x6 total thickness) - good
two separate 2x4 walls with a space between them - better

Would it be better still to use two separate walls with the studs staggered? If the studs of both walls line up, the space between them is very narrow. With the studs staggered, you have the space + stud thickness between them. And, I never thought of 24" OC stud spacing. Would that be worth while in double wall too? I suppose a double wall with 24" OC staggered stud spacing wouldn't need RSIC clips, would it? Or multiple layers of drywall?

Of course, all examples are insulated.

I have started this thread: http://www.avsforum.com/avs-vb/showt...hreadid=489669
which outlines my quest for a dedicated room. I have received numerous tips from bpape on how to handle some of the sound issues. This thread really helps explain why he made the recommendations he has.

Craig

My Media Room Construction thread. Work began 2/15/05, finished 7.1 install 6/2005. Sold house 7/2007.
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hey Craig, i'll check out your thread, should be a fun read.

i think your idea about not aligning the studs has merit. Another gent brought that up in a thread a while ago, and i think the concept is just great - no wood-small air-wood path, everything has to go through absorbing material.

The flip of that coin, though, is that this will improve primarily mid/high frequencies - and that type of wall is so fantastically good at those frequencies already that there is essentially a 99% chance that performance is being limited by "flanking" noise - being limited by noise going through doors, outlets, floors, ventilation systems, etc.

so you'd probably improve things in a lab test, but you would probably not gain much in the real world - because no wall/floor, etc. can perform better than the weakest link, and in the case of a double-stud wall the weakest link is almost sure to be something other than the wall at mid/high frequencies.

where that wall type will probably be the weakest link is in the subwoofer region around the resonance point. so lowering the resonance (deeper air cavity, more mass on BOTH sides) or damping the resonance are the best improvements that you can make.

or both of those.

Brian

Understanding sound isolation
That link may be helpful
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i should clarify too:

two separate 2x4 frames with drywall on one side of each is great. two separate 2x4 walls with drywall on both sides and a small, sealed air gap between them is not.

Great:

drywall/stud+insulation/air gap / stud+insulation /drywall

not great:

drywall/stud+insulation/drywall / small air gap / drywall/stud+insulation/drywall

the first makes 1 large air cavity, the second makes 3 air cavities, one very small. It's always wisest to make one big air cavity -vs- many small ones. The reason is that with each air gap comes a resonance, and you don't really gain much by the two gaps on the 2x4 walls because of the direct connection between the two drywall layers. Also, that central gap would be small, and might yield a high frequency resonance of the worst sort.

If you need to frame in front of a normal 2x4 wall and can't/won't rip out the existing drywall on one side these are great steps for success:

1. make the air gap in front of the existing 2x4 wall as large as possible
2. damp the 2x4 wall before putting the new framework up


now i don't gather that this is what you had in mind, Craig, just commenting in case anyone misinterpreted the discussion above.


24" stud spacing can be advantageous at times. On conventional 2x4 walls, this will lower the systems resonance a bit, but that does not amount to an actual meaningful improvement - the problem remains just as severe if you don't controll it.

With a double-stud construction, the reduced stiffness of 24" spacing -vs- 16" spacing may lead to a lower system resonance in some cases, which can be an advantage...

Brian

Understanding sound isolation
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Brian,

Thanks for the added information. For the new wall, I was thinking about it in terms of the one large air cavity. No drywall on the inside.

But, I'm glad you pointed that out. i have the existing walls to deal with, and I'm not sure how I am going to deal with them. I don't necessarily want to clutter up with thread with specifics on my project, so feel free to comment in my thread. There are many different space considerations in that thread, but I think the space my wife and I are biased in using is the 2nd floor Activity Room--so if you want to contribute there, that's probably the room worth discussing.

Craig

My Media Room Construction thread. Work began 2/15/05, finished 7.1 install 6/2005. Sold house 7/2007.
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Infinite Mass as a good way to sound proof a room.


Great!! I just finished the calculations for recreating a black hole in my living room and I look forward to being finally able to soundpro.....
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ok, i revised a couple of the posts above, i think they are clearer.

the last principle of sound isolation is condution.

conduction plays a direct role in the performance of the common single-stud wall. the vibration in the drywall panels reaches the studs, transfers through, and performance is inhibited as a result. if the drywall could not conduct vibration, a whole lot less could make it to the other side... simple enough.

the other area where the "conduction" of mechanical vibration causes harm with respect to sound isolation is flanking noise. Some sound enters a floor, and transfers via the structure to the next room. The same can happen with ceilings, walls, and any other structure that is mechanically connected to another room.

to eliminate or mitigate the conduction of energy from point A to point B through all these myriad paths, you can do one or more of these:

de-coupling the wall/ceiling/floor panels1. you can de-couple the walls, floors, ceilings, etc. resilient channel, sound clips, a "room within a room", etc.

The fundamental limitation of this is at the low frequency mass-spring resonance. At the resonance point and below, all forms of de-coupling fail, there is no exception. But at mid/higher frequencies, this can be very effective (or into the midbass).

start cutting things 2. you can cut things. cut mechanical paths. This isn't always practical or even possible. For example, cutting the sub-floor will not have much effect if the joists are perpendicular to the cut line. The path is not broken.

in general, the more complex a path is, the less problematic it becomes. A subfloor that is continuous is a big culprit. A continuous concrete slab might cause problems, a continuous drywall surface going up, or continuous studs going up from a lower level, and so forth.


so, de-coupling will fail around the low-frequency resonance and below, but is tremendously helpful at higher frequencies. breaking mechanical continuity between point A and point B is helpful, but not always practical or possible. do what you can if you really want to go the extra mile.

damping the other thing that can deal with conduction is mechanical damping. Raise the damping of a floor, wall, structure, and it will dissipate the energy as it tries to move from piont A to point B.

constrained layer damping is your best bet, but paint-on damping materials can work wonders on thinner, more flexible things like ductwork.

the capacity of damping to dissipate energy as it moves from point A to point B depends on how much damping is present. the more the merrier, so pick a damping material wisely. damping materials are available as pre-damped sheets and liquid "glues" from various sources.


and that's the low-down on the 5 basic things that can make or break your isolation project.

Understanding sound isolation
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ok, let me try to finish this guy up by summing everything into some basic rules for getting good sound isolation

rule #1: The rule of the boat - your room should float when the doors are closed. the most important factor of sound isolation is sealing your room. If you are on a budget, and have 200 bucks to spend, spend it on caulk and dealing with any potential ductwork paths. This will ensure that your wall isn't a horrid failure.

rule #2: How to get a good seal. A good method for sealing a room (not a conservative method, mind you) is to put 1-2 generous beads of caulk between the floor, other strucures, and the framing members (bottom plate and/or top plate if applicable). Then lay a bead of caulk around the perimeter (top/bottom, and sides on the end sheets of drywall) before putting in each sheet of drywall. Then go as wild as you wish with additional beads when you are done. Just make sure that your beads don't space the layers if you are using double drywall.

rule #3 - the rule of resonance. If you build a decoupled wall (staggered stud, double stud, room within a room, resilient channel, sound clips (like RSIC or IsoMAX), follow these 3 rules:

1. put as much mass on both sides of the wall as possible
2. make the wall as deep as possible
3. fill the entire cavity with normal fluffy fiberglass

rule #4 - the rule of insulation. use insulation, any type will do. thicker beats denser.

rule #5 - the rule of stud spacing. Use 24" or 16" on-center studs, wider probably isn't legal, closer isn't helpful, either one or the other of those will not have a paramount impact on any common wall type, although things will change, you coudln't definitively say one was better, one was worse.

rule #6 - the rule of gluing. If gluing a wall, things to the studs, or sheets together, and if available to your budget, flexible glues are preferable. Dennis Erskine of this forums recommends gluing with something to prevent rattles that can occur from high volume subs.

rule #7 - the triple leaf rule. NEVER, EVER, EVER make a wall with more than one air cavity. if, in traveling from front to back, sound has to go through something solid, then an air cavity, then something solid, and then another air cavity... there is basically a 100% chance that the results are not ideal. USE ONE AIR CAVITY, AS BIG AS POSSIBLE

rule #8 - the air cavity rule. NEVER make a de-coupled wall with a small air sapce. 4" should be considered the absolute minimum (that's RC + a 2x4 stud)

rule #9 - the decoupling point rule. Calculate the depth of your air cavity by taking 1.4*thickness of insulation + thickness of dead air space. convert to millimeters. take [(mass 1 + mass 2)/(mass1*mass2*depth)]^0.5*1900. When you get that figure, multiply by 1.4 and you will have an estimate of the frequency at which your wall de-couples. Remember, de-coupling will make it worse below that point unless the wall is very heavily damped.

rule #10 - the rule of "what's best". Some sub-rules here

10a- what type of resilient mount? sound clips appear, based on the preponderance of available data, to allow (perhaps?) lower de-coupling poitns for a given amount of air space than RC. The sound clips available on the market LARGELY increase the volume of air space in the wall, which only aids in this.

10b- what type of ceiling? i do not think that (assuming damping materials are not used) a sound clip ceiling will perform worse than a room within a room, whch might save soem construction trauma.

10c- modifying LF resonant behavior. (assuming no damping material is used). Using sound clips, which are generally spaced broadly and a bit randomly, will allow essentially any given wall type (including stagg studs or double studs) to more closely approximate the mass-air-mass resonance theory, and it is certainly feasible that you will improve performance by adding them. The effect would be to reduce stiffness, basically.

10d - modifying LF resonant behavior. (Assuming damping material is used). strutural damping has been shown in various lab and field studies to have a nice to paramount impact on low-frequency performance, and if your budget will allow, use some form of structural damping.

10e - how many layers? if using a decoupled wall, you should never use just one layer of drywall on both sides. You should use at least 2 layers on both sides, ideally.

10f - the rule of the floor. whatever you do to your ceiling, if you leave your floor a single layer of wood, your results will be alot less than they could be.

Rule #11 - the rule of the common wood stud wall. You will never get a high degree of isolation on a wood stud wall without adding alot of structural damping.

rule #12 - the rule of the aquarium. You have to treat all the sides of your room for best results. Floor, ceiling, and walls. Think of the sound in your theater as the water in an aquarium. Put one hole in the aquarium, anywhere, and it doesn't matter where, the water will get on the floor. So it is with sound, a weak wall that is facing, say, the garage can allow sound to enter the structure of your home and cause problems despite your nice efforst elsewhere. this is called flanking noise, thanks to DE for the analogy.

rule #13 - the rules of doors. some sub-rules

13a - communicating doors (two doors must be opened to enter the theater) will outperform any conceivable single door

13b - an exterior steel door is the best starting point for DIY work

13c - adding mass (or damping while you are in the process of adding mass) is never a bad idea

13d - if using communicating doors, put some form of absorbing material in the space between

13e - the most important aspect of door performance are the seals


rule #14 - the rule of ductwork. ductwork that is exposed to the room should be lined, bent (so the sound has to travel around a complex path), and the distance to the next opening should be as long as possible. shieling ductwork with soffits is wise

rule #15 - the rule of outlets. don't line them up back to back from one side of the wall to the other, same for lights or other openings where performance might be weaker.

rule #16 - the rule of the weak link. your overall performance will never be better than the weakest link. Great wall, but bad door, lots of noise in the next room, etc.

rule #17 - the rule of reason. not every situation needs the ultimate level of isolation, and while too little can ruin the fun of your theater, it's possible that you don't need limitless levels.

rule #18 - the rule of this post. it's time to hit submit and check later to see if it is all in good form

Brian

Understanding sound isolation
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ok, rule #19 - don't bother using 2x6 studs if you are not going to decouple or damp. There is, in essence, no advantage to deeper air space if you just screw normal drywall directly to the studs. the reason is that deeper air space doesn't lower resonance much in this type of wall, because the wall's resonances are wholly mechanical. The other reason is that the thicker insulation won't help much (or at all) because the sound can bypass it by traveling through the studs.

rule #20 - odn't bother with 2x6 studs at all now that i think about it. use staggered studs instead.

rule #21 - if you have stud or joist spacing that is very close (say <12"), you really are forced to use resilient decoupling to get good isolation above about 100hz.

rule #22 - there are no mystic space typhoons in sound isolation. Little things like GG or rubber pucks or cool engineered strips between studs can help, but they will not in and of themselves make a great stand-alone isolation scheme (unless you don't need that much, which leads to...). strips, goop, and beads of caulk aren't RSIC clips or RC or GG between layers of drywall. Cool add-ons or boosts (Especially in the speech range), but in and of themselves that will not yield what you seek, if you seek alot of quiet.

rule #23 - every situation is different, and every perception is different.

rule #24 - if a plane crashes about 50 yards from your lab, it doesn't matter if you just put Green Glue on all the walls, it's gonna be loud (happened the other day)

Understanding sound isolation
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