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

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.

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

Thanks guys - plenty of other crazy ideas to come, for sure!

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

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

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

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

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.

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

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

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.

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:

http://entercaraudio.com/asimplespring.GIF

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.

brianr820:
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.

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

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....

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.

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

http://audioalloy.com/resil2.gif

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/showthread.php?s=&threadid=464702&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

Excellent stuff. This should be a sticky.

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.

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

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.

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/showthread.php?s=&threadid=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

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

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

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

Infinite Mass as a good way to sound proof a room. :D


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.....

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.

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

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)

ok, folks, here are some numbers for noise reduction from the biggest, baddest study on wall behavior ever done, and my all-time single favorite piece of lab work from any discipline, IR-761

see here: http://irc.nrc-cnrc.gc.ca/fulltext/ir761/

they were calculated according to the noise curve that i present below, which was based directly upon the work of Tennekes (which is presented in 1 octaves, and i expanded to 1/3 octaves) which i referenced in the Green Glue thread. I extended the low frequencies to be -3dB at 25hz (the original curve rolled off below 63hz, and was for disco/club/bar music). And i bumped down the midbass levels just a dB or two to simulate the boost expected in most hteaters around the subwoofer region. it's gotta be noted that results will change a little bit with different full-band curves, but this is a pretty good starting point.

i 1000% welcome all alternate comments and thoughts

all noise reduction values given as dBA reduction, results would vary for dBB or equal loudness (systems that take into account human hearing and compensate for it), and systems with good low freq performance would perform even better.

2x4 wall, no insulation, single 5/8" drywall: 22 dBA reduction
2x4 wall, same, add fiberglass insulation: 23 dBA reduction
2x4 wall, min fiber, 3 total drywall layers: 25 dBA reduction
2x4 wall, fiberglass, 24" stud spacing, single 5/8": 24 dBA reduction
2x4 wall, no insulation, 12" stud spacing, single 5/8":

note: estimate adding 4-5 dBA per DOUBLING of layers, so 27-29 for double drywall both sides, ESTIMATED, assumes insulation and good seal, etc.


below all with fiberglass batts, results vary from selection to selection
steel stud wall, 90mm studs, single 5/8": 26 dBA reduction
steel stud wall, 90mm studs, 3 layers 5/8": 30 dBA reduction
steel stud wall, 90mm studs, 4 layers 5/8": 30 dBA reduction

note: DON'T read that to think that adding the 4th layer didn't help, it's just one of those things


note: all the below have full-cavity fill with fiberglass
resilient channel wall, 1 layer 5/8 each side: 26 dBA reduction
RC wall, 1 layer on studs, 2 layers on channel: 28.5 dBA reduction
RC wall, 2 layers on studs, 1 layer on channel: 29.5 dBA reduction
RC wall, 2 layers both sides: 31.5 dBA reduction


soundboard wall, 1 layer soundboard, 1 layer 5/8, w/fiberglass: 28.5 dBA
as above, but 2 layers 5/8 each side, 6 total layers: 31dBA


double-stud wall, 1 layer 5/8" each side, 7" fiberglass: 32.5 dBA
double stud wall, as above, add 2nd layer 5/8 to one side: 37 dBA
doub stud, as above, but double 5/8" both sides: 40 dBA
double studs, 3 total layers, but one in the center: 32 dBA

note that here, the more de-coupled wall (who's resonance is below the 50hz cutoff of this analysis) gives larger boosts for more mass, 8dBA instead of 4-5. Also note that putting one layer (soundboard in this case) in the wrong place causes a very large loss of performance (5dB). don't split air cavities.


stagg stud, single 5/8 both sides, fiberglass: 29.5
stagg stud, as above, add second layer of 5/8 one side: 33.5
stagg, as above, double 5/8" on both sides: 35.5

note that the stagg stud walls in this study perform bettre than RC or steel, despite having lower maximum STC's. darn that 125hz cut-off


stagg stud, double 5/8" boht sides, RC, 3.5" glass: 36.5 (note that this, with deeper air cavity, is alot higher than the nromal RC wall. deeper air cavity plus additional decoupling and flexibility due to the staggered studs)


concrete block, 190mm: 40dBA reduction

note: the block has the worst voice-reduction performance of any of the above save probably the single stud 2x4 walls. it's worth noting that the above values reflect lots of bass coming through, this value reflects voice frequencies, and may or may not be more annoying

Concrete block + 3" steel studs, fiberglass, and 5/8" drywall: 41 dBA reduction
concrete block + 2" steel studs, fbrglss, 5/8": 32dBA reduction

note that the resonance of the smallish cavity caused a horrid loss of performance. follow the 3 rules: mass, depth, insulation


so basically, concrete is super-duper-heavy, and so gives good airborne isolation in the sub region, but resonance problems make it mediocre at best over much of the speech range. stud walls with their decoupling can give supremo mid/high frequency performance (and midbass), but limited mass and resonance problems give lower sub-region performance. that's about it in a nutshell.


anyway, might be helpful. i won't put any figures for commercial product tests here, i'll just mention that the (Diff lab) RSIC performed better than RC by quite a bit ONCE YOU GOT 2 LAYERS ON BOTH SIDES. load those thing sup, boys, they make a nice wall :)


one more edit: i picked the best examples (highest STC, eyeball) of steel, RC, staggered, etc. there weren't that many normal 2x4 walls to cherry-pick. one of the steel stud single drywall cases scored ~20, some of the staggered stud examples would have scored lower to much lower too, same for RC, etc. there were only 4 soundboard results,i just took the two they had for those configurations.

finally, results will vary depending on the noise curve you use to analyze the stuff as well, keep that in mind. Also, while these single-numbers are a whole heckuva lot better than STC, they are still just a single number and DO NOT NECESSARILY MEAN THAT'S HOW ANNOYED YOU WILL BE, although you may reasonably anticipate large changes to mean something. Concrete walls have been associated with flanking nosie problems on this forum by various experts.

below 50hz the performance will be

2x4 wall >= RC or steel stud >stagg stud > double stud

for a given amount of mass.

Great stuff Brian. Thanks!

Andre

Brian. Your posts have been a great help to me in my quest for the ultimate home multi-tainment complex. This information is much appreciated to say the least!

Originally posted by ebrigham
Brian. Your posts have been a great help to me in my quest for the ultimate home multi-tainment complex. This information is much appreciated to say the least!

hey, i'm very glad. i always hope that whatever i lop onto a board somewhere is useful, but you never know, i guess.

i was going to re-visit this thread this evening and talk about sub-region sound isolation a bit.

subwoofer region sound isolation

ok, this post is intended to specifically address subwoofer region sound isolation. The idea is to basically outline the factors that play a role here, and sort of put to rest a few (not always irrational) misconceptions about sound isolation in general, and the low-end in particular.


There are basically 2 factors, and in more aggressive constructions a third, that affect sound isolation on the low end. There are additional factors, but for the most part they can be simply disregarded from the design/planning process.


factor #1 - MASS The first factor is mass. It works as simply as this: heavier is better. The potential sound-stoppage due to mass can be calculated via the "Mass Law":

20*log(mass*frequency)-48, where mass is in kg/m^2

or, if you want a more specific formula:

log(1+(pi*M*F/415*cos(A/57.3))^2)*10

The key points are this: doubling the mass buys you 6dB extra performance, and halving the frequency costs you 6dB. in other words, if a wall had (via mass-only) the potential to stop 25 dB of sound at 80hz, it would have 19dB of potential (via mass-only) at 40hz. doubling the weight of the wall would raise that 25dB @80hz to 31dB, etc.

for easy reference, you can calculate the lbs/square foot by taking the weight of one sheet and dividing by 32 (for a 4' x 8' sheet). that will give you lbs/ft^2. multiply that by 4.88 to convert to kg/m^2



factor #2 - resonance. The second factor is resonance. Resonance is sort of like anti-mass. the more mass the higher the performance.... mass adds to the performance of the wall. resonance deletes from the performance of the wall.

As a general rule, resonance beats mass. even an extremely heavy wall, like concrete block or whatever, can have mediocre or poor performance at & around a strong resonance point.


factor #3 - spring isolation in a decoupled wall. Before we touch on this, let's define a de-coupled wall: a wall where the two "leaves", or sides, are not mechanically connected in any rigid way. resilient channel, sound clips, double-stud walls, (and sort of staggered stud walls & steel stud walls qualify here.) any single wood-stud wall does not qualify here, period.

The next important thing is to define a special type of resonance called the mass-spring resonance. You may have heard someone mention a "mass-air-mass" resonance, this is a grossly simplified calculation of the location of the mass-spring resonance.

This mass-spring resonance point is HUGELY important because it determines where de-coupling begins. Remember, at the resonance point, performance will be bad, and below it it won't be one blooming bit better than with no de-coupling.

but starting maybe 1/2 to 2/3 octave above it, the wall has the POTENTIAL to outperform a non-decoupled wall. Mechanical resonance issues and various factors not generally considered in the calculations used by most acousticians will generally keep the actual frequency where the wall starts to outperform higher than predicted, sometimes alot higher.




so those are the three basic factors that you can plan for in designing your theater (or studio or whatever).

next, some myths & clarifications, after that some peeks at how these 3 factors interact.

first, a couple of good-fortunes when it comes to stopping low-frequency sound. For the most part, everything stacks up to work against us when trying to stop sub-region sound. Mass law predicts that walls get worse the lower in frequency they go. The worst resonances in almost any wall occur at low frequencies, and it's lots of fun to have LOUD low frequency...

which all adds up to a mess in the sub region.



but, one slice of good fortune that we have at low frequencies is that the limited size of realistic partitions gives us a boost. typically, walls tested in labs are somewhere in the vicinity of 8x8 to 9x14, which is enough to cause them to be several dB above the mass-law prediction in the sub region. If you have something even smaller, like a window, the effect can be even larger. Small "weak spots", like a window or door, will get a bit of benefit at low freq's from their modest size.

there is no point in taking this into account during your design phase, however, because your wall size will generally be determined by anything but your isolation needs. :)


the second slice of good fortune that we have at low frequencies is that all the million-and-one ways that sound can get from point A to point B (flanking, through the smallest of cracks, through outlets, yada and so on) don't generally tend to affect low frequency noise as much as mid/high.

air gaps will have LESS effect on sub-region performance than higher frequency performance. generally, flanking noise WON'T be the limiting factor in the sub-region sound-stopping of your wall, and things like outlets may simply have no effect below a few hundred hz. THAT DOESN'T MEAN YOU SHOULDN'T CAULK YOUR WALL, CAULK CAULK AND CAULK AGAIN

in general, sub-region sound goes through the wall more directly (because, in general, it's not that hard for sub-region sound to go through a wall directly), which is kind of comforting.



now some misconceptions that come to mind as i sit here and yap:

stiffness. it seems that everywhere a fella turns someone is touting stiffness (or the lack thereof) as beneficial at low frequencies. And the term "stiff" gets grossly abused. Folks, stiffness is how much something bends when you push on it, ok? adding a second layer of drywall w/o rigid adhesive will have only a fractional stiffening effect on a wall. You often here folks talking about how "making stiff walls makes modal problems in the room worse". what they mean is "making massive walls", not "making stiff walls".

stiffer is better: then there is the conception that bracing a wall, and making elaborate cross-check pattersn of reinforcement between the studs will have a supremely beneficial effect on low frequency sound-stoppage. "the walls are so stiff that they actually resist bending from these low frequency sound waves"

makes sense, doesn't it? but it ain't so. I am aware of no test data demonstrating that stiffer walls do better at LF, and much to the contrary (all things at least somewhat equal). Attempts to build a wall so stiff as to capitolize on this in our lab failed as well.

There was considerable discussion on this topic on another forum not long ago, i'll try to find a link. stiffness CAN be a factor in low-frequency sound stopping, but NOT ON REALISTICALLY SIZED REALISTIC WALL ASSEMBLIES. even on a 9' x 9' slab of 12" concrete, stiffness appears to only be a factor below ~30hz. and your wall is bigger and one heckuva lot less rigid than that, hence FORGET ABOUT STIFFENING YOUR WALLS TO IMPROVE LOW FREQ TL



softer is better Then there is the 180 degree opposite camp. the view that "limp" materials have supreme low frequency sound stopping because "their limpness allows them to move with the sound waves, absorbing it"

if i had a dollar for every time i've heard that logic i'd at elast be able to get all liquored up and barf somewhere that i'm not supposed to... (hee hee)

but what you would find if you measured a wall made of pure asphalt or whatever, is that it just followed mass law. or mass + size effects, or mass + size effects + quirks of any given measuring facility.

for goodness sake, folks, the ability to stop sound is 110,000% related to RESISTING vibration from sound, not allowing it. and it's mass, NOT STIFFNESS, that provides this resistance. put an accelerometer on a slab of drywall and a same-weight slab of MLV or asphalt at 100hz (or wherever) and what will you find? the stof stuff vibrates more, right?

nope, it would be the same, mass-controlled. (at least if the hard stuff isn't resonating at that frequency)

the real advantage of those materials is that they are non-resonant (too floppy to be resonant at frequencies of interest). Use limp materials, just don't say that the mystic limpness causes enormous absorption at bass frequencies or i might have to fast for a month in protest.



#3 - insulation & low frequencies. this insulation is better, especially at low frequencies. blah blah blah blah blah

try this: put a batt of the insulation that is going to go into your wall in front of your main speakers or surrounds. it'll totally mess up the sound and wipe out most of it.

put it in front of your sub. it won't do spit, or not much of anything, anyhoo



#4 - obsessive compulsive decoupling saves the performance at the low end things like a bead of caulk between the studs and the plate, the plate and the slab, rubber gaskets here. heck, i've even seen people post about using rubber shanks of some kind around screws in an effort to "keep the bass from going through the screws".

eek! none of these kinds of things (although they can be fun for people to do, satisfaction of going all the way) are going to do anything at 50hz. not one steenking thing.


#5 - the mystic properties of viscoelastic materials i swear that if i read one more time (sometimes from people that should really, really know better) that VE materials "totally decouple the layers" or "the damping resists motion in the wall" or any of these odd sort of fantasy-world dream-schemes that pepole propagate about VE materials i'm going to fast for a second month, eating only Scooby Doo brand Fizzy childrens vitamins and milk.

a VE material isn't some mystically powerful anti-vibration layer, it's a damping material. damping is good, mysticism causes ulcers.



see, low-end performance of any given wall is alot more to do with big things, big, well-documented things, big pre-plannable things, big-obvious things, than it is with oddball theories and wild decoupling schemes involving rubber screws.

ok, so the next part is how all those factors interact. but that'll have to wait for the sunshine, folks, see you tomorrow.

Brian,
Thank you for the considerable effort as well as information conveyed to educate the "masses". Sounds as if there are a few myths that deserve a "BUSTED" stamp on them. (puns not intended)

Good stuff.

Best regards
Don

hey Don, that's an awesome signature line!


folks: i don't ever mean to say that putting limp mass in cavities is bad. i simply mean to say that it's 110% known that putting stiff mass in a cavity is very bad, and splitting air cavities isn't generally a good way to get to your desired point B WRT isolation.

limp materials have some obvious advantages: they, by themselves, are not resonant, and they have extremely low sound-velocity due to the flexibility/lack of stiffness. a 2x4 wall made out of (equal mass) of MLV only with no drywall would perform supremely at all mid/high frequencies. at some low frequency, a resonance would occur and due to the lack of stiffness of the MLV, this would perhaps be primarily an air-spring resonance. presumably the resonance would be damped nicely, and below that the two walls would perform according to mass (so they'd be basically equal, the drywall assembly and the all-MLV assembly).


WRT the stiffness issue in the other extreme, i got some data on a wall of solid 12" concrete, and it no apparent rise in performance that could be attributed to stiffness down to at least 31.5hz.

To put some perspective to that, 12" of solid concrete is approximately as stiff as nearly 2 feet of rigidly bonded drywall, and about 80,000 times stiffer than a sheet of 1/2" drywall.

fugget about it!

and to add some more (and i 110% realize this is counter-intuitive, and i absolutely beg you to not take an attitude of "well if it won't hurt the low end that much, i'll just skip caulking" due to this post)

but another test on a cocnrete wall that had a seal problem in the lab shows

-no effect below about 80hz
-only slight effect below about 250hz
-enormous (15+ dB) effect at higher frequencies
-15 point loss of STC

strange, but true. the behavior of seals is complicated, involves resonance, and i guess we should just be thankful that this doesn't manifest itself most severely at the low end (because most factors involved do their worst in the sub region)

Excellent information Brian, thanks for taking the time.

Just so I understand: Take a basement with 8" poured concrete walls - all below grade save for the top 12" or so. Not a lot of vibration in that wall.

Then, build a 2X4 wall 2 inches in front of the concrete wall. Said wall is 8' high and 15' long. Insulate between the studs with FG, glue and screw 1/2 drywall to the studs. Said wall is going to vibrate considerably - especially at LF, correct?

Next, take the same two walls, except this time snugly fit and glue blocks between each stud and the concrete wall, roughly 4' off the floor. Would this not move the vibration UP to a higher frequency?

Ok-ok - it IS my basement. If I do a very basic test, hit one of the un-blocked studs with my hand or my hammer the thing shakes like a Chihuahua in freezer (don't ask). Hit a stud that IS braced and, well, not much there.

Seems logical - perhaps just to me, that such an assembly would help reduce LF vibrations.

What am I missing?

Dan

Originally posted by ss3964spd
[B]Excellent information Brian, thanks for taking the time.

Just so I understand: Take a basement with 8" poured concrete walls - all below grade save for the top 12" or so. Not a lot of vibration in that wall.

well, at frequencies not involving resonance or "coincidence" (a sort of resonance-ish phenomenon), the raw mass of the concrete will resist vibration.

Then, build a 2X4 wall 2 inches in front of the concrete wall. Said wall is 8' high and 15' long. Insulate between the studs with FG, glue and screw 1/2 drywall to the studs. Said wall is going to vibrate considerably - especially at LF, correct?

Next, take the same two walls, except this time snugly fit and glue blocks between each stud and the concrete wall, roughly 4' off the floor. Would this not move the vibration UP to a higher frequency?

well, assuming your studs are 16" OC, bonding the studs to the block will convert teh system from a traditional mass-spring resonance (determined by the air cavity dpeth & the mass of the stud wall) to the type of resonance exhibited by a 2x4 wall. The frequency of the resonance will move up, yes, which in general isn't necessarily good. (if a wall bleeds wild sound at low frequencies it TENDS to be less annoying that if it bleeds wild sound at higher frequencies. It is important to note that for home theater, with alot of bass via our dear Velodynes, this general paradigm is questionable, and maybe higher frequency resonances are preferable). if you can get a resonance to <40hz it probably holds that this is by far and away preferable as <40hz becomes increasingly inaudible.

however, you'll also ruin the decoupling (which will yield magnanimous gains at mid/high frequencies) aspect of putting the drywall in front of the concrete.

Concrete isn't innately good at midrange frequencies due to coincidence (a phenomenon involving the speed of sound in air and the concrete)

Ok-ok - it IS my basement. If I do a very basic test, hit one of the un-blocked studs with my hand or my hammer the thing shakes like a Chihuahua in freezer (don't ask). Hit a stud that IS braced and, well, not much there.

Seems logical - perhaps just to me, that such an assembly would help reduce LF vibrations.

not illogical at all, but i wouldn't recommend putting the blocks in place. instead, put another layer of drywall on the studs and drive the resonance down as much as you can.

the phenomenon that you are observing on raw studs is related to a purely mechanical resonance of the stud itself, and the bracing has a direct, systemmatic, predictable result of pushing the resonance frequency up, and reducing the magnitude of the vibration. but that won't necessarily tick back to gains WRT airborne sound.

I'm going to try to finish out my low-frequency blather, and then i'll enter some guesstimates of the impact on airborne sound of those pegs.

Brian,

Honestly, I have yet to read all of the "epics" you've wrote above, but the printer is working on it....

I have a similar situation with concrete walls in my basement that I'll build stud walls over for the HT. It appears that it's preferred to leave a couple inches between the concrete and studs. Is this correct?

Thanks!
Jerrod

yes, i recommend these rules when putting things in front of concrete block:

1. as deep an air cavity as possible
2. as much mass on the studs as possible (i.e., multiple drywall layers)
3. use insulation

the deep air cavity is moot if you couple the studs to the concrete

Originally posted by jerrodshook
It appears that it's preferred to leave a couple inches between the concrete and studs. Is this correct?

If any of these concrete walls are exterior walls, you would want to keep wood off these wallls for moisture problems as well. Many people use a treated 2x4 for the floor for the same reason.

I'm having a terrible time getting head around this - perhaps I'm noodling with it too much. Please be kind if I'm the only one who doesn't get it!

Let me state that I do get the concept of a large air space - it takes more movement of the wall surface to compress a large volume of air. I also understand that the more mass you can add to the wall the less it's going to vibrate (or it will take more energy hitting it to make it vibrate).

From what I've read here I think I've come to understand that controlling the LF vibes is the most difficult task. A wall assembly that moves back and forth actually reproduces frequencies. Therefore (in Dan's small mind anyway) if one can prevent a wall assembly from moving back and forth by making the wall as rigid as possible I would have guessed that would cut the LF vibs the wall reproduces and would therefore cut the vibes transmitted to the rest of the structure. The forces acting on the walls will be the same but the amount the wall moves in reaction to those forces will be reduced.

So it seems to me that the more rigid you can make the wall the less important the volume of air in the cavity becomes. Same thing with adding mass.

Bob, agreed - untreated wood against concrete or block would be bad.

Thanks for letting me think out loud folks....

Dan

ok, i made a handful of graphs based upon data from IR-761, National Research Council of Canada. This data is copyright them, used with permission.

The graphs are intended to bascially outline the nature of resonance ,mass, decoupling, and how they all interact.

What they are intended to demonstrate are these:

1. below the fundamental resonance of some given wall, mass (in theory) defines the performance

2. all double-leaf walls will drop well below the potential defined by mass around the spring resonance point.

3. decoupling can only help well above the resonance point

data below 50hz is simply invented by me, but is reasonable. the NRC's data only went to 50hz, i include the extra data to outline the basic jist of it. The adjustment for size is done by me, and shouldn't be considered either absolutely accurate or unreasonable.



to start: the common 2x4 single-stud wall

http://audioalloy.com/2x4wallavsthreadaboutairborne.gif

you see here the basic areas of interest.
1st: a strong mechanical resonance in the midbass (125/160hz in tyhis case).
2nd: performance only a bit above mass law higher than this resonance
3rd: performance gravitating to a mass-defined condition well below the main resonance


next, a resilient channel wall:

http://audioalloy.com/RCwallmasslawdecoupleresAVS.gif
1st: the resonance occurs lower in frequency (and is a totally different resonance)
2nd: decoupling provided by the channel allows performance to be vastly better than mass law at higher frequencies
3rd: at very low frequencies, like a wood stud wall, performance will gravitate towards a mass-defined condition



now a steel stud wall:
http://audioalloy.com/steelstudsresdecoupleAVS.gif

same basic behaviors as a resilient channel wall. However, this resonance is a different sort of beast than that in an RC wall, but that's for another day


a staggered stud wall:
http://audioalloy.com/staggstudwallresdecoupleAVS.gif

This wall has a deeper air cavity and thicker insulation, and a somewhat lower frequency at which decoupling allows the assembly to outperform raw mass. As with all the other walls, well below it's spring resonance it will gravitate towards a mass-defined performance, enither better nor worse than a single 2x4 wall.


a staggered stud wall + resilient channel
http://audioalloy.com/staggstudplusRCforAVS.gif

Here we see some interesting things:

1. the spring resonance appears to shift down somewhat (the curve sort of rotates up at ~63hz), the point at which de-coupling benefits us shifts down a bit, and the wall is a tid-tad bit better overall. you can also see the gains at higher frequencies due to the channel.

By and large, this gain in the stagg stud assembly due to RC is minimal, but i include it to issue this basic lesson: the less stiff wall exhibits a lower "decoupling point", which i define herein as the point where the overall assembly outperforms mass alw

at very low frequencies, the two walls (with and without RC) gravitate to the same mass-defined condition



a double-stud wall
http://audioalloy.com/doublestudAVS.gif

a double stud wall is real decoupled, has the deepest air cavity, and exhibits the lowest de-coupling point of any of these assemblies, and also the best overall performance. Nonetheless, it will be lower in performance than would be expected for a same-mass 2x4 wall across much of the sub region due to the spring resonance and overlapping mechanical resonances.



a highly damped double stud wall
http://audioalloy.com/masslawhighdamped2bstud.gif

this is a well damped double-stud wall. Controlling the various resonance behaviors yields performance far above mass law to the low-end of the test, 40hz. The basic lesson seems to be that if resonance is controlled, then de-coupling is a win-win situation, not a tradeoff.

Below the resonance point, the performance of the damped double stud wall would gravitate to a mass-defined condition, just like everything else.



To sum: that's how walls work at the low end. Mass, resonance, and de-coupling, with de-coupling playing any kind of postitive role in the sub region (defined by me as <=80hz, is that reasonable?) only if the resonance is controlled.

however, overall, a steel stud wall IS prefereable to a normal 2x4 wall, an RC wall IS preferable, etc. Sub-region sound stopping is an issue, the biggest issue, but not the ONLY issue, and a normal 2x4 walls resonance is both severe, and still reasonably low in frequency (and you'd hear it as a low-frequency noise), and miserable all-in-all.

not that a normal 2x4 wall wouldn't be good enough for you, that all depends.

and FWIW, these walls tend to behave consistently in lab assemblies, but the myriad of variations intrinsic to real world constructions change things around. However, it's reasonable to consider lab tests reflective of the general state of things.


how was that? did that do a better job of explaining LF wall behavior than my previous jabber or what?

the "correction" to the NRC data was proposed by them in one of their publications, i use their adjustments. their discussion of this warns that this shouldn't be done blindly, so you must note that this is an approximate thing, and no official anything, just there to make a point.

ack, i see i forgot to correct the stagg stud data, pardon me for that, i'll edit the pic another day

Should I remove the plasterboard in the walls of the adjoining rehearsal rooms and replace it with 2 layers of 200mm block. Will this adhere to the Mass-Air-Mass law,and provide better low end isolation between rooms.

I should point out that the proposed construction consists of

Room3 2 layers 15mm drywall-studs-5inch gap with rockool-200mm block

6inch air gap between rooms

Room 4 200mm block-5 inch gap with rockwool-studs-2 layers 15mm drywall

Each inner stud leaf floated on a 6 inch concrete slab.

I have just got planning permission for 6 rehearsal rooms in Glasgow City centre and was convinced of the suitability of proposed construction. And then stumbled across this thread. I had thought the stiffness of the drywall coupled with the mass of the block would control low end transmission between rooms. So my question is will the doubling of the block wall provide better low end isolation.between rooms.And does the construction as proposed constitute a 3 layer seperation.

Brian,

Great posts. I'd probably have to read it a few times to really understand everything, or at least sort of understand everything.

I like those numbers on the damped double stud wall. That's what I built. :) Sometimes I regret having to cut a 51" opening in that wall for the double doors. :(

Hopefully in a few weeks when construction is finished and the door seals and automatic door bottoms are on I'll be happy with the results.

I really can't wait to take some SPL measurements in the hallway.

hey fellas, i'm on the case next time i stop by the forum, just stopping by to say i wasn't ignoring anybodies conversation or anything, but now sleep and i need to get re-aquainted.

Originally posted by ss3964spd
I'm having a terrible time getting head around this - perhaps I'm noodling with it too much. Please be kind if I'm the only one who doesn't get it!

Let me state that I do get the concept of a large air space - it takes more movement of the wall surface to compress a large volume of air. I also understand that the more mass you can add to the wall the less it's going to vibrate (or it will take more energy hitting it to make it vibrate).

From what I've read here I think I've come to understand that controlling the LF vibes is the most difficult task. A wall assembly that moves back and forth actually reproduces frequencies. Therefore (in Dan's small mind anyway) if one can prevent a wall assembly from moving back and forth by making the wall as rigid as possible I would have guessed that would cut the LF vibs the wall reproduces and would therefore cut the vibes transmitted to the rest of the structure. The forces acting on the walls will be the same but the amount the wall moves in reaction to those forces will be reduced.

So it seems to me that the more rigid you can make the wall the less important the volume of air in the cavity becomes. Same thing with adding mass.

Bob, agreed - untreated wood against concrete or block would be bad.

Thanks for letting me think out loud folks....

Dan


ok, your thought: the stud can flop back and forth resonably easily and resonante, bracing the stud mitigates this motion and should be of assistance - is intuitive and good.

however, there's a bit more to the situation, and i'll do my worst to explain.


with the studs decoupled from the concrete (and for the sake of this discussion, i'll assume they are actually decoupled) you create a situation where a mass-spring-mass resonance is in effect. The air cavity acts as a spring, will resonate at some point, and somewhere above that point the wall will decouple and performance will soar.

with the studs coupled to the concrete, you DO kill that MSM (mass-spring-mass) resonance, but you couple to the concrete wiping out alot of your gains, and you replace the MSM with the type of resonance that afflicts a 2x4 wall.


decoupled, simplified sketch of resonance and guesstimation of performance:

http://audioalloy.com/decoupledfromconcrete.gif

compared to the coupled guesstimation of performance

http://audioalloy.com/coupledtoconcrete.gif


ok, so the sound-reductions for these guys in dBA would be

for music & theater (pretty bass heavy, bass to 31.5hz, no calculations below that)

raw concrete: 41
single drywall coupled to concrete: 40
single drywall decoupled from concrete: 44
double drywall, coupled: 43
double drywall, decoupled: 49

and there you have it, not formal numbers by any wild stretch, but calculated in good faith and within reason. i'll take my calcs/guesses over software simulations, whatever that's worth.

for vocal-range reduction, by enormous margins the decoupled systems will be sueprior.

a note: the resonance with the coupled wall doesn't look as bad as the other one. it's probably worse, it's just that the resonance of the concrete block example that i entered falls in the same frequency range, which results in a situation where two different types of resonance overlap, and i don't know what to do with that...

by and large it's reasonable, but no more.


you're experiment might be one tad-bit misleading in that it looks at the studs motion, which is constrained by the bracing, and not the drywall motion, which becomes the culprit in the braced scenario.

Originally posted by midgeybin
I should point out that the proposed construction consists of

Room3 2 layers 15mm drywall-studs-5inch gap with rockool-200mm block

6inch air gap between rooms

Room 4 200mm block-5 inch gap with rockwool-studs-2 layers 15mm drywall

Each inner stud leaf floated on a 6 inch concrete slab.

I have just got planning permission for 6 rehearsal rooms in Glasgow City centre and was convinced of the suitability of proposed construction. And then stumbled across this thread. I had thought the stiffness of the drywall coupled with the mass of the block would control low end transmission between rooms. So my question is will the doubling of the block wall provide better low end isolation.between rooms.And does the construction as proposed constitute a 3 layer seperation.

sorry for the slowish response, i got back-logged


is that a true floating slab that you show? spring-decoupled with properly low resonance point? the thing is a bit too small to make out the text.

it is likely that the mass of the concrete will pay off over the drywall, yes, but it's also alot more expensive.

I planned to float the slab on this stuff.Crown floor slab by knuaf.Wont let me post the link.Just put www in front
.knaufinsulation.co.uk/cgi-bin/files/pdf/Crown Floor Slab.pdf

Is the compression data quoted up to taking 4x2 studs plus ceiling plus 2 layers of gyproc.A variant on the puck compression discussion here and elswhere. It quotes a weight of 6KN/m2. An alternate method I am considering is pour the slab direct on subsoil and float a 2 inch cement screed on the Crown slab above the concrete.Putting the studs on the unfloated concrete slab thus bypassing the problems of resilient layer compression.

midgeybin:

Is the purpose of the Crown Floor thermal or acoustic/soundproofing ?
What is the application for the rehearsal rooms: Home Theater or Drums or Piano or Choir?

Originally posted by Brian Ravnaas
is that a true floating slab that you show? spring-decoupled with properly low resonance point?
Brian is concerned because whatever frequency the floor resonates at will AMPLIFY the sound going into the next room whenever the floor is energized at that frequency.

So if you have a home theater with subwoofers cranking out noise from 15hz through 50hz, and the floor resonates at 30hz, then frequencies near 30hz will be louder in the next room than if the room were just on top of concrete.

If you have a piano, and people are playing the lowest A, B, C, D keyboard notes, and the floor resonates at 30hz, then those notes would be louder in the next room than if the room were just on top of concrete.

Whereas a choir or speaking might not have any energy low enough to resonante a heavy floor.

To properly calculate resonance you really need the manufacturer to do it for you, and you need to tell them what will be in the room to the nearest 100 lbs, and where it will be (so they can put more material under the walls and other heavier spots). Resonance is a function of weight over the spring (or rubber or whatever), the height of the 'spring' material (taller is usually lower frequency resonance), and the shape of the 'spring' material.

This page
http://www.earsc.com/HOME/engineering/TechnicalWhitePapers/Vibration/index.asp?SID=61
shows some of the math involved, and more importantly Figure 2 shows the amplification point at resonance. Basically you want the resonance to be between SquareRoot(2) lower and an octave than the lowest frequency you intend to create in the room (target frequency). With resonance frequency SquareRoot(2) lower than the target frequency, the TL at the target frequency is zero.

BTW, wood floor over rigid mineral wool apparently bows (lower at the walls, high in the middle). But with 4" to 6" of concrete you shouldn't have that problem.

Originally posted by Brian Ravnaas



a double-stud wall
http://audioalloy.com/doublestudAVS.gif

a double stud wall is real decoupled, has the deepest air cavity, and exhibits the lowest de-coupling point of any of these assemblies, and also the best overall performance. Nonetheless, it will be lower in performance than would be expected for a same-mass 2x4 wall across much of the sub region due to the spring resonance and overlapping mechanical resonances.



a highly damped double stud wall
http://audioalloy.com/masslawhighdamped2bstud.gif

this is a well damped double-stud wall. Controlling the various resonance behaviors yields performance far above mass law to the low-end of the test, 40hz. The basic lesson seems to be that if resonance is controlled, then de-coupling is a win-win situation, not a tradeoff.

Below the resonance point, the performance of the damped double stud wall would gravitate to a mass-defined condition, just like everything else.


Great posting, Brian!

Should the caption on the upper graph read something like "A decoupled (double stud) wall without damping"? This is a double stud wall, not a staggered stud wall like the caption says, right?

How is the highly damped condition created? Is this from the NRC data as well, and simply due to insulation in the cavity?

If so, it looks like GG will offer no additional advantage for well-insulated double stud walls, because there is no significant resonance left to dampen.

- Terry

hey terry, thanks for noticing the label error, it's corrected to show double stud wlal. That's (undamped double stud) NRC data, and the damped wall is a GG wall, taken at Riverbank (see the GG thread by japanesegeek for details)

both walls have 7" of insulation, w/o insulation double stud walls show severe cavity resonance effects across alot of hte midband (room modes inside the cavity, basically).

FWIW, i think that GG wall is the best overall stud wall ever tested.

Originally posted by midgeybin
I planned to float the slab on this stuff.Crown floor slab by knuaf.Wont let me post the link.Just put www in front
.knaufinsulation.co.uk/cgi-bin/files/pdf/Crown Floor Slab.pdf

Is the compression data quoted up to taking 4x2 studs plus ceiling plus 2 layers of gyproc.A variant on the puck compression discussion here and elswhere. It quotes a weight of 6KN/m2. An alternate method I am considering is pour the slab direct on subsoil and float a 2 inch cement screed on the Crown slab above the concrete.Putting the studs on the unfloated concrete slab thus bypassing the problems of resilient layer compression.

OK, WRT your original question of subbing block for the stud wall on the interior, it is likely that this will raise sound isolation. it's just so heavy, and the cavity/double wall arrangement yields great vocal range reduction (the basic weakness of concrete)

in tests at the NRC in days gone by, double-block walls

1. benefit every bit as much as stud walls from insulation
2. perform remarkably worse when coupled. the NRC tested nominally identical block walls where one was fully separate from the other, and where they were both on the same "wythe", and the change was big. like 10-20dB big, still 5 dB or whatever down under 100hz,

i imagine the raw mass of a block wall on the floating floor makes for a complex engineering situation, and you can certainly attain good results with a stud wall. adding more layers of drywall can be of aid, various products (GG is one, there are others) can be of aid, and high TL can be anticipated.


now, floating floors come in a few flavors. the "tenderloin" is to use springs (actual springs or more commonly calibrate rubber pads) and calculate the mass/load/dispersion of mass and get the rsonance very low (<20hz)

all other systems are less than this - rubber mats, foam mats, rigid fiberglass and others, but can still be of aid.

A thought that comes to mind are U-boats from Auralex and using studs on the floor (presuming you can spare the space.

when floating a floor in that manner, the basic guidelines of as deep an air cavity as possible and use insulation should be respected.



your initial question was "would double block be better", and the probable answer is yes. The rest of the answer is that over most of the band, flanking noise will limit the assembly, and the floating floor would have to perform at an extraordinary level to reap the full potnetial of block / air / block.

i presume the block construction is alot more costly.

setting studs into the knauf mat that you describe may well prove successful as well.

whatever you opt for, you hvae clearly outlined a plan that has huge potential for excellent success.

here:

this table & procedure may be of aid in estimating the performance of a floating floor:

procedure:
1) estimate the total load (Weight per unit area)
2) place the proper load (weight/area) on the "soft" floating material in question
3) measure how much it "sinks"

this is called "deflection"


this table & formula gives the approximate resonance frequency with only the amount of deflection as input:

formula: 15.8/(deflection^0.5)

or 15.8 divided by teh square root of the deflection = resonance frequency in hz

table

deflection resonance
0.5mm = 22hz
1mm = 16hz
1.5mm = 13hz
2mm = 11hz
3mm = 9hz

if deflection is too small to measure, you can double the load, and divide deflection by 2



this system is not the same as engineering something all thorough-like, and springs are not all linear, and so the doubling-the-load that i proposed can sometimes mislead, also damping and other factors can afefct the dynamic behavior of various types of "spring", especially some elastomeric materials can cause deviations from the expected.

but it's a great starting point, and the theory is sound.

a truly proper floating floor is perhaps the most challenging and expensive undertaking in sound isolation.

Brian

here is mass law -vs- behavior for a single slab of 4" concrete


the big dip at ~250hz is the coincidence dip (which occurs much higher in frequency in drywall).

this combined with the lack of decoupling/absorption (wildly effective at mid/high frequencies) keeps it's vocal range reduction well below all but the 2x4 wall shown above.

which is a bummer

http://audioalloy.com/4inconcreteAVS.gif


i cannot tell you whether a panel resonance problem is likely to occur, although i suspect it will be low in frequency, i can only say that i saw one TL plot once that showed this (dramatic) effect in a masonary wall.

the blue and white lines are made up to outline two points. the first is that this wall is mass controlled (blue line) at LF. the second is that the (not so likely to be a problem in the real world) low end may be affected by a panel resonant behavior.

Thank you for the replies. Thank you also for an excellent forum. I really appreciate the time taken to provide the detailed analysis of my plans and the suggestions for optimum sound isolation.

Brian,

If I may be so bold to ask-

Knowing what you know, what kind of room would you build? (ie. list the layers).

If you have a couple different answers based on budget, please list as a Low cost, mid-range, and money no object range. :D

If you have read the reviews of the 'music room' for the guy up in Toronto (room with-in a room, 7 JB Labs Utopias as the 7.1 surround system, budget $2 million +) I guess that would be an idealized setup for sound isolation.

By the way, you have some of the most interesting and knowledgable posts. Always an enjoyable read, although sometimes they go over my head. :)

Originally posted by FusionRx

If you have read the reviews of the 'music room' for the guy up in Toronto (room with-in a room, 7 JB Labs Utopias as the 7.1 surround system, budget $2 million +) I guess that would be an idealized setup for sound isolation.


Who is this guy in Toronto, and would he like to adopt a 28 yr old with a family? :D

Originally posted by FusionRx
Brian,

If I may be so bold to ask-

Knowing what you know, what kind of room would you build? (ie. list the layers).

If you have a couple different answers based on budget, please list as a Low cost, mid-range, and money no object range. :D



sure, that sounds like a fascinating project. how about the rates given at www.get-a-quote.net as a standardized labor rate to allow the guesstimations of price to mean more than just whatever my local dude tells me?

Brian,

I was meaning that what would you design/have you designed for your own personal pleasure?

Many people always say "If I had an Unlimited budget..."

Originally posted by Brian Ravnaas

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.


Hi Brian,

What a great thread! Thanks.

In reference to the aquarium analogy I asked the following question in an other thread. (http://www.avsforum.com/avs-vb/showthread.php?postid=5430384#post5430384) Unfortunately it was overlooked.

Would you please respond to the following:

As a layperson I find the aquarium analogy useful, however, an aquarium with even a small hole, is useless while, even the best of home theaters will have some "holes" and still can be used.

Sometimes the analogy is invoked when an enthusiast discovers his/her theater has "leaks". His well-meaning, fellow-enthusiast says well it's too late to retrofit the theater because the existing framing will "leak" too much no matter what "soundproofing" is added to the walls.

I find myself as the "well-meaning" layperson whose friend wants to retrofit a room to isolate it, and all that comes to mind is leaking aquariums. The only sound isolation measures he's taken was to put insulation in the walls. At this stage, he has the shell of the room completed, are there any measures he can reasonably take now to reduce some "leaks" that wouldn't be a waste of time?

I've told him it's too late to prevent bass from traveling through his house, but perhaps an additional layer of drywall, and a suitable door might have some merit. I'd greatly appreciate any advice, is retrofitting too little, too late?

Thanks.

Larry

quote:
--------------------------------------------------------------------------------
As a layperson I find the aquarium analogy useful, however, an aquarium with even a small hole, is useless while, even the best of home theaters will have some "holes" and still can be used.

Sometimes the analogy is invoked when an enthusiast discovers his/her theater has "leaks". His well-meaning, fellow-enthusiast says well it's too late to retrofit the theater because the existing framing will "leak" too much no matter what "soundproofing" is added to the walls.

I find myself as the "well-meaning" layperson whose friend wants to retrofit a room to isolate it, and all that comes to mind is leaking aquariums. The only sound isolation measures he's taken was to put insulation in the walls. At this stage, he has the shell of the room completed, are there any measures he can reasonably take now to reduce some "leaks" that wouldn't be a waste of time?

I've told him it's too late to prevent bass from traveling through his house, but perhaps an additional layer of drywall, and a suitable door might have some merit. I'd greatly appreciate any advice, is retrofitting too little, too late?
--------------------------------------------------------------------------------




hey larry, i'm glad you asked again, i never mean to not answer questions, but sometimes it happens, time is short, and all of these.

he has an existing wall w/insulation... you mentioned adding more drywall, so i guess that means the walls aren't all treated/fabriced/beyond modification?

for the pain of ripping out the inside layer of drywall, he could add the whole woiks, whatever his pleasure (assuming floor space was available). clips, channel, make it a staggered stud wall byadding a 2x2, or add another stud row for double studs.

w/o ripping out the wall, he could add more drywall, or green & drywall, or quiet rock or something

all of those options have whatever cost associatd with them, of course, and whatever floor space penalties.


Dennis's aquarium analogy (as far as i know it's his) relates to flanking sound, i've seen him use it to say "build a great ceiling, and that's dandy, but if everything else is poor, you may still be disturbed" or somethin glike that

WRT your friend, ... you never know, not all situations and people need alot of isolation, and he may well be ok

?

good luck

hey again, larry, if the drywall cannot be modified, (assuming i've made sense out of them) the 5 basic principles issued above have to be observed for the isolation of th ewall to be largely improved.

make sure it's well sealed (so you at least get the max performance potential of what's in place), and don't resort to solutions involving small air cavities...

Very interesting thread. We are planning our theater now, working with a contractor. Won't be able to incorporate all of the suggestions and advice, but at least should be able to avoid major mistakes.

One area I haven't seen discussed is the floor in the room(s) above the theater. Lots of discussion about the theater ceiling, but isn't the floor above equally important? I would be interested in Brian's, Dennis' and others' comments on this. What are the relative merits of

3/4" hardwood over tar paper over 5/8" or 3/4" plywood
carpet over underlay over 5/8" or 3/4" plywood
rock tile (granite, marble or limestone) over 1/2" concrete/mesh over 5/8" or 3/4" plywood
ceramic or porcelain tile over 1/2" concrete/mesh over 5/8" or 3/4" plywood


This has probably been discussed somewhere, but it would an interesting consideration to add to this already comprehensive thread.

hey kabilly,

you can apply the basic principles

1. mass
2. decoupling
3. absorption
4. resonance


heavier is innately good, but what you supply is only part of the picture, the ceiling below is the other part.

good luck!

Originally posted by Kabillyhop
I would be interested in Brian's, Dennis' and others' comments on this. What are the relative merits of

[1] 3/4" hardwood over tar paper over 5/8" or 3/4" plywood
[2] carpet over underlay over 5/8" or 3/4" plywood
[3] rock tile (granite, marble or limestone) over 1/2" concrete/mesh over 5/8" or 3/4" plywood
[4] ceramic or porcelain tile over 1/2" concrete/mesh over 5/8" or 3/4" plywood


This has probably been discussed somewhere, but it would an interesting consideration to add to this already comprehensive thread.

OK, applying the 5 basic principles:

3&4 would be the heaviest. with that would come the greatest low frequency sound-stopping potential. add RSIC clips (or room w/in a room or whatever) to the ceiling w/double drywall and you'd have a great partition, period.

now, for impact noise, carpet has no peer for quieting footsteps, and the hard-surface (3&4) would probably be worse despite the higher mass, all other things equal. the heavier (3&4) things w/carpet would be just sick for impact noise performance.

to estimate the overall gains due to mass you can calculate the weight of each on a per-square foot basis, take the lighest one and use it as a reference, and then add 6dB for each doubling of mass, and 3dB for each 50% more mass

so if one was 10lbs/square foot and another was 30 lbs/square foot, the 30lbs/square foot would have the potential for 9dB more lowfreq isolation DUE TO MASS ALONE.

low frequency isolation isn't mass alone, other factors like resonance play a role, and decopuling can be a big additive factor on something as deep as a ceiling (low resonance point), especially if heavy (3 or 4 + double drywall ceiling). resilient channel will limit how low the rsonance point can go.


in general (and i always fret that what i post isn't maximally useful) applying the five basic principles can get you a pretty good loose idea of how things stack up. :)

Originally posted by FusionRx
Brian,

I was meaning that what would you design/have you designed for your own personal pleasure?

Many people always say "If I had an Unlimited budget..."


with this question posed, and myself (and i'm sure others) getting asked so often "what's best", and with 3rd party GG data in (that is basically the same as previous data taken in our labs) for 2x4 walls and double stud walls...

i thought what the heck, i'd do a big old analysis and offer it here. I know this forum is full of sharp people, so all i have to ask is "read with a clear head". This is a big, long, blabbing, but potentially very interesting and useful set of posts, folks, handle with care.

Analysis like this can cause confusion, and relies heavily on a couple of factors that can shift the final results quite a bit.
These factors are

1. the choice of noise curve. If one selects a curve that does not represent the noise that is actually present in some given situation, you risk doing 12 hours of calculations that simply don't mean anything.

the curve chosen here is shown below:

http://audioalloy.com/noisecurves.gif

Thoughts and rationale behind this selection

A) the Tennekes based curve that i have previously utilized rolls off speech range frequencies alot. This is convenient for marketing 2x4 GG walls, BUT, first it was based on noise found in discotechs, and second speech range isolation is so critical (if that's bad, then everything is bad) that this is simply not reasonable. NOBODY wants to hear dialogue through the walls.

B) it is abundantly clear that the bulk of raw energy in theaters comes from those big, boisterous subs. And typically they are rolled in at 63-125 or 80-100 hz (i'm sure YMMV) - below the speech range. and typically there is some region of overlap between front&center and sub. hence the curve rolls up towards its final low freq level

C) the absorption in the rooms fails at low frequencies, BUT WILL reduce the reverberant energy ALOT at higher frequencies. i used ASTM E90's calculations as a basis for estimating the impact of this on the reduction of HF noise, and many thanks to Mysphyt (Chris Collins) for supplying some theater measurements... it's important to remember that it doesn't matter what the noise at your listening position is hsaped like, it matters what the noise at the wall is shaped like.


I think this curve is reasonable, and i think if anything it is conservative WRT the importance of low frequency noise. If anybody wants to contribute to the generation of a truly excellent theater-noise curve i'd be so grateful that i'd just pass out.




2. the importance (if any) of data below 50hz. some walls (double wood stud, stagg stud with resilient channel, sound clip walls, etc.) might exhibit the lowest level of performance below 50hz, while some walls (common 2x4 wall, steel stud walls, RC walls) will exhibit their weakest performance point above 50hz. More or less zero data exists below 50hz (audio alloy and RSIC are pretty much it). The old USG reports with data to 31.5 hz are CALCULATED DATA BELOW 50HZ, something that is not always noted when discussing them.

in conversations with various folks from the theater industry, including DE, they have always expressed the need to analyze to 31.5hz, or desire anyhoo. to handle this <50hz data i did the following:

A) for wall types with the primary low frequency resonance INSIDE the 50+hz range (steel studs, 2x4 walls, resilient channel walls) i simply set the 40 and 31.5 hz data points high enough to not affect the values at all

B) for wall types who's primary low freq resonance (and performance minima) is PROBABLY inside the 50 hz range (double stud with single drywall, staggered stud) i set the <50hz values so that they had <0.5 dB of effect on the final ratings

C) for wall types who's weakest link (the low freq resonance) is CERTAINLY in the <50hz range (double stud double drywall) i set the values so that the final results would not be lowered by more than 1dB. this is probably conservative, and overly generous to these wall types, but since it's always risky business to just make up numbers, i think this is the best policy.


so, although the calculations were done down to 31.5hz, in essence all numbers are defined by data in the 50-5000hz band.







and that's the background and disclaimer for this analysis. It's presented in good faith, and it took half a work week, BUT, if my curve selection is good, then it's well, well worth it.

i can re-generate these numbers in a few seconds for a different curve at a future time, kind of handy.

Total analysis included ~550 walls



one final note: i add the calculations for GG tests, and averages & standard deviations for results for other wall types. results vary from lab to lab and test to test, and as such the GG walls may have gotten lucky and done well, or unlucky and done worse than their potential.

see? same for PAC walls or whatever (i don't include PAC data as that's probably not polite, their heavy walls rule & justify the expense over RC for this application, even if you got the RC insatlled perfectly)



ok, next post is the data

ok, wall type by wall type, here are the calculated single-number performances


higher is better, just like STC


normal 2x4 single wood stud walls:

this wall category has less data than any other generic category, probably because performance is bad, and all other things are aimed at improving upon it

it also has the largest variation of any wlal type from data set to data set, probably because alot of the data is ancient. and perhaps because assembly details affect the resonance behavior alot.


sound reductiosn range from 27-32 dBA for a 2x4 wall with single drywall and insulation. In the precious few cases showing the effect of adding progressively more layers, we find an average of about 3dB for adding one more layer, and 5dB for adding a layer on both sides.

the best 2x4 wall i have full-band data for is 34 dBA of reduction with double drywall both sides.

so (i just dno't have enough data to actually generate a meaningful average) we can perhaps say that a typical 2x4 wall will offer

2x4 wood stud wall: 28dBA / 29 EqLoud
add one layer : 31 dBA / 32 EqLoud
add two layers: 33 dBA / 34 EqLoud


Resilient channel: averages of dozens of data points for each possibility:

one layer each side: 33. dBA / 35 EqLoud
one / two layers: 36 dBA / 40 EqLoud
two layers each side: 38.5 dBA / 43 EqLoud

So resilient channel (despite the factual reality that it worsens performance <100hz or so) if properly installed could be expected to give some performance gains... about 5dB, perhaps. The reasnos being vastly superior midbass and up performance (assuming it's properly installed).


Steel studs (thin guage 25ga, 3 5/8" studs)

one layer each side: 32. dBA / 33 EqLoud
one / two layers: 35 dBA / 37.5 EqLoud
two layers each side: 37 dBA / 40.5 EqLoud

On average steel studs were highly similar to RC, both in the shape of the curve and overall score. as you use thicker gauge of metal, the performance will shift towards thato f a wood stud wall.


staggered studs:

one layer each side: 35. dBA / 38.5 EqLoud
one / two layers: 38.5 dBA / 43.5 EqLoud
two layers each side: 41 dBA / 47 EqLoud

it is worth mentioning that although staggered studs don't necessarily (or hardly ever) yield HF performance spuerioer to RC, what datas i have show better LF performance (Deeper air cavity being a big factor). And the staggered walls rank higher.


Double Wood Stud walls

one layer each side: 38. dBA / 43.5 EqLoud
one / two layers: 42 dBA / 49.5 EqLoud
two layers each side: 45.5 dBA / 55 EqLoud

With a deeper air cavity, more insulation, and the most decoupling of any of the above, mass for mass the double wood studs easily outperform. Insultation, BTW, is important MORE for lowerig resonance frequency than for absorbing sound. ;)


the first GG 2x4 wall gets 42.5 dBA / 49 EqLoud
first GG one one side, regular drywall on the other side of a 2x4 wall gets 39 dBA / 44 EqLoud
the first GG double stud wall gets 56dBA / 66 EqLoud

estimates from our work with no 3rd party data to substantiate (yet)
GG staggered stud wall ~50 dBA
GG steel stud wall ~similar to wood stud GG wall


EqLoud calculations are based upon the equal loudness curves given in ISO 226. Those are like modernized Fletcher Munson curves.

Other equal loudness assessments that are much more famous - like Fletcher-Munson or Robinson-Dadson weight the low frequencies more heavily, and give very different results.

for fun: ALL WALLS DOUBLE DRYWALL BOTH SIDES

Robinson-Dadson calculations
2x4 wall: 28
steel studs: 32
resilient channel: 34
staggered studs: 37
double wood stud: 43
GG 2x4: 39
GG double stud: 56


Fletcher-Munson calculations

2x4 wall: 34
steel studs: 33
resilient channel: 35
staggered studs: 40
double wood stud: 50
GG 2x4: 49
GG double stud: 76


the wild differences with Fletcher-Munson are not calculation errors, but relate to the very different low frequency shape of those curves, grain of salt and all of those.



and about sound clips: BY A MILE, the RSIC test data justifies their cost relative to RC for home theater applications WITH HEAVY WALLS having multiple layers of drywall on at least one side.

the heavier the RSIC walls get as they move from 1+1 to 1+2 to 2+2, the wider the performance margin gets between them and RC

the reason being that RSIC clips seem to allow the low frequency spring resonance to go lower than RC, which means they effectively decouple lower. Mass is a critical component of getting this decoupling point down, the extra air space that RSIC clips add is another component, and don't expect them to perform the same with a low-profile 3.5" cavity depth -vs- the as-tested 5.25" cavity depth (or so)

clips are good, that's all there is too it.

ok, important to the equal loudness calculations:

sound level was 105 dB / 95 dBA (roughly)

is that a bit too loud? hmmmm. all equal loudness scores would IMPROVE if we turned the volume down, but the order wouldn't change.



the above is FYI only and is not intended to reflect "what you will get", nor can it possibly or remotely reflect what you will get in a complex real world situation with ductwork and doors and flanking noise galore

i guess cost/convenience calculations are the other relevant parameter, no?

it's a dirty wrotten bugger to estimate labor costs, if it's pure DIY work and you have limitless capacity to endure the hours, then material costs can be calculated more easily.

using the labor rates for california given at www.get-a-quote.net (for some attempt to standardize labor, which is not remotely standard i'm afraid)

so, forget the grain of salt and just eat the whole shaker & get all thirsty like:

regular 2x4 wall $6.03
steel stud double drywall $6.08
RC double drywall $6.88
stagg studs double drywall $8.00
double wood stud $8.40
GG double stud wall $9.90
GG 2x4 wall $7.53
GG one side of 2x4 wall $6.01
2x4 wall, no acoustic sealant,
single drywall, no insulation $3.32

assumes 80% coverage of GG as spec'd and caulk-tube labor rates (total = 75 cents/sq foot). assumes that only one layer of drywall is mudded/taped/sealed. caulk rates and material costs and all others taken blindly from get-a-quote.net. FWIW, they had drywall cost at about 12 bucks/sheet which is high for my area (that's delivered in the peak of the season + a buck or so), labor for drywall was ~ the same as it's cost.

lost floor space is not included, and could skew the analysis wildly, making double stud walls foolishly expensive. i have utterly no intent of presenting the above as related to what any given assembly WILL cost, i just calculated it based on the web site given above. check it for yourself.

and let's be serious, manufacturers cost analysis are usually roughly as slanted as the sinking titanic. heck, it's been proven that the most expensive sound isolation products i'm aware of are cheaper than resilient channel for the love of petie.



soundboard is the cheapest and lowest labor of anything, BUT: i don't think there is any evidence that full-band noise reduction gains are had with soundboard at all, really. with 6 total layers (soundboard + double 5/8") on each side of a 2x4 wall in one test it got 38.5dBA/42.5EqLoud, in other tests it's less.

maybe a dB or two on a 2x4 wall, it simply must lower performance on decoupled walls due to lower mass than the alternative (drywall). you gotta realize that stuff has been supported from up on high by the wood industry itself, and has 40 years of marketing & hundreds of lab tests and a really, really brilliant name - soundboard.



and finally, the STC=30-36 single wood stud walls that everybody talks about in markeing ARE NOT WHAT YOU HEAR IN A CHEAP MOTEL when you can hear speech and TV through the wall clearly.

that's an unsealed horrid pile of miserable construction practice that probably has an STC of 15 or something.

do you need a gg double stud wall? how do i know? very possibly not, and that's all there is too it. caulk, caulk, caulk, caulk, caulk and you might be as pleased as punch with a thoroughly average wall.

Great posts, Brian.

I wonder if any of us will 2nd guess our double stud GG walls. :)

hey toe, how is yours? still have those double doors in the floorplan?

i hope everybody reads the various explanations and disclaimers (although i know they are burdensomely long-winded :( ).

Originally posted by Brian Ravnaas
hey toe, how is yours? still have those double doors in the floorplan?


I'm finishing up mudding/sanding this weekend. My new deadline is Wed. I will be priming/painting starting on Thursday.

The double doors (48" total) are installed. I haven't installed seals/door bottoms/etc yet, and there's still a 2" hole bored in one door w/o hardware, yet with the door closed, performance seems pretty good.

I'll know more in about 2 weeks, I guess.

Brian, thanks for the indepth analysis. Long winded, sure... but a great read.

Yes, excellent posts Brian. Well worth reading through in their entirety. Thank you!

- Terry

Brian:

There's a wall (ceiling) assembly I see often asked about but have never seen a definative answer. I'm woried it has the capability to be a problematic assembly and wondered if you could offer an opinon on a good treatment for it?

The assembly is the wood truss beam system. It's what I have and mine in particular is designed to be pretty stiff. The trusses are 16 O.C. 21" tall made 100% of 2x4 web members (typical wood truss).

This presents a large, yet difficult to fill cavity. I'm concerned about this cavity air spring effect, in particular with low freq. transmission.

The original plan was to use a suspended ceiling until I read about isolation in your informative posts. Now I think the best solution is to use drywall attached to RC and RISC clips on the bottom of the trusses.

Presuming this method, do you have an opinion on the best treatment for the airspace created by the 21" trusses? One can stand normal "fluffy" fiberglass in the space between the trusses, but it would require heroic efforts to get that type insulation into the trusses (in the trianglular spaces created by the diagonal members) themselves. One possibility is blown in insulation which would more easily fill the spaces but I don't know if this will held in the isolation effort or not. And does the cavity need to be filled 100%? How detrimential would leaving the top 50% (~10") unfilled?

A not to difficult installation woudl be 16" wide 10" thick batts standing on their edge between the trusses. Everyhere except where the HVAC ducts for the floor above exist. But thsi woudl leave a lot of airsace in the trusses themselves and some on top of the batts, too.

The only other option I know of is sparyed in iceyne open cell foam. Again, I don't know if this would help or not, not much mass.

Since this space also has unmovable ductwork for the living area above, I'm also concerned all of the effort may be moot due to flanking infiltration picked up through those ducts?

Thanks for any suggestions you can offer related to this ceiling/wall assembly.

Best,
Scott

I had to bump this thread as it's such an awesome information filled resource.

If you're planning to do some heavy isolation work, this thread is a MUST READ. Start reading at page one and read it all the way thru. It certainly makes a huge case for RSIC clips, GG, and double stud walls. It also brings to the forefront what the hazards of flanking can be, and the importance of a Mass/air/mass assembly.

This thread really cemented in my mind the things that are important to maximum isolation.

Brian went out of his way to compile a lot of very useful info in one place.

I think a Mod should make this a STICKY.

Marcel

hey, thanks Marcel, i'm glad you liked it

i think about this thread sometimes and wish for the chance to thoroughly go through it all to make sure it's sensible... and maybe to have a chance at condensing it

Shameless bump! Can we get this made sticky? Please, please, please.

Hi, I THOROUGHLY enjoyed the STATISTICALLY driven posts you made, especially w/ lab work to back it up ;)

http://www.avsforum.com/avs-vb/showthread.php?t=452667

I have a couple clarification questions:

a double-stud wall


a double stud wall is real decoupled, has the deepest air cavity, and exhibits the lowest de-coupling point of any of these assemblies, and also the best overall performance. Nonetheless, it will be lower in performance than would be expected for a same-mass 2x4 wall across much of the sub region due to the spring resonance and overlapping mechanical resonances.



Does doulbe stood (looking down from the top plate) look like this? || or this
|
|

in other words, do you butt the 4" surface face to face or the 2" surfaces face to face? Maybe, yet another way to say it is, does your resulting wall become 2x as thick or does the stud you're aiming dry wall screws @ get 2x as wide? :)



a highly damped double stud wall


this is a well damped double-stud wall. Controlling the various resonance behaviors yields performance far above mass law to the low-end of the test, 40hz. The basic lesson seems to be that if resonance is controlled, then de-coupling is a win-win situation, not a tradeoff.

Please forgive me, I read ALL of this post and it was some of the GREATES 2-3 hours of my life - as it CLEARED up a TON of the "hear-say" from these boards - which is still sorta helpful, but THIS was like DEFINATIVE! Okay, so disclaimer in place, is r-13 & r-19 dampening?

I guess I'm just confused about what dampening is...

again, thank you for your posts...I'll put this one in the thread in case you want to reply there...but I'm getting ready to frame w/ the whole family this weekend - all the guys gonna help - and I wanna make sure the plan is solidly in place and that I can explain WHY as I'm CERTAIN that they will ask! :)

THanks again! :)

Mounds of information in here

So I've read/skimmed this thread... and have a theoretical question about an ultimate wall.

How about a double wall, concrete, air space, concrete. The air space acts as a spring, so say we fill the space between those two concrete walls with sand. I believe this would act to damp the cavity.

This would give a tremendous amount of mass, decoupling, and damping.

So I've read/skimmed this thread... and have a theoretical question about an ultimate wall.

How about a double wall, concrete, air space, concrete. The air space acts as a spring, so say we fill the space between those two concrete walls with sand. I believe this would act to damp the cavity.

This would give a tremendous amount of mass, decoupling, and damping.

Hi,

Your bunker-like walls might resist sound from passing through them, but how do you prevent the sound from leaving a room through the floor and ceiling? What exotic arrangement would you propose to anchor the walls in place while retaining tons of sand that wouldn't result in providing a means for bass energy to leave the room?

Practical and cost consideration aside, even if you could come up with decoupling between between the floor, walls and ceiling that wouldn't result in a structural failure, what do you think the internal acoustics of a room with concrete walls thick enough to retain such a load would be? Certainly not an ideal room for a dedicated home theater.

More than likely more conventional isolation techniques would prove to be more practical from the standpoint of balancing, isolation, acoustics and cost.

Larry

Hi, I THOROUGHLY enjoyed the STATISTICALLY driven posts you made, especially w/ lab work to back it up ;)

hey, you are welcome. it looks like i'm a few months late with a reply, i do apologize.

http://www.avsforum.com/avs-vb/showthread.php?t=452667

I have a couple clarification questions:



Does doulbe stood (looking down from the top plate) look like this? || or this
|
|

in other words, do you butt the 4" surface face to face or the 2" surfaces face to face? Maybe, yet another way to say it is, does your resulting wall become 2x as thick or does the stud you're aiming dry wall screws @ get 2x as wide? :)

2" surfaces, to make a deeeep air cavity. typically 8" cavity




Please forgive me, I read ALL of this post and it was some of the GREATES 2-3 hours of my life - as it CLEARED up a TON of the "hear-say" from these boards - which is still sorta helpful, but THIS was like DEFINATIVE! Okay, so disclaimer in place, is r-13 & r-19 dampening?

I guess I'm just confused about what dampening is...

again, thank you for your posts...I'll put this one in the thread in case you want to reply there...but I'm getting ready to frame w/ the whole family this weekend - all the guys gonna help - and I wanna make sure the plan is solidly in place and that I can explain WHY as I'm CERTAIN that they will ask! :)

THanks again! :)

damping is energy dissipation. decoupling is sort of a break in the mechanical system - like the gap between studs in a double stud wall - that break makes it harder for sound to move via the mechanical paths.

damping doesn't stop sound from going anywhere, it dissipates it along the way - turns it to heat.

Also, at resonant points in any wall, damping can make a huge difference, which is what leads well-damped walls to generally have very favorable low-freq behavior relative to other options.

So I've read/skimmed this thread... and have a theoretical question about an ultimate wall.

How about a double wall, concrete, air space, concrete. The air space acts as a spring, so say we fill the space between those two concrete walls with sand. I believe this would act to damp the cavity.

This would give a tremendous amount of mass, decoupling, and damping.

that's a good thought.

however, if i had two concrete walls, seperated by a space, id' just fill the space w/fiberglass and leave it at that.

And the reason is that the spring between the two masses isn't going to be an issue with a double wall of concrete. The insane weight of the concrete would drive the resonance frequency down so low (the weight on either side of a wall + depth of air space + insulation determine the resonance frequency in a decoupled wall) that it won't be much of an issue.

What is said to be the highest performance isolation room/building in the world is the Galaxy Studio in Europe, designed by one Eric Desart & a partner. It is a very thick concrete bunker floated on calibrated springs inside another very thick concrete bunker (plus attention to all the details to an extreme degree).


... before sand, or worrying about the resonance, in a construction like that you'd have to worry about structural connections. you see, concrete is extremely stiff, and extremely poorly damped, and as a result it can conduct vibration like crazy. And if you mount two concrete walls on the same surface, performance would probably drop 20-30dB relative to mounting them totally decoupled (as in one room on springs inside another room) and this penalty would extend well into low frequencies. mount them rigidly bonded to the same concrete slab and it might be even more of a penalty.

concrete is heavy, that's its advantage. basically everything else about it is bad.

Brian

More than likely more conventional isolation techniques would prove to be more practical from the standpoint of balancing, isolation, acoustics and cost.

Larry

That's why I said theoretical. :D

The more I read here, the more I believe the ideal room is an underground bomb shelter with lots of in room reflection treatments.

That's why I said theoretical. :D

The more I read here, the more I believe the ideal room is an underground bomb shelter with lots of in room reflection treatments.

if the concrete isn't attached to the concrete of the rest of the house, like seperated by some dirt.

connected concrete ultimately limits things. not necessarily to a "bad" level, but well short of the ultimate.

:)

Brian-

Very helpful thread!

I'm strongly considering a 2x4 staggered stud wall insulated and covered with double 5/8" drywall+GG on the theater side; single 5/8" drywall on the other side of the one wall that's not an exterior wall (ie bricked externally). On the other side of that single wall, there's only my A/V room, a pantry, and a hallway that accesses only the theater. I'm not too concerned about sound transmission to the otherside of that wall as no one will likely be in those areas during HT use (except for possibly a wet bar slipped into the hallway on the otherside of th HT wall and adjacent to the HT doors). The HT room is an addition on the back of the house and lowered 4 steps from the first floor.

My double doors to the HT, however are my question....I have two options for this "side-entry" HT. Either fit the doors flush with the HT walls when closed (& therefore open out into the hallway) OR have the doors open the opposite way (toward the HT interior) but be hinged a "door-width" away from the HT walls and open into a little alcove/vestibule on the side of the HT (so they don't open into the "rectangle" of the HT). Either way takes up about the same floor space...in the first way, the"unused area for opening of the doors" is in the hallway outside the HT, and in the second way the area communicates with the HT as a recessed area in the HT side wall.

How does one handle the issue of the door frame w/r/t the double drywall/GG isolation (does the frame touch the drywall or is it caulked, etc.?

Is there any problem decoupling/isolating around the outside of a corner (as into the little alcove)?

Do you think the room acoustics will be less affected by the doors flush to the side wall or recessed in a "door sized" alcove on the side of the HT?

Thanks!!

Harry

Brian-

Very helpful thread!

I'm strongly considering a 2x4 staggered stud wall insulated and covered with double 5/8" drywall+GG on the theater side; single 5/8" drywall on the other side of the one wall that's not an exterior wall (ie bricked externally). On the other side of that single wall, there's only my A/V room, a pantry, and a hallway that accesses only the theater. I'm not too concerned about sound transmission to the otherside of that wall as no one will likely be in those areas during HT use (except for possibly a wet bar slipped into the hallway on the otherside of th HT wall and adjacent to the HT doors). The HT room is an addition on the back of the house and lowered 4 steps from the first floor.

My double doors to the HT, however are my question....I have two options for this "side-entry" HT. Either fit the doors flush with the HT walls when closed (& therefore open out into the hallway) OR have the doors open the opposite way (toward the HT interior) but be hinged a "door-width" away from the HT walls and open into a little alcove/vestibule on the side of the HT (so they don't open into the "rectangle" of the HT). Either way takes up about the same floor space...in the first way, the"unused area for opening of the doors" is in the hallway outside the HT, and in the second way the area communicates with the HT as a recessed area in the HT side wall.

Hi Harry,

if by double doors you mean communicating doors (2 doors forming an airlock), then alot of what i type below may not apply to your situation.

i hope my mental camera is working this morning. In one scenario, the wall would be flat, in the other a small "entry" would make the room deeper on one side of the theater.

the alcove/vestibule idea is better for sound isolation. by far and away the double doors will be the weak link to this wall, and however complex/long/twisted & turned you can make the path that sound has to take the better.

sound isolation through an open airway isn't zero. Its defined by the absorption of sound (dissipation of sound via carpet/wall treatments/etc.) as it travels. Imagine a typical apartment building with a hallway and many doors... play music in one apartment with the door open, and see how loud it is in an apartment 2 doors down... much quieter. And quieter still if the hallway had carpet/pad, quieter still if you lined the hallway with fiberglass room treatments. indeed, if the hallways was very absorptive, you might find that at some frequencies (low) structure-borne flanking outweighed airborne sound.

same principle as lined ductwork, and adding absorption to this vestibule will help the overall isolation, though what the ultimate performance would be i don't know.

ultimately, you will also have to pay special attention to the seals of the doors.

now, for communicating doors (airlock), the bigger the air space between the doors the better.

perhaps a sketch of the two concepts is possible>?





How does one handle the issue of the door frame w/r/t the double drywall/GG isolation (does the frame touch the drywall or is it caulked, etc.?

Whatever the manner of handling of the door would have been if gg / double drywall wasn't present. There is no need to take special care to decouple the frame of the door from the structure of the wall.

Did i interpret that question correctly?






Is there any problem decoupling/isolating around the outside of a corner (as into the little alcove)?

no, building gg walls around corners (or conventional walls) isn't an issue. some level of mechanical contact at a corner also won't prove to be a huge issue in a GG wall such as this.



Do you think the room acoustics will be less affected by the doors flush to the side wall or recessed in a "door sized" alcove on the side of the HT?

Thanks!!

Harry

It might have some effect on room acoustics to have the room asymetrical as you mention (alcove).

I would perhaps raise that question in the acoustics master thread (the sticky on the first page of this forum) or seperate thread, as room acoustics aren't something i should be considered an expert at (sound isolation is my area, not the sound in the room)

Thanks, Brian, for that extensive response! I think that you understood my questions; just for clarity, I've attached a pdf sketch of the HT addition. (It's my latest revision for my draftsman, so it's not cleaned up yet.)

The double doors open into a little alcove; the other option would be to have them flush with the HT lateral wall and open outward into the hallway (ie backwards from the way they are in this floorplan, but using the same space for the doors). I think I prefer the way it's shown here, though.


Thanks for the input; I'll probably post the floorplan on the acoustic sticky,too (after I've gotten it more "presentable"!)

Harry

Hi again, Harry,

i see, you mean double doors, not communicating doors/airlock. The doors will be the limiting factor for isolation through that side of the room. When you have a door that will be leaking some sound, treating the path from this door to somewhere else that you need to be quiet like ductwork is a good policy - put absorption in the path.

As for the alcove -vs- flush... From an isolation standpoint, i'd put absorption on the walls in the alcove if it opted for an alcove (you'd want to anyway for room acoustics reasons).

i do apologize for not being of more concrete help,

Brian

This is an incredible thread. I have a question for Brian:

I'm not looking to completely sound proof my theater room (in the basement), but I am looking to isolate it to a reasonable level. Given my basement layout, this is the cross-section of the theater room (linked in to save bandwidth):
Basement Plans (http://gallery.avsforum.com/data/507/Basement_Plans_2.png)
Basement Plans - Theater Cross Section (http://gallery.avsforum.com/data/507/Basement_Plans_2_-_Theater_Cross_Section.png)

My plan was to build a floating floor using DRICore. Then build 2x4 staggered walls around the entire room with the walls sitting on the DRICore and a foam gasket. The walls would be constructed of a
16" on-center 2x4 stud -> sound clip -> 5/8" Drywall -> Green Glue -> 1/2" Drywall.

That should isolate most of the sound coming through the side walls and help isolate the bass from shaking the entire house... correct?

My problem mostly lies in the ceiling. Given the annoying supply and return ducts located roughly in the middle of the theater room and given that I prefer a drop/suspended ceiling, what is overkill because of that and what can be done to aid in isolating the high frequency sounds to the theater room/basement? Or am I basically stuck with what I got and the wife won't get a good night sleep if I'm in the basement watching a movie or playing games?

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
My TV room (not worthy of HT title) in the basement will be on a wall opposite to the furnace room, and surrounded on the other 3 sides by the concrete wall. Because sound will be able to travel through the vents to the rest of the house (inlcuding the bedrooms on the second floor, 2 floors up - the main concern) I imagine I want to make this wall pretty soundproof. So I'm trying to predict at what point the low frequency sound will be heard in the furnance room. As I understand it, the resonant frequency is the point at which the wall decouples, and below that the stagerred stud approach offers no benefit in terms of sound isolation than a standard 4" wall.

I was going to attempt the above calc until I realized that I don't know the mass of the walls themselves. So hopefully someone knows in practice, down to what frequency can I expect a 6" staggered stud vs an 8" stagerred stud wall to peform positively? (I could build it a bit wider and sacrifice the space if need be).

Also, the outside 3 walls are already framed (by the previous owner). Without doing a "room within room" approach for the ceiling, will attaching the staggered stud wall (and the outside 3 walls) to the ceiling joists eliminate the benefits of building the staggered stud wall against the furnace room? Are there any other options besides fitting in different joists? I have a fair amount of ducting that would get in the way of that. But obviously I have to attach the walls to something....

So hopefully someone knows in practice, down to what frequency can I expect a 6" staggered stud vs an 8" stagerred stud wall to peform positively? (I could build it a bit wider and sacrifice the space if need be).
Never mind... Found a great response to this Q here... http://www.avsforum.com/avs-vb/showthread.php?p=8361508&&#post8361508

Still wondering about what to attach that wall to, and if I just use the current joists, does that defeat the original benefits of the wall.

My TV room (not worthy of HT title) in the basement will be on a wall opposite to the furnace room, and surrounded on the other 3 sides by the concrete wall. Because sound will be able to travel through the vents to the rest of the house (inlcuding the bedrooms on the second floor, 2 floors up - the main concern) I imagine I want to make this wall pretty soundproof. So I'm trying to predict at what point the low frequency sound will be heard in the furnance room. As I understand it, the resonant frequency is the point at which the wall decouples, and below that the stagerred stud approach offers no benefit in terms of sound isolation than a standard 4" wall.

Yep, basically. Except at the resonance point the wall is worse than it would be with no decoupling, and its at frequencies a bit above resonance (about 1.4 times the resonant frequency) that the decoupling helps.

So if you had a resonance of ~40hz, you could expect the decoupling to help at around 60hz and hurt from 30-55hz, with basically no effect below 30hz.

I was going to attempt the above calc until I realized that I don't know the mass of the walls themselves. So hopefully someone knows in practice, down to what frequency can I expect a 6" staggered stud vs an 8" stagerred stud wall to peform positively? (I could build it a bit wider and sacrifice the space if need be).

The resonance point of the 2x6" wall (5.5" deep) -vs- 7.5" deep 2x8 wall will both depend on how much mass is on each wall.

With single 5/8" drywall, resonance would be:
69hz (lowered by insulation) for the 2x6
59 hz (lwoered by insulation) for the 2x8


With double 5/8" drywall on each side, resonance would be:
41hz (lowered by insulation) for the 2x8 wall
48hz (lowered by insulation) for the 2x6 wall
For curiosities sake, moving to a 2x10 would lower resonance to about 37hz

Insulation will lower the resonance frequency, and most of the reason that i always recommend low-density common building fiberglass for insulation is that it seems to be the most effective insulation type at lowering resonance. Other types are better absorbers, but the boring common stuff is best at lowering resonance.

Also, the outside 3 walls are already framed (by the previous owner). Without doing a "room within room" approach for the ceiling, will attaching the staggered stud wall (and the outside 3 walls) to the ceiling joists eliminate the benefits of building the staggered stud wall against the furnace room? Are there any other options besides fitting in different joists? I have a fair amount of ducting that would get in the way of that. But obviously I have to attach the walls to something....

Well, no. The staggered stud wall will still help. But you may benefit from a less rigid connection to the structure above. You can use different products like some offered by Kinetics or Pac for the attachment (Sway braces and the like), or other options.

If you opted for the 2x8 wall, you might consider just using a double stud wall (even if you don't complete the true room within a room), as the depth would be the same.

If you can't do seperate ceiling joists, the next closest thing on ceilings are Kinetics or PAC sound clips or spring ceiling hangers from makers like Kinetics and Mason. They are far preferable to resilient channel.

Attached shows double drywall on both sides of a staggered stud wall on a 2x6 plate. The wall has 3.5" fiberglass insulation.

double 1/2" on both sides and double 5/8" on both sides are shown. The heavier 5/8" gives better overall low-freq performance.


The interesting thing you can see in this graph is that other resonances (dip at ~80hz) have a negative effect and keep performance lower than you'd expect from mass to frequencies higher than the basic predictions offered above would reflect.

Damping these resonances can greatly lower the frequency to which deocupling is effective, and these reosnances (at ~80hz) aren't caused by decoupling, and would still exist without decoupling and would be more severe if the decoupling wasn't present.


Thank you very much for the compliment, it does mean alot to me when my jabber helps people out.

Thanks so much for the response.

Page 292 of the large PDF on that other thread shows a 6" stagerred stud wall with double 1/2" drywall and 5.5" of fiberglass insulation. Except for the insulation depth it should match the wall you're describing from what I can tell. The performance graph in the PDF shows no dip around the 80Hz mark. Is that expected from just the extra insulation or could the difference just be in testing methods?

(at first I was wondering why the pdf showed no dip on the very low end, until I realized that the chart you posted goes below 50 hz) :)

Edit: Oh, I see from the Kinetics site what you mean... My project is coming together (in my head). :) What does something like the Kinetic product cost for say, a 10 ft span?

Thanks so much for the response.

Page 292 of the large PDF on that other thread shows a 6" stagerred stud wall with double 1/2" drywall and 5.5" of fiberglass insulation. Except for the insulation depth it should match the wall you're describing from what I can tell. The performance graph in the PDF shows no dip around the 80Hz mark. Is that expected from just the extra insulation or could the difference just be in testing methods?

(at first I was wondering why the pdf showed no dip on the very low end, until I realized that the chart you posted goes below 50 hz) :)

Edit: Oh, I see from the Kinetics site what you mean... My project is coming together (in my head). :) What does something like the Kinetic product cost for say, a 10 ft span?


That's very observant, Felg.

But it may show a dip, we just cna't tell. Please let me explain. In a lab something that should have a nice smooth TL curve (like a simple panel) may have a very choppy looking curve at low frequencies. This is due to room modes and limitations in the measurment process and things causing any given lab to give somewhat boosted or somewhat depressed TL values at different low frequencies.

So lets say, for example, that some given lab has a "boost" at 80hz. And a wall has a dip at 80hz... then you might just look flat at 80hz.

In aother case, lets say a lab has a dip at 63hz, and the wall has ad ip at 63hz also, they might add together to make a super-dip.

This doesn't make low-frequency testing bad or useless, or even really lower the value of low-frequency testing at all. But it does make it important to compare things in very methodical ways. Same lab, same construction protocol (wall size, etc.).

Even changes in the same lab can affect low-frequency values. The NRC's lab (who published that big 350 page report) made some changes to one of the rooms in their lab and some of their dips and peaks shifted around and other things as well. Its best in this field to always compare things as "apples to apples" as possible.


Outside of that, and also interesting, the location of resonances can vary from assembly to assembly somewhat. In the real world, things like doors and screw spacing and so forth shift them around a bit. MEchanical resonances are more prone to this variation than air-spring related ones as a general rule, but they are all subject to some variation from construction to construction.

I hear what you're saying... Dam low frequency resonances. I spent about a week playing with my SW in my old place until I found a floor position and settings that were (barely) acceptable. :)

One more question that ties into sound damping and low frequencies... Is it a function of insulation depth vs wavelength that determines how well a damping material will absorb sound? A 50 Hz wave is over 20 ft long; is there some percentage of that depth needed in order for insulation to effectively absorb the sound, or is it something else altogether that enables low frequencies to pass through insulation?

Well, for room acoustics, insulation depth is really important, but in walls adding insulation doesn't have so much effect on low-frequency resonances beyond lowering the frequency at which the primary resonance manifests. Some damping effect is observed, but it isn't extreme.

Inside a wall, at low frequencies, insulation doesn't absorb sound, as the wavelengths are so long that ... well, its more just pressure in the cavity, and not waves of sound as would exist in a room.

At low freqs, the air in the cavity (and other factors which vary from wall to wall) forms a "spring", and as sound moves the wall this spring is compressed and extended. The mass on either side of the wall + the spring "connecting" these masses create something not so far from a simple mass-spring system, and the resonances calculated above reflect the resonance point of this mass-spring system.

Other resonances also come into play. Think of the drywall screwed to all those studs. Each stud space, say, 16 or 24 inches apart, makes a "panel" out of the drywall between the studs, and this creates another resonance that manifests at low-freqs. In a conventional wood stud wall, the air-spring resonance is essentially absent, and this mechanical reosnance dominates. In a staggered or double stud wall, both types exist.

Insulations damping effect is limited to a mild effect at the air-spring resonance (and lowering of reosnance frequencies). Introducing damping into the mechanical system can have a larger effect at low-freqs, and that's what i was referring to.

A picture might be in order for this discussion, i'll see if i can concoct one.

The concept of really just changing air pressure within the wall as a result of the 'air spring' action makes complete sense to me.

For the depth vs frequency question I was thinking more in terms of blocking low frequencies in the open air, or basically about the theoretical design of bass traps. Like, surrounding your subwoofer with 4" of insulation doesn't have much effect. Why not?

And is there a relationship between insulation depth and frequency that can be used to predict how well a certain frequency sound will be absorbed. For instance, if the insulation is 10% of the depth of the frequency, we expect 2% sound absorbtion (or some similar formula).

Because I don't know what I'm talking about I fear my question is not clear. Sorry 'bout that...

I think perhaps i don't quite understand what you're asking. If you are talking about acoustics inside a room, you could check the master thread at the top of this forum in the "sticky" section. Room acoustics aren't my forte, and all the comments entered herein are for sound isolation (reducing sound escaping from a room).

4" of fiberglass around a sub doesn't do much because 4" of fiberglass doesn't absorb much sound at sub frequencies... There can't be a generic formula to show absorption -vs- thickness, because different absorbing materials will have different levels of efficiency. For room acoustics, generally really low density materials like common fiberglass are inefficient for a given thickness. But in walls, for sound isolation, this is not relevant to low-frequency performance, and denser materials are generally disadvantageous.

Ok, thanks Brian. Yeah, the question is much more related to room accoustics; really to the fundamental nature of sound waves. I'll check out the sticky.

All of your help incuding the very terrific posts at the start of this thread have really gone a long ways for me. Much appreciated!

that's a good thought.
concrete is heavy, that's its advantage. basically everything else about it is bad.
Brian

Yep. Concrete rotten fast and it burns easy as paper too. Concrete isn't strong and therefore can't hold must weight so thats why there is no concrete bridges in the world... blah blah...

To be serious; The list of good things about concrete is *long*. The only bad thing is the price. Basically everything else is goodcompared to other materials.

Now I bet someone will mention flanking problem. Well, steel and wood has a lot flanking problems too if talking about low frequencies ( 20-150 hz). In fact everything that is not soft has serious flanking problems no matter material used.

What I would like to get (which I haven't found anywhere) is a frequency curve from 20-20000 hz which show sound transmission between two concrete walls on same concrete fundament (high flanking transmission). I think flanking problems increase with the frequency. 50 hz would problably have much less flanking problems than let say 500 hz. If i'm wrong prove it with finding test results.

Another problem is the ceiling type. Is it better to add a lot mineral wool (like 10 inches thick) and floating concrete floor on top (at least 2 inches thick) or is is better to add this 10 inches below the main floor with lightweight ceiling if the problem is to stop low frequency sound getting down to the floor below? I guess most ppl will say one is better than two because of the resonance problems more than one "spring layer" will introduce. Very well, but what effects does 10 inches mineral wool have compared with let say 2 inches for floating concrete floors, and how think should the floating floor be? 4 inches is a lot better than 2 or what?

Yep. Concrete rotten fast and it burns easy as paper too. Concrete isn't strong and therefore can't hold must weight so thats why there is no concrete bridges in the world... blah blah...

To be serious; The list of good things about concrete is *long*. The only bad thing is the price. Basically everything else is goodcompared to other materials.

hi dets, welcome to the forum.

Concrete as a construction material certainly has a myriad of virtues and so on. Although fire and strength may play a role in any given construction requiring sound isolation, they aren't relevant to sound isolation in a specific sense.

Though i've not re-read the entire thread, my comment as you quoted is very reasonable from a sound isolation standpoint. Concrete is very stiff (which works to lower the frequency of coincidence dip all things equal, and also works to raise the speed at which sound is transmitted through the material), and very low in damping.

Concrete is heavy (good), but stiff and very low in damping (bad), hence the comment about its heavy and everything else about it is bad is more than reasonable from a sound isolation standpoint.


Now I bet someone will mention flanking problem. Well, steel and wood has a lot flanking problems too if talking about low frequencies ( 20-150 hz). In fact everything that is not soft has serious flanking problems no matter material used.

Please let me observe that nobody i've ever seen on AVS has disputed that wood and drywall and steel have flanking problems. Different systems have different causes, paths, and so forth for flanking, and probably the most troublesome form of flanking outside of doors or the occasionally bad duct system is flanking at the primary low-freq resonance of typical double leaf constructions. If all the rooms in a house have homogenous construction, and you're makign noise in a theater, then

-the freq at which the theater walls vibrate most intensely is the freq at which other walls in the house are most easily stimulated
-you can wind up with a whole house rattling

Which is why often worthwhile gains in isolation can be had by ensuring that the theater room is different in construction than the rest of the house. Avoids this flanking trap to a degree.

What I would like to get (which I haven't found anywhere) is a frequency curve from 20-20000 hz which show sound transmission between two concrete walls on same concrete fundament (high flanking transmission). I think flanking problems increase with the frequency. 50 hz would problably have much less flanking problems than let say 500 hz. If i'm wrong prove it with finding test results.

well, the factors affecting flanking noise are several or many, and include these:

-damping
-wave speed

those two work together to help define how far vibration can travel through a simple panel. Imagine a really long, unbroken concrete strip that you strike repeatedly in the same location. As you travel away from that location, the intensity if the waves decreases. The stiffness/mass of the concrete work to define how fast the waves travel away, and damping works to define how rapidly the waves decay.

Damping is typically expressed ina per-cycle basis. So if waves of different frequencies are moving at different speeds, higher freq waves will dissipate faster over distance.

In structural situations, higher speed waves move faster than lower speed waves, but not fast enough to compensate for the difference in frequency, and you still find that high freq waves dissipate more quickly over distance than lower freq waves.

That is counter to your intuition, but your thoughts are good, and i hope that helps.

With respect tot esting the scenario, that would require a very specialized type of lab and as of now i'm not aware of a suitable facility. real world facilities would be the best bet, but considerable challenges exist there as well.

at www.nrc.ca the NRC of Canada has several documents discussion flanking noise in massive detail, and among their commentary are comments that the higher damping and so forth of wood floors work to make flanking less problematic on floors not involving gypsum concrete or concrete. They certainly don't say that flanking can't always be a problem. That site also has documents with very mathematical theory about flanking noise.



Another problem is the ceiling type. Is it better to add a lot mineral wool (like 10 inches thick) and floating concrete floor on top (at least 2 inches thick) or is is better to add this 10 inches below the main floor with lightweight ceiling if the problem is to stop low frequency sound getting down to the floor below? I guess most ppl will say one is better than two because of the resonance problems more than one "spring layer" will introduce. Very well, but what effects does 10 inches mineral wool have compared with let say 2 inches for floating concrete floors, and how think should the floating floor be? 4 inches is a lot better than 2 or what?

The concept of floating floors and how to build them is really well researched both in theory and practice, and discussed in many text and 'web threads.

Nobody is likely to argue that thicker "floating" layer in a floating floor is basically always better as it works to lower the resonance frequency of the mass-spring system. STructural and support and longevity problems and many, many other considerations exist, but a thicker layer of mineral wool in a floating floor - just like a deeper air cavity in a wall - will work to lower resonance.


hope that helped,

Brian

Ok, thanks Brian. Yeah, the question is much more related to room accoustics; really to the fundamental nature of sound waves. I'll check out the sticky.

All of your help incuding the very terrific posts at the start of this thread have really gone a long ways for me. Much appreciated!

you're sure welcome, post again if you think we can be of help. :)

So I finally posted my accoustic theory question in the Accoustical Treatment sticky... http://www.avsforum.com/avs-vb/showthread.php?p=8489929&&#post8489929 If you have any thoughts I'd love to hear them.

So I finally posted my accoustic theory question in the Accoustical Treatment sticky... http://www.avsforum.com/avs-vb/showthread.php?p=8489929&&#post8489929 If you have any thoughts I'd love to hear them.

i can take a peek, but i'm not a in-room-sound-quality guru, just sound isolation.



with respect to concrete: i don't mean to belittle the value of its weight, and if you have a room with concrete block all around, you have a very good starting point for a very successful isolation scheme.

To best answer your post, dets, i'd have to re-read the context of the conversation above. For now i'll just hope my post earlier today clarified the comment. :)

Okay, so let's say I wanted to build a room/box where the entire purpose of said room/box is to contain low frequency sound (80Hz and below) within the room/box. I do not care a whit what it sounds like inside this room/box. What would be the best way to construct this...?

Is this a decent alternative?:

2x4 single leaf wall with 2 layers of ??? with GG in between.

Better ones within reason? I don't have the room or inclination to use concrete or blocks so only within materials like plywood, OSB, drywall, etc. Thanks.

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.
...

CAC is computed exactly like STC, except that the specific test method is defined in ASTM E 1414. It is expresses as an integer point value, like STC, not as a fraction.

Regards,
Terry

Brian, I was wondering if you could clarify the statement you wrote.

"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.:

I am building a HT in my Townhouse basement which has concrete block walls. I built the studs (wood, 16 inch center) 1.5 inches in front of the the block, then added 3 inches of Roxul insulation, and (very Soon) 5/8 drywall. The ceiling is decoupled with hurricane brackets and filled with roxul as well. I plan on using 2 layers of 5/8 drywall on the ceiling. I have attached some pictures if this helps. I am wondering before it's to late, if you see any mistakes.

Thank you

Tom

Hi Tom, I'm not Brian but I think I can clarify Brian's statement effectively. Note Basement Bob's next post which tells this story:
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.
The basic concept is shown in the graph in Brian's first post. At the resonant frequency (30 Hz for person A, and 135 Hz for person B, having a decoupled wall will actually make that frequency less isolated than a basic attached wall. That is the peak shown in Brian's graph. Below the resonant frequency the performance will about match the normal wall, and above the frequency, the decoupled wall will perform much better for isolation.

Since your studs are 5" away from the concrete you have an air spring that probably matches pretty close to what Person A had above. I think Bob's calcs are correct, which means that your resonant frequency will be about 30Hz. It's low enough that a) not much sound is played that low by your HT, and b) it's at the threshold of hearing and is probably not that bothersome. So I think your fine Tom.

My studs are 1.5 inches away from the concrete wall unless you were referring to the drywall being 5 inches away from the concrete wall. Is that what you meant? Hope so.

Yeah that's what I mean. What's important is the depth of the actual air gap, because it's the air that acts as a spring in order to create the resonance.