AVS Forum banner

1 - 20 of 120 Posts

·
Registered
Joined
·
635 Posts
Discussion Starter · #1 ·
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
 

·
Premium Member
Joined
·
3,136 Posts
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.
 

·
Registered
Joined
·
2,114 Posts
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
 

·
Registered
Joined
·
635 Posts
Discussion Starter · #4 ·
Thanks guys - plenty of other crazy ideas to come, for sure!
 

·
Registered
Joined
·
2,114 Posts
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
 

·
Registered
Joined
·
2,114 Posts
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
 

·
Registered
Joined
·
2,114 Posts
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
 

·
Registered
Joined
·
2,114 Posts
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
 

·
Registered
Joined
·
635 Posts
Discussion Starter · #9 ·
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.
 

·
Registered
Joined
·
2,114 Posts
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
 

·
Registered
Joined
·
844 Posts
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
 

·
Registered
Joined
·
91 Posts
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.
 

·
Registered
Joined
·
2,114 Posts
hi guys, and thanks.


with respect to low frequencies...


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


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


here's a picture:




3 areas of interest


1. a resonant frequency. at and around the resonant point performance is grossly worsened

2. below the resonant frequency, you gain nothing.

3. at ~1.4 times the resonant frequency performance improves(so if the resonant frequency was 100hz, at about 140hz performance starts to improve)


air is a spring.


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


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


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


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


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


etc.


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


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


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

·
Registered
Joined
·
5,002 Posts
brianr820:
Quote:
where to take this post from here...? i could throw up some data or graphs to illustrate examples of spring-related resonances working their mischief... i'll think about it and come back

My favourate example of resonance is the shared wall.


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


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


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


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


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

·
Registered
Joined
·
147 Posts

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

I never thought of that. Thanks Bob!


Always learning

Andre
 

·
Premium Member
Joined
·
3,136 Posts

Quote:
Originally posted by BasementBob
My favourate example of resonance is the shared wall.

Ha, that's a great example, BB. I have experienced similar situations....
 

·
Registered
Joined
·
2,114 Posts
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.
 

·
Registered
Joined
·
2,114 Posts
de-coupling the two sides of a normal double-leaf (one air cavity) wall is probably the most familiar means of improving sound isolation to many, and the results can be over-whelming.


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


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




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


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


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


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


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


to push the resonance frequency down these basic rules are helpful


1. use as much mass on both sides as you can

2. use as deep an air cavity as you can

3. use insulation in the air cavity


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

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


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


take care all,


Brian
 

·
Registered
Joined
·
7,655 Posts
Excellent stuff. This should be a sticky.
 

·
Registered
Joined
·
2,114 Posts
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.
 
1 - 20 of 120 Posts
Top