Just how much do damping layers increase the transmission loss of a wall partition? I'm still looking for more quantification of this phenomenon.
In another thread, Brain R wrote:
Quote:
materials such as the roofing products really do not fare well at imparting damping to a system involving something quite stiff - like a layer of MDF or drywall. no better than normal drywall adhesive, often worse.
Brian, is your statement based on any tests?
If gaffa tape can damp a taut drumhead to a useful degree, I would think that a limp mass material like roll roofing or vinyl should help damp drywall to a useful degree.
I've got an AC unit outside with a housing made of metal, but the housing makes no discernible noise. I was told the designers, who wanted to keep the noise down, made it of two different kinds of metal sandwiched together. Since the two metals expand and contract at different rates, each effectively damps the other. Is there anything to be learned from this that can be applied to constructing lightweight, space-saving, yet still cost-effective high TL walls?
materials such as the roofing products really do not fare well at imparting damping to a system involving something quite stiff - like a layer of MDF or drywall. no better than normal drywall adhesive, often worse.
Brian, is your statement based on any tests?
yes it is. i labor in a lab which has tested the damping of most combinations of conventional materials we could think of for damping. you can find some measurements here:
http://forum.studiotips.com/viewtopic.php?t=813
that file also includes data on the rigidity (affects location of the coincidence dip) of the various materials used to glue the sheets.
not today, but sometime soon (at least in geographic time), I intend to upload the whole data set of normal materials with photo's of the impulse responses on Studiotips to act as a reference if anyone is interested.
_________________ All posted information copyright Brian Ravnaas
thoughts on the data in that file will be added to that thread shortly.
regarding the question of damping and transmission loss...
for a single-panel (no air cavity) mass law defines the limit of performance potential. various resonances delete from this potential (the coincidence dip, and panel resonances)
so raising the damping of the panel can only bring us closer to mass law, it can never allow us to surpass it (in theory).
in practice, typically we see a bit higher than mass law, especially at low frequencies (even for ultra-low damping matrials such as raw drywall, see IR-811) but this should not lead us to conclude that mass law has been broken, just that the measurment system and finite panel size, etc., cause deviation
some other things are described about the impact of damping on sound properties of things. for example, theory would express that impact noise in a floor would fall by:
10*log(new damping/old damping)
when we raise the damping of a floor. the best online references i have been able to find in a quick search are
very nice document, outlining the two predominant methods of damping, the pros and cons of each, and offering charts reflecting this.
it is worth noting that this leaves raising panel damping the only means of improving low frequency impact noise for a floor i am aware of (outside of shear additional mass). in theory, doubling mass reduces impact noise by 6db.
with respect to double leaf panels... simple physics is very clear that any time you raise the damping of any system, the peak/dip of resonant problems is mitigated. how much it's mitigated depends on the damping, and is readily predictable for various resonant systems (closed box subwoofer, mass-spring isolator, etc.)
so, if you could definitively say that the damping factor of a wall as used (NOT a single panel, walls are different as the air cavity is involved) raised, then you could rest assured that whatever resonant problems involved would be reduced. an extreme example: a piston in a frictionless cylinder... to raise the damping of the piston (inside the piston, NOT piston sliding in cylinder) will not affect resonant behavior. that would be a true mass-air-mass resonance.
with all of the above - impact noise, capacity to mitigate problems in a single leaf panel, and performance in a dual-leaf system - the impact of damping is TOTALLY dependent on how much damping you have actually imparted. again, elementary physics is very clear on that.
damping doesn't equal damping any more than one 12" woofer = all 12" woofers. some 12" speakers are midrange drivers that couldn't shake anything, and some are monstrous beasts of boom.
finally, the two other effects that the literature is emminently clear on are these:
1. decay of energy in a given system over time(the rate of decay of energy in a bell, for example) varies according to damping by:
27.3*damping factor*frequency
so decay is linear to damping, double the damping, double the decay rate. experiments are satisfyingly close to this.
2. decay of energy over distance
that could, again provided you actually achieved a high level of damping, tremendously mitigate flanking noise problems which are often just as problematic as wall failure out here in the real world.
we would predict the same pattern we see for decay over time in a single panel/bell/structure - decay rate linear to damping, damping factor correlating to decay per cycle, but we have to substitute time for distance by determining the wavelength at the frequency in question. in this case i think we should concern ourselves primarily with the bending waves in the materials, so we'd have to calculate distance based on bending wave speed.
but, TOTALLY disregard ANY nonsense from ANYONE that goes along the lines of "it converts the sound to heat". damping materials convert vibrational energy to heat, but...
they can only act on stored energy. if the panel stores no energy at a given frequency, there is nothing to damp. see?
resonant modes in panels, and energy in transit are the basic forms of stored energy. It's worth noting again that the performance of virtually all walls are resonance limited (dig through IRC-IR-761 and look for what limits STC and what causes the biggest dip in TL), and that energy in transit (flanking noise, energy going from one leaf to the other but NOT through the air) is pretty prevelant as well.
that's what i can offer that has sound foundation in the general theory.
_________________ All posted information copyright Brian Ravnaas
Technical Director, Audio Alloy
Last edited by Brian R on Fri Sep 10, 2004 5:11 pm; edited 1 time in total
curses... Tom Irvine had an excellent file called dcross.pdf outlining the relations between various methods of expressing damping. regrettably, he now charges for this
document:http://www.vibrationdata.com/damping.htm
pepole have tried to express the effect of damping on the coincidence dip, both by calculation and by looking at real test data. i can't find references online, so i must (sorry) give a book reference:
Norton: Fundamentals of Noise and Vibrational Analysis for Engineers
INSUL software uses what i suspect is an approach to modeling this based on interpolation from test data, and as such it allows us to wildly exceed mass law when changing damping. in fact by 30+db at times! wish it were so simple. all a fellow would have to do then is find a way to make something with a critical frequency of 20hz and damp the heck out of it and we'd be set. could weigh nearly nothing and get high STC, lol
finally, discussion of the effect of damping on spring-mass systems exhibiting resonance is outlined very well in myriad text across the net. i'll try to find a page speaking english and not technobabble and post a link. a firs-level college physics book would cover this nicely.
so to answer: people know alot about damping, but i have never seen formal tests for transmission loss where data was shown with and without damping published for public view.
there are elaborate theorum on damping and flanking noise and this and that, but they are beyond the scope of my lunch-break posting
nice page with pics about impact noise and damping:
there is considerable confusion in forums about the nature of damping and viscoelastic materials in general, let me elaborate here:
1. a viscoelastic material is not a mystical vibration barrier when used in a thin film. they DO NOT have the same effect as a spring-mass isolator (ala resilient channel or sound clips) when used in thin films. there are pro's and con's to that, that spring-mass behavior causes harm as well as aid.
2. (from a fellow who stated to work with VE mateirals, i can't find the link so this is from memory) ~"but the layer of <VE material> totally isolates the layers of drywall". that is not correct or even possible. if you made a panel with a thin, bonded, film in between and measured vibration on both sides you would find no such effect (which, again, is not bad, just trying to clarify).
i should certainly mean no offense to the gentleman who made this comment, i'm sure he's a fine fellow with good intent.
i make damping materials for a living, i should say, and another comment on that in a week or two.
_________________ All posted information copyright Brian Ravnaas
Technical Director, Audio Alloy
jazzman_in_pa Recording Org Pro Audio Forums
Joined: May 12, 2003
Posts: 796
Location: Philadelphia
So Brian, let's see if I understood the conclusions to which you have come.
I'm primarily concerned about the potential value of using some sort of damping material in a relatively lightweight wall assembly, specifically in a double stud-frame wall with two layers of drywall on the outer sides of the two sets of framing, since we already know that that's the most cost effective wall to build for purposes of isolating spaces intended for music from one another (i.e. better LF TL).
The main question I have is this: is it worth it to attempt to introduce ANY damping material between the two layers of drywall per side, and if so, what?
For such a wall, are you claiming (on pages 14 and 15 of "bonding drywall layers r.pdf") that the ONLY effect of inserting ANY damping material between the layers would be to drop the coincidence dip to a lower frequency?
When you speak of "full coverage," with an adhesive like Liquid Nails for Subfloors with its relatively high damping factor for a cheap and readily available product, how thick or thin a layer of adhesive are you talking about, and do you really mean a continuous layer preventing drywall from touching drywall?
On the basis of your research, how would inserting a 1/8" thick layer of mass-loaded vinyl between the two sheets of drywall affect the TL at various frequencies in such a wall? (Cost: $1.50/sq ft)
Same question for a 1/8" thick layer of a modified bitumen product such as GAF Liberty SBS (i.e two 1/16" thick layers of http://www.gaf.com/Content/Documents/20351.pdf). (Cost: 40 cents/sq ft, self-adhering)
Are you working on a commercial product that might help in this area at a reasonable price?
Yeah! Very interesting and understandable post Brian.
Lee, if ya wanna damp drywall, screw another layer of drywall to it!
I'm currrently piling on the zillion layers of drywall in my Bomb Shelter Stdio Build.
Bear in mind that the drywall layers are screwed to studs at 16" ( 400mm) centres, and are also against 4" ( 100mm ) of rockwool slab ( between the studs ).....
One layer rings like a mofo when struck with a fist. Two Layers seems more damped, and even more so when I've done 3 layers.
When I've finished all 7 layers ( 5layers 1/2 drywall + 1layer 3/4MDF + 1layer drywall ), I fully expect a dull thud ( .. and broken fingers ), when I punch it!
I've never made a Drywall/Stud wall before, so after finding 2 layers of drywall still' booms' considerably when hit ( all kinds of frequencies predominently between around 60Hz to about 500Hz, depending on where you hit it ) I have started to wonder about the effectiveness of the normal 2 or 3 layers of drywall for studio walls.
Surely if a wall is gonna resonate at such freqeuncies easily, then surely the vibration of the drywall panel will vibrate the air on the other side and reduce transmission loss?
Finally, I've been reading through thousands of 'DIY Isolation' posts on acoustics forums for a few years now, and I still haven't found a solution that seems as good as simply lots of layers of drywall. There's been mass loaded vynils, roofing felt, and even paint - eek!
But drywall is so DAMN CHEAP! And multiple layers of it seem to give ( from what I've read ) more benefits than just the equivelant mass of ONE THICK drywall panel.
I can't wait to test the transmission loss of my bomb shelter, and see if my educated guess is correct or not
I'd also be interested to hear what Eric or Brian have to say, about my thoughts
Paul
jazzman_in_pa Recording Org Pro Audio Forums
Joined: May 12, 2003
Posts: 796
Location: Philadelphia
So Brian, let's see if I understood the conclusions to which you have come.
I'm primarily concerned about the potential value of using some sort of damping material in a relatively lightweight wall assembly, specifically in a double stud-frame wall with two layers of drywall on the outer sides of the two sets of framing, since we already know that that's the most cost effective wall to build for purposes of isolating spaces intended for music from one another (i.e. better LF TL).
The main question I have is this: is it worth it to attempt to introduce ANY damping material between the two layers of drywall per side, and if so, what?
For such a wall, are you claiming (on pages 14 and 15 of "bonding drywall layers r.pdf") that the ONLY effect of inserting ANY damping material between the layers would be to drop the coincidence dip to a lower frequency?
no, Lee, not at all. that file was not so much intended to be a formal piece of research or release as a follow-up to a debate started by a gent on another forum. Using INSUL, it was predicted that STC would fall by 10 or 15 points if we bonded the layers of drywall.
those graphs were generated as described in that thread (most of the way down that thread). those are not measurements, but predictions.
if you can damp any resonating system, you will mitigate the depth of it's resonance.
however, remember that for two pistons resonating back and forth in a cylinder, damping the piston won't help - because 100% of the resonating behavior is governed by the air column. going theory suggests that the big low freq resonance in ALL walls is air-column governed.
i disagree with that and will elaborate at some point in the future.
Quote:
When you speak of "full coverage," with an adhesive like Liquid Nails for Subfloors with its relatively high damping factor for a cheap and readily available product, how thick or thin a layer of adhesive are you talking about, and do you really mean a continuous layer preventing drywall from touching drywall?
ALOT. i showed an S pattern in that file that was for a normal sized bead. for full coverage i would have squeezed out much more and then smashed the two test pieces together.
that file was somewhat informal, and there are limits to what a fellow should conclude based upon it - see the last post in that thread. if i didn't mention it before, it's my hope to release a much broader and more formal report on the damping behaviors of different commonly available materials in the reasonably near future.
Quote:
On the basis of your research, how would inserting a 1/8" thick layer of mass-loaded vinyl between the two sheets of drywall affect the TL at various frequencies in such a wall? (Cost: $1.50/sq ft)
Same question for a 1/8" thick layer of a modified bitumen product such as GAF Liberty SBS (i.e two 1/16" thick layers of http://www.gaf.com/Content/Documents/20351.pdf). (Cost: 40 cents/sq ft, self-adhering)
releasing test data for the damping of MLV in various configurations is something that i have mixed emotions about. it's a commercial product, and a branded one, for so many people, and i simply don't know if it's my place to do so.
now GAF doesn't care about the damping properties of their stuff, and the studio world is a goldfish in the ocean of the roofing material market, so i'd very much like to test that if i could find any.
i have data for Home Depot's version of that sort of stuff, for good old fashioned tar paper, and the various things i can find in my area. what i can't find is a smooth-on-both-sides bitumen type roofing thing.
Quote:
Are you working on a commercial product that might help in this area at a reasonable price?
yes, sir, i guess i've been making damping materials since 1998, and i will be part of a commercial entity that will release a product for this purpose soon, although the product is having demand issues that keep it from being released. i'll change my siggy then, etc.
_________________ All posted information copyright Brian Ravnaas
But drywall is so DAMN CHEAP! And multiple layers of it seem to give ( from what I've read ) more benefits than just the equivelant mass of ONE THICK drywall panel.
yes, very cheap! at least if a fellow is doing the installation himself.
Quote:
Finally, I've been reading through thousands of 'DIY Isolation' posts on acoustics forums for a few years now, and I still haven't found a solution that seems as good as simply lots of layers of drywall. There's been mass loaded vynils, roofing felt, and even paint - eek!
SOME types of regular old latex paint make fair to nice extensional damping materials. the catch is that they are so low in solids (mostly water in there) + extensional damping requires ALOT of film thickness to effect the damping of something stiff like drywall...
so... we wind up needed many gallons per sheet of drywall to raise the damping to a useful degree.
Quote:
Lee, if ya wanna damp drywall, screw another layer of drywall to it!
yep, that'll help. the going theory on why layers of drywall raise damping a bit is friction between the layers. generally, though, the amount of damping you get with this method isn't so mcuh, but with your 6 layers it might add up to something, i have never tried it. generally, a firmly screwed (like to meet building code) drywall sandwich yields a little less damping than a glued sandwich (with ordinary glues like drywall adhesive)
Quote:
Some studios are built with 2x6s (or deeper), 12" on cn center. That will do a lot to keep the panels from flopping around.
that'll stiffen the wall, but stiffening something does not inherently damp it. damping, by literal definition, is the dissipation of energy. just stiffening something doesn't give us means of making energy go away.
_________________ All posted information copyright Brian Ravnaas
on the very bottom of the page, glued in between drywall offers MARKEDLY superior damping to any of the roofing or tar products i have found around here.
call those fellows up and see how cheap it is in a big roll.
and it is very possible that some roofing product out there also performs very well, but i haven't found such a beast.
_________________ All posted information copyright Brian Ravnaas
Technical Director, Audio Alloy
jazzman_in_pa Recording Org Pro Audio Forums
Joined: May 12, 2003
Posts: 796
Location: Philadelphia
that'll stiffen the wall, but stiffening something does not inherently damp it. damping, by literal definition, is the dissipation of energy. just stiffening something doesn't give us means of making energy go away.
As true as that may be, a stiffer wall sure SOUNDS like it's damped more. I've got some wooden stud frames with an inch and a half of paneling on them. One section of the framing is 2x4s, 16" oc, with 2x4 cross-bracing every two feet apart. Another section is 2x12s 12" o.c. When I bang the 2x4 section with the heel of my hand, I barely hear a "boing", but when I hit the 2x12 section, the sound is so low and short that it's obvious that something fortuitous is going on. Let's consider what happens when energy from the hand is transferred to the wall section. In the 2x4 wall, the paneling itself can move more freely, and does. In the 2x12 section, the movement of the panels is inhibited, so the energy is distributed over the entire mass of the wall assembly. At least that's my guess.
That asphalt-based product at the bottom of the page is $1.80 a square foot in small sheets. It happens to match the description of the stuff I was tallking about, GAF Liberty SBS: asphalt-based, self-adhesive, and a 1/16" thick.
Quote:
i'd very much like to test that if i could find any
I found some by calling major roofing distributors in the yellow pages. Anybody who buys from GAF can order it. In fact, I found out about this Liberty SBS stuff by calling customer service at GAF, asking about the regular stuff they sell at Home Depot ($39 a roll, non-granular), and the rep suggested that Liberty SBS would be easier to install since it's lightweight and self-adhering.
Good luck with the damping product you're working on.
that yields a damping factor of ~0.08-0.09 when sandwiched between 1/2 drywall. based on a solitary test, so more should be run, but it is plainly superior to any off-the-shelf material we have stumbled across to date. for reference, typically 5/8" drywall is 0.006, 1/2 ranges from that to .01 or a little higher.
_________________ All posted information copyright Brian Ravnaas
As true as that may be, a stiffer wall sure SOUNDS like it's damped more. I've got some wooden stud frames with an inch and a half of paneling on them. One section of the framing is 2x4s, 16" oc, with 2x4 cross-bracing every two feet apart. Another section is 2x12s 12" o.c. When I bang the 2x4 section with the heel of my hand, I barely hear a "boing", but when I hit the 2x12 section, the sound is so low and short that it's obvious that something fortuitous is going on.
another interesting example is simple steel. if you get a large ball bearing and hit it, it sounds very dead. but it has the same ~.001 damping typical of steel.
concrete block is another nice example. very low damping factor, but if you hit one with a knuckle it sounds very dead.
impact noise (hitting something) is different than airborne noise as well, in that the maximum velocity that the panel reaches from an impact is DIRECTLY related to stiffness.
so stiffen a wall, and if flexes less when you hit it. makes sense, right? flexes less when you hit it, and it makes less noise, as it has less initial vibration amplitude.
however, airborne sound is a bit different at almost all frequencies.
an interesting analogy is loudspeaker drives. some subwoofers have very loose suspensions, you can easily push them in and out and in and out, just a tap from a fingernail and the cone moves alot.
some are very stiff, have to push HARD to move them at all. car subwoofers are often very stiff.
interestingly, if we changed the stiffness of the suspension from very light to very very stiff, obviously us tapping the cone would have less of an effect...
however, the efficiency of the loudspeaker DOES NOT CHANGE AT ALL from varying suspension stiffness.
only the resonant frequency does.
more on that in a moment, my young son has tired of his pals and come back in.
_________________ All posted information copyright Brian Ravnaas
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