My experience with Green Glue

[Rod Gervais]

[pardon me, i don't think i fully understood the latter half of your post, seems like a jab at cost-effectiveness of something or other. not to be a pest, but as AA has only distributed the test data for GG on request, i don't think there's grounds for conclusion. :)

Brian,

I like you - and am not taking a jab at either you or your product............

I agree that there isn't ground for conclusion yet - and more testing has to be performed, but at some point in time this will surface as making sense from a cost point of view - or not making sense from a cost point of view.

Not a jab at the product - but it has to work (in the long run) from both perspectives.

When I headed up the design team for a movie studio I built - the acoustic consultant came up with a beautiful design using a major manufacturers isolation products...........

This in response to a design I sent him to analyze to verify my analysis.

His design would have worked - but I still forced him to do the analysis.

When it was all said and done we went with my design - and saved the client over a 1/4 million dollars in labor and materials (real estate stayed the same, 8) ) just by going with standard construction products with some specialized personal details - and provided the same isolation.

If I had no data to do a cost analysis this money could very well have been thrown away.

When we speak of pennies a square foot - it doesn't sound like much one way or another - but I do this for a living - and am getting ready to join a project that will develope 34 movie studios in the north east over the next few years.

pennies a square foot mean something in a 15,000 s.f. studio with 50' walls to the underside of roof.

Multiply that times 34 studios and we're talking a LOT of money........

I hope you can see my point.

Sincerely,

Rod

[Brian R]

hi again, Rod. i like you too, Eric as well. you guys are very kind above with the compliments. I was just saying don't be sure of a conclusion. :)

in any case, i'm going to enter 3 or 4 or 5 posts about the effect of damping on various structures here, more when i can.

the goal is to outline the effect of damping in as plain a manner as possible. you will find nothing that i enter below to be in err in a prudently run lab test.

I'm going to post about damping, and it's impact, i'm going to resist the temptation to counter the overbearing implications that damping can't be a cost effective answer. I'm going to avoid talking about the pros and cons of damping relative to other options. And why doesn't everybody just join that bandwagon. rather than a patent bicker-fest, let's just talk science here. :) let's just chat about damping.

[Brian R]

ok, imagine an enrmous sheet of drywall or plywood or whatever with no studs, just a huge wall of continuous drywall...

something called "mass law" will define the performance of that wall over almost the entire frequency range below something called "coincidence"

coincidence will occur at high frequencies, maybe 2-3000 hz, and cause a big dip in performance there.



that coincidence dip looks pretty dramatic, but in alot of situations it isn't that big of a deal. the reason is because it occurs at such a high frequency... calculated data for undamped and perfectly damped.


in other assemblies, that coincidence dip can occur MUCH lower in frequency, and there it is a more important factor in the overall performance.


the effect of damping relative to no-damping for a single leaf non-studded panel is huge at that coincidence dip, but because that dip isn't all that important, the most you could expect from drywall would be perhaps 1-2dB of actual improvement.


now, note this, and this is critically important: DAMPING CAN ONLY RAISE A PANEL TO MASS LAW. the ever-popular software simulations are incorrect. anyone with alternate comments to that is incorrect. trust me.

in a huge, unbounded panel, low-frequency resonances will occur, and be incredibly severe, but they will be so low in frequency as to be inconsequential.



in some structures - such as perhaps a gypsum concrete over OSB floor - the effect of damping may be several dB. why? because the coincidence dip may occur at 400hz, not 3000... see?


so the important things:

1. in an unbounded panel, mass law rules

2. damping cannot raise the performance beyond mass law (neither could resilient channel, etc., not with a single panel)

3. the value of damping is mostly related to where the critical frequency/coincidence dip lies. in a drywall assembly, this is high in frequency, and damping is not so important



edit: to add some calculations:

for a sheet of 1/2" drywall, the gains for critically damping the sheet would be

2-3 STC points
~0 OITC points
fraction of a dB of music or theater noise gain

clearly this is a complete waste of resources


for something with a lower coincidence dip, like 13/16" plywood maybe, the impact is higher (theoretical gains shown below for moving to critical damping):

8-9 STC points
~3 OITC points
1.5-2 dBA music or theater noise

still modest improvement.



for something like, say, 4" concrete with a still-lower coincidence dip, the impact is pretty large (again, theoretically going to critical damping)

9-10 STC
8-9 OITC
~8 music noise


NOTE: YOU CANNOT CRITICALLY DAMP ANY OF THE ABOVE STRUCTURES. on the lightweight (drywall or plywood) structures you could add enough damping to get within a stones throw of the predicted gains for critical damping.

on 4" of concrete it is not likely you could add enough damping to get closer than a few dB of the critical damping gains. talk to someone long and hard before spending money on damping a concrete wall, there are technical challenges to that of notable size.

[Brian R]

ok, now a more realistic single-leaf


in JASA, 1977, Lin and Garrelick described the behavior of a single wood stud wall. based on extensive work on airplane wings, i think, they showed the basic behaviors that i'll outline in my next post.

basically, a series of mechanical reosnances and anti-resonances, NOT RELATED TO the air cavity.


they also outlined that these same resonances will occur in a single-leaf studded assembly as well, but far less severe in magnitude. And indeed, this is found in the precious few such tests that can be found, including some from the NRC. you can even find commentary on this in some NRC discussions, i think IR-693. :)


as such, the impact of damping is larger. the graph below is 100% hypothetical, i've never actually measured a studded single leaf, so i have nothing to offer.



the value of damping will be directly related to how severe the resonances are, and how much damping whatever treatment is applied yields.

if you look at the wood-floors-over-joist examples from IR-811, you can see considerable resonance problems.

YMMV. but in any case, the overall effect of damping is not likely to be more than several dB. maybe 5dB on broadband noise or so, and YMwillV, depending on the existing resonance conditions.


the critical points:

1. this is still a signle leaf, so mass law is still applicable, and still sets the limit

2. the presence of studs or other mechanical constraints creates, in essence, smaller "panels" out of the large panel, pushing resonant behavior up into the relevant frequency range

3. overall gains will vary from system to system, probably the largest gain to be had in a common system is on a wood sub-floor, where the wood is screwed so tightly down.

4. - and this is critical - Lin and Garrelick outline that the same series of behaviors occurs on all studded assemblies, double leaf or not


some estimated gains, as above, assuming you move to critical damping:

on the most-resonant subfloor-over-joists example from IR-811, 6-7 dB of broad-band noise reduction are feasible by moving to critical damping. perhaps 5 on an extremely rolled-off curve like Tennekes.

on drywall over studs this will generally be a bit less


so the gains are much more notable, but not nearly as high as on double leaf assemblies.

dig around in IR-811, have some fun.

[Brian R]

ok, those are pretty simple. mass law sets the limit, you can easily estimate how much gain is possible due to damping by estimating how far below mass law you are. remember, no attainable damping system will be perfect, and how close you get to mass law will depend on how good your damping system is.

[Brian R]

ok, now for a bit of a more complex system, and then i'm off to lunch.

the most common wlal in america by 10x. the single wood stud wall.

Lin and Garrelick modeled this wall nearly 30 years ago... nobody seems to have paid much attention to their work,

their basic predictions:



notes:

1. far below primary structural resonance this sytem is mass defined

2. primary structural resonance is mechanical

3. these walls will exhibit anti-resonance (in theory damping would make a panel WORSE at anti-resonance)

4. this series of resonance/anti-resonance occurs at all frequencies, but the normal coincidence dip dominates at higher ones...


so 30 years ago these guys made this model. essentially every single TL test every run on wood stud assemblies reflects these basic predictions. they were dead-right.





so, back to the topic at hand, what effect can damping have on a single wood stud wall. the graphs below show data taken in my lab, and in 3rd party labs. This data is as follows:

1. difference between damped and non-damped panels are subtracted
2. if mass was different (say comparing single layer -vs- double layer on the same construction, later adding damping and more mass), the curve is adjusted down to compensate for the higher mass



the basic regions are observed above

1. theoretically no effect in the mass controlled region

2. (depending on damping level) very large effect at primary structural resonance

3. a minimal effect at the first big anti-resonance

4. (depending on damping level) large (10db+) effect at mid

5. (depending on damping level) enormous effect at coincidence

6. nowhere is it worse, no trade-off, no alternate resonance at some lower frequency, etc.

7. this dissipation-of-energy-over-distance creates a sort of decoupling effect and raises mid/high frequency performance. this cannot happen in a single leaf, but can/does in a double leaf single stud.

#4 & #7 might be suprising to anybody with some technical knowledge. the reason this occurs is the dissipation fo the bending waves ocross the panel, before reaching the stud. at high levels of damping, this effect can be immense, >50dB/meter...


now, these things must be noted:

1. location of the primary resonance depends on stud spacing, stiffness of the panels, and insulation & cavity depth (mostly stud spacing and stiffness)

2. thinner drywall will give higher performance in combination with damping, mass-for-mass, due to lower bending wave speed yielding higher rates of decay over distance, and the lower stiffness yielding a lower primary structural resonance, all things equal

3. wider stud spacing (for a single stud wall) will give somewhat higher performance in combination with damping, due to longer distances to travel before reaching studs and lower primary structural resonance. However, depending on the shape of the noise curve, this may or may not add up to gains in dBA calculations.

4. as damping becomes very high, the systems wind up limited by the mass-defined region, not teh resonant and higher regions. kind of fun, that



and there you have it.

-some to large gains across the mid/high frequency range due to decoupling-by-dissipation

-some to enormous reduction of resonant dip at the primary structural resonance

-reduction of coincidence

those are the effects damping can have on this type of wall. It has those effects due to the combinatoin of damping (resonance reduction), and energy dissipation over distance.


FWIW, adding raw mass has typically yielded predicted (like with Erics spreadsheet) gains about proportional to the change in mass. in other words, doubling the # of layers raises performance about 4-5dB (this doesn't actually double mass, as studs and insulation have some weight, etc.). adding a single layer yields about 3dB.

the net gains from adding one layer + damping have shown to yield
8-13 STC points
8-10 OITC points
8-10 dBA with the music and theater curve i use sometimes
similar for dBA with other heavy-LF curves

YMMV, results will vary with different levels of damping, i only have results for a few systems, of course.

for adding 2 layers + 2 dampinmg layers,

12-17 STC points (larger variance in STC w system parameters)
11-14 OITC points
11-15 dBA with M&T
blah blah blah

you must, of course, adjust those down for mass. the bulk of experiments have been build wall, add damping and more mass. if anybody cares, there is 3rd party data in line with all of the above for both possibilities. others likely have data of their own as well. in any case, it is not unreasonable (and indeed substantiated by 3rd party blah blah blah) to expect a contribution of 7+ (or as high as 10 or 11) dBA on broad-band calculations for the addition of a nice level of damping. on this type of wall, other walls are different. :)



YMMV, check existing tests to verify the ~5dB for doubling layers with conventional materials. ~3dB for adding 1 layer.



ok, lunch time, i hope those were A) interpretable, B) interesting, and C) etc. Feel free to ask for additional input.

take care,

Brian

edited after some calculations. obviously it is VERY possible to add damping to a wall and get 0-1 or 3-5 or whatever dB gain, it all depends on how much damping, i report what data i have, some is taken in my lab, some is 3rd party. :)

[AndrewSteel]

Hi Brian,

was that the right link?? I had a look but didn't see what you referred to.

Andrew

[Brian R]

andrew,

there is a picture about 1/2 way down the page that shows a graph of the change in reverb time - higher on the graph is more absorption.

absorption coefficients ... i didn't attempt to calculate them, and they would have alot of uncertainties anyway, i mention a couple of things in that thread.

damping seemed to reduce absorption at mid/high frequencies, didn't seem to much at the "panel trap" frequency, why i can't imagine.

Brian

[z60611]

Brian R:

DAMPING CAN ONLY RAISE A PANEL TO MASS LAW

I was thinking about that a bit this morning.

As phrased I think it's correct -- i.e. the word DAMPING. Damping to me means that graph you have at your website where a damped vibration lasts less time than an undamped one.
e.g. vibration: http://www.audioalloy.com/commdamp3.gif (from here)

But viscoelastics do more than just DAMPING. They also resist deflection (elastic). Which possibly means that any deflection will be resisted, including the first impact of the wave (not just vibration).

A wall probably moves a lot more (accelerometer, laser interferometer) at resonance and coincidence, and the more deflection (i.e. distance), and the more deflections (i.e. repetition/vibration), the more efficent the viscoelastic would be at grabbing the energy. But wouldn't it add just a bit of TL regardless of frequency, because it's resisting movement even a bit?

There's a nice description of coincidence at
http://www.squ1.com/index.php?http:/...nsmission.html
(also the bit about SRI of materials was interesting there - if Plywood is any indication, then MDF in walls might be worse than Gypsum for more than just cost)

a) Resonance causes a lot of movement.
b) With the animation about coincidence it's obvious how the traveling wave in the panel is getting more energy from the sound wave that's moving at the same speed, thus getting a lot of movement
c) but even without coincidence or resonance, by definition, for sound to get through the wall, the wall has to move and flex.

for (c), I know a large mass can move a little and induce a large movement in a lighter mass. e.g. large mass = gypsum, ligher mass = air. Unfortunately viscoelastics require more movement to be efficient.

Nevertheless, how do you feel about:
DAMPING CAN ONLY RAISE A PANEL TO MASS LAW
VISCOELASTICS CAN ONLY RAISE A PANEL LESS THAN A dB ABOVE MASS LAW

(BTW, great post)

[jazzman_in_pa]

I don't want to interrupt the general discussion of damping here, but just want to interject a couple of things in response to Eric, Rod, Z, and Brian:

You guys rock! You're certainly raising the right questions: is GG worth it? Or is it solving a non-problem?

As Z pointed out, if AA's tests are right and there is substantially improved TL at 50/63 Hz in the classic double stud wall with 2 layers of 5/8" gypsum per side, then the GG is worth it to me. Was it necessary in the ceiling? I can't know, but I do know that it sure sounds nice and dead, and didn't add more weight to those overhead floor joists--which is an advantage over adding more drywall, plus the application of the glue doesn't take much time compared to measuring, cutting, trimming, futzing with, and screwing another panel to the ceiling. It may also be worth the extra $100-200 or so for the wall in question between the control room and vocal booth for the same reason that Paul has built a sonic bomb shelter: a little bit more domestic tranquility. I love my wife and I love my music. (I love my kids too but not necessarily their music. ; - )

I've been mulling over Eric's and Rod's experience that well-built walls don't ring to the point of a problem. Here I'm wondering about two potential differences between Rod's walls and mine: I'm sure you build a helluva wall, Rod, and secondly, your studio rooms are larger than mine.

1. My wall may well be TOO stiff. Two years ago when the MDF went up, I imagined that stiffness was a good thing, limiting the wall's movement and the ability of sound waves to pass through. But since then IRC-IR-761 and you guys have made clear that the increased "give" (i.e. movement) of panels on studs that are 24" apart improves TL, and Brian has helped make clear just how important it is to consider resonant frequency. Learn something new every year. (Don't you hate that/love that?) So maybe my problematic wall needs some GG just to compensate for the excessive stiffness and resonance.

2. Suppose for a moment that my said non-GG wall and a normal professionally built non-GG wall were to "ring" for about the same number of milliseconds. In Rod's and Eric's world, those walls are farther away from the listening position, and the ratio of sound coming from the speakers to the sound coming off the walls will be higher and better in a larger room than in my smaller room. In other words, is it possible that what isn't a problem in a larger room becomes a problem in a smaller room, or do you think the difference between large and small rooms don't matter in this regard?

Eric, in talking about the damped versus undamped metal booth, raised the question of the psychological versus actual effect of damping. Let's think about this. When we rap the side of some object to gain a sense of its solidity, the brain interprets the sound and evaluates the object on the basis of our previous experiences. Rap on a 12" diameter wooden pier support and you associate certain positive values of massive strength to the object. Pound on an 8" thick concrete wall and you do the same. IOW, our brain imputes a certain amount of mass to an object on the basis of the sound we hear. So when we knock on the side of a damped metal sound booth, we may impute greater mass to it than it actually has. My good buddies here are reminding me (in the nicest way, you gentlemen) that my brain has fooled me. They could have said, "It's the MASS, Stupid" and not what sounds like mass, that counts.

One issue I'd really like to clarify for myself one day is: To what extent, at what point, or volume level and frequencies, does a vibrating object (other than the loudspeakers and air molecules) in our room clutter/muddy the listening environment through phase cancellation, and what, if anything do we need to do about it? (Then, there are membrane/panel bass traps which work precisely because they vibrate out of phase with a problem frequency.)

Lee