Panel Resonance

[Eric.Desart]

Brian,

I really should like to see such effects (2 different thickness layers).

It's very interested to see such isolated phenomena.
However it's also interesting to relate this to real live circumstances.
What I mean is, If I have a free vibrating steel plate I can get sensational effects with damping.

Important is what is the gain in mounted circumstances causing the damping from the whole system to be significant higher as from bare plates or panels.

Brian I do believe that you're one of the best to be found in studio environments on the net.
What's important to me that one finally can transplant those ideas towards gains in practical circumstances.

While important to study the individual phenomena, one of the classical things I experienced in studio groups is that individual phenomena become more important than the overall picture.
What I mean is.  If a wall can improve with 1 to 2 dB music noise, by complicated, expensive or time consuming measures than one should make that very clear.

I personally NEVER heard a rattling normal good drywall, and I also measured with a thousand watt noise source (I can really be a noisy guy).  Of course one can excite whatever resonance frequencies, but that the same for wine glasses which start to travel. I never suggested anyone that he should glue them in his cupboard.
I try to imagine this. One has 2 panels without an airlayer. They almost suck to one another. If it should rattle what does happen then?  In any case I never heard this as a problem mentioned for a normal good construction.
Does anyone else here ever heard or encountered a problem with ratling walls (standard good build of course)?

This standard gluing has also it's counter arguments.  So I really should like to see somewhere an objective measurement comparison.

Does a home theater use higher levels than disco's and studio's?

I wonder why one only sees this for home theaters and not for studios, this argument.
I also remember Dennis' comment to the lateral and upward instability of a drywall ceiling when hung on those Kinetics spring hangers. Such a double drywall ceiling is massive as hell, if you want to push (even when balancing) such a ceiling aside away from his gravity equilibrium one needs a serious amount of force.

I should like to see the different thickness effect on damping.  It's still the easiest and cheapest manner in those rare cases that it should matter.

Many thanks
Eric

[Brian Dayton]

hi eric,

a few things to reply to here...

i'll post some measurements when i can, always fun to talk about those things.

It's very interested to see such isolated phenomena.
However it's also interesting to relate this to real live circumstances.
What I mean is, If I have a free vibrating steel plate I can get sensational effects with damping

raw metal, and metal in general, is by far the most dramatic application of damping, yes.  because it starts with such incredibly low internal damping, to even attain a little bit of damping - even a poor damping material can have stunning effect if applied to a bell or steel bowl, etc.

no mystery there, steel has damping factor of ~~.0005 to .001, raise it to .04 (still poor damping) and the difference is excellent!  40 times higher damping, ...

While important to study the individual phenomena, one of the classical things I experienced in studio groups is that individual phenomena become more important than the overall picture.
What I mean is.  If a wall can improve with 1 to 2 dB music noise, by complicated, expensive or time consuming measures than one should make that very clear.

it's always important to isolate phenomenon so we can figure out why the whole is as it is.  

i'm not sure what the 1 to 2db music noise and complicated expensive measures are?

I personally NEVER heard a rattling normal good drywall, ...  In any case I never heard this as a problem mentioned for a normal good construction.
Does anyone else here ever heard or encountered a problem with ratling walls (standard good build of course)?

This standard gluing has also it's counter arguments.  So I really should like to see somewhere an objective measurement comparison...

Does a home theater use higher levels than disco's and studio's?

i skipped some of that quote to keep this post of reasonable size.  

i'd like to see a comparison of normal drywall to conventional adhesive as well, i never have.  i expressed my ponderings on this long ago.

i have heard a rattling wall on two occasions.  the first is the old lab room for tests like the ones above.  buzzed at 88hz.  additional screws/tightening resolved the issue in that case.  the second was with a TL specimen that make an awfl racket when stimulated with anything from 80 to 200hz in the chamber.  

dis-assembly revealed a fracture in the gypsum which was not visible under the paper.  came hat way from the drywall supply place, apparently.

i haven't a clue about relativ volumes.

the phenomenon outlined by the NRC (which is measureable) suggests that resonance between unbonded layers can/does occur.  i can post some pics of tests i ran in an attempt to isolate that phenomenon as well.

I wonder why one only sees this for home theaters and not for studios, this argument.
I also remember Dennis' comment to the lateral and upward instability of a drywall ceiling when hung on those Kinetics spring hangers. Such a double drywall ceiling is massive as hell, if you want to push (even when balancing) such a ceiling aside away from his gravity equilibrium one needs a serious amount of force.

DE has posted on various occasions dating back, i think, years about the necessity of very good constructon practices to avoid potential rattle problems.  he seems to have found that utilizing adhesive eliminates this possibility.  considering that he's built alot of theaters, and i have probably never encountered anyone who gets such universally rave reviews from his customers, i posted that i thought his recommendation should be taken seriously.  if he finds that liquid nails prevents rattles, i have no basis to argue with.  your logic about lateral motion is very sensible, of course, but i can't bend a layer of concrete block, but sound can.

anyway, you seem perturbed in your post here again.  again i'm at a loss for why.

take care,

Brian

[Brian Dayton]

anyway, to add

i offered to jazzman, and his lack of reply made me wonder if he found my comment dissapointing, that the modest gains in damping resulting from liquid nails or something were likely trivial.

and they likely are, and for the reason eric outlined above.  real constructions will almost always exhibit damping a bit higher than the materials they are constructed from.  screw locations, friction between layers, and the viscous-piston-shock-absorberish damping effect of air leaks all contribute to this.

i would offer ~0.03 to 0.04 as damping of a normal drywall wall, and i'd ask to be allowed to collaet all of our recent data from testin various non-lab walls before being pinned to that.  

and that's several times higher than the raw drywall, but again is not a meaningful amount of damping.  and my opint to Jazzman was just that the various quirks and interactions in normal constructions tend to drown the effect of incremental changes due to this's or that's

[Eric.Desart]

Hi Brian,

Thanks once more.

Don't worry about how things sound. Otherwise I'm more busy with language and how how it feels than the content. I don't feel, hear are talk with English ears and must express myself with a limited vocabulary.

I feel uncertain.
At one hand there is the physics of different phenomena, which is very interesting to study the impact.

At the other hand there are studio groups searching withing often limited budgets the best solution. In the case of TL the highest TL with the emphasis towards the lowest frequencies.

So one build walls.  What is the most cost effective manner to obtain this goal.
I still must check lot of your messages back.

When, why and how is there a reason to deviate from standard building practices.
Damping a coincidence is good within a degree it causes trouble.

Please Brian. I get the feeling sometimes that I repeat myself.

There is Eric the acoustician interested in physics and all those phenomena.
There is Eric the acoustician who hates that people like to complicate things, making it more expensive to obtain their goals.
This are 2 different guys combined in one.

I try to get things to there basics. If somebody tells to glue panels because a wall rattles, then I want to know why standard building practices shouldn't do the job.
I've measured a lot in labs where we use maximum power to get maximum dynamic range.
I've done lot's of jobs in practice not specific (but also) for home theaters but related to Horeca, Studios, and whatever. So I really am not without real live practice.

If I never felt the need to glue something to prevent rattling, or heard any of my colleagues talk about it, and the home studio group is the first I hear about rathling walls, then I wonder why (assuming build alright).
Correct is that I'm more used with metal framing.  Wood in drywalls is more limited here to the DIY world.  This has nothing to do with good or bad. It's just our culture and availability.

I can judge the arguments DE uses in function of this upward deflection. A steel spring works close to linear.  
I know this lateral stability: I have a very hard time buying that argument.

An easy manner to damp coincidence should be this thinner drywall, ONLY needed when it matters which it not often does and in some degree, but it's cheap and easy to apply.
I indeed should like to see some more objective data about that.
Important what does it do for low frequencies (don't think much).
How do such walls compare? (additionally assuming same total mass)

Warm regards
Eric

[Bob]

Eric:

Although Dennis Erskine used to recommend gluing Gypsum to studs, and Gluing gypsum to gypsum, this is now obsolete.

These days he recommends using Green Glue (cheap DIY viscoelastic compound) between layers of gypsum.

[Brian Dayton]

hi again all,

evolving topic here...  coincidence, damping, and rattles.  didn't this start with rattles?

i'll start with rattles.

eric, your practical experience is one of your most marvelous attributes, i sure never called that into question.  but DE has epic experience as well in the home theater arena.  do other theatre builders concur?  i haven't even a foggy notion.  but i did run home to bring my young son a coat, and playing loud 1/3 octave warble tones in my living room set off buzzes in the 80 (just a wee bit) & 100hz bands (stronger).

to be FULLY CLEAR:  that's not my argument, my philosophy, or my area to argue.  i've built not so many walls outside of the lab, and lab walls don't rattle.  all i can say is that it can happen and it seems to have happened in someones experience, and i don't think liquid nails will cause the world to end, hence my comment.  

take it up with DE, i'm sure he could much better explain the how's and why's than myself, i am not the construction guru here by a long shot.  and to be honest, i really don't care outside of making sure that anything i say doesn't lead someone building themselves a theater astray.

now concidence.  i've attached graphs of direct comparison (no other variables) for going 5/8 / 1/2" drywall instead of  5/8" / 5/8", and some other examples with similar assembly.  there is no reason based on the NRC's work to conclude that this effect is even particularily significant, much less sufficient to wipe it out.


now damping.  if i was gong to sit down and ponder the merits of damping a wall (i did once sit down and ponder that, for about a year, lol, more on that early next week), i wouldn't fret about the coincidence region at all.  in a floor i would, particualrily in a floor with concrete or gypsum concrete layer.  but not in a wall.  

no, i'd turn a lustful eye towards the mass spring mass resonance - the bane of most walls existence - and see if there was a way to mitigate that guy.  the first thing that i'd do after that is realize that the concept of a mass-air-mass resonance is folly in many wall constructions, and ideally correct in none.

to make that unconventional statement intuitive:  drywall on a resilient channel would have some resonance pont, as resilient channel is not infinitely limp.  hence the resonance in an RC wall would be air spring + channel spring.  in some constructions the channel spring may be <<< than the air spring and neglectible, but in many constructions... etc., etc., etc.  just think about it, for any conceivable wall to resonate, something has to bend, and whatever is bending adds another measure of stiffness.  channel has to bend, the rubber puck on an RSIC clip has to smush, studs have to bend, drywall panels have to bend, etc.  something always has to bend + air, not just air.

after that i'd look at conduction over distance through structures - at how much damping can reduce the abillity of a material to transmit vibration over distance, if i was building a floor i'd drool over the theoretically predicted reductions in impact noise, and then i'd ponder the possibilities of reducing resonant activity outside of the primary mass-spring-mass resonance.

then perhaps say "oh, and the coincidence dip is mitigated as well, which is nice"


eric said "I can judge the arguments DE uses in function of this upward deflection"

was that intended to mean all statements by DE can be judged based upon this single one?


ok, more later, and nice discussion/cause to think.  always a pleasure to have something to think about, for me.

take care,

Brian

[Brian Dayton]

in more carefully reviewing the data offered by the NRC, i think it would behoove me to change my response from "use all heavy stuff" to "it really doesn't matter at all"

lol  

[Eric.Desart]

Hello Brian,

I check Google to-morrow for messages of Noral and Angelo.
The difference is rather small between 4/8 and 5/8.

Also strange is that one should expect the coindicence to shift ca 1/3 octave for the thinner panel..

I agree, a measurement is a measurement.  However it looks as the panels respond as 1 massive panel.

Certainly in the second this doesn't broaden coincidence.  Also hardly in the first.

I'll check tomorrow.  Any answer causes new questions.

Many thanks Brian

[Brian Dayton]

those examples are for 5/8" + 1/2" on a wall side.

so perhaps one would expect a "blend" of 2000/2500 and 3150hz Fc's?

a comment aimed at nobody, just a general thought.  damping seems to be the strangest thing.  first year physics topic, but perhaps the largest mystery in all of sound isolation.

resonance next in both counts.  lots of super-smart people who don't seem to even grasp what a damping factor is or that damping is measureable or even that damping = energy dissipation, and only energy dissipation.  that is the most literal definition, and all the other things - resonance peak, transmission over distance, etc. - flow from that solitary phenomenon.

yet damping = isolation to some.  spring isolation is referred to as damping.  damping = stiffening to others.  damping is called dampening, damping materials are reported as having mystic properties at times, and as being the devils work at other times.

the only thing everybody agrees on is that laminated glass has superior sound transmission to regular glass.  and that's with a thoroughly mediocre level of damping.  thoroughly mediocre.  and also in constructions where the worst of the resonances really can't be controlled by panel damping.

anyway, that's just something which has struck me in my time on these forums.  the vacuum around damping as a topic.

and anyway, perhaps the most discouraging isn't that this very straightforward concept is very misunderstood.  it's that everyone is an expert on it.  i worry about that document i posted a while back about the damping of different ordinary glues.  

someone is going to come across it and proclaim OSi drywall adhesive a "very high damping" glue.  this roofing felt business...  touted as mystical damping material #1.  but i wonder if any lab on the planet but ours has ever tested it?  i'd guess not, considering that it's 1) not cheap (40 cents/sq foot around here for the ~0.8lb/sq foot type) , and 2) not very effective.  but that doesn't stop it from being the solution.

there was talk of wasting money above.  now those CAN really be a waste of money from a damping perspective, at least with most variations on that material type i have encountered.

lead!  lead is touted as allowing the construction of panels with "really high internal damping".  lead itself isn't well damped (insul software reports <0.1, other sources between .05 and 0.1, i've never tested it), although it's contributions are perhaps notable and worthwhile.

sound board causes huge gains in isolation by "damping out resonances", so one can read.  well, the highest damping factor soundboard i have ever measured has damping of ~0.045, high for a construction material, but a VERY LOW level of damping nonetheless.  pre-fabricated damped boards are sold.  but i wonder if the local vendor would like to see the damping factor or impusle response of the sample he gave me?  lol

and people forget that what they say is taken as gospel by many who read it.  someone with a question in his mind finds someone posting this same thing, and viola, it's truth, and he preaches it to half the planet.

as long as i'm (cheerfully, i mean malice to nobody, and i aim my comments at nobody as well) venting here...


the textbooks need some considerable work here as well.  

nobody feels the slightest bit constrained to be accurate when speaking on this topic, it seems, or very few.  maybe at best they made their felt panel and tapped it with a knuckle and it sounded better.  sort of like talking into an egg-crate, concluding that it sounds different, and using this as proof that egg crates are marvelous.

this different thickness thing...  damping = ability to dissipate energy.  put something with no dissipation ability next to something with no dissipation ability and you have two somethings with no dissipation ability next to each other, not a modern marvel of damping.  it CAN create a more chaotic, less focused resonant region.  it CAN raise damping via friction between panels, but so can same thickness.  

alas, my sandwich arrived and i copmletely forgot the rest of my ramblings while staring at the meatbals...

but i swear i had something useful to say.  guess it will have to wait.  

oh yeah, i remember now.  i was going to ramble about spring-mass isolators and how half the posting planet seems to think isolation ISN'T frequency dependent.

but another time, another time.

and i would ALWAYS be pleased to see some test data that supported the different thickness theory.  i simply never have, and that includes plenty generated by self.

and i hope my post was enjoyable and didn't sound terse, for i am certainly not upset with anybody.

[Brian Dayton]

Also strange is that one should expect the coindicence to shift ca 1/3 octave for the thinner panel..

I agree, a measurement is a measurement.  However it looks as the panels respond as 1 massive panel.

Certainly in the second this doesn't broaden coincidence.  Also hardly in the first.

the regions do broaden a bit.  but just a wee bit.  you said above:

I try to imagine this. One has 2 panels without an airlayer. They almost suck to one another.

so perhaps if they are sucked together we'd expect them to behave as a single mass?  maybe this all fits?


i wonder sometimes if this mystic damping capacity of different thickness layers stemmed from anyones test data or from just thinking about it...  and it makes sense superficially, so it was propagated across the years.

another thing that i wonder about (WONDER, not making great issue here):  show me test data where the low frequency behavior of a wall is superior with LESS mass because of this "if the leaves have different masses, they resonate at different frequencies" business.  i see systematic improvements with addition of mass in IR-761.

even in the resilient channel section, it's worth mentioning that 1 layer of drywall on teh channel and 2 on opposing studs has WORSE low frequency behavior than 2 layers on the channel but 1 on the opposing studs.

the masses are more equal in the superior case, not the differing case.  the superior case has 2*drywall opposing drywall+studs

the inferior case has 2*drywall+studs opposing 1*drywall, broader mass ratio.

i'll post some graphs of that if/when i remember/have time.

look at the small # of 2x4 examples...  no mystic gains from uneven mass on the two sides.


i have many books on this topic, and have permused more.  i respect the contributions of their authors.  but frankly i think IR-761 should be course 101 when teaching sound isolation.  truly it is a gift to understanding.  and i really, really think that prediction software running MAM calculations should be taken with a spoonful of salt, and not treated as gospel and reason for indictment/approval of anything.

take care all and good night

Brian

[Paul Woodlock]

Very Interesting thread - thanks for letting it all out Brian


some layman comments...

so perhaps if they are sucked together we'd expect them to behave as a single mass?  maybe this all fits?

I've just been thinking about the layer thing/different thickness layer thing...

If two panels are close enough ( infinitely close enough ) together then they merge into 1 panel which has different properties.

However in practise the 2 panels will never behave exactly as 1 panel becuase of the very small airgap and other imperfections. In other words the panels are trying to behave as 1 panel, but the inperfections make them try to behave as 2 panels. And in normal buildign construction they are constructed accurately enough to make the 1 panel properties be the far dominant.

Does that sound like it makes sense

i wonder sometimes if this mystic damping capacity of different thickness layers stemmed from anyones test data or from just thinking about it...  and it makes sense superficially, so it was propagated across the years.

Probably from thinking about it. It's the conclusion I came to a cople of yeaers ago when I first designed my bomb shelter layers.

Originally I'd specified multiple layers of not only different thickness drywall, but also different densities.

My layman brain came to this conclusion.....

"let's say we have two panels. One resonantes at 30Hz, the other at, say 50Hz. SO when we layer these two panels together the 30Hz panel won't be able to resonate so much at 30Hz becuase the 50Hz panel placed against it won't vibrate so easily at 30Hz, thus preventing ( dampening ) the 30Hz panel from doing so. And of course vice versa. Conclusion....  Placing these 2 panels together will make them 'dampen' each other of their offending frequencies"

I am starting to see that this way of thinking is flawed when you consider the multiple panels acting as one panel.

So do we start designing multi-drywall layer walls with an 0.5mm airgap between each differing thickness drywall layer?

And here's another layman question...

Just how much  relative to the total TL does the 'friction' between layers contribute?

Any dissimilar vibration of seperate panels in the direction perpendicular to their face will cause impact rather than friction. And any dissimilar vibration in a direction parrallel to the panel face will of course cause friction, but wouldn't the screws prevent most of this and try to make the panels vibrate as one?

Has anyone ever had a special 2" thick drywall panel made so they can test the difference between that and 4 1/2" drywall panels?

Right, I better get back to studio designing now, lest this post get's too long

Paul

[Brian Dayton]

Just how much  relative to the total TL does the 'friction' between layers contribute?

Any dissimilar vibration of seperate panels in the direction perpendicular to their face will cause impact rather than friction. And any dissimilar vibration in a direction parrallel to the panel face will of course cause friction, but wouldn't the screws prevent most of this and try to make the panels vibrate as one?

well, the friction between drywall layers will typically raise damping a bit, but typically less than using a normal drywall adhesive.

in your case, i think we have so many layers that damping might be raised quite a bit, and i've meant to test that since realizing the extent that you've taken your project.  i will do so someday.

the NRC outlined a phenomenon whereby two layers of drywall together yields alot less than the predicted gains over a range, and they hypothesized that this was due to a minute air spring between the layers.

whatever it's from the same phenomenon is present in various NRC and Riverbank data that i've found over the years.

but...  i don't think that would affect your situation as much, as i don't think 6 of the same resonance in series is necessarily worse than just one.

i also think that the raw mass of your constructions and superlative decoupling measures would yield TL so high at 800hz (about where that phenomenon was outlined) that the point is reasonably moot (this mini-resonance for between layers air space)

ok, my little guy is up so i better run, but i guess the answer to the gains of friction damping can probably be best divined by analyzing going from one + one to two + two layers from IR-761

i posted a document with curves reflecting that long ago, and Ido brough the thread up to the top recently

it's my considerable hope to identify a DIY damping solution, but thus far none have been identified  which are a match for the thing i posted at RO

[Brian Dayton]

hot off the press...

we've located a commodity asphalt material that exhibits good damping properties

damping of ~0.11 was found in a sandwich between 2 layers of 5/8" drywall with the asphalt stuff from Home Depot with adhesive on one side.  

couple of quirks:  the stuff has a gravely surface, so you have to bond it in with a pretty thick layer of some stiffish glue.  

anyway, the stuff was ~40 cents/sq foot + full coverage with adhesive leaves us 60 cents/sq foot at least (to fill the gravel you need a slug of glue)

so a pain in the rear, but the best we have found to date.  most of the asphalt based materials we've found do not exhibit good damping properties in this configuration for a variety of reasons.

so, i guess 0.11 is a useful amount of damping, and perhaps my previous waste of money statement would have to be retracted, especially if we could find a smooth grade that was useful.  who knows, maybe the gravel/grit digging around as the panel flexes is responsible for some of the damping...