call me stupid...

[whatismisophonia]

I'm still having alot of difficulty with a paradox here; if floppy materials absorb more sound because they give with pressure and absorb energy, wouldnt it take more energy to move a more rigid wall?  Wouldnt a wall that behaves like a membrane be a bad thing?  For example:  If you had a container of water with a hard barrier cutting the water in half, and dropped a rock in one side, the rigid barrier would stop the wave from being reproduced on the other side;  Conversly, if the barrier were flexble and slightly limp, it would give when the waves hit it, slightly reproducing the waves on the other side of the barrier.  I mean, it wouldnt make sense for your walls to be floppy right?  Or is it that soft matterials resonate differently than stiff ones?

[whatismisophonia]

"Barrier materials are used to block airborne energy. Structural materials, such as the metals used to build heavy-duty trucks, can serve as sound barriers, but these stiff materials also support structural resonances within the audible frequency range of 20 Hz to 20 kHz and can add to the total noise problem.

When a material resonates at or above the critical frequency—the point at which the speed of sound moving through the structure matches the speed of sound in the air—it fails to impede sound. This resonance diminishes effective transmission loss—the ability of the material to stop the flow of airborne sound energy through it.

Limp mass barriers, such as flexible, weighted sheets of vinyl or urethane, will not support resonances in the audible range. Instead, they block and redirect the flow of the sound waves."     - http://www.earsc.com/HOME/engineering/TechnicalWhitePapers/Heavy-DutyNoiseControl/index.asp?SID=69

This seems to make alot of sense to me, at least.

[whatismisophonia]

useful posts by Brian Ravnaas at  http://www.avforums.com/forums/home-cinema-building-diy/284503-diy-sound-proofing.html

Ok ok, i think i get it...
stiffer materials resonate at higher frequencies, limpid materials at lower ones.  In terms of soundproofing construction, the goal is to make the resonance as low as possible, because above the resonance, decoupling allows walls to vibrate seperately; at the resonance, sound is perfectly reproduced from one wall to the other; below the resonance, individual wave lengths are more than long enough to pass through the full thickness of the wall, such that both wall sections behave as one (a single solid mass).  Stiffer materials may resonate at a higher frequency, but because below that frequency they act as a solid mass not decoupled, they kind of suck a blocking most frequencies we need them to.  If a material were to be made limp enough that the resonance is low enough to be inaudible, it's individual constituents act more independently, regardless of whether or not it is incorporated into a decoupled wall system.
So then, in the case of the containers of water, i would hypothesize that what waves are being more easily reproduced through the barrier would depend on how the masses of different rocks (larger ones creating larger wave lengths and smaller ones creating smaller wave lengths), interact with different stiffnesses of barrier; that is, if we assume these barriers are made of similar materials, putting aside other variables.  For example, comparing a much softer plastic to a much stiffer one at similar densities and thicknesses.  That might be an interesting test.

Look at my @$$ up in here; i make the questions and anwser them too    with research baby (which almost entirely involves believing what people i dont even know tell me)

[whatismisophonia]

so here's what im trying to figure out now... why the hell would someone recommend adding 2x6 studs rather than 2x4's, then say the added "stiffness" is of benefit?  Also, the greenglue website says that, for example, solid wood is better than more flexible metal studs for lower frequency.  Am i going insane or does this contradict all the soundproofing principles ive understood so far?  It kinda goes back to my original question, "if floppy materials absorb more sound because they give with pressure and absorb energy, wouldnt it take more energy to move a more rigid wall?  Wouldnt a wall that behaves like a membrane be a bad thing?" barring the resonance issue, the question would still seem valid when thinking about how heavy a limpid matierial would have to be in order to not vibrate at low fequencies; plus the fact that youd have to have too much of it to be cost effective... jeeze, right back at the beginning.  Im gonna stop talking now...

[whatismisophonia]

would love a reply

[johnbergstromslc]

You just way overthink things....

This forum is not 'theoretical acoustics', it's devoted to practical soundproofing.  Mass, decoupling, absorbtion, sealing.  Pay attention to all four and you'll get a good result.  If you build an STC 60 wall and only end up with STC 55, well, that's about par for the course.  It's good enough!

You strike me as a curious person with a lot of time on his hands, but not much practical experience in actually building these kinds of walls you're discussing....

I used to be like you, an idealist - I wanted my walls 'optimized' (as you would say).  But it's a waste of time and money trying to reinvent the wheel.  Stick to the tried-and-true solutions!

[whatismisophonia]

completely missed the point....-_- I'm not trying to reinvent the wheel, I'm just trying to UNDERSTAND what people are saying here and there.  I don't have personal experience, but those who do often sharply disagree, even on practical application.  By the way, I'm acually talking about practical application, not theoretics.  If floppy materials are better, why do some advocate a stiffer wall? It's a simple question, I just don't know the answer personally, and am just trying to work it out.

[johnbergstromslc]

A stiff wall is not a bad thing - most homeowners will insist on it!  As long as it's somewhat decoupled from one side to the other.  Drywall on resilient channels is not really 'stiff', but 'floppy' is not quite accurate.  'Spongy' is the term I would use.

If you're refering to MLV, it isn't exactly 'floppy' either.  MLV is classified as a viscoelastic material, which means it has both elastic and viscous characteristics.  When sound waves hit it, energy is transformed into small amounts of heat, dissipating the energy.

What is comes down to is this:  MASS is still, by far, the best way to block sound energy.  To try and build a partition using only 'floppy' materials is a futile effort, doomed to failure...

[whatismisophonia]

here's a lengthy bunch of posts you might check out (post 6 talks about stiffness vs softness) :  http://www.avsforum.com/avs-vb/showthread.php?p=5405261&&#post5405261

Evidently stiff materials are good, but the differences between floppy (or spongy) materials and stiff materials, as I understand them, are not astronomical enough to make a crap.  The main issue with rigid materials is their propensity to resonate and have more than one coincidence dip.  Less ridid materials simply don't resonate so badly (at the level of stuff we can hear). 
That being said, I am curious as to how MLV works on more than just the mass law (and helping with resonance).  You say it has functions to disapate energy as heat, since it is visoelastic, but I'm a bit confused, and this really is important as to whether I would ever consider using MLV:  visoelastic polymers rely on solid materials sandwiching them in between.  In fact, these companies specifically say that these polymers cannot serve as a coating without a solid mass on either side, so that together they help to compress and stretch the polymer as pressure is placed on wall components.  How on earth would this principle work with MLV, when the defining factor is not the weight, but the proper application of solid masses?  And of course there is also the fact that MLV doesn't do crap when cemented between two layers of wallboard.  the greenglue website does show an improvement at low-frequency (which is what you see with damping compounds), however, this may also exist due to the uneven thickness of wall panels used in that specific example.

[johnbergstromslc]

That being said, I am curious as to how MLV works on more than just the mass law (and helping with resonance).  You say it has functions to disapate energy as heat, since it is visoelastic, but I'm a bit confused, and this really is important as to whether I would ever consider using MLV:  visoelastic polymers rely on solid materials sandwiching them in between.  In fact, these companies specifically say that these polymers cannot serve as a coating without a solid mass on either side, so that together they help to compress and stretch the polymer as pressure is placed on wall components.  How on earth would this principle work with MLV, when the defining factor is not the weight, but the proper application of solid masses?  And of course there is also the fact that MLV doesn't do crap when cemented between two layers of wallboard. 

What you are talking about is constrained layer damping (CLD).  In order for a drywall/MLV/drywall sandwich to be effective at CLD, you would need to contact cement all the mating surfaces and roll it to get a good bond.  Major pain in the ass, major amount of work, major potential for expensive mistakes.  I thought about doing it once, but I also wanted to make a profit on the job, so I passed on that idea....

I wouldn't say MLV 'doesn't do crap' when sandwiched in between drywall - at 1/8" thick, it has a density equal to a 1/2" thick piece of wallboard, so it's to effectively add another layer of wallboard without increasing the wall thickness much.  It's just a damn expensive way to do that, at $1.50+ per sq/ft...  Drywall is less than 30 cents/sq. ft.

[whatismisophonia]

I am aware of the terminology(cld) and usage, but the point is, that's how visoelastic compounds work.  I don't see how mlv would function specificaly as a visoelastic compound, when it is not suppose to be used in such; the whole thickness gives.  It sounds alot like the expensive use of cotton, whether as an acoustic panel or not.  I mean sure, if you got the money for it...

[johnbergstromslc]

I am aware of the terminology(cld) and usage, but the point is, that's how visoelastic compounds work.  I don't see how mlv would function specificaly as a visoelastic compound, when it is not suppose to be used in such; the whole thickness gives.  It sounds alot like the expensive use of cotton, whether as an acoustic panel or not.  I mean sure, if you got the money for it...

Actually, that's exactly how MLV works...

If you hang MLV as a limp curtain, it flexes slightly as sound energy hits it.  The internal friction of the material absorbs and dissipates some of the energy.

As for the use of cotton insulation, well, I've already gone a few rounds on this forum with that topic.  I'm in business to make money, and I am sticking with fiberglass.  Readily available at my local home center, and costs 80% less than cotton sold online.