John Sayers' Recording Studio Design Forum Archive

maybe i can contribute something here. i'm pretty sure both of you guys already are aware of these things, but what the heck.

check out figure 2 of this page here

http://irc.nrc-cnrc.gc.ca/cbd/cbd239e.html

it shows that below the resonance frequency, the airgap doesn't matter. you can't affect TL below resonance by making the airgap bigger.

below resonance, the two leaves act as one (like no airgap exists).

also here is an interesting paper on RC

http://irc.nrc-cnrc.gc.ca/fulltext/nrcc ... c44692.pdf

it shows that the presense or RC in a wall system can limit the usefulness of enlarging airgaps beyond a certain point.

although you decrease the stiffness of air by making the gap larger, the stiffness of the RC then becomes the dominant factor affecting mass-air-mass resonance frequency.

so, if you use RC, there is a built-in limit to how low your MAM freq can get, regardless of how big your airspace is.

and again, below MAM freq, airspace is irrelevant.

granted, this paper is a bit difficult to understand, and as i'm an amateur noob etc i don't have a firm grasp of all of this.

i'm not sure how this stuff applies to double stud constructions or staggered stud constructions. i think the tests in the latter article were done with single stud.

i've seen or read that single stud constructions have different inherent characteristics than double stud, but i think this gets a bit complicated.

but, i think the principles that those articles outline make sense to keep in mind. it seems to me that they should apply in double stud constructions.

is this wrong? i'd appreciate any comments.

dan

"you can't affect TL below resonance by making the airgap bigger. "

- Say what? Air gap is one of the main controlling factors of resonance, so widening that gap would LOWER the resonance. Am I still half asleep, or are we in "catch 22 " land here?

"although you decrease the stiffness of air by making the gap larger, the stiffness of the RC then becomes the dominant factor affecting mass-air-mass resonance frequency."

This I agree with... Steve

Dan Fitzpatrick wrote: it shows that below the resonance frequency, the airgap doesn't matter. you can't affect TL below resonance by making the airgap bigger.

below resonance, the two leaves act as one (like no airgap exists).

Dan,

You have to take this in context. 1st off - they're dealing here specifically with STC ratings. They are speaking here of an assembly resonance (hence center frequency) of 80hz.

They are also dealing with a simple frame assembly - and thus not only does the "spring" (air space) couple the wall - but the frame does as well.

2nd - there does reach a point - even with multiple frame assemblies in this range - that reasonance takes over - this has been proven through tests at Riverbank. But (and this is a big but) it isn't a case of no gain - it's that the gain is not one that you would expect through strictly mathematics. For example - the riverbanks study of the Mason Industries Floating COncrete slab showed an increase of 54 STC to 57 STC by simply pouring a 4" concrete slab over an existing 6" elevated concrete slab. Raising this slab 2" brought the STC from 57 to 79. This due to the air space plus decoupling. Doubling this to 4" would (mathematically) raise this an additional 5 STC - however the gain is only 3 STC due to the control that resonance has in the equation.

So it isn't that it doesn't give a gain - it's that the gain is not what one would expect - and at some point in time - the cost of the real estate becomes so expensive that the gain isn't worth the cost incurred.

so, if you use RC, there is a built-in limit to how low your MAM freq can get, regardless of how big your airspace is.

Once again - you take this out of context - it isn't proven by this study to be true in all cases. In fact - it can't be true in all cases.

Suppose I had 7 layers of mass on a single stud frame of 2x6 @ 16" centers, and then a one mile air space - and another 2x6 16" stud frame with rc and 3 layers. Are you telling me that the one mile air space would not grant me any isolation value over the value of the mass?

Once again - the cost of the real estate would prohibit me from the actual construction - but if money were no object - the additional isolation would be tremendous.

and again, below MAM freq, airspace is irrelevant.

'

Wrong.

but, i think the principles that those articles outline make sense to keep in mind. it seems to me that they should apply in double stud constructions.

In this case you are mixing apples and oranges.

The test study was not done to provide information on wall cavity depth - but rather was specifically designed to compare different types of Resilient Channels - which happened to have different depths.

The effects of decoupling of these channels proved that they all basically provided the same amount of attenuation - regardless of designed shapes or additional air depth provided in the test.

Once again - this was performed on walls with single stud framing - not seperated frames, and it was never intended to provide information that one could use to extrapolate any conclusions regarding the effect of the air cavity on the isolation value of the assembly.

You cannot use these tests of simple frame structures with leafs directly attached to each side to conclude anything regarding staggered or seperate 2 leaf systems.

Heck - at some point the simple fact of flanking within the structure takes place which can't be overcome through mass or air space. But because of that you couldn't conclude that this would establish a limiting factor for all isolation assemblies where decoupling is in effect.

I hope that helped.

Sincerely,

Rod
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Steve, yeah you caught me being a little stupid there, and a little unclear. of course the increased airgap lowers the MAM resonance frequency.

BUT the frequencies BELOW 1.4 x the new lower MAM frequency WON'T be helped by the increased airgap. Those frequencies WILL be helped by increased mass. that's the point i think i was trying to make.

Rod, the principle that performance reverts to mass law below resonance has nothing to do with the example partition (the 80hz center frequency wall you refer to) mentioned just above the graph.

The graph states that it illustrates the "Effect of air space on ideal double walls with 0.5 mm steel on each face, sound absorbing material in the cavity and no rigid mechanical connections between the faces.

As for the second link i provided, i still think that the conclusion i refer to is interesting, and relevant. In fact, the original premise of the article, to investigate teh different types of RC, turned out to be uninteresting (there was no difference between types). The quote below sums up the conclusion i find facinating (especially for it's implication for floating floors):

"For larger cavity depths the stiffness of the resilinet channels becomes relatively more important compared to the stiffness of the enclosed air cavity. Although increased cavity depth can be used to lower the frequency of the modified mass-air-mass resonance, the result is limited by the presence of the stiffness of the resiliet channels."

"Thus at some point increasing the cavity depth will have little additional effect because the total system stiffness will be mostly determined by the stiffness of the resilient channels. ... The need to consider the stiffness of both the air and the resilient channels is similar to the problem of resiliently mounted floating slabs discussed by Ungar."

It is true that these tests were done on single stud constructions. Are you suggesting that if the tests were done with double stud construction the results would be different? then why?

both a double stud construction with RC and a single stud construction with RC have the same exact factors determining MAM frequency. The size of the airgap, the mass of the leaves, and (as this paper shows) the stiffness of the RC.

The only odd thing i know of with single stud constructions is that if there is not RC, then there is no true MAM frequency, because the wall resonates as a whole. but that does not apply here.

the conclusion i draw is that RC will limit performance in walls with large airspaces. Specifically there is no point in making an airgap over about 160mm (about 6") if you are using RC. unless, as you suggested, you make the airgap a mile wide

earlier you wondered, "So with a staggered stud wall we improve low frequencies attenuation in the 30 - 300Hz range - but still have those coupled plates....... would we gain anything from the installation of RC to effectively decouple at least one face from the plates in those ranges? "

i think after reading this paper my answer would be no, that adding RC to a staggered stud wall with 6" airgap would be certainly a waste of time and perhaps actually be a detriment.

well i think i've beaten this horse to death, sorry about that. but i really think that my points were worthwhile, and felt i should defend them a bit.

and no disrespect meant, i know you guys know a lot more about this than me, but i always was and still am one to challenge the teacher

dan

Edit 10/3/05 ... the lack of any response from these very smart guys led me, quite foolishly, to believe that i was right.

unfortunately, the truth as i now understand it (three months later) is that a single stud wall of any kind -- whether 2x4, 2x6 or 2x12 -- has almost the same performance due to the hard coupling of the leaves thru the studs. that is, CRAPPY performance.

therefore, it seems that a single-stud wall of ANY thickness would be better if RC was used.

the only time when my point above has any merit would be, that a 2x6 single stud wall WITH RC will perform about as well as, say, a 2x10 or 2x12 with RC. THAT is probably true. BUT, a 2x12 wall with RC is better than a 2x12 wall without RC.

on a staggered stud wall, because of the partial decoupling already offered by the staggering, what would happen if you added RC, i don't know. would RC help or hurt a 12" wide staggered stud wall? who knows. that's probably one for an experiment.

sorry for any confustion this has caused.