Flanking advice...

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Re: Flanking advice...

Postby chconnor » Fri Aug 06, 2010 6:58 pm

Thanks Rod - I did consider all the points you raise, and I concede the probable uselessness of it all. My tack at this point is to build it, see what my crazy design reaps, and if I'm not satisfied (and if I can afford it) do a second layer of GG/drywall on the ceiling in room A. This still leaves all the girt-borne flanking issues present, but it helps with the unaddressed cavity flanking through the totally open ceiling. (And some say that the GG also helps a tiny bit with the structural flanking?) At least then I'd just have the coupling to worry about (which admittedly would be almost impossible to deal with at that point, short of re-drywalling room A entirely.)

(normal message ends here; begin tiresome speculation:)

All this does raise a question that I've been wondering about a lot, recently, which is the nature of structural flanking noise. It seems like the experience and knowledge of all the professionals I've heard from (or read, in your/Rod's book) implies that structural flanking noise doesn't care about the path it takes: e.g. whether it goes through a girt into the drywall, or through a girt into the post into drywall, or through the girt through a post through another girt into drywall, it's more or less the same, unless that structure is very massive (drywall, concrete, etc; wood doesn't qualify).

Further, it's implied that "directionality" of flanking noise is either nonexistent or irrelevant for all practical purposes (e.g. that once noise "gets in" to wood, it doesn't matter how or at what angle anything is connected to it: once it's shaking around, if it's connected firmly to something else, that other thing will shake around as well, in all directions, equally.) ...A corollary being that the natural tendency of wood to vibrate along or against it's grain...effectively irrelevant.

Further, it sounds like the properties of wood, regardless of how massive or the degree to which it's made rigid by framing (e.g. the 6x8 post), the way it's nailed or supported, etc, are also basically irrelevant (as evidenced by your opinion that attaching to the post gains nothing by having the flanking noise move through more mass before it gets to drywall.) Obviously, at some point enough wood will isolate respectably, but apparently the amount needed makes it impractical for that use. I'm sure that in some contrived case any of these factors could be demonstrated to be relevant, but basically they seem not to be concerns.

In terms of structural flanking, the only concern is whether things are coupled to other things, and how firmly. I assume that's because the vibrations we're talking about are on a scale (frequency and amplitude) that make all those other considerations moot.

I did some tapping on things with my ear to the wood... this is patently unscientific, obviously, and probably way more directional than normal radiated sound would be, so I draw no real conclusions from it, but I did notice a large difference between, for example, the sound apparently radiated from one side of the partition wall to the other via the girt (loud) vs. one side of the partition wall to the other via that girt and then through the post (much quieter). Similar tests made the splitting of the end stud seem like > 0, though small, in value. Since tapping is probably more about super-low-frequency compression waves and less about natural sound, I tried simply rubbing my finger on the wood and listening for the higher frequency components... same results... of course it's probably the mids or low-mids that are more relevant in structural flanking...

Anyway, while I have little-to-no hope that my "experiment" means much in the face of the many years of experience and knowledge represented in forums like this, it makes me curious about the purely academic question of this structural flanking and whether the tiny details (directionality, etc) are ever relevant in the sound isolation world?

Does that question make any sense? :-)

-c
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Re: Flanking advice...

Postby bert stoltenborg » Fri Aug 06, 2010 11:38 pm

Are you Fitz?
If you view life with the knowledge that there are no problems, only opportunities, you are a marketing manager.......this is my personal philosophy
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Re: Flanking advice...

Postby Rod Gervais » Sat Aug 07, 2010 2:31 pm

Let me try to answer that by exaplaining to you an experience I had with flanking.

In Hartford CT I was involved in a project called Adriane's Landing.....

This particular project was a 500,000 sf convention center with a detached high rise hotel immediately adjacent to it.

There is a plaza between the 2 buildings, with a full width set of steps that rise a total of 3 stories. The plaza was a piece of the convention center construction.

The hotel was separated from the convention center construction through the use of a 5" air space at the closet point (7" at the furthest) This large due to the tip action of the hotel (which was 18" max under 110 mph winds) combined with the tip action of the convention center, adjusted for the location of the joint. We had to make it that large in order to make certain that under the maximum wind load the 2 structures could never touch.

The air space between the 2 building was connected only through the use of a special rubber expansion joint to allow free movement of the buildings.

The convention center construction at that point was a concrete wall that created the end wall of the plaza, it was about 16" thick...... the building structure is carried on a series of concrete piles - with probably (in that particular location) anywhere between 10 to 16 piles per pile cap.
The pile caps were probably around 10' square and maybe 8' thick.

Pretty massive.

Now - 2 (and only 2) of the pile caps shared bearing (in the particular area I am speaking of) between the concrete structure for the convention center and a steel super structure for the hotel.

The steel for the hotel was huge - not only due to the gravity load of the structure above it - but also due to the fact that it was part of a brace bay that took the brunt of the wind load from the structure above.

The hotel and convention enter were constructed on earth that did not have suitable bearing capacity for the structures (which is why we used either driven piles or caissons (with socketed concrete caps) to provide bearing) thus the slabs were a minimum of 14" thick.

ok - so not you have this picture of this massive structure sitting on top of massive pile caps - with a massive slab on grade.

As is usual with these types of projects we are running around crazy trying to get done - and in the process some of the marble tiles in the lobby get damaged...... and the contractor sent down a worker with a very small electric chipping hammer. When I say "small" picture a 6.5 amp hammer that weighs about 7 pounds.........

I was in the lobby and had someone show up that needed to speak with me - to get away form the noise I took him up onto the plaza and we're standing outside talking - 3 floors above the work - about 70 or 80 feet south of the lobby location where the work was taking place........

So here is where it gets interesting.

We're 3 floors up - doors to the plaza are closed (think commercial glass doors - with a 8' deep airlock...... standing out on the plaza talking - and I realize that I can hear as plain as day the work taking place 3 stories below us............

This machine is making a mid range to low HF sound - no I do not know the exact frequency.......... but it is light weight - they were not running it at it's top speed - which is around 3500 bpm - probably around 1500 to 2000 I would guess....... and it is not a heavy hammer head or a blunt point we're talking about here - it is a thin edge chipping blade - made for getting small objects off floors when you do not want to damage the good material adjacent to it.

And we can hear it plain as day.........

It had enough energy to excite that 14" structural slab - travel through the slab into the common pile cap - excite the concrete wall sitting on top of that pile cap = and then make the 2" schedule 80 guard rail on the plaza sing........... that is where we could hear it as it exited the structure..... we were probably 40 to 50 feet from that when we first heard it - it took us a minute or so to be able to focus on exactly where the sound was - and the the closer we got the more clearly we could hear it..... that is the potential for flanking noise....

Green glue is good - but it still does not provide a lot of isolation from flanking caused by low frequencies for walls with hard connections between drywall faces.........

OK - great story - very true - and from just one simple common connection point..........

In your case there are a series of weak points.....

If all other things were perfect - the girts would just be something you had to live with - this because there is no way to make them go away..........

But all other things are not perfect......... you have the issues with the top of the walls - the common plates top bottom and mid span - plus the girts......... I am still of the opinion that some manner to decouple from that structure is going to be your best bet to make this work.......

When everything is said and done - adding mass will not overcome flanking if flanking is the weak point........

BTW - flanking does not have to be a hard connection - it can also be take place through the air cavity of an isolated assembly.... basically - flanking (picture the term from the military perspective) is any path other than the direct path through the assembly in question......... the same way that a flanking action is placing part of your army to help through means other than a direct frontal attack...... However flanking through hard structural connections is typically the path of least resistance.
Rod

If you view life with the knowledge that there are no problems, only opportunities, you will find the load to be a lot lighter then it might be otherwise......... this is my personal philosophy
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Re: Flanking advice...

Postby chconnor » Sat Aug 07, 2010 7:36 pm

Rod Gervais wrote:Let me try to answer that by explaining to you an experience I had with flanking.


Wow, great tale, thanks. In the course of this project/research the most interesting thing has been having preconceived notions and intuitions challenged and overturned.

Rod Gervais wrote:BTW - flanking does not have to be a hard connection - it can also be take place through the air cavity of an isolated assembly....


Yeah, given that decoupling is the best bet, but that I can't really make that happen, I was deciding between decoupling the ceiling to address structural flanking, vs adding mass to address to the ceiling to lessen the flanking through the cavity; since the cavity flanking seemed like a bigger deal than the structural flanking, I was leaving adding mass as my after-the-fact potential improvement plan. Maybe I'm confused about that: I was assuming that the cavity-borne flanking through the ceiling would be best addressed by adding mass, but maybe if the ceiling isn't decoupled, the sound just goes through the wood and into the cavity anyway...

Now for the 50th time I'm churning over how much money/time/energy would be involved in putting off the drywallers, relocating receptacle boxes and light fixtures, ordering clips, installing.... Thanks for the incitement. :-)

-c
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Re: Flanking advice...

Postby Rod Gervais » Sat Aug 07, 2010 9:40 pm

Tomorrow I will provide you with a detail that will minimize the issue with the hole at the roof.......
Rod

If you view life with the knowledge that there are no problems, only opportunities, you will find the load to be a lot lighter then it might be otherwise......... this is my personal philosophy
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Re: Flanking advice...

Postby chconnor » Sun Aug 08, 2010 2:22 am

Thanks, I will eagerly await... :-)
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Re: Flanking advice...

Postby Rod Gervais » Tue Aug 10, 2010 3:16 pm

Try this - even though there is an opening for air passage at the top of the drywall on the top left where it meets the roof - it will minimize your leakage - the side parallel to the rafters should run tight to the underside of the roof deck - there is no reason to do anything else there..........
Attachments
CEILING ISOLATION DTL.pdf
(47.99 KiB) Downloaded 199 times
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Re: Flanking advice...

Postby chconnor » Tue Aug 10, 2010 3:54 pm

Rod - first, thanks so much for taking the time to draw that up, much obliged.

I believe I understand the idea. So even though there's a gap, that would still help, huh? Good to know. Given all the attention about sealing everything airtight, I'd have thought a 1-2" gap would render it useless. Anyway, thanks again... -Casey
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Re: Flanking advice...

Postby Rod Gervais » Tue Aug 10, 2010 7:30 pm

Casey,

one of my favorite sayings is that "we live with that which we must live with"

You have no choice in the matter of the ventilation of the space beneath the roof deck - it's a code requirement.......

So you must live with that......

This does not (however) mean that nothing else matters.......

You need to make the body of this as tight as you can possibly make it - then you simply live with what remains because you cannot change that.

If viewed the other way - then you could just do without any walls.

One extreme to the other...

Just because you can't make it perfect does not mean you should make it any less than you can - the best you can get is still much better than the worst.......

Decreasing the size of the whole helps (a lot actually)
Rod

If you view life with the knowledge that there are no problems, only opportunities, you will find the load to be a lot lighter then it might be otherwise......... this is my personal philosophy
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Re: Flanking advice...

Postby chconnor » Tue Aug 10, 2010 11:30 pm

Rod Gervais wrote:Decreasing the size of the whole helps (a lot actually)


Yeah that's the part that's good to learn. I had it in my head that transmission, through an opening in a theoretically perfect isolating wall, so rapidly approached 100% as the opening gradually increased in size that after you passed an inch or two you might as well be wide open. Good to know that (especially in a desperate situation) a partial closing counts for something.

Thanks again,
-c
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Re: Flanking advice...

Postby Rod Gervais » Wed Aug 11, 2010 3:25 am

When you are working with a perfect assembly the losses can be drastic - but when you are working with a bad assembly the increases can be just a dramatic
Rod

If you view life with the knowledge that there are no problems, only opportunities, you will find the load to be a lot lighter then it might be otherwise......... this is my personal philosophy
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Re: Flanking advice...

Postby homestudiobldr » Wed Aug 18, 2010 4:41 pm

Rod wrote:
The answer to that question is relatively easy to answer - and that is because a pole barn does not have a foundation to speak of - the poles are buried in the ground and form the full support for the structure - the "girts" are typically installed a little above the slab that is poured (just a thickened edge at the outside - no frost protection there) to allow for free movement of the slab. It has little to nothing to do with saving money on framing

8O :bang DOH! How dumb of me. :lol: Of course...even I have a pole barn(no walls, no slab).
the savings is on excavation and concrete.
Untill you decide to put a slab in afterthefact. :mrgreen:
This however poses a potential issue


Speaking of "potential issues"..try this one on for size. Holy moly...two years ago, my brother in law had a slab poured within the pole boundarys of his pole barn. Unfortunately, the previous owner(with no permits of course :roll: )built "infill" walls between the posts, via standard "stick" construction, with the wall plates DIRECTLY on grade...no footings..no foundation..no nothing..zilch 8O :roll:, then sheithed it with T111 ply.(it was for his motor home). But now, my brother-inlaw decided he wanted to make it into a shop, so he hired a concrete contractor to pour a slab. But also unfortunatly, the grade from front to back was sloped over TWENTY inchs downward 8O
To pour a standard 4" slab, the contractor excavated the front area 6" deep to allow a bed of gravel to be "leveled" front to rear. But imagine a contractor doing this. What this meant was, the gravel at some point front to rear, began to climb higher than the wall "plates" on grade, and then began to enter the stud cavities. At the rear, this gravel was 20" thick plus or minus. THEN..he poured a 4" slab that actually entered the stud cavities up to the shiething, and covered portion of the studs!! Unbelievable. Before the contractor did this, I suggested they should at least build a permenent form(either against the plate/studs, or spaced about an inch)out of Pressure treated 2x. But nooooooo...they decided I didn't know what I was talking about. :bang :cry: :evil: ...that is...untill I showed my brother-in-law why. Now all the T111 is buckled out around the rear perimeter and beginning to rot from water weeping. Oregon rain is very unforgiving. He finally cut away some sheithing to expose..yup, you got it...rotted studs..which now are imbedded in CONCRETE! :mrgreen: So much for my suggestion.

Are you Fitz?
Have you missed me? :mrgreen: I wouldn't hide my identity bert. If I have something to ask, you'll know its me. Which btw I did...above :
homestudiobldr wrote:


geeeez bert, why would I be asking myself a question? :bang :roll: :wink:

But for what its worth, I've pretty much exhausted my curiosity in acoustics, at least to the point of starting threads. I decided to put my energy into finishing my studio instead. But I am always interested in Rods(and others) replys and still have a few nagging gaps in my understanding of TL. So I still lurk here. I just grew tired of trying to pry open little "secrets" of the pros, which even Rod has suggested exist regardless of Scotts scoffing view that they don't. :roll: BTW, this place has become pretty boring huh?. :lol: So I've found other places to direct my curiositys, which at the moment is Computer modding(remember my computer enclosure?..2 new i7's with watercooling in the works) and getting back to playing/recording music. Anyway, I'm keeping busy and having fun doing it. Lottsa new toys to keep me from getting bored too. Especially tools. Just bought a Oxy/Acetylene setup to mod my console...for the umpteenth time. :mrgreen: Goin for the "Harrison/Fairlight" look ..
Image
Image
Image



Just kidding..but I like the level of audio display these are capable of..especially what you can do via this little goodie in the Fairlight. Its available standalone...

Image




Unfortunately...that keyboard/fader assembly is $18k 8O :roll: :cry: But I'm still heading in that direction now via ITB multiple video/monitor/computer multicore processor stuff. And yea Eric...I still like complicated look stuff. :mrgreen: But sooner or later..now it all comes down to this...even recording :roll:

Image

:roll:


although, it might help if its interfaced with something along these lines...

















Image
Image


I can imagine all the eyes rolling now. But thats ok...I still have fun imaging stuff. I can't wait to get the i7 graphics rendering computer with a dual Quadro card setup finished.


:mrgreen:

to be brutally honest with you - I see little to no advantage to even bothering with the staggered stud wall system in your construction.
Hmmm, that was my feeling from the get-go..but I figured it was best left to the experts to point out. In this case, I guess andres standard "footnote" is the best hindsight. :mrgreen: However, thats why I decided to look at Rods solution. His insight and caveats never ceases to amaze me. :8

Ok, again, my apology for interjecting my off the wall stuff in the OP's thread. Sometimes I just get carried away. :mrgreen:
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