Question about reading the specs on rigid fiberboard...

[tubedude]

I'm comparing specs on the Owens Corning, the Knauf, and the Johns Manville rigid fiberboard products and I noticed something.

The stuff seems to be pretty differetn down low and up top when its backed and when its not.
My questions:
1) are the ones with backing showing better bass absorption because the backing is denser?
2) is it showing less high end absorption because the backing is reflective and the specs are with the backing facing forward?

If so, then 3) If I place the backing on the backside of the panel, then it will effectively absorb the high end the same as the un-backed stuff, right?
Which is better then? Backed?
And last, is there a drawback to using 6PCF instead of 3lb, or vice versa?

Thankee.

 

[Ethan Winer]

Dude,

> are the ones with backing showing better bass absorption because the backing is denser? <

No. The backing itself helps the fiberglass absorb to a lower frequency.

> is it showing less high end absorption because the backing is reflective and the specs are with the backing facing forward? <

Yes, exactly.

> If I place the backing on the backside of the panel, then it will effectively absorb the high end the same as the un-backed stuff, right? <

Yes.

> Which is better then? Backed? <

Depends whether you want to absorb lows more or highs more.

> is there a drawback to using 6PCF instead of 3lb, or vice versa? <

I'm not sure. I believe the denser types are better, but I haven't yet measured that so I don't know for certain.

--Ethan

 

[RICK FITZPATRICK]

No. The backing itself helps the fiberglass absorb to a lower frequency.

Ethan, to what can you attribute this to? Because of 1/4 wavelength reflection off the backer?
Although acoustics is something I will never thouroughly understand, it must obey physics in some way or other. I ask these questions to better understand. For instance, I was under the impression that, IF sound enters 703 with no backing of any kind, SOME of the wave will be absorbed simply by resistance, as if you had it in front of an open window, and that which was not absorbed simply keeps going never to return. IF you put the 703 in front of a barrier, such as a wall, this will lower the absorption frequency, by reflection back through the 703, and the dimension from the barrier to the face of the 703, determines the frequency of which it absorbs, as this dimension is the length of 1/4 wavelength,correct? IF so, this backer, is doing what a wall does, correct? IF so, then the thickness of the board, is the only dimension governing the 1/4 wavelength principle. IF the specified frequency of absorption is lower than this principle predicts, what other principle is at work? It couldn't be membrane/cavity resonance, and it couldn't be hemholtz resonance, so what other mechanisims are in play here? This is also my question in regards to statements regarding placing a poly film in front of the 703 in slat resonator units, which now I understand lowers the absorption also. Just trying to understand the principles, thats all.

fitZ

 

[Ethan Winer]

Rick,

When a paper facing is bonded to rigid fiberglass the facing becomes a sort of membrane. So that then becomes the absorber, and the fiberglass serves to damp its vibrations. Does this help?

--Ethan

 

[guido #2]

Fitz
You didn't get enough of this the other day? ;^P

 

[knightfly]

For in-wall use, you can "dial in" your performance (sorta) - higher density for better high frequency loss, lower density for better low frequency loss (Transmission Loss, that is) - here's a link to USG's take on this -

http://www.usg.com/Design_Solutions/2_3_7_insulationperf.asp

If you don't want to read it all, just use 3 PCF stuff. The 6 pound stuff would weaken isolation in the bass range according to Mr. Roller, the author of the above article... Steve

 

[RICK FITZPATRICK]

You didn't get enough of this the other day? ;^P

Hey guido #2, everyone around here knows I'm a glutton for punishment self flogging with strips of 703 is mandatory... daily. I know, but shit, got nothin better to do than bother the experts.....I mean, what the hell else am I going to do? Can't stand bowling, fishin sucks, working on ANYTHING outdoors sucks in this weather.. well, maybe I'll have some energy left this weekend to finish my console. Work kinda wears me out by the weekend anymore...just want to sit on my.. er..drafting chair And then I stare at the computer....and WHAMMIE!...a question comes to mind. Sooooooo....here I am gain. Askin dumb questions.

Ethan, Steve, thanks gents for puttin up with my questions. Anytime you get tired of me, just don't answer. Even I can take a hint. and btw Steve, I'm not done on Erics thread.
Had to think about your reply for a few days. I know I was out of line, but on the other hand, after spending so much time trying to understand the small room diffusion thing, it made me freaking CRAZY! And mad. Thats my only defence...I'm INNOCENT by reason of insanity I tell ya...hahaha! But I know I owe Eric an apology....damn, you'd think I'd get tired of that wouldn't you? I do. But I'm my own worst enemy sometimes. It probably wouldn't make a damn bit of difference to him anyway, but just the same, I'll do it. Thanks for still sharing Steve.
fitZ

 

[guido #2]

LoL!
Went crazy from the reflections, huh?
Kinda like working in a house of mirrors....... ;^}

 

[knightfly]

Rick has finally, through his tireless attempts at making sense from gibberish, succeeded in transforming himself into a Tangential Mode - This is two steps above Pie ala Mode, one step below an Oblique mode (which Mr. Fitz may NEVER reach, since he is anything BUT oblique :=) and two steps below "keeping your lawn mode" - which is just a half step below "dem mode" - which in itself is a prerequisite to an actual "release", which may only happen upon a "comm-mode" -

Sooo, as you can see, our Mr. Fitz,
while he strains and strains to take a --itz,
and although he has reached a new plateau,
still has a very long way to go.
And that's the end of this pathetic rhyme,
I could do more but I'm out of thyme -
So instead of screwing with Rick's head,
The hell with it - think I'll go to bed... Steve