More on ridgid fiberglass...

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pdlstl

pdlstl

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Obviously, OC 703 enjoys great favor on several different recording forums and is searched out like some sort of Holy Grail.

My choice for ridgid fiberglass products is the Johns Manville 814. Why? Because it's easily available, priced right and has slightly better specs than the 703.

As I've researched the different fiberglass products, I've hit upon a question I don't have an answer for. This regards density.

The chart below is for the JM products. I buy the 814. It shows to have a density of 3 pcf (pounds per cubic foot). When compared against the 817, which has a density of 6 pcf, the only real difference is down at 125 Hz. From there upward, the 814 compares almost identical to the 817.

Question, would the difference at 125 Hz (814=0.24, 817=0.38) justify the obviuos price difference between the two products? Or more simply, does density (pcf) make that much of a difference? How much of a difference is going to be noticed at 125 Hz.?

Earl

jm814.jpg
 
Earl,

> would the difference at 125 Hz (814=0.24, 817=0.38) justify the obviuos price difference between the two products? Or more simply, does density (pcf) make that much of a difference? <

Yes and Yes. This has been discussed in other web forums, with some self-proclaimed "acousticians" claiming that density is not related to low frequency absorption. In fact, as the data you posted proves, the difference can be substantial.

I recently measured this myself in a real lab, and at low frequencies the denser type is up to 40% more absorbent. Even more important is what happens below the 125 Hz octave band. When I tested this the absorption was measured in third octave increments all the way down to 25 Hz. So to me there's no question that the denser type is worth the extra cost when low frequency performance is important.

--Ethan
 
Ethan,

I was very happy to see you weigh in on this.

Thanks for the clarification!!!!

Earl
 
Ethan Winer said:
Earl,

So to me there's no question that the denser type is worth the extra cost when low frequency performance is important.

--Ethan
Click to expand...

Don't you think its obvious?

with some self-proclaimed "acousticians" claiming that density is not related to low frequency absorption

Anyone who says this is arguing against the basics of mass/density to a certain extent.

SoMm
 
I don't understand how a product can have an absorbtive coefficient greater than 1. :confused: :confused:
 
Michael Jones said:
I don't understand how a product can have an absorbtive coefficient greater than 1. :confused: :confused:
Click to expand...

Michael,

I've wondered this same exact thing many times.

Earl
 
I'm actually using the Knauf 2" 3PCF (plain faced) simply because its readily available here.
Compare its absorbtive coefficients here:

http://www.knauffiberglass.com/index.cfm?fuseaction=prd.dspProdDetail&ID=12

For 125Hz the coefficient is 0.29 - Decent, but I may run into a problem at frequencies above 1000Hz as the absorbtive coeff. goes way up. I understand you can counter this by wrapping the board in a plastic membrane, which, supposedly won't interfere with the low frequency absorption.

My head hurts - They said there'd be no math here. :(
 
Michael Jones said:
<snip>
but I may run into a problem at frequencies above 1000Hz as the absorbtive coeff. goes way up. I understand you can counter this by wrapping the board in a plastic membrane, which, supposedly won't interfere with the low frequency absorption.
(
Click to expand...

I hoping that I'll overcome this issue by leaving a certain amount of wall space untreated. Am I thinking correctly?

Earl
 
I bought the Knauf stuff myself except it is the foil faced type. I left the foil on in the corners and took it off on the stuff on the walls. I figured the foil would reflect some of the highs.

I also left each oppsite wall untreated.
 
HangDawg said:
I also left each oppsite wall untreated.
Click to expand...

Good idea.

When I get home this evening, I'm going to take a couple of pics and post 'em in this thread. I hope you folks will give me some feedback on what I'm doing treatment-wise.

Earl
 
I initially mounted it right to the wall, but I'm going to make some spacers and get it off the wall some.
 
SoMm,

> Anyone who says this is arguing against the basics of mass/density to a certain extent. <

Yeah, well, they argue that exact point and also tell me I'm an idiot. Go figure.

--Ethan
 
Michael,

> I don't understand how a product can have an absorbtive coefficient greater than 1. <

It's very simple: The edges of a rigid fiberglass panel absorb, yet the edge surface is not included in the calculations. Also, if a panel is mounted spaced away from the wall or ceiling, sound striking the nearby surface can bounce into the rear of the panel and be absorbed.

The complete story is in the Acoustics FAQ, second in the list on my Articles page:

www.ethanwiner.com/articles.html

Look for the sidebar "Measuring Absorption."

--Ethan
 
HangDawg said:
I bought the Knauf stuff myself except it is the foil faced type. I left the foil on in the corners and took it off on the stuff on the walls. I figured the foil would reflect some of the highs.

I also left each oppsite wall untreated.
Click to expand...
If you look at the absorbtive coefficients, the faced panels apper to be better. The have the same coefficient for the lower frequencies, but a lower coef. for the higher frequencies.
So you can get a handle on the lows, without wipping out the highs.
Good stuff!
Had I known better at the time, I probably would have bought the faced panels.
 
Thanks... :(

I just bought mine yesterday. But....after reading Ethan's bass trap info, I'm going to build them which wouldn't work as well with the faced stuff.

My room is 16x14x9 tall with drywall and tile floor. Plenty of highs to say the least.

Earl
 
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