In the case of the fiberglass traps that go in corners, I gather that the basic principle is that "more is better?" In other words, a layer of rigid fiberglass from floor to ceiling that is 4" thick would be better than 2" across all frequencies? But does it also follow that the best would be a triangle of fiberglass that basically filled the space in that corner out to the same surface?

I'm hearing that filling the corner with a uniform triangle of rigid fiberglass might not be best, or even really gain anything over a couple of inches across the same span with space behind it. True?

I also am wondering about another potential design. In designing barriers for sound, one of the better ways of dealing with noise is to vary the density. Foam/barrier/foam designs work better than foam/foam/barrier designs of the same thickness and material.

So...I'm wondering if, instead of filling the corners completely with fiberglass, forming a sort of horn with alternating layers would help? Picture a cross section of a corner, viewed from above...the fiberglass pieces are sort of like the pages of a book when the book is opened up, a splay of the rigid fiberglass. Has this design every been tried or tested?

Or is it just as good to simply buy some 2" 703, and span the corners across two feet?

Absorb of all of the above and add to that this summary of thicker material absorption

4" Material Absorption

The both easiest and cheapest is to use a material like Roxul "Safe 'n'Sound" (see my post "Why I have a headache").

It is the easiest because all you have to do is cut the 2'x4' sheets into 4 triangles that you stack in the corner.

It is the cheapest because of the pricing of the product. 703 is as low as 53 cents/ft^2/inch. "Safe 'n' Sound" is as low as 12 cents/ft^2/inch. These prices are based on the purchase prices in

So how much are we paying

In terms of doing a corner, 4"703 straddling will cost $2.12/ft^2. The "Safe 'n' Sound" filling the corner will cost $1.44/ft^2.

If you want to find equivalents relatively quickly, have look at

Bob Golds' Absorption Data

I plan triangular slabs of Rockwool RW3 (pretty much eqiv. to the rigid slabs you use in the States). RW3
2" (50mm) gives 0.11 at 125Hz. I'm not sure how you calculate four thicknesses of this stacked as 2ft x 2ft triangles, and only wrapped in a thin material - i.e. no wooden panel sides.

Again, I really appreciate everyone's patient explanation...please don't take this post wrong. I'm just still confused because I don't have the right data, but I do have some opinions from people that I can't reconcile.

The missing piece of the puzzle for me is something along these lines, as an example: For a given corner trap size (i.e. there is one surface area that is constant across a corner 18" to either side of the corner) what are the different absorption curves for

a) 2" 703 only, across the front

b) 4" 703 across the front,

c) the entire triangular cavity filled with 703,

d) three layers of 2" 703 with 1" gaps between them, parallel to the hypotenuse,

e) 6 layers of 1" 703 splayed out from the junction of the walls like the pages of a book.

Originally posted by Ethan Winer:
George,

> is Ethan saying that filling the whole is "better" in the sense of smoother overall? <

Great question, and my FAQ may be the source of clarity or confusion. I can't say for sure until I do more testing.

The notion of a hole in the absorption at some "middle" frequencies caused by a large air gap came about as a result of tests I did in a real acoustics lab. However, Eric Desart, a pro acoustician I respect, insists the drop in absorption is caused by a different mechanism, probably membrane resonance since I was testing FRK fiberglass.

So all I can say for now is further tests are needed to really know how significant the dropout is. I will tell you that an air gap equal to the material's thickness is always good. As far as I can tell, all that's in dispute is what happens when the gap is significantly larger than the material thickness.

--Ethan

Originally posted by Ethan Winer:
George,

> In the case of a corner trap, then, how do you calculate the "gap" behind the material? <

Are you still talking about a fiberglass trap, or a wood panel trap? They're entirely different and work on opposite principles.

--Ethan

Originally posted by Ethan Winer:

> how do you calculate the "gap" behind the material? <

There's nothing to calculate. Standard rigid fiberglass comes in 2x4 panels, so you place them across the corners and you're done. The air gap varies, of course, but that's a good thing.

--Ethan[/QB]

Originally posted by Ethan Winer:
George,

> I could make a 2x8 panel, or a 4x8 for that matter <

With bass trapping, the more the merrier. But the room size and ergonomics are a factor too. If you really are willing to give up the space for a 4x8 sheet of rigid fiberglass straddling every corner, I say "Go for it."

> I could double the thickness to get 6" ... I still haven't heard anything about the relative effects of length and width. <

Again, the only way to know for sure is to measure all those combinations. I've compared two inch thick rigid fiberglass to three inch thick, but never three inch to six inch. If you mail me $1,000 I'll gladly have both thicknesses tested at a lab. Otherwise, all I can suggest is to buy the ETF software and measure the change from 3 inches to 6 yourself in your own room to see how much better that is.

Let us know what you find out.

--Ethan