Black hole of sound?

[laptoppop]

Is this stuff really as good as it seems?

The Knauf 4" 3.0 PCF Plain fiberglass insulation board says it has the following absorbtion characteristics:
125 Hz - .95
250 Hz - 1.11
500 Hz - 1.17
1000 Hz - 1.07
2000 Hz - 1.07
4000 Hz - 1.06

http://www.knauffiberglass.com/index...odDetail&ID=12

I've heard of the 2" size being used in higher end absorbers, but it seems like this stuff can be used for both higher end, and some applications where folks are using slot absorbers. Of course, you won't get the high end diffusion that the slot absorbers give you.

Am I reading this right?
Thanks,
-lee-

[John Sayers]

Am I reading this right?

yup - I think so

That's why I can't understand why people spend heaps on foam!!

cheers
john

[Michael Jones]

That's pretty darn good!
How does that compare to 703?

[laptoppop]

Originally posted by Michael Jones
That's pretty darn good!
How does that compare to 703?

I can't find 703 in the 4" thickness on the OwensCorning website -- I'm not sure they make it that thick. The 2" version compares to this board pretty closely -- a bit worse at 125, a bit better at 250 compared to 2" of Knauf. *But* thats quite a bit worse than the 4". The numbers for 2" plain 703:
125 Hz - .17
250 Hz - .86
500 Hz - 1.14
1000 Hz - 1.07
2000 Hz - 1.02
4000 Hz - .98
Source:
http://www.owenscorning.com/comminsu...s700Series.pdf

Does it come in 4" version, or is Knauf the only supplier for this?
-lee-

[John Sayers]

Michael - here's a comparison of them all....I've highlighted the 2" comparison.



cheers
John

[Michael Jones]

John, you ARE the man!
Thanks for the charts.
The reason I ask is that the Knauf products may prove more readily available.

[laptoppop]

Great page, John - thanks!

I know that leaving a space between the insulation and wall improves the low-end response. What I haven't been able to find is ...

How much does leaving a space between the insulation and the wall improve the low end absorption? For example, if you mount a 2" thick piece 2" away from the wall or backing, what happens to the response numerically?

Thanks,
-lee-

[John Sayers]

Lee - when a wave reflects back off a wall it always starts from zero. It reaches it's highest energy point at 1/4 wave length which is where you want to put the absorption.

So if sound travels at 330m/sec at 100hz the wavelength is 3.3m (sorry about the metric it's just easier ) which is around 11 feet. So 1/4 wave length is 2.75 feet.

So if you place the absorption at 2.75feet you will absorb 100hz.

cheers
john

[laptoppop]

Ahhh, OK. So mounting the fiberglass board 2" away will improve absorption at higher frequencies, not lower. If my math is right, 2" would be 1/4 wave for about 1650 Hz.

For lower frequencies, it looks like we need either the thicker material, or resonators.

Thanks again,
-lee-

[TexRoadkill]

Since 703 is already so good at high end absorption doesnt that make it pointless to give it a few inches of space then? I've read people recomend that but it doesn't seem like there is any point to it based on the math.

[John Sayers]

Yes Tex - those figures are based on it straight on the wall and it's pretty good there. Lifting off the wall is only for extending the lower frequencies, the highs are fine.

cheers
John

[TexRoadkill]

Originally posted by John Sayers
Lifting off the wall is only for extending the lower frequencies, the highs are fine.

But a few inches doesn't help the lows, right? We need a few feet to effect the lows?

I just want to confirm that leaving a few inches of space is pretty much worthless because it does not affect the lows and the highs that it would absorb are already being absorbed at the highest possible factor.

Is that correct?

[knightfly]

Tex, I know you've been playing with your new Radar, but you gotta stop thinking "digitally" - the further you move the stuff off the wall, the lower the freq. it applies to. In order to know whether this is necessary, you need to know your room. Some of the spreadsheets I've mentioned will help, but there's nothing like reality checks over theory -

I found a great tool for this, check it out here -

http://www.etfacoustic.com/

been meaning to download the demo for a few months, finally did yesterday.

BTW, for those without a copy of Master Handbook of Acoustics, the REAL absorption values max out at 1.00, which is perfect absorption. The elevated #'s result from edge effects of the sample, making the sample seem larger than it actually is. Some mfg's round back to .99, others report the #'s they got as is... "The nice thing about Standards, is there are so freakin' MANY of 'em..."

Happy/safe holidaze all... Steve

[TexRoadkill]

Lol, okay. I've got the room stuff down somewhat. What I'm really trying to decide is if a few inches of space are really that effective.

IF the standard absorption of the 701 is sufficient for your highs when placed directly against the wall. Then placing it a few inches away will only affect the highs some more and be superfluous.

When people say to give a few inches of space to help the lows they are really mistaken because in order to get down to 100hz you really need over 2 feet of space between the wall and the 703.

Can I get an amen?

[John Sayers]

When people say to give a few inches of space to help the lows they are really mistaken because in order to get down to 100hz you really need over 2 feet of space between the wall and the 703.

amen

before the introduction of products like 703, and rigid fibreglass the material only went down to 800hz and rolled off. It that case lifting off the wall would extend it down lower.

cheers
JOhn

[TexRoadkill]

Originally posted by John Sayers
amen

before the introduction of products like 703, and rigid fibreglass the material only went down to 800hz and rolled off. It that case lifting off the wall would extend it down lower.

cheers
JOhn

Bless you my son.

I was just trying to decide if building fancy frames to give me some space was worth the trouble and it looks like it probably isn't. I have more mid low issues than high end issues and that seems pretty typical of most situations.

Ever notice how you go through all these fancy room calculations and it always comes out to 'fix the low mids'.