Panel bass absorbers modification question

Posted: Sat Aug 26, 2006 9:42 pm

I have a question about the standard homemade sealed panel absorber with 1/4" plywood membrane... would a 4" cavity behind, overstuffed with fluffy pink insulation (so that it is touching the membrane as well as the wall/ceiling behind) work.

And by "work" I mean to ask how my deviation would change performance from 703 mounted "close, but not touching the membrane" (a la Ethan Winer) or the "glued to the membrane" (a la John Sayers).

As I understand* the panel absorber, this option is OK, probably not quite as good as the 703. However, when I factor in my untrained ear, the results may be remarkable...

I'm finally taping the drywall in the studio and I'm anxious to start treating this room.

Thanks in advance,
CHAD

*understand is a relative term

Posted: Sun Aug 27, 2006 9:54 am

Chad,

> would a 4" cavity behind, overstuffed with fluffy pink insulation (so that it is touching the membrane as well as the wall/ceiling behind) work. <

When I was shown how to build those traps it was explained that the fiberglass must not touch the wood. But you know what? I can't say for sure that's correct because I never built a bunch of them that way and compared them to traps built the standard way.

That said, why not just build them the way we all know for sure they will work correctly?

--Ethan

Posted: Mon Aug 28, 2006 8:32 pm

Ethan, thank you.

I ask because my resources are
1) time
2) construction ability
3) wood & tools
4) two leftover bails of fluffy r-19, 24" wide just sitting here

My only liability is the $180 investment in 703. I have searched and can't find any suppliers in the Kansas City area. Most insulation companies haven't heard of the OC 703, and the rest I've spoken with don't (won't) carry it.

Perhaps another recording.org member has had better luck from the Owens Corning website and the KC Yellow Pages.

Out of curiosity (and although I know I've seen something referenced about it in another post...) do you know why the link on John Sayers' website suggests to glue the 703 to the panel? (click "Absorbers" on the left, then the "Low Frequencies" tab)

It seems I read another post on here saying the only change would be a slightly lower resonant frequency, while dampening and absorption would still occurr...

CHAD

Posted: Tue Aug 29, 2006 10:22 am

Chad,

> I have searched and can't find any suppliers in the Kansas City area. <

This place has outlets all around the country:

www.spi-co.com/servicecenterdirectory.mv

They list Kansas City as one of them.

> do you know why the link on John Sayers' website suggests to glue the 703 to the panel? <

There are a lot of ways to make bass traps. I've never tried that one, though.

--Ethan

Posted: Sun Sep 03, 2006 9:03 am

On the SAE / John Sayers site it's a damped membrane design. As far as I understand, it's all about making sure while vibrating the membrane won't radiate the sound back into the room. Also, it lowers the center resonant frequency. A better design I guess Rolling Eyes

Much more difficult to predict the resonant frequency though. I've never seen a formula for this design.

You'll better ask this question on John Sayers forum I suppose ?

All the best,

Michel

Posted: Mon Sep 04, 2006 8:07 am

msikio wrote:

Also, it lowers the center resonant frequency. A better design I guess Rolling Eyes

Much more difficult to predict the resonant frequency though. I've never seen a formula for this design.

It seems to me as if it would broaden the value of the attenuation - and I haven't seen a calculation for this either.

Good advice to go to John and ask him the question directly.

Sincerely,

Rod

Posted: Mon Sep 04, 2006 12:36 pm

Rod Gervais wrote:

msikio wrote:

Also, it lowers the center resonant frequency. A better design I guess Rolling Eyes

Much more difficult to predict the resonant frequency though. I've never seen a formula for this design.

It seems to me as if it would broaden the value of the attenuation - and I haven't seen a calculation for this either.

I believe it does both.
The insulation increases the effective depth by 1.2 to 1.4 for how ever much insulation you have in there (lowering the frequency), and the insulation lowers the absorption coefficient and widens the bandwidth (lower Q). I have no measurements for that.

Recently it was explained to me that the usual formula for this is probably wrong -- at least the way most of us would use a panel trap (i.e. against a gypsum based wall).

d = 28900 / (f ^2 * m)
where
d = depth in inches
f = resonance frequency in hz
m = surface density in pounds per square foot

http://www.johnlsayers.com/phpBB2/viewtopic.php?p=49275#49275

Quote:

As far as I understand, it's all about making sure while vibrating the membrane won't radiate the sound back into the room

I wonder if green glue or some limp mass might damp the resonance well -- of course it would chop some more off the absorption coefficient, but might reduce the ring. Although 703 "glued to the membrane" (a la John Sayers) would probably damp the same.

Posted: Sat Sep 16, 2006 5:58 pm

Ethan, the SPI-CO place was the one. At the KC branch on Friday, a wonderful lady, Becky, gave me 14 pieces of 1.5" 705 that they were going to throw out. Plus she gave me 7 boxes of OC rigid pipe wrap for 5/8" pipe with a 1.5" insulation thickness, so my panel absorber question is now mute. Much smaller link is a (poor) picture of what I'm talking about. Scroll to the bottom of that picture to see the tubing.

Does anyone think these tubes could be set up "bowling pin" style in the corners, floor to ceiling, and also in the ceiling/wall corners for broadband absorption? I think that would work. They're 36" long, 4" diameter with the 5/8" hole down the core, and appear to be insulation based on 703.

CHAD

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