To RC or not to RC in this case?

[jazzman_in_pa]

I've read the theories, but I also did an experiment that seems to fly in the face of the theories, so now I'm asking the experts. The case is a ceiling, and at this stage I'm mostly concerned with minimizing bass bleed from downstairs to upstairs.



FIRST LEAF:
==== top layer: 1/8" PVC vinyl flooring
==== 2nd layer: 1/8" plywood
==== 3rd layer: 3/4" plywood
==== 4th layer: 1/2" sheetrock (cut to fit between joists and attached to bottom of floor)

|| 2"x10" solid wood joists + 10 more inches of wood framing, with 8" fiberglass insulation batts between the joists, plus 10" ventilation ducts or 8 more inches of fiberglass insulation

SECOND LEAF:
===== 5th layer: 5/8" plywood directly attached to wood framing
===== 6th layer: 1/8" heavy vinyl, held up with 3/8" thick wooden strips


Now it's time for the final layer(s): one or two sheets of 5/8" drywall. But how? What is better:

A) Attach resilient channel to the strips of wood, fill air cavity with 1" absorbent, and attach plasterboard to resilient channel.

B) Remove resilient channel and attach plasterboard directly to wooden strips, leaving a 3/8" air cavity.

C = ?

I turned the radio on upstairs to a bass-heavy song and held an SPL meter directly to the downstairs ceiling's wooden strips. Then, for comparison, I shoved a little bit of closed cell foam behind the resilient channel to dampen it, then held the SPL meter directly
to the resilient channel. The meter (C weighted, fast response time) recorded lower SPLs when it was touching the resilient channel than when it was touching the wood. That makes me think that less low frequency sound energy will transfer to below using A, but theory says B is better. So I'm confused.

What do you recommend?

I'm not worried about high-frequencies, because all that insulation and vinyl seems to be blocking
a lot already. I'm just interested in blocking as much bass as I can, and intuition tells me I'll block a few dB more with RC than without. But, as we all know, in acoustics intuition often leads us astray.

Removing the 5/8" plywood which is just above the vinyl is not an option, as it's holding a downstairs partition wall in place. In my NEXT studio I'd do this differently, but how can I best salvage what I have? Any takers?

Thanks in advance,
Lee

[Rod Gervais]

Lee,

having a board directly below the joins - and then an air-gap between that and another board decreases the rating....... at this point i would reccomend that you remove the wood furring and apply directly to the bottom of the vinyl....

If we had spoken about this prior to your application - it would have been - RC...... drywall - Vinyl (seeing as you want it - i don't know that it's really worthwhile) and then drywall over that....... but the air-space between boards - even with insulation - is a bad thing......

Rod

[jazzman_in_pa]

Yeh, that's what I was afraid of.

It's too bad that Auralex recommends a similar method here, in the section "Existing Walls":

http://www.acoustics101.com/room_its...0Room%20Itself

Oh well, it's not too bad. At least I'll gain another inch of headroom in a tight space.

Regarding the vinyl, I'm a fan of it. It's thin mass that provides some damping to adjacent surfaces.

Lee

[Rod Gervais]

Lee,

I'm confused - when i look at their detail - it shows:

2 strips of sheetblock attached to the bottom of joist - then RC - then the coverings - how did we wind up where we are now?

Rod

[jazzman_in_pa]

Here's a better link to the part of the page I was talking about, and especially the details of this diagram, where they recommend attaching furring strips plus RC to the surface of an existing wall.



I know now that it would be better to attach RC directly to ceiling joists. But in this location I had additional concerns. I had HVAC ducts sitting below the ceiling joists that had to be boxed in. In the same area, below the ductwork, I wanted to add a wall, and needed a stable place overhead as an attachment point for that wall. After boxing out the ductwork with a framework of 2x2s and 2x3s, I looked at the framework and wished it could be more stable. Then I got the idea of adding a continuous sheet of plywood to the bottom of the whole thing for added stability. Plus, the plywood would offer easy attachment points for the vinyl seal. Hey, the vinyl works well both to add some mass and as a seal, but it's a bitch to put up, and this seemed an easy way to do it.

Had I known then what I know now, I probably would have still used the plywood for its added stability but would have cut out all unnecessary sections to allow the whole assembly to work as a double leaf system instead of a triple leaf. Maybe I still can, if it's really worth it.

Lee

[eric_desart]

[ December 31, 2003, 07:13 PM: Message edited by: Eric Desart ]

[Rod Gervais]

Lee,

Take it from me - regardless of their claim - they are wrong...... you will create problems with this approach - that 1 3/18" air space is going to decrease your isolation - not increase it........

This is one of the problems i have with a company that sells product getting involved in anything other than the testing of their product.... they screw things up badly when it comes to the real world ......


Rod

[jazzman_in_pa]

Thank you, gentlemen.

Lee

[knightfly]

From their picture and comments, I think I know where the misunderstandings come from - They are talking to you as if you're NOT going to remove anything that's already there, but just ADD more. I've gotten a LOT of requests for ways to "get away with" this, and I ALWAYS try to talk people into "doing the right thing" and removing material back to the frame, so that they can end up with a double leaf system. It's a pain in the ass, but worth the effort.

The company whose information you posted needs to be more upfront and tell you what's BEST, not just what SELLS their product for them. Had they been totally honest in their literature, they would have admitted that you'd get much better results by peeling back to the frame and putting RC and 2-3 layers of mass.

Of course, if they WERE being honest, they'd also have to admit that their high-priced vinyl won't do noticeably better than a thinner layer of wallboard between the other two layers, it just COSTS 5 times as much... Steve

[jazzman_in_pa]

In all fairness, Steve, to the company we're referencing, I took the time to write them an email, and one of their top guys took the time to write me back in detail, saying he prefers to avoid drywall + RC8 + drywall because of the resonance issues. When it's too problematic to rip everything back down to the frame, he suggested going beyond the furring strips + RC as it appears in the diagram and adding 1" of mineral wool to control the resonance and serve as a physical damper on the adjacent materials. "This is about the best you can do with an existing wall without either tearing out the original drywall or building a full double wall."

They shouldn't let that diagram stand.

I also asked if the company had done any tests of such a system, and he said the same thing Ethan says: he'd love to, but it's very expensive to run tests on every idea you come up with, so no, they haven't.

I may still remove the vinyl, but cut away parts of the plywood to extend the air space to the subfloor above (about 20"), and add back the RC. Then the remaining plywood would still give me most of the ridigity I want.

Lee

[ October 29, 2003, 06:22 AM: Message edited by: jazzman_in_pa ]