Noob ceiling Q's: beams, ducts, and tiles (oh my!)

[robman]

Hi all,

Please indulge a few newbie questions--I am sure you have answered them before, but my searches didn't turn up answers--at least none that I could understand.

I want to build a basement room with an isolated ceiling (ideally, two layers of drywall suspended from RSIC-1 clips). A supporting beam transverses the entire room-to-be below the joists, and in several places HVAC ducts and plumping pipes do so as well. In other words, a cross-section of the suspended ceiling might look like this (solid lines = drywall):

----- ___________________ ------------------- _____________________
pipe |------------- ---------| beam ---- duct |-------------------------| duct
____|---------------------- |______________|-------------------------|_____

(1) From the little I know about room acoustics, it seems to me that the "boxing out" of these items should be part of the suspended ceiling itself, and not connected to them or the joists in any other way. In other words, the whole ceiling, boxes and all, would be one suspended unit. Is that correct?

(2) For ventilation, my plan is to tap the first floor HVAC system, since both the outgoing and return conduits run through this space. I am guessing that the best way to connect the main conduits to the vents in the suspended ceiling would be to install 180-degree loops of lined flexible ducting between the joists. Will that work?

(3) There are a couple of water meters above the ceiling-to-be that I will need to read occasionally, so I will need to install access panels beneath these. Any tips on how to do this while minimizing increased noise transmission through the ceiling?

(4) Finally, my wife would like to hang an Armstrong tiled ceiling below the drywall, into which we could place recessed lighting cans. Would this create a "three leaf" problem? The "default" tiles (Sahara) have a CAC of 35. I don't know how CAC values relate to STC values, so I don't know how much of a "mass" this is.

Thank you!

 

[Bryce_H]

(1) From the little I know about room acoustics, it seems to me that the "boxing out" of these items should be part of the suspended ceiling itself, and not connected to them or the joists in any other way. In other words, the whole ceiling, boxes and all, would be one suspended unit. Is that correct?

Yes, you want to suspend the entire ceiling. I did this in my basement. I had an I-beam that held up the first floor run across my theater from left to right about 1/2 way back. So the from the front wall to just behind the I beam is a suspended ceiling using DX-02 clips. From the I-beam back I used RISC-1 clips to attach the drywall.

(2) For ventilation, my plan is to tap the first floor HVAC system, since both the outgoing and return conduits run through this space. I am guessing that the best way to connect the main conduits to the vents in the suspended ceiling would be to install 180-degree loops of lined flexible ducting between the joists. Will that work?

I was origanially going to do a similar setup but once I contact a HVAC contractor he said I should branch off the main truck rather than off the 1st floor ducts. I verified this through some DIY book on HVAC. The reson is then you will be cutting the 1st floor air volume in half (read cold 1st floor, where as if you come off the main trunk the impact is less. In nterms of running the ducts in the joists - that would work fine. You shouldn't need lined ducting as I would guess you are going to stuff the space above your drywall with insulation.

(3) There are a couple of water meters above the ceiling-to-be that I will need to read occasionally, so I will need to install access panels beneath these. Any tips on how to do this while minimizing increased noise transmission through the ceiling?

Nope

(4) Finally, my wife would like to hang an Armstrong tiled ceiling below the drywall, into which we could place recessed lighting cans. Would this create a "three leaf" problem? The "default" tiles (Sahara) have a CAC of 35. I don't know how CAC values relate to STC values, so I don't know how much of a "mass" this is.

Not sure

 

[WmAx]

(1) From the little I know about room acoustics, it seems to me that the "boxing out" of these items should be part of the suspended ceiling itself, and not connected to them or the joists in any other way. In other words, the whole ceiling, boxes and all, would be one suspended unit. Is that correct?

Correct.

(2) For ventilation, my plan is to tap the first floor HVAC system, since both the outgoing and return conduits run through this space. I am guessing that the best way to connect the main conduits to the vents in the suspended ceiling would be to install 180-degree loops of lined flexible ducting between the joists. Will that work?

[/quote]

If you tap into the room your trying to isolate as you just described, you'll completely breach the low frequency isolation if you do not use specially designed ducting and baffle system. But on the bright side, you could use the very flexible duct to connect to the room as you described and have a solid dense panel that could be attached over the vent when actual noise is being produced or needs to be attenuated.

(3) There are a couple of water meters above the ceiling-to-be that I will need to read occasionally, so I will need to install access panels beneath these. Any tips on how to do this while minimizing increased noise transmission through the ceiling?

Air tight dense solid panels. Build a strong recessed frame into the suspended ceiling to accomodate and air tight seals. The choices of seals and fasteners, or however you want to attach them is up to you.

(4) Finally, my wife would like to hang an Armstrong tiled ceiling below the drywall, into which we could place recessed lighting cans. Would this create a "three leaf" problem? The "default" tiles (Sahara) have a CAC of 35. I don't know how CAC values relate to STC values, so I don't know how much of a "mass" this is.

Sorry, I can't answer this. I'm not an installer, I just know the basic physical rules governing your situation.

-Chris

 

[Rip Van Woofer]

The baffle WmAx speaks of is what I'm considering, along with flex duct. You could maybe build the baffle into one of your beams. Here's a quickie schematic drawing. Not to scale, it's merely schematic.

The baffle box is rigid, and lined with rigid fiberglas insulation for ducting (forget the exact name). There might be an all-in-one insulated sound abosorbing material designed specifically for building ducts too -- I'm still finding out about this stuff myself. Hopefully other responders will help us out. from The three lines are baffles across the width of the baffle box of the same material to further block the sound. Its kind of like you folded the baffle box, but easier to do.

You could build two baffles of different sizes to attenuate different low frequencies (the effect is over a pretty broad range). There are formulae to calculate the affected frequencies if you want to go that far.

Basically, this is how a car muffler operates!

Ignore the thin line at the baffle outlet (right side), just a goof.

I was thinking of starting a thread along these lines too. Glad you did. We'll all learn something.

If you haven't already, get your hands on the Master Handbook of Acoustics by Everest. Another good one (though he has an axe or two to grind concerning speakers, and I'm dubious about his advice to use rope for suspending a ceiling) is Premium Home Theater by Earl Geddes. Self-published, available at his company Website: http://www.gedlee.com I've actually met the guy and been in his HT and the proof is in the pudding.

 

[WmAx]

That's not a flex duct attaching the baffle to the isolated room outlet, is it? That's what it looks like in the picture. The duct connecting the room outlet to the baffle needs to be air tight and rigid(standard sheetmetal is not correct). It should be adpated from solid materials such as plywood(and lined with a skin that will not easily harbor fungus/mold growth).

-Chris

 

[Rip Van Woofer]

That's not a flex duct attaching the baffle to the isolated room outlet, is it? That's what it looks like in the picture. The duct connecting the room outlet to the baffle needs to be air tight and rigid(standard sheetmetal is not correct). It should be adpated from solid materials such as plywood(and lined with a skin that will not easily harbor fungus/mold growth).

-Chris

Yes, it is flex duct. Wanna make something of it? Oh, you did...

Thanks, actually. I'm still hazy on some points and corrections/suggestions are welcome. I'm sure robman agrees. I assume a mildew-resistant paint like Zinsser PermaWhite would do for the inside of the rigid ply duct?

But I have to ask: if the flex duct is isolated by the drop ceiling or other massive material from the HT room., I assume that it will not transmit sound. At least the kind of flex duct I'm thinking of, which is basically soft fiberglas sandwiched in wire-reinforced plastic.

Kind of a tangental anecdote: Many years ago I used to work with my Dad on commercial/industrial construction projects. One building had a big duct (I mean, big enough to walk upright in!) that was lined with (if I recall) a sort of felt like sound absorbent. It was still open and unfinished. It was the closest thing to an anechoic chamber I can imagine - you went in a few feet and any ambient noise disappeared as if in a black hole. Spooky. I do not remember the kind of building it was to require that kind of ducting.

 

[WmAx]

But I have to ask: if the flex duct is isolated by the drop ceiling or other massive material from the HT room., I assume that it will not transmit sound. At least the kind of flex duct I'm thinking of, which is basically soft fiberglas sandwiched in wire-reinforced plastic.

I am familar with the material. This will cause a huge breach. The duct is not rigid enough. The low frequency pressure will be offered no resistance, as if it was an open gap. The density and thickness of fibergalss in it will have zero(well, maybe 0.000001 ) effect on low frequencies. You must have a rigid, air-tight connection.

-Chris

 

[ScottMayo]

In my opinion, if you put a hole in the ceiling, no matter what sort of connection you make to it, it will transmit bass. A baffle will not stop deep bass. Two layers of sheetrock will barely stop deep bass.

On the other hand, so what? Air conditioning is good, and a little bass leakage isn't the end of the world - some is going to leak no matter what you do. The 8Hz "sound" effects in some movies will go through concrete, let alone wallboard. It's not a question of stopping bass - it's a question of what frequency you can live with in your leakage.

My advice (having done what you are doing) is to use the fattest duct and grate you can - small ducts increase the flow rate of the air, which means it makes more noise when it exits into the room. You want the air to move as slowly as possible when entering and exiting the room, which means a big fat pipe. I had the HVAC guys line the space between two ceiling beams, creating a lined duct, and had it vent into a ceiling soffit with sides of OC 703 and burlap (quite a bit of surface area). The result is that the air diffuses in through the OC 703, across the whole room. It's just about silent. The soffit doubles as a bass trap.

 

[WmAx]

In my opinion, if you put a hole in the ceiling, no matter what sort of connection you make to it, it will transmit bass. A baffle will not stop deep bass. Two layers of sheetrock will barely stop deep bass.

The majority of bass is transmitted through direct mechanical/rigid connection through the house substructure. Two layers, or 3 layers, or 4 layers of sheetrock will do little for LF, since the energy would be directly deflected into the wood frame to which it is attached, which in turn is attached to the rest of the house's substructure. Additionally, the primary frequency band that is transmitted is directly related to the resonant frequencies of the substructure/frame of the house. If you have an isolated, dual air tight set of rigid[or semi-rigid] structures isolating the system, it will attenuate very significant levels of LF energy, dependant on the corner frequency of the isolation system and the frame design of the barriers[they should be made as rigid as is feasible]. The user here is not going to use a floating isolation system(which is ideal), but has under-earth basement with block walls. He intends to mount the two air tight layers to this structure. This dense, under earth/dampened structure will serve to dampen much of the bass energy that is delivered into the first air-tight layer, preventing it from transmitting at a high amplitude to the second layer which is directly attached to the house substructure/frame. As I stated, it is not ideal as a proper isolated/decoupled system, but it is far superior to a system that is mounted directly to the house's wood substructure/frame system.

On the other hand, so what? Air conditioning is good, and a little bass leakage isn't the end of the world - some is going to leak no matter what you do. The 8Hz "sound" effects in some movies will go through concrete, let alone wallboard. It's not a question of stopping bass - it's a question of what frequency you can live with in your leakage.

The user has not specified '8Hz' sound effect. This is a very low frequency, and is very difficult to dampen or otherwise remove by any means. Fortunately, the frequencies of primary interest are usually much higher in frequency, even in HT. Being that the user here is wanting to remove the majority of SPL from a drumset that will be played by someone in the basement, the system planned will be very effective if executed properly. It is a tried and tested method of isolation that is used to remove extraneous LF noise for recording studios and test labs.

The HVAC baffle may be a problem, but I am not versed in the specific construction/calculation of these baffles. I'm not sure how large the baffles would have to be to effectiely muffle low frequencies. But a simple air tight panel could be used over the vents when maximum attenuation is required.

-Chris