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That is the norm. However, regular 3/4" sub-floors are nailed down across joists, such that there's no gap between the joists and the sub-floor. So, in your case the potential problem is you're elevating the new sub-floor every n". Does that make sense? The pressure of whatever is sitting on the sub-floor + the sub-floor material itself are applying gravitational force at only the points where your 1x4's contact the underside of the sub-floor.

So, first if you strengthen the thickness of the sub-floor, then the sub-floor itself becomes more rigid and is better able to handle what are called point loads without flexing. Secondly, you will have a consistent load on the sub-floor above its outer edges, created by the bottom plate of your inner stud wall. By utilizing 1x4's beneath the edge of the sub-flooring, you would reinforce the ability of the sub-floor to transmit that vertical load down to the concrete foundation.

I hope that's helpful and not confusing.
I'm a little confused simply because I may have misunderstood what to do with my riser. For the riser, I had thought that the drywall should go all the way to the concrete floor (with a quarter inch gape that will be acoustically caulked to the concrete), with the riser added later to the interior of the room after the double drywall is up. I was thinking of my contraption in the front of the theater as a mini-riser of sorts, which is why I was assuming it should be installed the same way. Is my plan for the big riser correct? What is the advantage of attaching the drywall to the baby riser?
 

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I was wondering how to handle the heating/ac vent placed in the ceiling of the HT room. The bottom of the boots (straight or elbow) seems to be designed for one layer of drywall. When going with clips, hat channel, and double drywall, I would like to know how you handled the extra thickness of the ceiling?
I custom built the air return and supply vents into my room.

Depending on the design of the boot/register/grille, you may have to create a short flue of some sort they attach to. Or you may be able to modify the OEM parts to fit in 2x thick drywall. Another option would be to install the 1st layer of drywall, install the boots, then install the 2nd layer of drywall.
 

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I'm a little confused simply because I may have misunderstood what to do with my riser. For the riser, I had thought that the drywall should go all the way to the concrete floor (with a quarter inch gape that will be acoustically caulked to the concrete), with the riser added later to the interior of the room after the double drywall is up. I was thinking of my contraption in the front of the theater as a mini-riser of sorts, which is why I was assuming it should be installed the same way. Is my plan for the big riser correct? What is the advantage of attaching the drywall to the baby riser?
You're on the right path. The riser gets built after the room's floor/wall/ceiling inner shell. Leave a 1/4" or so gap between your riser and the drywall. Don't connect the riser to the walls directly (e.g. screws, etc.). Gravity will keep it in place.

In your case - with a raised floor - this will be another area where you need to plan your sub-floor carefully. You will want to ensure there is sufficient support beneath your riser. That likely will just mean a few extra 1x4 boards placed strategically to ensure even load support of the riser above them.

To re-cap, ideally you'll build the sub-floor, your wall and ceiling framing (inner stud wall), then stagger your ceiling/wall drywall layers, and then create your riser. Leave 1/4" or so gap in corners where the various surfaces meet. For example, where you have a corner between drywall wall and sub-floor, and drywall wall and ceiling. Caulk the gaps. The purpose of the gaps is to provide some decoupling between neighboring surfaces by creating a very small separation. The acoustic caulk seals the gap to prevent flanking sound leakage. Acoustic caulk is more flexible than regular caulk so even though you are technically recoupling the surfaces, it is with a material designed to give a little and bounce back.

So, there's two factors at play here. One is physically supporting the inner wall you're building. The other is retaining as much flexibility as possible in your inner wall so when sound wave pressures hit them they are able to flex a little while absorbing energy at the same time.

If you build your new sub-floor as I've indicated, it will also help keep the column speaker backer boxes isolated from the main structure, as they'll also be on top of your floating floor.
 

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Short answer is, yes - you're right (about more mass being better). The point I apparently didn't get across too well is I don't believe a small area of the floor lacking an additional layer (mass) of solid material will significantly affect your outcome. I was attempting to suggest focusing your attention and efforts on other areas of your room and not worrying about that too much, particularly since I got the impression your choices are limited. I'm sure you could come up with a way to fill the area, but my suggestion is the minimal gain is probably not worth the effort required to do so.

So, bottom line - yes, you're right a bit more mass in the gap area would help with sound absorption. However, I don't personally believe it's worth the effort given your circumstances. Purely a personal opinion though. :)
I've read your other post and I get that caulk is better than no caulk. What I'm having trouble understanding is how the caulked area doesn't create a weak spot that undermines the soundproofing. I realise that this is largely academic as we have to seal the gap and caulk is the best/only option but still the question bothers me.

I've attached three drawings to illustrate my thinking. In the first one, there's no caulk and I've drawn arrows to show where sound could be rising up from the floor (and then travelling through the insulation) and coming out of the wall (which has less plaster, or even none, on the area below the wall). These sounds would then be free to travel up into my room and equally sound could travel from my room down into the wall and the concrete slab.

In the second drawing I've added the double drywall walls, with the clips+channel indicated in black with insulation between. I imagine in some scenarios people only need to treat the floor but I'm almost certainly going to have to do the walls as well, so I'll assume that's the case here. Without caulk, there's still no real impediment to sound travelling in either direction (other than the size of the gap, which as we've already discussed would be too small for LFE to travel through).

In the third drawing I've filled both the floor perimeter gap and the gap under the wall with caulk. This obviously improves things considerably compared to having those gaps open but what I'm querying is whether the relatively low mass of caulk can sufficiently block mid->high range frequencies? It doesn't matter whether caulk blocks those frequencies less than two layers of OSB, all that matters is whether caulk blocks them enough that no human would notice the difference. In the case of the sounds emanating from under the floor, there's two separate layers of caulk between them and my room which no doubt helps, but only one layer between the above floor section of wall and my room.

If caulk does sufficiently block mid->high range frequencies, then presumably if they were all that needed addressing and there was no LFE to worry about, then a single layer of something light-weight and low mass like plywood would be sufficient on the walls (still using clips+channel to decouple), as I imagine even the lightest wood has more mass than caulk? Obviously one would have to use something thicker for the floor as it has to support the load but still only a single layer would be needed.

As for the perimeter tape, I should clarify that it's not a choice of caulk or perimeter tape. As shown in that Cochlear drawing I linked to previously (screenshot attached), there's still caulk at the top, just perimeter tape underneath that.

So the perimeter tape doesn't substitute for caulk as a means of closing the gap at the top. I presume it helps to prevent noise from the wall getting into the underfloor space and also to keep the subfloor from shifting. With caulk, how much depth would you fit, just equal to the top layer of 18mm OSB or even less and how do you control the depth of the caulk as there's nothing to stop it going all the way down to the bottom? Would 18mm depth of caulk be sufficient to stop the subfloor shifting?

If the perimeter tape is completely unnecessary to stop the subfloor shifting, then I'd be inclined to fit a piece of wood to the wall in it's place to seal it off, or maybe just add some extra plaster, making sure that whatever I use doesn't touch the subfloor as obviously anything solid would transfer sound. I wouldn't have thought it would matter if the wood/plaster touches the caulk as the caulk is already attached to the wall anyway and that would provide a base to build the caulk up from.
 

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In the third drawing I've filled both the floor perimeter gap and the gap under the wall with caulk. This obviously improves things considerably compared to having those gaps open but what I'm querying is whether the relatively low mass of caulk can sufficiently block mid->high range frequencies? It doesn't matter whether caulk blocks those frequencies less than two layers of OSB, all that matters is whether caulk blocks them enough that no human would notice the difference. In the case of the sounds emanating from under the floor, there's two separate layers of caulk between them and my room which no doubt helps, but only one layer between the above floor section of wall and my room.

If caulk does sufficiently block mid->high range frequencies, then presumably if they were all that needed addressing and there was no LFE to worry about, then a single layer of something light-weight and low mass like plywood would be sufficient on the walls (still using clips+channel to decouple), as I imagine even the lightest wood has more mass than caulk? Obviously one would have to use something thicker for the floor as it has to support the load but still only a single layer would be needed.
The caulk simply completes the process of creating a sealed chamber of sorts. It will help dampen flanking sounds that could otherwise enter via a small gap. It's purpose is to cover gaps. The purpose of using acoustical caulk is it's designed to flex. Your walls will be flexing a bit from the sound pressure applied to them. Small gaps are normally left between adjoining surfaces (e.g. wall and floor) so the surfaces can flex independently of each other.


As for the perimeter tape, I should clarify that it's not a choice of caulk or perimeter tape. As shown in that Cochlear drawing I linked to previously (screenshot attached), there's still caulk at the top, just perimeter tape underneath that.

So the perimeter tape doesn't substitute for caulk as a means of closing the gap at the top. I presume it helps to prevent noise from the wall getting into the underfloor space and also to keep the subfloor from shifting. With caulk, how much depth would you fit, just equal to the top layer of 18mm OSB or even less and how do you control the depth of the caulk as there's nothing to stop it going all the way down to the bottom? Would 18mm depth of caulk be sufficient to stop the subfloor shifting?

If the perimeter tape is completely unnecessary to stop the subfloor shifting, then I'd be inclined to fit a piece of wood to the wall in it's place to seal it off, or maybe just add some extra plaster, making sure that whatever I use doesn't touch the subfloor as obviously anything solid would transfer sound. I wouldn't have thought it would matter if the wood/plaster touches the caulk as the caulk is already attached to the wall anyway and that would provide a base to build the caulk up from.
It's up to you if you want to use the perimeter tape or not. I just don't believe it's going to have a significant impact on your overall sound proofing results.

You just want the caulk in between material surface layers on the inside of your room, where there's a gap. If your gap is larger than about 10mm or so, you may need to use what is commonly referred to in the USA as "caulk saver."




IMHO, I don't believe there's going to be a significant difference in sound proofing results whether you plug the holes on the edges of your flooring plan with OSB, drywall, etc. or not as it doesn't seem to be a lot of space. It's possible that I'm wrong, but as I see it the concern there will be flanking noise from below. Since any sound from below will already have to travel through the other floor areas, it's already going to be damped. If you feel the need to fill the space, I would suggest something flexible such as stuffing it with loose fiberglass insulation. That would have the benefit of further reducing sound transmission and it would be a relatively easy process to implement.

I did not intend to suggest filling the edge holes/space with caulk. It seems as if that would be quite a bit of caulk! The caulk only needs to in seams between surface materials.
 

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"caulk saver."
We call that "backer rod"
It's available in several diameters from 1/4" to 1".


Note that caulking that covers a wide gap will fail. Googling for "caulk two point adhesion" "caulk three point adhesion chinking", or "caulk why joints fail" will turn up details. Basically caulk should only fill gaps between 1/4" and 3/8", with no side nor depth more than 1/4" from any point on the caulk. Backer rod is good for reducing that gap down to something caulking will stay with.
 
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I couldn't keep up with all the back and forth but as for this discussion of caulking and mass. The gap should and usually is very small. As HTGeek pointed out, a surprising amount of sound can get through a crevice. You want to avoid having the wall sitting right on the floor and the floor sitting right against the wall. It helps decouple them from each other. That gap should be filled with caulk. Typically this gap is tiny so the loss of mass is minor.

I too made a big deal about this. I was really worried. All the pros and everything was reading said don't worry and just caulk. I can honestly say my end product bore our that advice. Just don't lose sleep over it. The issue is minor.

In fact many people have far greater problems with their door frames, hvac, etc in this regard.


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No. You should run the drywall from ceiling to the sub-floor. Build the sub-floor first, then the inner wall on top of that. So, your construction order is:





  1. Inner stud wall
  2. Raised sub-floor
  3. First ceiling layer
  4. First wall layer
  5. Second ceiling layer
  6. Second wall layer

HTGeek what makes you feel this way? I'm not sure I've read a definitive on this before. In my mind there are advantages to running the walls closer to the concrete and caulking it. Then adding the subfloor and caulking the perimeter again. My thinking is that since the floors typically aren't built to be soundproof over concrete this avoids a possible flanking path through the floor.

I didn't address this in my build because I don't have a subfloor. I put a sub on the riser and have butt kickers for the tactile sensation.

While some may use MBM's for tactile bass I did it for smoothing the bass. I have three subs currently used for optimal bass coverage. I actually used my MBM's up front, the lower frequency sub (ULF) on the riser, and I'm currently finishing a fourth. I think the most important reason for having subs like that is smoother bass rather than the feel.

At a future date I plan to commit the cardinal sin and penetrate my shell to duct bandpass subs into the room. I have a very large crawl space around the outer shell and want to hide them in that space. I have some ideas on how to duct the bass without impacting soundproofing much.



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HTGeek what makes you feel this way? I'm not sure I've read a definitive on this before. In my mind there are advantages to running the walls closer to the concrete and caulking it. Then adding the subfloor and caulking the perimeter again. My thinking is that since the floors typically aren't built to be soundproof over concrete this avoids a possible flanking path through the floor.
Thanks for quoting part of my post, as I realized I had juxtaposed the order of the first 2 list items. They should have been in the order of my preceding text comment. I've edited my original comment (above).

Anyway, to answer your question: My thinking was if the goal is to get vibrations through the floor, then it may be better to allow any vibrations in the walls to travel down to the sub-floor instead of the concrete. However, I'm not sure it matters a whole lot either way. I agree with you either caulk and/or the thin insulation that goes under the footer (the name of which escapes me atm) should be used.

There shouldn't be a flanking issue if all the gaps and seams are caulked.


At a future date I plan to commit the cardinal sin and penetrate my shell to duct bandpass subs into the room. I have a very large crawl space around the outer shell and want to hide them in that space. I have some ideas on how to duct the bass without impacting soundproofing much.
That should be interesting! :)
 

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Thanks for quoting part of my post, as I realized I had juxtaposed the order of the first 2 list items. They should have been in the order of my preceding text comment. I've edited my original comment (above).



Anyway, to answer your question: My thinking was if the goal is to get vibrations through the floor, then it may be better to allow any vibrations in the walls to travel down to the sub-floor instead of the concrete. However, I'm not sure it matters a whole lot either way. I agree with you either caulk and/or the thin insulation that goes under the footer (the name of which escapes me atm) should be used.



There shouldn't be a flanking issue if all the gaps and seams are caulked.









That should be interesting! :)


Hey I didn't invent the idea! Seems to be working for a variety of manufacturers. I won't cut holes until I'm confident in the designs. At the moment the bandpass subs have undersized ports and overload easily. The plan is to use an hvac duct boot or possibly a 6" flares duct end. A rubber isolation collar, semirigid flex duct, and then another rubber collar and take off. The semirigid flexduct is stiffened and damped with window flashing tape. I have the materials just waiting on a friend with a router to stop by and help me cut some holes in the boxes. I'll test them behind the false wall for now.

I'm going to reinforce the entry point into the room with a third layer of drywall, caulk it really well, etc. even if it does compromise the soundproofing a little, it's leaking sound into a craw space under the stairs that is fully dry walled itself.

I actually drew up plans for these bandpass subs using 18" or 21" B&C drivers. They really will be nutty. 130db at 1 meter in a through wall subwoofer. The 18" driver I want should be available any day now and so once it is I'll order one to test. If it works out I'll go from there.


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Brainstorming Stage

Hi, I'd appreciate any feedback on my soundproofing concept.

For starters, I already have a room within a room, so I'm simply going to do the following to the interior drywall:


  • 1st Layer: MLV or Peacemaker (6.4 mm)
  • 2nd Layer: Audimute's Acoustic Tiles
  • 3rd Layer: Acoustic Wedges
  • 4th Layer: Base Traps (in corners)
  • 5th Layer: Sound Absorption Sheets/Blankets (Hanging)
I'm hoping to affix the first layer with some kind of spray adhesive. Then I'll use the same adhesive to affix the tiles and wedges onto the MLV/Peacemaker. Not sure what I'll do with the ceiling, though.

So...instead of decoupling and installing sheet rock, will this alternative route help me achieve my goal of "soundproofing" the space (despite my using an inordinate amount of sound absorption techniques)? If so, by how many decibels (theoretically)?
 

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Hi, I'd appreciate any feedback on my soundproofing concept.

For starters, I already have a room within a room, so I'm simply going to do the following to the interior drywall:


  • 1st Layer: MLV or Peacemaker (6.4 mm)
  • 2nd Layer: Audimute's Acoustic Tiles
  • 3rd Layer: Acoustic Wedges
  • 4th Layer: Base Traps (in corners)
  • 5th Layer: Sound Absorption Sheets/Blankets (Hanging)
I'm hoping to affix the first layer with some kind of spray adhesive. Then I'll use the same adhesive to affix the tiles and wedges onto the MLV/Peacemaker. Not sure what I'll do with the ceiling, though.

So...instead of decoupling and installing sheet rock, will this alternative route help me achieve my goal of "soundproofing" the space (despite my using an inordinate amount of sound absorption techniques)? If so, by how many decibels (theoretically)?
I believe you are going to be creating a very dead sounding space if you follow through on your plans. Many of those products you have listed are used to help absorb sound inside the room and prevent sound from reflecting and causing issues inside the room. They are not used to help keep sound from escaping the room.

I would wager that if you already have a room within a room and the inner room is properly decoupled from the main structure of the house you are already pretty well sound isolated once you hang one or two layers of drywall. Depending how "sound proof" you want the room to be you could use green glue between two layers of drywall and skip the clips and channel.
 

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Hey I didn't invent the idea! Seems to be working for a variety of manufacturers. I won't cut holes until I'm confident in the designs. At the moment the bandpass subs have undersized ports and overload easily. The plan is to use an hvac duct boot or possibly a 6" flares duct end. A rubber isolation collar, semirigid flex duct, and then another rubber collar and take off. The semirigid flexduct is stiffened and damped with window flashing tape. I have the materials just waiting on a friend with a router to stop by and help me cut some holes in the boxes. I'll test them behind the false wall for now.
Well, why not take advantage of the properties of flanking, eh? :)

Have you considered an infinite baffle? If you're going to poke a hole in your shell and you have the space, it may be worth considering. Not sure it would work in your case (space-wise). I don't see why you couldn't also do it via your duct concept.

What is an infinite baffle subwoofer? [HT Shack]

IB build project w/pics, REW data
 

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Hi, I'd appreciate any feedback on my soundproofing concept.

For starters, I already have a room within a room, so I'm simply going to do the following to the interior drywall:
How would you describe the construction of your existing room-within-a-room? Double stud, staggered stud, clips & channel, etc.?? Are the interior walls/ceiling/floor decoupled from the home's structure? Which floor of your home is the room on?


So...instead of decoupling and installing sheet rock, will this alternative route help me achieve my goal of "soundproofing" the space (despite my using an inordinate amount of sound absorption techniques)? If so, by how many decibels (theoretically)?
I believe you are going to be creating a very dead sounding space if you follow through on your plans. Many of those products you have listed are used to help absorb sound inside the room and prevent sound from reflecting and causing issues inside the room. They are not used to help keep sound from escaping the room.
+1


... inner room is properly decoupled from the main structure of the house you are already pretty well sound isolated once you hang one or two layers of drywall. Depending how "sound proof" you want the room to be you could use green glue between two layers of drywall and skip the clips and channel.
Agreed. Your base should be 2x layers of drywall, either with or without a viscoelastic material in between the layers (e.g. Green Glue). The GG lowers the resonance of your walls (good thing). With the exception of MLV, all the other products you've identified are designed to tackle particular issues present in a room that has already had a 2+ sheet drywall shell built. MLV is normally applied to the studs before drywall is attached. MLV is not very useful as a sound dampening tool. It's awkward and difficult to apply to the studs, and you're looking at minimal benefit in most circumstances.

To illustrate what I'm talking about, here's a selection of lab results with various combinations of drywall layers, Green Glue (or not), and MLV (or not). The examples that don't have clips are single stud walls (not room-within-a-room). This is just for comparison purposes, to illustrate the benefits of extra drywall layers, drywall thickness, MLV, and Green Glue. All are 2x4 wood stud walls, 24" O.C. with R13 fiberglass insulation, unless indicated otherwise:


  • 1x 5/8" Drywall | 1x 5/8" Drywall; no insulation: STC 38
  • 1x 5/8" Drywall | 1x 5/8" Drywall [with R13 insulation]: STC 40
  • 2x 5/8" Drywall | 1x 5/8" Drywall: STC 42
  • 2x 1/2" Drywall + MLV | 1x 1/2" Drywall: STC 44
  • 1x 5/8" Drywall + MLV | 1x 5/8" Drywall: STC 45
  • 2x 1/2" Drywall + GG | 1x 1/2" Drywall: STC 52
  • 2x 5/8" Drywall + GG | 1x 5/8" Drywall: STC 52
  • 1x 5/8" Drywall + RSIC-1 Clips | 1x 5/8" Drywall: STC 56 [16" O.C. studs]
  • 1x 5/8" Drywall + RSIC-1 Clips + 1 psf MLV | 1x 5/8" Drywall: STC 57 [16" O.C. studs]
  • 1x 5/8" Drywall + RSIC-1 Clips + 2 psf MLV | 1x 5/8" Drywall: STC 58 [16" O.C. studs]
  • 2x 5/8" Drywall + RSIC-1 Clips | 1x 5/8" Drywall: STC 60 [16" O.C. studs]
  • 2x 5/8" Drywall + GG + IB-1 Clips | 1x 5/8" Drywall: STC 67
  • 2x 5/8" Drywall + GG + IB-1 Clips | 2x 5/8" Drywall + GG: STC 71
At first glance, looking at what I like to call the 'headline number' (STC), it appears using MLV is a no-brainer, right? 5 STC points better than no MLV on a single drywall layer on both sides of a wall. Well, unfortunately it's not that straightforward. Comparing the results ONLY to a standard 1x/1x wall, MLV does provide significant benefits in most frequency ranges (+5 STC; only 63-80 Hz, 250-315 Hz get worse). However, look at what happens when you decouple one side of the wall and then add MLV. Hardly any difference. What does this tell us? MLV is quite useful on a standard, single shared stud wall with no decoupling. However, decoupling is more effective, and decoupling plus MLV results in minimal gains (+1 STC) versus decoupling only.

You can also infer from the results above that Green Glue is more effective at sound proofing than MLV (+10 STC for GG; 52 vs 42). I haven't seen any lab reports that used both MLV and GG. It's possible the combination would be even better, but even if that is true, we already know when decoupling is added into the mix, MLV is likely to only provide +1 STC at best. And we know that swapping out MLV and replacing it with GG on a non-decoupled wall gives us another +8 STC (52 vs 44).

Point is most people don't use MLV because unless you're going to have a standard 1x/1x drywall wall, it's really not worth the work and cost. It is true that the heavier the MLV, the more effective, but beyond 2 psf it's going to be quite difficult to install and could begin to have an adverse effect on the load bearing capability of whatever one is attaching it to.
 

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I don't know if this was already said. T MLV should also not be adhered to drywall. That isn't how it was designed to be used nor most effective. It should be adhered to the studs directly with the drywall over that. This allows it to act as a limp mass between studs. It also was originally specified on 24" studs for this reason. When you glue it to the drywall the mass is made rigid by coupling.

If the room within a room was done right and drywall is already hung, then a second layer with green glue would be more effective.


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I don't know if this was already said. T MLV should also not be adhered to drywall. That isn't how it was designed to be used nor most effective. It should be adhered to the studs directly with the drywall over that. This allows it to act as a limp mass between studs. It also was originally specified on 24" studs for this reason. When you glue it to the drywall the mass is made rigid by coupling.
Good points.

You've also reminded me one of the issues with MLV is its nature tends to make for inconsistent installation techniques. There's very few lab tests with it, and I only recall one with details of how it was hung on the studs. I still don't get how it's possible to hang it 'limply' as you said. I know it's the recommended method, but how does one interpret "limply?" How does one retain limpness while mashing a sheet of drywall onto it?

Btw, I'm not questioning you... rather, trying to underscore your comments. Personally, I think MLV is just so much more likely to create a problem versus solve one that it's not worth considering.
 

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I apologize for the double post. I have a question for the experts on something I'm not entirely sure on after some of my research.

HVAC pipe. I have three runs of HVAC pipe between the overhead floor joists of my theater. None of these runs will be fed or output from the same air space as my theater since the theater will have it's own cooling/heating system. I'm trying to decide for sound isolation if I need replace the pipe runs with flex duct to help reduce sound transmission to rooms upstairs. The only sound that would enter the pipe would be sound transmitting through (from theater up to HVAC pipe) drywall, green glue, OSB, 25-gauge furring channel, clips and the dead space between the floor joists filled with Owens Corning Thermafiber or Roxuol Rock Wool. I've also considered keeping the pipe but wrapping the pipe with Dynamat or a similar product to help insulate them a little more. I'm just not sure how much I need to worry about sound making it into the HVAC pipe before it gets a clear path to the output vents in the rooms above.

Here is a picture of the pipes I am considering replacing with flex tubing. Thoughts?
 

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As I understand limp hanging just means it's hung on the studs. The drywall won't add much stiffness over the mlv because it isn't adhered to the drywall.

While I've never seen true lab tests of this I have seen builds of studios where a half inch of material was added on the studs after hanging the mlv to give a half in of air space.

MLV is still far more widely used than is greenglue. It doesn't get tested like Green Glue partly because it has been tested and the findings published in 100's of journal articles, books, etc. if you search the literature you will find more information. My understanding is that there are also a number of lab reports floating around that give the info you suggest. The limp vs sand which approach was tested and that report is available. I forget where I found it but if I find it I'll share.

I'm not as anti-MLV as some. I actually think MLV used in conjunction with CLD has merit. They each operate on slightly different principles and so I think they would provide coverage where the other is weakest. I have a feeling that it would extend the TL to lower frequencies and would also add 2-3 stc points on average across the board. I think it's biggest negative is that it's really expensive and hard to work with. For the same cost you could add a third layer of drywall with green glue. In fact for the same cost you could probably add a 4th layer.


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I apologize for the double post. I have a question for the experts on something I'm not entirely sure on after some of my research.

HVAC pipe. I have three runs of HVAC pipe between the overhead floor joists of my theater. None of these runs will be fed or output from the same air space as my theater since the theater will have it's own cooling/heating system. I'm trying to decide for sound isolation if I need replace the pipe runs with flex duct to help reduce sound transmission to rooms upstairs. The only sound that would enter the pipe would be sound transmitting through (from theater up to HVAC pipe) drywall, green glue, OSB, 25-gauge furring channel, clips and the dead space between the floor joists filled with Owens Corning Thermafiber or Roxuol Rock Wool. I've also considered keeping the pipe but wrapping the pipe with Dynamat or a similar product to help insulate them a little more. I'm just not sure how much I need to worry about sound making it into the HVAC pipe before it gets a clear path to the output vents in the rooms above.

Here is a picture of the pipes I am considering replacing with flex tubing. Thoughts?
Don't change those to flexduct. Leave them alone. I think you may be mixing up concepts a bit here. The reason to use acoustic flex duct is because it has a microperforated inner layer that absorbs sound that is transmitted through the duct. Transmission loss of flex duct is next to nothing (sound readily travels through flexduct). Transmission loss of steel ducting is actually fairly high due to the mass and stiffness. If you dampen the steel ducting (which dampens the resonance spike at the critical frequency) you actually get a value on par with a single layer of drywall or better.


I'm not sure of your plans, but if you are building a soundproof theater, then the ceiling itself will largely keep sound from transmitting through to the ductwork. The steel ductwork will further mitigate transmission. You should have very little sound that makes it to the duct work.


To maximize this, you will want to insulate around the ductwork. The insulation will help absorb some of the sound, help dampen the steel, etc. If you really are worried there are spray on damping materials that will increase TL. You would spray it on all the ductwork before insulating and hanging drywall. Me personally, I wouldn't bother. I did it to mine largely because I did run the ductwork into my theater and I did need to maximize the damping and TL of the ducting. You don't have the same issues.
 

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I have a feeling that it would extend the TL to lower frequencies and would also add 2-3 stc points on average across the board. I think it's biggest negative is that it's really expensive and hard to work with. For the same cost you could add a third layer of drywall with green glue. In fact for the same cost you could probably add a 4th layer.
Yeah. It's a P.I.T.A. to work with, which is primarily why I'm not a big fan of it. I agree that adding another drywall layer is easier. GTK regarding your other comments above.
 
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