Ductwork, soffits, registers, stc...

what exactly am I missing here? I've read Rod's book, Jeff Cooper's book, Everest, etc, etc.. and I still can't get my head around one thing regarding HVAC.. So, at some point, you must punch a hole in the sealed-up acoustic shell of your rooms to get HVAC in and out. Building soffits in to maintain the integrity of the wall is ideal - I get that, too. But here's what I don't get: At some point along the soffit - you must have the actual register/return - at that point, haven't we just punched a hole in the inner leaf? Now there's nothing between the instruments in the room and the outer leaf except some insulation and ductwork. Shouldn't this bring the STC of the wall down dramatically? Am I missing some key construction detail here? Thanks!

Am I missing some key construction detail here?

Yup! You are missing about six key details, apparently! :) 1. Multiple 90 degree corners in the ducts 2. Insulation on the INSIDE of the ducts (not the outside). 3. Large silencer boxes in line with the ducts. 4. Extra long ducts 5. Oversize ducts 6. Low speed, high volume air flow. All of the above helps to keep the acoustics under control. There might be other factors that I forgot, but those are the ones that come to mind. - Stuart -

biggator wrote:

But here's what I don't get: At some point along the soffit - you must have the actual register/return - at that point, haven't we just punched a hole in the inner leaf? Now there's nothing between the instruments in the room and the outer leaf except some insulation and ductwork. Shouldn't this bring the STC of the wall down dramatically? Am I missing some key construction detail here?

This was something that was hard for me to figure out too... I mean, it all seems for naught, right? Well, it is and it isn't. In addition to what soundman touched on, this is also why you take great care in the technique of penetrating the envelope. You need to seal every bit of the actual penetration... caulk, overlap seams, filling screw holes, etc... Lemme see if I can s'plain it this way... Yes, you work like hell to make as massive of an air tight wall to get the STC as high as possible, because you are going to punch some great big holes in the envelope. There's going to be sound leakage from the duct work. However, just like everything else, you work to minimize that leakage... and the best way to do that is to prevent air leakage from further reducing the effectiveness of the room envelope. Caulking the seams and edges around all the surfaces of the opening is crucial. As is making sure that all of your ductwork is sealed at each junction. The same applies to windows and doors, actually.

I'm aware of all the sealing of openings, 90o bends, etc that Soundman mentioned.. I understand all that as necessary for keeping both AC system noise, and 'other room' noise from getting through the ductwork.. what I'm missing is how we keep the wall itself at STCxxx where the registers are actually installed. See below - I'll try to draw it. There's a diagram in Rod's book that shows the detail of a soffited duct - I just did a really crappy quick redraw of my understanding of it (soffit with duct at a wall/ceiling corner). Here it is: Now, here's how I understand it to work once you actually put in a register or return: Unless I'm missing something - wherever you have a vent, the inner leaf is now just insulated duct... so, I'm assuming I'm missing some detail here, otherwise - the STC of the entire wall should drop to that of the duct (obviously that isn't the case, or this forum wouldn't have much to talk about!) "-)

I guess the way to think about it is to realize that a room is a system of parts that act as a whole, in ways you would and would not expect. When you penetrate the envelope with an HVAC duct, door, window or even a nail hole, you are correct in that the entire STC of the barrier is diminished. Yes, a single nail hole will reduce the STC of a wall, but is that single nail hole going to allow as much energy out of the envelope than if the envelope didn't exist? Obviously not. In theory, you are correct in surmising that the entire wall is reduced to the STC of the duct. But what is the surface area of the duct, in relationship to that of the envelope? STC is a valuable part of the equation, but it isn't the be all/end all of building a studio. If it was, you wouldn't need to evaluate the amount of absorbtion/reflection or ratio of L:W:H either. A studio environment is a system comprised of many different disciplines. They work together and against each other. So, you giveth in one aspect and taketh away in another. What you try to end up with at the end, is a compromise that you hope will be acceptable to a minimum of standards you have established.

So - my drawing is fairly correct? Does that mean that in an example like in Rod's book (the 'putting it all together' section where he lays out the basement studio) - that example studio has 5 vents each in the control room and live room... how much of a difference in isolation does this make? It would seem pretty significant, yet isn't covered in good detail in any book I've read - I just keep going over it thinking 'what am I missing here?'. Using the example from the book - that's 5 holes in the shell - each of which is going to be at least 12"x6" (probably bigger).. that's the equivalent of having a 5'x2.5' spot in the ceiling where instead of 2-3 layers of drywall, you have ductwork. While the ductwork isn't an STC of 0, it's also not an STC of 54.. maybe I'm overthinking it, but it's bugging me! "-)

Based on the little I have on this part of the build, the hvac will pressurize the room if all the specs are properly achieved. It[this pressurization] is most effective when all the baffles, proper ducting, etc., are in place and if so, the breaches made by having air in and out are reduced. I wish I had a better explanation, because I need it for my bookmarks:)

xSpace wrote:

Based on the little I have on this part of the build, the hvac will pressurize the room if all the specs are properly achieved. It[this pressurization] is most effective when all the baffles, proper ducting, etc., are in place and if so, the breaches made by having air in and out are reduced. I wish I had a better explanation, because I need it for my bookmarks:)

That does help.. The reduction in STC due to lower mass is not as much as a pure air-gap - that makes sense. Though, it begs the question if building the rest of the wall to a higher STC is worth it or not.. or more specifically - how to figure out how high of a level to build to so that it matches the weakest link (the ductwork).

Gator, I can completely understand your question as I am equally concerned about the reduction in TL from HVAC penetrations as well. It’s hard to accept reductions from HVAC when you hear phrases like "your isolation will only be as good as your weakest link". One thing I know for sure is that each and every studio project out there is completely unique and no one solution will work for them all. For my project, I need to keep as much sound as possible from escaping through penetrations. No matter how many bends you put in your duct, no matter how much insulation you put in them or how big you make them, will negate the effect of sound entering the envelope through the duct at the actual point of penetration. The only way I can see to combat this is to install your silencer box directly at the point of penetration through the inner leaf. Here’s a couple crude drawings. Ex.1 is just the plain duct, be it rigid fiberglass or lined metal duct. The red arrow is the sound path. Ex.2 is the same thing but with the duct replaced with a silencer box with 1" duct liner. Check out this link and look at the "sound damper" drawings that Rod attached. Max, Did you incorporate these into your studio?

wow - now THAT helps a ton. I had planned to put some kind of plenum/baffle box in-line for noise purposes.. never thought to put it right at the point of room penetration. I think the lightbulb just went off for me. THANKS!!

From that other thread, Rod posted this about using silencer boxes to shield the penetration:

Frame out an opening (in your ceiling joist where the drop is) about 16" wide and then enclose all 4 sides of it with drywall. Make sure to caulk the corners and stagger the joints (and use backer rod of course). SO now your drop will pass from the baffle through a sound stop...... Reinstall your drop and outlet......... insulate around that well. When you install your ceiling make certain to caulk against the drops of the drywall box you installed above. This way the baffle you built will serve dual purpose - stop air (and mechanical) noise through the duct itself - and stop sound transmission from room to room.

Does anyone have a drawing or pictures of this type of installation? I think I get how to do it, but when he says 'enclose all 4 sides of it'.. the silencer box should have 6 sides, it's a cube (well, a rectangular box).

I want to add my technique on this. Once I made the penetration into the room, I continued running the duct inside the room and built the soffit around that. This way, the penetration is behind another layer of drywall (or two) and the actual vent into the room is a ways away from that initial penetration, like at least another 90 turn away. Have a look: viewtopic.php?f=2&t=10533&start=30 I should mention that I saw this used in a build in Rome that Walters-Stork did. They just punched a hole in the wall and kept running the duct around the room to the ideal spot to have the air come in and then covered it all up. -ashley

Thanks, bassman.. reading through your build thread was great stuff - the build looks awesome! I had thought about your way of doing it... but had concerns about building a sealed up drywall-covered soffit for ductwork because I wasn't sure if that would create a 3-leaf wall in sections. I'm most concerned with keeping sound out of the upstairs (my build will be in the bottom floor of my house - want to be able to play while the family is upstairs). The silencer boxes will need to be built anyway - so installing them at the penetration point is actually a pretty good idea. I get the additional isolation, and I get to get the silencer box out of the way. DO me a favor - check out my 'hvac - exchange chamber' thread.. see what you think. "-)

I had thought about your way of doing it... but had concerns about building a sealed up drywall-covered soffit for ductwork because I wasn't sure if that would create a 3-leaf wall in sections.

Just a quick comment here: I don't think that the ductwork would create too much of a third-leaf problem, if any, mainly due to it not being that big an area, as compared to the entire wall. It would only be, what, maybe 10 or 15% of the entire wall? Also, if you have drywall around both sides of the duct, then it would be a four-leaf issue, not three leaf, but you will also have a fair amount of extra mass in there, which compensates to a certain extent. I'm no expert on that, but it seems to me that any multiple-leaf effect you might get there would not be that big, and probably not worth worrying about. IMHO! - Stuart -

bassman wrote:

I want to add my technique on this. Once I made the penetration into the room, I continued running the duct inside the room and built the soffit around that. This way, the penetration is behind another layer of drywall (or two) and the actual vent into the room is a ways away from that initial penetration, like at least another 90 turn away. Have a look: http://johnlsayers.com/phpBB2/viewtopic ... 3&start=30 I should mention that I saw this used in a build in Rome that Walters-Stork did. They just punched a hole in the wall and kept running the duct around the room to the ideal spot to have the air come in and then covered it all up. -ashley

Here's another example: The ducts are installed inside the room, surrounded with insul, and covered w/ 1 layer drywall forming a membrane absorber: http://www.gearslutz.com/board/photo-di ... nna-2.html

That is exactly the thread I referenced when doing my ducts. Bingo! -ashley