HVAC baffle box location

[DasProductions]

Hi everyone,
Building a room within a room for isolated studio and live room together in the basement. I think I have the right dimensions for the baffle box sizes but after all the reading I’m still confused about the installation location. My goal is to prevent sound traveling out to the HVAC system and then to the rest of the house.

So far I’ve seen the following:
1.First, is it ok to use one baffle box per line in a room within a room design? one for inlet, one for return, one for clean air coming in and one for air return.

2. Install the baffle box between the existing ceiling joists, between inner and outer room, extend the inlet/outlet using wood and flush to the inner room ceiling drywall, not touching it, adding acoustic sealant from the wooden inlet/outlet to the drywall.

3.Or install the baffle outside of room, extend the inlet/outlet using wood and flush to the inner room drywall, not touching it, adding acoustic sealant from the wooden inlet/outlet to the drywall.

4.Or install the baffle outside of room, use flex pipe between inner and outer room, and direct it
to the inner room. I would think this way is not sufficient because sound would travel in the flex
pipe and in between inner and outer walls. This I think requires a second baffle box in the inner room.

Any recommendations and suggestions from the experts greatly appreciated.
Thank you,
Jim

[Soundman2020]

So far I’ve seen the following:
1.First, is it ok to use one baffle box per line in a room within a room design? one for inlet, one for return, one for clean air coming in and one for air return.

Depends on how much isolation you need, basically. If your needs are for high isolation, then you really need one box on each wall penetration. In other words, one on the inner-leaf and another one on the outer leaf, for each duct. If your isolation needs are more modest, then you can get by with just one box per duct, but you still need to isolate the duct where it crosses from inner-leaf to outer-leaf.

2. Install the baffle box between the existing ceiling joists, between inner and outer room, extend the inlet/outlet using wood and flush to the inner room ceiling drywall, not touching it, adding acoustic sealant from the wooden inlet/outlet to the drywall.

That's also a good option, if you have enough space to do that. Silencer boxes tend to be pretty big, so they often don't fit in those spaces, but if you can make yours fit, then that's a good option.

3.Or install the baffle outside of room, extend the inlet/outlet using wood and flush to the inner room drywall, not touching it, adding acoustic sealant from the wooden inlet/outlet to the drywall.

Another way is to break the wooden "duct" in the middle of the wall, leaving a gap, and wrap that gap with rubber (Neoprene, EPDM, Sorbothane, etc.) That way, you don't need to worry about isolating the duct and register from the inner leaf: each part of the duct can be attached to its own leaf, and the rubber decouples them.

- Stuart -

[DasProductions]

Thank you very much Stuart, really appreciate for taking the time and answering my questions.

[andy_eade]

Depends on how much isolation you need, basically. If your needs are for high isolation, then you really need one box on each wall penetration. In other words, one on the inner-leaf and another one on the outer leaf, for each duct.

Hi Stuart. I'm hoping you can help me better understand your above statement as I'm in the throws of designing my HVAC solution. I was under the impression that I needed only one silencer for each room penetration, which in my case would be one for each return, and one for each supply. I'm confused where you're describing having one silencer on the inner leaf and one on the outer leaf. In my situation (adding a zoning system to my HVAC) there will be flex duct runs coming off the main supply and returns (these will be rebuilt with acoustic lined rigid duct) that will run in walls and ceiling joists and hence will be between the inner and outer.

I know I'm missing something here and no matter how many times I retread your post I can't quite fathom it.

Many thanks and wishing you a joyous Easter,

Andy

[amac2673]

Let's say for the ease of describing the scenario that you decided to mount your return silencer on the inside of your inner leaf, what Stuart is describing is adding another silencer for that return on the on the outer leaf penetration also.

So basically you have register grill-silencer-inner leaf wall- air gap- outer leaf wall- silencer-return duct/register grill.

You are just adding a silencer at each penetration for each leaf, two leafs, wall penetrations, two silencers.

[amac2673]

Keep in mind though that the silencer does not "have" to be on the inside of each leaf as long as the silencer is attached directly to the penetration, ie: it could be attached directly to the outside of the of the inner leaf penetration as long as there is enough room for it.

[Soundman2020]

It's basically an issue of how much isolation you need, and what is inside/outside the point where the duct penetrates the leaf. If you don't need a high level of isolation, then you are fine with putting a single silencer box on each duct, provided that you put it on the "correct" side. By "correct" I mean the side that actually requires the isolation, not the one that doesn't need it so much.

The issue is basically that noise will get into the duct / leak out of the duct on the side of the wall where there is no box.

For example, if your priority is to keep external noise out of the tracking room, then I would put the silencer inside the tracking room, not outside. Thus, any sound that does leak into the duct, either outside the wall OR IN THE CAVITY BETWEEN THE LEAVES, will be silenced within the box, which is inside the room. If you had put the silencer on the outside of the outer leaf, then any noise that got into the duct within the cavity (such as cavity resonance, for example, or noise from other rooms that share the same cavity space) would make it into your tracking room unattenuated.

Conversely, if you were trying to keep your drum noise from getting to the outside world and annoying the neighbors, I would put the silencer outside the room, where any noise within the cavity that was picked up by the duct, would be attenuated by the box on the outside.

The basic problem here is that your MSM isolation wall has two leaves, and each of those is supposed to be made of a continuous unbroken layer of mass. If you cut a hole in one leaf then you have removed that "unbroken mass" at that point: there is no longer continuity. You have breached the continuous mass, so you have to do something to make up for that again: the silencer box accomplishes that, since it is made from materials that provide roughly the same mass as the piece you removed from your wall. Yes, the box is also a "hole", since it provides a passage for the air to flow, but that passage is twisted through several corners and faced with several barriers that also have a surface density similar to that of the leaf. Air can get easily around those corners and make it past the baffles, but sound can't travel that path so well. Sound travels mostly in straight lines (or at least, it's easier to think of it doing that), so at each turn the sound hits a massive barrier head-on, which attenuates it greatly. Some sound does get past of course (since sound is also pressure-driven, not just velocity-driven), but that sound immediately hits another baffle, and is attenuated yet again...

So for high isolation, you need to make up the "missing" mass on BOTH leaves, not just one leaf. And you do that by putting a silencer box on each leaf. Or, as amac2673 suggested, by building a single silencer WITHIN the cavity, if there is enough space to do that.

There's one other way of doing it: Since the issue is partly about noise being picked up by the duct that passes through the cavity, that can be eliminated by NOT using a duct to pass through the cavity! Instead, use a massive extension of the silencer box to pass through the cavity. So for example, you would build your silencer box on the outside of the outer leaf using plywood, then create a "duct" out of the same plywood that goes completely through both leaves and terminates in the register, on the other side of both leaves. That plywood "duct" would still need to be broken in the middle (to prevent sound flanking along it) by simply sawing all the way through it, then wrapping the gap with thick rubber (EPDM, Neoprene or Sorbathane) to seal the gap and make up the missing mass.

But you can only do that in cases where you need the register fairly close to the point where the duct penetrates the wall. If you have a large ducted HVAC system where most of the ducting and plenums are located within the cavity space with fairly long runs (for example, where the only point you can bring in fresh air into the cavity is 20 feet away from the place where you need to get it into the control room), then you really should have silencer boxes on all penetrations (one on the inner leaf, and another on the outer leaf).

So that's the general concept. And the decision of "one box or two", depends mainly on "The Number One Big Question": How much isolation do you need? If you are looking for 40-something dB of isolation then you'll likely be fine with just a single box. But if you are shooting for the rather high goal of 60- or even 70-something dB of isolation, then you really do need boxes on every penetration.


- Stuart -

[andy_eade]

Thanks so much Stuart and Allen. I have yet to start drafting my HVAC plans in Sketchup and so I'll share them when I do to make sure I'm not breaking any golden rules or missing anything else. I'm definitely shooting for the best possible isolation given that I have a wife and two very young kids upstairs

Happy new year to you both and thanks as always for your time and insight.

Andy

[Mastiff]

There's one other way of doing it: Since the issue is partly about noise being picked up by the duct that passes through the cavity, that can be eliminated by NOT using a duct to pass through the cavity! Instead, use a massive extension of the silencer box to pass through the cavity. So for example, you would build your silencer box on the outside of the outer leaf using plywood, then create a "duct" out of the same plywood that goes completely through both leaves and terminates in the register, on the other side of both leaves. That plywood "duct" would still need to be broken in the middle (to prevent sound flanking along it) by simply sawing all the way through it, then wrapping the gap with thick rubber (EPDM, Neoprene or Sorbathane) to seal the gap and make up the missing mass.

I realize this is an ancient thread, but I'll ask anyway.

With the concept above, I mount my silencer box rigidly on the outer leaf, the heavy duct penetrates through to the inner leaf and is mounted rigidly there. But as you say, I put a small gap in the duct and seal it heavily so I don't have a rigid path from inner to outer leaf. It still seems that sound has an easier path through though, since it can go through the duct and out the back side of the box (not through the muffling switch-backs) and only have to penetrate a single leaf vs. two, right? What am I missing?

[Gregwor]

Any penetration through your leaf sheathing must be accomplished using a "sleeve" as they are referred to around here. Sleeves must be made out of a material that has the same surface density as your silencer box and of course, the sheathing itself. 1" thick MDF is a popular choice for many reasons. In example above, a flexible connector such as flex duct or DuroDyne is used to connect the two sleeves together within the inner and outer leaf cavity. This means that the sound only has one place to go and that is into and through the silencer box itself. By the time it zig zags through the box and comes out the other end, with a properly designed box, the insertion loss will be great enough that it the sound will be attenuated enough to match that of the leaf sheathing. Fresh air and "silence"!

Greg

[Mastiff]

Here's some Powerpoint art of me trying to understand. In the no HVAC case, the sound (red arrows) needs to penetrate two surfaces to get from left to right. With two boxes, it still either needs to penetrate two surfaces, or get through the air path in the box in lieu of passing through a surface. In the third case, with only one box, it appears the sound can get through by only penetrating a single surface. Isn't that worse? Or did I draw the picture wrong?

hvac.png

[Gregwor]

To make your illustration easier to understand, I would draw the first image (no HVAC) with no red arrow to indicate that the sound is stopped all together (I know, this is not real life, but for the purpose of the image I think it would be best).

Then, for the second image I would draw the red line getting more transparent as it goes through the silencer indicating the insertion loss.

The third image is drawn wrong in that you wouldn't have a flexible membrane isolating the two chunks of the sleeve. The sleeve would have to be a single piece and decoupled using caulk at the penetration through the sheathing. I think the only way to maintain a decent amount of isolation would be to use very very thick material for the sleeve (at this point entirely relying upon mass law). Furthermore you will indeed have structural transmission through the sleeve. Again, this method is in fact a compromise but better than no fresh air!

Greg

[Mastiff]

I was going off of this part of the thing I quoted previously:

That plywood "duct" would still need to be broken in the middle (to prevent sound flanking along it) by simply sawing all the way through it, then wrapping the gap with thick rubber (EPDM, Neoprene or Sorbathane) to seal the gap and make up the missing mass.

But then it doesn't really seem like it would matter exactly how it's implemented, the point being that there is a direct path from the inside room to the box mounted on the outside, with an attempt made to physically isolate this path from the inner wall using something flexible yet heavy. It sounds like your choice is caulk. I was imagining a 1/2" gap in the heavy duct wrapped with a heavy material.

I'm going to try not to have to do it this way. Still working on options to make the middle one work...

[Gregwor]

That plywood "duct" would still need to be broken in the middle (to prevent sound flanking along it) by simply sawing all the way through it, then wrapping the gap with thick rubber (EPDM, Neoprene or Sorbathane) to seal the gap and make up the missing mass.

This applies to designs where there are both inner and outer leaf silencer boxes. This is because by the time the sound hits the MSM cavity, it should be attenuated the same amount as if there was no HVAC whatsoever.

It sounds like your choice is caulk. I was imagining a 1/2" gap in the heavy duct wrapped with a heavy material.

Caulk around the perimeter of the sleeve yes. This seals everything and in the case of a single silencer design, it's sleeve can stab through the inner leaf sheathing without having a rigid connection to the inner leaf itself.

Greg

[trodden]

Mastiff, thank you for asking these questions and bringing this thread up to my attention. I've been wondering the same things myself and trying to wrap my head around it as well. Also, thanks Gregwor for the info.