HVAC metal Duct sound isolation wrap and Flex duct

I am at the point in my build (See Ohio Basement Studio), where I need to sound insulate some existing metal HVAC duct. I am looking for a cost effective product to stop lateral noise to minimize transfer of sound to the rest of the house. I have seen a Paint-on solution, a metal foil type of solution, other variations on the Internet. Some items look like MLV. I am also looking for a good sound isolated flex duct. The Internet searches I have done have confused me more than helped. I don't know who to trust. I guess what I'm really asking is , can someone recommend a product that they have been pleased with? 1) An external sound isolation wrap for metal duct 2) Sound isolated flexible duct. I have combed through the forum looking for product names but have not been successful. I'm sure the knowledge is in this forum floating around somewhere. : ) In advance I thank you. Joe

I have decided to wrap my metal duct with MLV. It's thin and won't take up a lot of space in floor joist bays of the first floor. I need room to tuck the basement ceiling joists up in the floor joist bays of the rooms above. This way the rooms are physically isolated form each other. I was able to get MLV relatively inexpensively. So it fit the budget. I still need to determine where to get flexible HVAC hose that has sound isolation. The only thing I have been able to find online is Thermal insulated Flex hose which would not stop lateral sound, and would be like poking a big hole in my sound isolated room, even with a DIY silencer attached. Would someone please share their thoughts in this area? It would be much appreciated. Soon I will be stalled on my studio build if I don't have direction relative to the proper flexible HVAC hose to use. If I am overthinking this, I need to know that as well and why. Thank you. Joe

1) An external sound isolation wrap for metal duct 2) Sound isolated flexible duct.

Unfortunately there's no such thing! HVAC duct in itself is a really lousy isolator. It is usually made of either thin sheet metal, duct board, or plastic and insulation formed around a wire spiral. None of those are much use for stopping sound. Sound is best stopped by mass, and even then it has to be done properly. Just wrapping something heavy around the duct won't help a lot, since that basically leaves it as a single-leaf, and therefore isolation is limited by mass law. The correct way to isolate HVAC duct is to build a soffit around it, with the duct not touching the soffit at any point. The gap between the soffit and the duct should be filled with insulation.

I have decided to wrap my metal duct with MLV. It's thin and won't take up a lot of space ...

... and is also about the most expensive mass you can buy! What matters is the MASS, or surface density, expressed in terms of kg/m2. If you compare the surface density of MLV against common building materials, you'll find that the cost per kg of mass is very much higher for MLV. You'd also have a hard time attaching it to your decoupled frame, and keeping it sealed... Wrapping ordinary 1 pound MLV (5kg/m2) around your duct will get you about 20 dB of isolation, best case: that's practically nothing. Most likely 10 to 15 dB. (And that's assuming you manage to seal it airtight, and decouple it!) The normal way of isolating HVAC ducting is the same way everything else is isolated: build a simple wood frame around the duct that does not touch it, put fiberglass insulation in the framing, then put up 16mm drywall (plasterboard) on the framing, and seal it air-tight.

I still need to determine where to get flexible HVAC hose that has sound isolation

There is no such thing- Flex duct is made of thin plastic, mylar, or another type of foil, attached to a thin fiber insulation filler (often an inch of fiberglass), and with a wire spiral inside to keep the shape. There's no mass at all there, to speak of. Nowhere near enough to keep sound out (or in). Flex duct is also not a good way of ducting air. Installation isn't easy, as the duct tends to kink, and it only gives you the advertised characteristics if it is fully stretched out to the full length specified by the manufacturer. If you allow it to sag, kink, bend or droop, then you can lose a major proportion of the airflow, and also greatly increase turbulence and static pressure. Unless it is installed perfectly, it's not a good choice. Ordinary round or rectangular section metal duct is much better, but do make sure you get the stuff that is already lined inside with proper duct liner: trying to line it yourself is a major pain! If you do decide to go with flex duct anyway, then get stuff that is at least one size bigger than what your calculations show you need. What size duct did you calculate for your system? That's a very, very important point. You can't just guess here: the duct size should be calculated using the normal procedure.

and would be like poking a big hole in my sound isolated room, even with a DIY silencer attached.

Not if you build and install the silencer correctly! A correctly designed, built and installed silencer will allow plenty of airflow while greatly attenuating sound transmission. The insertion loss can be very high, and every bit as good as the rest of the isolation system.

Would someone please share their thoughts in this area? It would be much appreciated

HVAC is one of the more complex parts of studio design, since it is all about calculating the correct flow volume, flow velocity, static pressure, sensible heat load, latent heat load, fan efficiency, register size, turbulent flow, etc. HVAC is also probably the most neglected part of most builds, yet vitally important for the health and comfort of the people inside. It should not be taken lightly. If you don't know how to do it, then you should probably hire a local HVAC consultant in your area to do it for you. And that's without even considering the aspect of silencer box design, which is another thing entirely and beyond the capability of most HVAC consultants, unless they specialize in HVAC and acoustics.

If I am overthinking this, I need to know that as well and why. Thank you.

Actually, you are probably under-thinking it! There's a lot more to it than meets the eye at first glance... It would help if you would post the complete design for your studio, showing the HVAC system details that you are considering at present. - Stuart -

Stuart Thank you for the detailed response. I will post the plans that show duct position. I don't have the option to isolate my duct work by boxing it in. At least not in the CR. My CR ceiling joists need to nestle between the first floor joists above. My tracking room is a different story. I'm going to see if I can pull off the boxing and stuffing with insulation. I have started the duct calculation process based on predicted occupancy, equipment, humidity concerns, airflow etc.. My air handler is also going to have variable speed. I realize that lined metal duct will give better sound isolation. If I am more comfortable using flex duct will my isolation suffer? I plan on building the Silencers that many have built on the forum. Acoustically I don't see how they can stop sound from getting in and out of the room. In my mind, if it is connected to flex duct (my preference) it's still a hole in the room. If this is wrong could you point me to a reference that has a good acoustic explanation? I see how it could silence air from the HVAC system, but not how it could block sound. The silencer is still open on both ends. Thank you Joe

I don't have the option to isolate my duct work by boxing it in.

Then you don't have the option of isolating your room well! :) It's that simple. If you leave your duct-work in the MSM air cavity without boxing it in and also without silencer boxes on all penetrations, then you have a very good flanking path for sound to get out of your studio and into the rest of the house. Your isolation is seriously compromised. Even worse, if the ducts are in the MSM space, then they will not only transmit room sounds, but also potentially amplify sounds that happen to have tones close to the MSM resonant frequency of the wall. I'm not sure why you think that you don't have the option of isolating your room well: it can be done.

My CR ceiling joists need to nestle between the first floor joists above.

Why? That is only one of two possible ways of gaining headroom, and in fact it is the least effective of the two... Is there some reason why you cannot go with the other one?

I'm going to see if I can pull off the boxing and stuffing with insulation.

Hang on a sec! You just said that you CANNOT box it in, and now you say that it is ALREADY boxed in? That's confusing... :shock:

I realize that lined metal duct will give better sound isolation. If I am more comfortable using flex duct will my isolation suffer?

I think you missed the point I was making in my previous post. They are BOTH really poor at isolating sound! That's not what they are meant for: they are designed to move air, and they don't even do that too well... You can use flex-duct if you understand the issues and remember to oversize it by at least one size, preferably two. So if your calculations say that you need 6" duct, then what you really need is 8" duct, if you use flex, or 6" if you use round metal, or the equivalent cross-sectional area if you use rectangular metal. The biggest issue with flex is that the actual final inside diameter will be far short of the nominal, whereas with metal duct it will be exactly the same. So for flex-duct, the static pressure will be higher that it seems to be, and that's an important issue.

Acoustically I don't see how they can stop sound from getting in and out of the room. In my mind, if it is connected to flex duct (my preference) it's still a hole in the room. If this is wrong could you point me to a reference that has a good acoustic explanation?

You are not taking into account how sound actually behaves. Sound waves do not move at all like air flow does. Sound is waves, and waves do not behave like air. There are multiple acoustic issues going on inside the silencer, including gross impedance mismatch, phase cancellation, diffraction, absorption, reflection, and even diffusion, to a certain extent. An HVAC silencer box works in exactly the same way as the silencer (muffler) on your car engine. And if you have ever heard a car engine running WITHOUT the silencer in place, then you have a good idea of just how effective it is in reducing the noise of numerous high-intensity explosions occurring hundreds of times per second inside the engine, down to nothing more than a dull murmur coming out the tail pipe. In both cases, the gases flow through the silencer freely, but the sound is prevented from getting through, by means of the tricks of acoustics. Just the impedance mismatch at the entry port and exit ports is enough to knock off at least 10 to 20 dB, simply due to the sudden change in cross section. Add to that all the other things happening inside, and there's a large amount of acoustic isolation going on in there.

I see how it could silence air from the HVAC system, but not how it could block sound. The silencer is still open on both ends.

So is the silencer in your car! :) Yet it somehow manages to reduce that blaring scream of the engine down to a quite whoosh of exhaust gasses, in just a foot or two of length, and using nothing more than carefully shaped metal and perhaps a bit of fiberglass... The silencer boxes use exactly the same principles. You might find this interesting: http://auto.howstuffworks.com/muffler.htm - Stuart -

Stuart

If you leave your duct-work in the MSM air cavity without boxing it in and also without silencer boxes on all penetrations

I did not say I was not going to use silencer boxes. In fact I stated that I was. I just did not understand the theory behind it. BTW, Thank you for the excellent explanation. Very helpful. That is all I was looking for. "My CR ceiling joists need to nestle between the first floor joists above."

Why? That is only one of two possible ways of gaining headroom, and in fact it is the least effective of the two

I respectfully disagree. Having a room within a room design is better than having any part of that room attached to the house, including the ceiling. I wanted a completely isolated space. And that is what I have designed with two different acousticians. I realize everyone is going to have their own thoughts on how to do this. This is how I was advised and I respect the expertise of these gentlemen. Others on the forum have agreed with this approach as well, and many other builders on the forum have used this approach. I do not know why you consider it inferior.

I'm going to see if I can pull off the boxing and stuffing with insulation. Hang on a sec! You just said that you CANNOT box it in, and now you say that it is ALREADY boxed in? That's confusing...

You've taken this statement out of context. Please read the whole statement. "My tracking room is a different story. I'm going to see if I can pull off the boxing and stuffing with insulation." I was referring to a different room (I called it the tracking room). Probably not the best name to use, but not important. It is a different room that has different challenges limiting the height of the ceiling. The joists will not penetrate the cavities above, therefore leaving space to "box off" the bay where duct work resides. Stuart, I appreciate your advice on this particular topic and thread. If you don't mind I am going to copy my response here to my build thread to maintain continuity of that build. Thank you. Joe

"My CR ceiling joists need to nestle between the first floor joists above."

Why? That is only one of two possible ways of gaining headroom, and in fact it is the least effective of the two

I respectfully disagree. Having a room within a room design is better than having any part of that room attached to the house, including the ceiling. I wanted a completely isolated space. And that is what I have designed with two different acousticians. I realize everyone is going to have their own thoughts on how to do this. This is how I was advised and I respect the expertise of these gentlemen. Others on the forum have agreed with this approach as well, and many other builders on the forum have used this approach. I do not know why you consider it inferior.

Hoo boy! Don't get your knickers in a knot, buddy! And especially on a subject where you clearly don't understand what you are talking about... Yup, you managed to tick me off, and that's hard to do. :evil: So congratulations for that! Read what I said, instead of what you thought that you imagined I might possibly have maybe said! I said that there are TWO ways of gaining headroom with MSM construction, and the one you proposed is NOT the most effective. But you then came back with a comment basically calling me an idiot for saying telling you not to do MSM! HUH??? Where did I say any such thing???? Reading comprehensions doesn't seem to be your strong point. Here's an idea: go look up "inside out construction", then come back and apologize for basically calling me an idiot who doesn't understand acoustics. Maybe then you'll see how you put your foot in your mouth, and managed to insult someone who was trying to help you by showing you a BETTER way to do fully-decoupled 2-leaf MSM isolated ceiling. You berated me without even asking what I was referring to! You ASSumed that I was telling you to not decouple your ceiling, when in fact I was showing you another and better way to do it. And you have the bare-faced audacity to tell ME how to build a room-in-a-room studio? Niiiiiceeee.

I wanted a completely isolated space.

And you would have got it, if you would have read what I posted, instead of assuming something I never said....

Others on the forum have agreed with this approach as well, and many other builders on the forum have used this approach.

You just happen to be on the forum of John L Sayers, the guy who INVENTED the concept of inside-out construction!

And that is what I have designed with two different acousticians. ... and I respect the expertise of these gentlemen.

Then why bother asking us for advice on the forum, if you trust them so much? That seems rather strange.... Perhaps you don't trust them so much after all?

I realize everyone is going to have their own thoughts on how to do this.

Wrong. Any acoustician worth his salt will tell you the same thing: There's only one best way to isolate a room. There are variations on that same basic theme, and I was giving you one of them, which you somehow managed to totally misconstrue.

I do not know why you consider it inferior.

Because it IS inferior! Interlacing new ceiling joists between existing floor joints above is just one of several ways of building an isolated ceiling, and it is NOT the most efficient use of space. Inside-out construction is more efficient, as it can give you more headroom and a higher overall height for your inner-leaf than can be accomplished with interleaved joists. But what gets my goat is that you never even asked what I was talking about, or how there might be a better way: Instead you accused me of not knowing what I'm talking about, claiming that others know better. Instead of just saying "I don't understand what you mean. Please explain." you came along with a load of stuff about how everyone on the forum does NOT do it my way and DOES do it your way, when in fact you are absolutely wrong: many forum members have built their rooms "inside-out", with great success, and with superior isolation, not inferior as you claim. Including John himself, and several of my own paying clients. Search a few dozen build threads, and you'll learn how to do it. For ceilings it makes even more sense than for walls, since the floor joists above are usually very deep, thus allowing for a nice low MSM resonant frequency, and thus good isolation, with only a minimal gap between the bottom of the joists and the top of the inner-leaf. Use the search button.

Probably not the best name to use, but not important.

In acoustics, using the right name is critically important. That's why we don't use "soundproofing" and do use "isolation", for example. That's why we use "transmission loss" and not STC. That's why we use "joists" not "girders" or "rafters" to refer to the things that run under floors or over ceilings, to keep the leaf in place. That's why we use "vapor barrier" where appropriate and "vapor retarder" where that's appropriate. Etc. Using the right term and clarifying meaning carefully is vital. We all screw up with terminology every now and then, and being corrected is part of learning, but trying to tell off the administrator of one of the leading forums about acoustics on the internet, implying that he's clueless, wrong, and is arguing against other acousticians, really takes the cake.

If you don't mind I am going to copy my response here to my build thread to maintain continuity of that build.

That's what you should have done in the first place, since it is part of the forum rules...

Sorry to offend you Stuart. I am obviously Unworthy.

:roll: For the record, here's the BEST way to do this type of ceiling, which is by using the "inside-out" method developed by John and proven in practice both by many of his professional studios and also by numerous forum members, for the reasons given in each view below. First, a view of a typical room using "inside-out" ceiling construction: And the exact same room with an "interleaved joist" ceiling: Oh COOL! Take a look at that! My ceiling is higher with this one! So it must be superior, better, greater and more wonderful, just like the experts told me, right? WRONG! Take another look: the actual acoustic ceiling, which is the bottom surface of the inner-leaf, is NOT higher than for the inside-out case: it's actually lower. It only looks higher, but that's an optical illusion due to the joists not being visible any more. But in reality, the room has a worse acoustics now, simply because the acoustic boundary is lower, and the room volume is reduced. Ooops! Not only that, but there's no absorption under the ceiling now, and no diffusion either (those were being provided by the joists and the insulation between them). So now there's a major flutter-echo problem, between the floor and the ceiling. Damn! Didn't think of that before.... and my experts never told me about that.... But we can fix that with some acoustic panels hung from the ceiling! Yeah! That's the trick! Simple! Just like this: Problem solved! No more flutter! Wonderful! Except that there is now even less headroom, due to the panels... so the ceiling is not only acoustically lower, it is also very obviously aesthetically lower... Ooops! So let's take a closer look at these two... maybe the interleaved option is helping in some other way.... Another 2 views, first "inside-out": Then "interleaved": Hmmmm... but hang on a sec... there seem to be some strange looking issues with the "interleaved" version... Maybe we should take a closer look! Inside-out: Interleaved: But wait! I'm using MORE materials to do the interleaved version! The studs are longer, the drywall is higher, and I also need more insulation to fill the stud bays! So it's going to be more expensive to build this interleaved version, even though the ceiling ends up lower, and the acoustics are worse? DAMN!!! Nobody told me about that... Sigh! But maybe there are other advantages... let's keep looking. Inside-out: Interleaved: Hang on there... it looks like there's something strange going on at the top, where the ceiling sits on the inner-leaf wall. The inside-out version seems to be sitting there very firmly, with lots of contact surface, but the interleaved one is sort of floating.... Better take a closer look at that... Inside-out: Interleaved: Oh right! Now I see it! The edges of the drywall with the inside-out version is fully resting on the joists, but its impossible to do that with the interleaved version, because there 's nothing to attach it to! So what am I going to nail the drywall to?!?!?! Damn.... they didn't tell me about that either... There's no place to put nailing strips up there, unless I lower the ceiling a couple of inches... But gee! They told me this was the best way to get the HIGHEST ceiling, but now it looks like it isn't! Maybe they were wrong.... Inside-out: Interleaved: Whoaaa! There's something else strange going on there! It's impossible to put cross-bracing in there between the joists and also across the ends of the joists with the "interleaved" version, but dead easy to do it with the "inside-out version". So how am I going to provide sheer strength for my joists? How do I stop them from tipping over, warping, twisting, bending.... I can't see how to do that.... How come the experts never warned me about that? Maybe I could do "X" shaped bracing! Yeah, that would work! I could get my structural strenght back again like that! Bingo! .... Oh.... But then I'd have to lower the ceiling even more, by half the height of the joists, to do that.... So that means my ceiling would not be anywhere near as high as the experts told me it would be.... Darn.... But wait, I think I see another problem.... Inside-out: Interleaved: GOSH! It looks like the joists on my inner-leaf ceiling are pressing up into the insulation above, crushing it and destroying it's acoustic damping properties! DAMN! They didn't tell me about that either! Maybe I've been listening to the WRONG people.... Maybe I really should take a closer look at this "inside-out" thing. Maybe John Sayers is actually on to something here.... Maybe the people on this forum aren't so stupid after all, since they seem to have though of all the actual practicalities and benefits of their "inside-out" method. It just looks so much better all around.... I wonder what it would look like if I could make the insulation invisible? Maybe there's something hidden in there... Inside-out: Interleaved: DAMN! There's a HUGE amount or extra space up there with the "inside out" version, and there's no joists crushing my insulation! So there will be MUCH better isolation from doing it that way.... How come nobody told me about that? Inside-out: Interleaved: Gee, with the inside-out ceiling, it even looks like there's enough space up there for other things, like maybe HVAC ducts, silencer boxes, electrical wiring, and things like that: But there's no space for that with the "interleaved" version.... It looks like I could do other stuff too, like maybe box in my ducts for example, without any problem, and STILL keep my ceiling at the maximum possible height! I never thought of that... And neither did the experts I was relying on.... Inside-out: Interleaved: When I look at it like that, this "inside-out" thing just looks so much better, superior in every way! I get my ceiling higher up, I use less materials, I get better acoustics, I get better isolation, I don't crush my insulation, I get better structural integrity, I get more space to run ducts and things, it's easier to build, it's cheaper, it's faster.... Hmmmmm Now why didn't I just ask about this at the beginning, instead of pretending to know so much more about acoustics than the guys here, who are just trying to help me, and for free? Why did I have to go and blow it, by insulting them, telling them they don't have a clue, and then making things even worse, with a snarky comment about worthiness? If only I hadn't done that .... maybe... perhaps ..... posibly .... :oops: --- geee--- :roll: :| ------------------ Animations of both alternnatives: Inside out (the BEST way to do it): http://www.digistar.cl/sayers-forum/CEI ... t--S01.mp4 Interleaved (The not-so-good way of doing it): http://www.digistar.cl/sayers-forum/CEI ... d--S01.mp4