Duct silencers in between the inner and outer leaf?

Hey Quint, hope your project is going well.

Quint wrote:

So will a short piece of 4" flex duct be able to pull 45 cfm? I think you've more or less indicated that it can but I just wanted to double check. What is the maximum cfm 4" flex duct can pull over a short run like mine?

I would say the 4" flex duct would do the job but i'd rather go for 6" or 8". If your inline fan has a 4" in and outlet, I've heard going to 6" (or bigger, I guess) with an adaptor increases the performance of most units... But I'm no expert. Just another guy with no idea on HVAC or the concepts behind it trying to make my way through. :P I'd really love that some of your questions left to answer got attention. I'll be watching. :snack: This post has caught my eye since I am looking for info to find out how to calculate the correct sizing for my own silencers and ducting. Your post made me wonder... I thought on locating my silencers inside the air gap, vertically next to each other, and connect them with some kind of flexible rubber-like sheet, as Stuart has suggested, I was thinking on placing the inline fan right at the exit of the system in the outside wall (outter leaf) but I wonder: would it make a big difference if I duct the exit of the system and place the inline fan around 14 feet away? By the way, I'm going for 8" duct. Cheers!

Soundman2020 wrote:

A piece of 4" flex (12.56 sq in) duct between a pair of boxes with double the internal cross sectional area (25.1 sq in) should create an impedance mismatch in either direction, right?

Correct!

Surely 30 or even 45 cfm should provide enough fresh air/cooling, right?

For a small room like a vocal booth, that should be OK, yes.

I would think that any friction loss would be minimal given the short length.

Right. - Stuart -

So will a short piece of 4" flex duct be able to pull 45 cfm? I think you've more or less indicated that it can but I just wanted to double check. What is the maximum cfm 4" flex duct can pull over a short run like mine? Oh, and I have one other question. Instead of using 4" flex duct in between the boxes to create an impedance mismatch, could I use 8" flex duct and achieve the same thing? In that case, the duct would be the part of the flow path with double the cross sectional area (50.3 sq in) versus the cross sectional area (25.1 sq in) in the boxes. The advantage of this would be that I wouldn't run into potential limitations on maximum cfm with the 8" flex duct that I might with the 4" flex duct. Would this work? Maybe none of this matters because 4" flex duct will provide all the cfm I need but it would be good to know if the 8" flex duct would work? Thanks again for all the help.

With a 4" duct and 45 cfm, the air flow velocity in the duct will be 515 fpm. Not a problem. The cross sectional area of a 4" duct is is about 12.5 in2. You say the cross section inside your silencer is about 25 in2, so the air flow velocity in the silencer will be about 255 fpm. Not a problem. As long as the open area of the register is around that same size, or larger, you are dong fine. - Stuart -

Soundman2020 wrote:

With a 4" duct and 45 cfm, the air flow velocity in the duct will be 515 fpm. Not a problem. The cross sectional area of a 4" duct is is about 12.5 in2. You say the cross section inside your silencer is about 25 in2, so the air flow velocity in the silencer will be about 255 fpm. Not a problem. As long as the open area of the register is around that same size, or larger, you are dong fine. - Stuart -

But what about using 8" duct instead as an alternative way to achieve an impedance mismatch, as I asked about in my previous post? I may have a need to pull greater cfm than 45 so I wondered if 8" would also provide an adequate impedance mismatch while also allowing greater cfm capacity? The aforementioned part of my previous post is quoted below: "Oh, and I have one other question. Instead of using 4" flex duct in between the boxes to create an impedance mismatch, could I use 8" flex duct and achieve the same thing? In that case, the duct would be the part of the flow path with double the cross sectional area (50.3 sq in) versus the cross sectional area (25.1 sq in) in the boxes. The advantage of this would be that I wouldn't run into potential limitations on maximum cfm with the 8" flex duct that I might with the 4" flex duct. Would this work?"

Soundman2020 wrote:

With a 4" duct and 45 cfm, the air flow velocity in the duct will be 515 fpm. Not a problem. The cross sectional area of a 4" duct is is about 12.5 in2. You say the cross section inside your silencer is about 25 in2, so the air flow velocity in the silencer will be about 255 fpm. Not a problem. As long as the open area of the register is around that same size, or larger, you are dong fine. - Stuart -

But what about using 8" duct instead as an alternative way to achieve an impedance mismatch, as I asked about in my previous post? I may have a need to pull greater cfm than 45 so I wondered if 8" would also provide an adequate impedance mismatch while also allowing greater cfm capacity? The aforementioned part of my previous post is quoted below: "Oh, and I have one other question. Instead of using 4" flex duct in between the boxes to create an impedance mismatch, could I use 8" flex duct and achieve the same thing? In that case, the duct would be the part of the flow path with double the cross sectional area (50.3 sq in) versus the cross sectional area (25.1 sq in) in the boxes. The advantage of this would be that I wouldn't run into potential limitations on maximum cfm with the 8" flex duct that I might with the 4" flex duct. Would this work?"

I'm just about to commit to how I build and install my silencers based on the answers on this thread. I hope to build them this week. Watching with baited breath....

I'm just about to commit to how I build and install my silencers based on the answers on this thread. I hope to build them this week. Watching with baited breath....

Hi there, "Monkey-Engineering", and welcome! :) Feel free to start your own thread and post your design for your in-wall silencers! It would be good to see that, and we can also "look over your shoulder" so to speak, to see if there is maybe something you can do to improve the design. - Stuart -

Hi Stewart, thanks for the welcome! I've been silently following this forum for years. I've read Rods book many times. It can be difficult to find answers in here so, withough hijacking this thread, I'm also very interested in the effects of changing the cross sectional area of the linking between two silencer chambers. My build is quite different but I have a similar need. Its a single room with double leaf design on separate frames. On its own on a cement floor in a tin shed. The goal of my build is to reduce sound transmitted through from musos inside to neighbours outside. Same need as the vocal booth in the OP. I have the opportunity to build the silencers into the ceiling cavity. I have not yet chosen a fan. Cooling is by a separate split system. From what I can work out: It is just under 50m3 of air volume. (1765cf) With 6 changes per hour I need 300 m3/hr (10594 cf/hr) To get optimal velocity of 0.5m/s the exit needs to be 450mm in diameter (huge! ) To get good velocity of 1.0m/s the exit is 3 20mm in diameter (big, but doable) I can build the silencers as large or small as I want (within reason). Also I can build the reduced size links (or ducts in the OPs case) between the chambers any size. Is there a problem with them being too big? I understand having the chamber double the cross section of the link is desirable, but if you build a silencer bigger for a higher air flow than your fan, is there a problem or benefit? How big is too big? Surely this has an effect on the effective impedance. If it's detrimental can it be assisted by more turns in the baffles? Stewart, if I'm on track, I think your next answer to the thread should answer most of my questions. Cheers for the help lads, Steve

Soundman2020 wrote:

I'm just about to commit to how I build and install my silencers based on the answers on this thread. I hope to build them this week. Watching with baited breath....

Hi there, "Monkey-Engineering", and welcome! :) Feel free to start your own thread and post your design for your in-wall silencers! It would be good to see that, and we can also "look over your shoulder" so to speak, to see if there is maybe something you can do to improve the design. - Stuart -

Hi Stuart, Please, please take a look at my previous question about using 8" duct. I saw that you had posted in my thread today and thought that I had finally gotten the answer I was hoping for and then I saw that you were responding to somebody else. I think you missed the second half of my previous question about using 8" duct instead of 4" duct to create the impedance mismatch. I'm relatively certain it should work but I wanted to get some verification from you. I'm posting the question about 8" duct from my previous post below. This is the LAST piece of info I need to complete my build but I can't move forward until I'm assured it will work. Thanks. Here's the previous question: "Oh, and I have one other question. Instead of using 4" flex duct in between the boxes to create an impedance mismatch, could I use 8" flex duct and achieve the same thing? In that case, the duct would be the part of the flow path with double the cross sectional area (50.3 sq in) versus the cross sectional area (25.1 sq in) in the boxes. The advantage of this would be that I wouldn't run into potential limitations on maximum cfm with the 8" flex duct that I might with the 4" flex duct. Would this work?" So to reiterate, I simply want to know whether flex duct which is double the cross sectional area of the silencer boxes will create an impedance mismatch of equal quality to flex duct which is half the cross sectional area of the silencer box? In either case, you have a doubling and halving of the cross sectional area as air travels between each pair of silencer boxes. The only difference is that 8" duct doesn't restrict cfm like the 4" duct might. That's potentially a huge advantage to me. Can I use 8" flex instead of 4" flex and achieve the same level of impedance mismatch and corresponding sound proofing?

:snack: I'm following this threat with great interest.

Monkey-Engineering wrote:

Instead of using 4" flex duct in between the boxes to create an impedance mismatch

What I understood is that, between boxes, the ducting should keep the same cross sectional area than inside the silencer boxes and is best NOT to use regular flexible ducting. I'll do it with some flexible rubber material in between boxes. Maybe I understood everything wrong but I think it goes like this: Fan-flex duct-(through wall outter leaf)silencer box doubling the cross sectional area of previous ducting-rubberlike piece connecting both silencer boxes and keeping the same cross sectional area of the first silencer-inner silencer box-(through wall inner leaf) end of ducting to the room increasing the cross sectional area gradually. I don't know of the consequences of increasing, decreasing and increasing again the cross sectional area in your silencers. Anyone? Cheers. Correct me if I'm wrong, please!!

Hello, any update in your silencer's design?