I've read that having two 90 degree bends in your ductwork maintains the STC (or whatever measurement) of your room. I assume this means having this section of "duct" the same thickness as your walls.
How long does the ductwork with the 90 degree bends have to be before it can change to something with less mass (flex duct/ductboard).
Does the two 90 degree bends thing really not hurt the soundproofing? It seems unrealistic that if I put my ear at the one end, that I won't hear more sound coming from it than the rest of my walls. Not that i'm saying it can't be true :lol:
I want to make sure i'm not missing anything.
It looks like the silencers that people are making are more to silence the flow of air when in the room? But not to prevent losing isolation.
Am I missing something?
STC change from adding duct work?
Originally posted at johnlsayers.com, topic 8496.
Hmmm depends on what you mean by isolation I guess.
Do you mean:
- 1. Sound transferred from your room to other rooms via the ductwork.
2. Sound transferred from your room to an adjacent room, not directly through the ductwork, but in the general vicinity where ductwork lies (i.e. less insulation is in the wall where the ductwork is, so what is the reduction in STC.
3. Air handling noise of the ductwork
I'm speaking of your no. 1. I want as much isolation as possible from the rest of the house.
Isn't that 1 and 2 then Tom?
For example - if there's less insulation and a bigger space in the ceiling; more sound will travel up into the room above.
I know in my case for example this will be the weak link...
Tom, you misunderstand......... this is intended to stop mechanical or other sounds traveling through the duct itself - not from entering the duct from outside of it and then passing into the room. If that was the case - then it would fail because the would allow it between the 2 elbows. Take a moment to look at this: http://johnlsayers.com/phpBB2/download.php?id=16137 it is mounted directly at the point of passage through the celing in this case - but could be used through the wall as well. The idea is to create a safe passage directly at the point of penetration. Does that help? RodI've read that having two 90 degree bends in your ductwork maintains the STC (or whatever measurement) of your room. I assume this means having this section of "duct" the same thickness as your walls.
Hmmm...
Interesting topic. I was just about to ORDER 2 silencers, but I saw this post and a few others, so for the moment I'm in
S-T-O-P mode.
Ok, I've got 1 more shot at this before the construction gets to a point of "no-return".
(get it...no RETURN?? Ha ha! :lol: :) :shock: :? ...uh, anyway...
I've got 2 items of concern...
1.) I've got two flexduct return lines running to my air handler unit. One return is 8" coming from my Control Room. (The supply in the Control Room is also 8") The other return is 7" and it's coming from my Vocal Booth. (The supply in the Vocal Booth however is 8"...I simply couldn't get an 8" return to fit in the pathway I took to my Vocal Booth)
Below is a diagram of my HVAC set up. I would think since my air handler is a fair distance away from my Control Room & even further away from my Vocal Booth, noise would be fairly low from the air handler. I don't have the space to build one of the units I've seen on here, so I'm attempting to do the next best thing.
What I need to know is this...From reading Rod's comments here:
Is the position shown for my silencer location counterproductive? The silencer is NOT installed before entering the room, but instead is only 3-5 feet from the actual supply register/grill. The template Rod had in his post looked like it was 2' 9" which is wider than my joist width. Also, I would surely hope the silencer didn't WEIGH a whole lot! So instead, I was looking to buy the silencer model here... http://www.energyfederation.org/consume ... 86_744_680 My flex duct is traveling between my 24" OC ceiling joist so I would have no wall penetration anyway. The duct simply elbows off the trunk then travels to the register/vent. I wish I could have routed this a better way even using individual supplies, but the extra soffits it would have created just didn't work. I would think the position shown on the diagram below would be ok, but if not, there's no need to waste $$$. :wink: 2.) I tried to match my supply and return size for each room. (It didn't work for my Vocal Booth as I stated above). That led me to wonder about increasing the size of my supply line from my trunk to the register/vent in my Control Room & Vocal Booth. Ok, let's see if I have this right... If I use a larger supply from my trunk to each room (let's say a 10" supply duct) the volume for *1 VENT* would stay the same whether I used 8" duct or 10" duct but the VELOCITY would be lower if I used 10" duct with a larger register/vent. If I have this right shouldn't this somewhat help quiet the air flow "noise"? Once again, "IF" this is correct and the volume stays the same it shouldn't matter if I have a 7" return should it? Ok, ok...let me put it like this, I have an 8" supply right now and a 7" return. The return size "ain't gonna change", but the supply could. The volume of air out of the 8" duct "is what it is". If I put a 10" supply in, the volume stays the same but the velocity is lowered due to the size of the duct. Phew.... :roll: Ok...straighten me out...The purpose of the silencer it to quiet down mechanical noise- as well as the noise caused by air movement - and should be installed immediately before entering the room - because it can also act (in that case) as a shield to protect the actual hole you penetrated the membrane with.... systems I design have the silencer aqttached directly to the structure at the point of entry.
1st off - I hate the use of flex duct in lieu of regular duct - to me it is a plauge on HVAC systems the wy most people use it......... especially in long runs. I might make a long lined metal duct run - and then transition to an outlet with 2 or 3 feet of flex - but I would never ever use more than 3 feet - and even then only when the transition is difficult - AND I have other means of isolating rooms from one another. SO seperate HVAC systems for live and control rooms- AND carry the walls between them all the way up so the rooms cannot directly communicate with one another.......1.) I've got two flexduct return lines running to my air handler unit. One return is 8" coming from my Control Room. (The supply in the Control Room is also 8") The other return is 7" and it's coming from my Vocal Booth. (The supply in the Vocal Booth however is 8"...I simply couldn't get an 8" return to fit in the pathway I took to my Vocal Booth)
The first thing to do is to determine what you need - if you need anything at all....... Flex duct is good in the respect that (seeing as it's side walls have virtually no mass) it doesn't carry mechanical sounds for very long distances........ It is bad in the respect that due to it's basic design - and the fact that it can be easily deformed - even under it's own weight - it can produce it's own air noise. Buffeting air makes lots of noise all on it's own.Below is a diagram of my HVAC set up. I would think since my air handler is a fair distance away from my Control Room & even further away from my Vocal Booth, noise would be fairly low from the air handler. I don't have the space to build one of the units I've seen on here, so I'm attempting to do the next best thing.
Nope - the volume would only stay the same if you had a balancing damper so you could make an adjustment for volume. Let's picture the extreem for a moment - suppose you have a 16" x 40" main plenum............ and once it's installed it has 1" of static pressure - and volume of 6,000 cfm - which means I am moving air at 1500 fpm. Assume for the sake of this conversation that the fan can only produce 1" of static pressure or it goes into thermal overload. So if I install a 100' run of 12" round duct - the 1" of static (back) pressure is going to send about 675 cfm down my trunk - which is going to travel about 850fpm........ now I shrink that pipe to 8"........... that same static pressure will only send around 270 cfm @ a velocity of about 800fpm. 7" pipe will be 160cfm @ a velocity of about 600 cfm. These numbers (of course) assume a balanced return - removing the same amount of air that we are supplying...... Note how the velocity drops along wih the cfm - all while maintaining the same static pressure the fan is capable of producing........ BUT IN YOUR CASE, the problem is this - you're force feeding your 7" from an 8"...... and you have the air locked inside a room - so you're telling this 7" that you want it to take everything the fan is capable of - and if't it's 1" of static - the fan trips out on thermal overload - and will continue to do so until something is repaired. But now let's assume the fan is capable of whatever we need it to do..... My setup right now is creating 1" of static pressure on the system... but when I shrink down to 7" - and move the same volume - watch what happens...... Static pressure climbs to 2.6" and velocity climbs to almost 1100 cfm..... all this while putting a greater strain on the motor....... so installing a 10" supply will increase your problem unless you install a balancing damper on the supply line and lower the flow until the return is where you want it - but then again - you could do that with the 8". Your real problem will be if you NEED all of the air that the 8" is providing......... if that's the case then the only thing you could do to quiet down the return would be to install a larger return.......2.) I tried to match my supply and return size for each room. (It didn't work for my Vocal Booth as I had to use a 7" return instead of an 8" due to the "narrow pathway" to get to the Vocal Booth) Ok, let's see if I have this right... If I use a larger supply from my trunk to each room (let's say a 10" supply duct) the volume for *1 VENT* would stay the same whether I used 8" duct or 10" duct but the VELOCITY would be lower if I used 10" duct with a larger register. If I have this right shouldn't this somewhat help quiet the air flow "noise"? Once again, "IF" this is correct and the volume stays the same it shouldn't matter if I have a 7" return should it?
I wanted to use the Johns Manville superduct when the HVAC was installed a couple of years ago but I simply couldn't afford it, and even if I did there were problems with soffits in odd & low locations to get it where I needed it. Just one of those things... :?
One thing I need to make clear is that for each room the supply FLEX DUCT line is only as long as from the TRUNK to the register (approx 8-12ft in each room...see the diagram in my other post). The trunk is made out of ductboard. Are you saying that even at the distance from the trunk to the register the flexduct would not carry noises from the air handler thru the trunk, to the flex duct, thus, I wouldn't need the silencer?
From what you're saying, I need to stick with the duct size I now have, and may possibly need a damper for the Vocal booth to "balance out" the supply and exhausted air. The vocal booth is a small room
(approx 7'x12') I would think an 8" supply would be ample enough for this room as long as (like you said) I can remove the same amount I bring in. The control room is approx. 19'x 12'. I also guess larger registers are in order too...
Oh well...the best I can do is the best I can do...
Thanx Rod...
Is the system running now? Can you hear the mechanical noise through the duct? Metal duct carries a lot of noise a long distance - usually the flex duct is just the opposite...... the insulation wrapping the duct goes a long way towards quieting things down....... like I said earlier - the 2 biggest probable problems here are direct transer of room to room sounds - and air buffeting from the air passage through the duct itself...... andthe smaller the duct - the greater the possibility.......Are you saying that even at the distance from the trunk to the register the flexduct would not carry noises from the air handler thru the trunk, to the flex duct, thus, I wouldn't need the silencer?
I have to say - I came back this morning and studied this with a more critical eye- and for the control room I see a couple of potential problems...... the 1st is that just running the numbers quickly - if you only have about 800 watts in equipment - (which is not a whole lot - I have over 4300 watts of gear in my room and the studio I just finished design on has over 8000 watts of gear) but at just 800 you would have about 1/2 ton of cooling required for this room, and that idf you are the only person in it....... add 3 more people and it becomes almost 3/4 tons... and I don't know if you're going to move enough air with your 8" supply. But another concern I have is that your supply and return are one on top of another at the far end of the room from your desk and gear - and I get concerned about the ability of your set-up to move air throughout the room as opposed to short cycling and just moving air from your supply register directly to your return grille. ANd now you might have to throttle down your supply even more in order to quiet the system down because of an undersized return duct. If I were you I would be trying to think of some way to squeeze some more return into that room........ Sincerely RodFrom what you're saying, I need to stick with the duct size I now have, and may possibly need a damper for the Vocal booth to "balance out" the supply and exhausted air. The vocal booth is a small room (approx 7'x12') I would think an 8" supply would be ample enough for this room as long as (like you said) I can remove the same amount I bring in. The control room is approx. 19'x 12'. I also guess larger registers are in order too...
I don't think I explained myself clearly.
I'm not concerned with noise from one room to another based on quadruple leaves or less space inbetween leaves because of this duct.
I have an inner leaf and an outer leaf on the side walls (concrete that is underground). There is a good amount of space here and since the outer leaf is underground (basement), there is no concern about losing isolation here. So this is where I would be putting these ducts (not on the ceiling). They would travel outside of the studio into my ventalition system which will be located in the non-soundproof section of the basement.
My concern is purely the amount of sound from inside the studio traveling through the ductwork into the rest of the basement (the non-soundproof part) and then the rest of the house. This is where I thought the two 90 degree bends were supposed to help.
Thanks for the responses, any help now that I clarified?
Camistan,
What will you use where the duct goes through the wall? You want it to match your isolation, so will you box around the duct so that it matches the mass of the rest of your wall?
Hey Tom,
Originally, the only "wall" penetration was going to be my returns. My plan was to build a duct "chute" that would fill one of my stud frame cavities. That would enable me to have a decent size incoming duct and a register that I could have at whatever size I need. Inside the stud frame cavity I was going to put insulation around the duct "chute". The duct "chute" was going to be made of ductboard, but at this moment I'm wondering if I should just use metal duct and heavily insulate around it. (front back and sides). After that I was simply going to put up my drywall.
To be honest now Tom, based on Rod's comments I may have to totally re-do one of my walls to try and get more return in the room. The unfortunate thing is I can ONLY tap off the original return that was coming into the room. If I do this then I WILL have to look at how I'll bring this return in through the wall. The other return was going to come through my bass trap or corner absorber. Another thing is I have an ERV (Energy Recovery Ventilator-brother to an HRV). I'll have to figure out if it would be best to tap directly from the ERV return for both rooms instead of my air handler return. I could use some type of "Y" connection to have BOTH rooms tie into the one return on the ERV unit. Man-O-Man...this is getting kinda complicated, but I'm gonna push this thing thru as best as I can. I sure hope I answered your question. Hey I'll watch your back and you watch mine...maybe an idea for your studio would be more conducive for me and vice versa. Thing is, I need to make a decision on what to do soon as I've got carpenters and electricians lined up to come soon.
Thanx!
Rod, could you elaborate on why you think flex duct is a plague on hvac unless it's simply for the reasons you mentioned after...1st off - I hate the use of flex duct in lieu of regular duct - to me it is a plauge on HVAC systems the wy most people use it......... especially in long runs. I might make a long lined metal duct run - and then transition to an outlet with 2 or 3 feet of flex - but I would never ever use more than 3 feet - and even then only when the transition is difficult - AND I have other means of isolating rooms from one another. SO seperate HVAC systems for live and control rooms- AND carry the walls between them all the way up so the rooms cannot directly communicate with one another.......1.) I've got two flexduct return lines running to my air handler unit. One return is 8" coming from my Control Room. (The supply in the Control Room is also 8") The other return is 7" and it's coming from my Vocal Booth. (The supply in the Vocal Booth however is 8"...I simply couldn't get an 8" return to fit in the pathway I took to my Vocal Booth)
Or feel free to disregard if it's covered in your book. I haven't reached the hvac chapter yet and don't have it with me right nowIt is bad in the respect that due to it's basic design - and the fact that it can be easily deformed - even under it's own weight - it can produce it's own air noise. Buffeting air makes lots of noise all on it's own.
In this post, Rod wrote:
- . . . I do not recommend the use of flex duct of any type as it can kink and slow flow very easily - PLUS - becaus of the manner in which it's constructed it' prone to noise.
So by looking at Rod's attachment, I think I am understanding this correctly...
So it's true that the 90 degree bends will stop sound from traveling from the studio through the ducts to the outside.
Rod, can you verify this horrible drawing. Just to make sure I'm getting the idea accross right... and that the theory is correct. I'll need to figure out the details when I have exact dimensions.
So this is the basic idea. The boxes will be just as thick as one leaf to prevent loss of isolation inbetween leaves to the upstairs. I will also attach the insulated flex duct so that it's connected on the bottom of the box towards the concrete floor of the basement.
I did see that, just wondered from the way he put if there was more to it than that. I guess that's bad enough tho..In this post, Rod wrote:. . . I do not recommend the use of flex duct of any type as it can kink and slow flow very easily - PLUS - becaus of the manner in which it's constructed it' prone to noise.
--Keith :mrgreen: