I'm currently weighing options for ventilation to a vocal booth and I'm considering putting DIY duct silencers in between the inner and outer leafs of my room inside a room design (two layers 5/8 gyp on each 2x6 wall). These silencers (four in total, one at each wall for input flow and one at each wall for output flow) will be used for pulling air to and from the adjacent live room which will already be conditioned and have plenty of fresh air. I realize that, regardless of whether these silencers are located in between the two leafs or outside the leafs in the rooms themselves, you're creating a third leaf situation.
However, I've seen responses to this topic before in the context of the silencers being placed in the rooms themselves and the general response has been to not worry about it given the spacing between leaves in this scenario and their relatively minor influence. However, I haven't had much luck finding opinions on how a third leaf situation may affect things differently if silencers are placed inside the wall cavity, given the much closer spacing of the leaves. To be clear, each silencer will only be attached to its respective leaf and flex duct (or some other form of non-flanking-path-creating connection) will be used to transmit flow from one silencer to its companion silencer on the opposite leaf.
Can anybody weigh in on this? I prefer this option because it allows me to take advantage of the large cavity between my leaves (12") rather than having to soffit things in the rooms but I don't want to do this if it will majorly impact isolation because of a third (or fourth) leaf situation.
Thoughts? Thanks for any help you guys can give me.
Duct silencers in between the inner and outer leaf?
Originally posted at johnlsayers.com, topic 20320.
I just realized that I forgot to list my location (Austin, TX, USA) in my profile. It has now been updated. I also read the "before you post" section and I believe I've addressed those items that are applicable but if I've forgotten anything else I would greatly appreciate someone pointing that out.
Also, if this thread is more appropriate for the Studio Construction forum, Mods, would you please move it? Thanks.
Hi Quint, and Welcome! (And Happy New Year as well!)
I often do what you are talking about: to my way of thinking, putting the silencers inside the wall and/or ceiling cavities is good use of space.
Yes, in theory, doing that creates a 4-leaf system. In practice, it's not a big deal if you build it all correctly. The boxes are big, yes, but they only take up a small portion of the entire wall or ceiling, so the effect is limited. Build the boxes with very thick walls (I sometimes use two layers of MDF), leave a gap on each side of the box, decouple it from everything, line it with proper duct liner (NOT with ordinary insulation), and use ordinary insulation around it on the outside, between it and the actual leaves.
It works... :)
However, with 12" cavity depth, you don't have much space to fit a box in! Do the math: Assuming you'll be using 6" round duct (or 6" flex duct), you have 6" right there. You'll need an inch on either side for the duct liner, so you are up to 8". Two layers of 5/8" MDF is an inch an a quarter, on each side of the box, so 2.5 inches total.... that's 10.5" for your box.... With only 11" of air gap, you don't have enough space. I like to leave at least 2" on either side of the box, so a minimum of 15" is what you need. Your only other option is to use an adapter from your 6" round duct to a narrower size of rectangular duct, such as maybe 8"x4". With that, you could maybe make your box just 8.5" deep (2.5 + 2 + 4), and maybe squeeze it in to 11", but you'd only have an inch and a quarter or air around it... Not so good. And if the air space inside your box is only 4" deep, that implies a very long, thin box to keep the cross-sectional area of the air flow up high enough. You'd need to make the internal bays at least 15" long...
Of course, I'm assuming that you are aiming for high isolation levels: If your goals aren't so high, then you could do only one layer of MDF for the box, and save an inch and a quarter.
- Stuart -
You too. I've been lurking here for a while but finally got around to posting something.Hi Quint, and Welcome! (And Happy New Year as well!)
My plan had been to equal the mass of the walls when I built the boxes so, since each wall will have two layers of 5/8" sheetrock, I had planned to use two layers of MDF or possibly one layer of MDF and one layer of sheetrock on each box. What is the reason for the gap on each side of the box and, when you say "gap", are you referring to a gap between the studs (2x6 @ 24 O.C.) on either side or do you mean a gap between the box and each wall? If you meant the latter, I think you may be misunderstanding what I had in mind. There would be two boxes (four in total, two for input flow and two for output flow) in between the walls with each coupled (so it can't be decoupled from EVERYTHING) to its respective wall at and around the entry or exit point for air flow. It would look something like the Vocal Booth Ventilation System drawing that I attached below.Yes, in theory, doing that creates a 4-leaf system. In practice, it's not a big deal if you build it all correctly. The boxes are big, yes, but they only take up a small portion of the entire wall or ceiling, so the effect is limited. Build the boxes with very thick walls (I sometimes use two layers of MDF), leave a gap on each side of the box, decouple it from everything, line it with proper duct liner (NOT with ordinary insulation), and use ordinary insulation around it on the outside, between it and the actual leaves.
There won't be any round or flex duct within the wall other than a short piece of flex duct to connect the two boxes. My plan had been to use a fan somewhere outside the vocal booth and connected to the output silencer on the outside wall to pull air through the vocal booth. This will be a relatively low speed fan connected by 4" (12.6 sq. in. cross sectional area) flex duct to a transition (rectangular or possibly round) which will step it up to double the cross sectional area (25.1 sq. in.) right at the point where air flow leaves the output silencer and exits through the outside wall. All four silencer boxes will maintain this cross sectional area from one end to the other, thus keeping the velocity at half of that being pulled by the fan through the 4" duct. So the primary focus of my original question was, given the space constraints, how big of a deal is it if the inner wall of a silencer box is just a few (maybe 2" or 3") inches from the other wall ( which it is not attached to)? If you take a look at Silencer Cross Section drawing that I attached, you can see that each box is attached to one and only one wall and that there is a gap (labelled as "Air Gap" on the drawing) between the box and the other wall. This is where my concern about a third leaf derives from. Would the close proximity of the box to the other wall present a major problem here or is it, as you say, only minor because the box comprises only a small portion of the total wall area?However, with 12" cavity depth, you don't have much space to fit a box in! Do the math: Assuming you'll be using 6" round duct (or 6" flex duct), you have 6" right there. You'll need an inch on either side for the duct liner, so you are up to 8". Two layers of 5/8" MDF is an inch an a quarter, on each side of the box, so 2.5 inches total.... that's 10.5" for your box.... With only 11" of air gap, you don't have enough space. I like to leave at least 2" on either side of the box, so a minimum of 15" is what you need. Your only other option is to use an adapter from your 6" round duct to a narrower size of rectangular duct, such as maybe 8"x4". With that, you could maybe make your box just 8.5" deep (2.5 + 2 + 4), and maybe squeeze it in to 11", but you'd only have an inch and a quarter or air around it... Not so good. And if the air space inside your box is only 4" deep, that implies a very long, thin box to keep the cross-sectional area of the air flow up high enough. You'd need to make the internal bays at least 15" long...
I need to build this pretty soon. Anybody have any opinions on my previous post?
Beuller?
Did I commit a forum "faux pas"? Just trying to get some help here and I'm unsure why I'm not getting any responses?
I don't think it's going to create a 3 leaf system at all. The silencer box is not a sealed thing...it is in fact a somewhat direct part of the internal or external room (when you think about air flow etc.). So I'd say you can confidently build that with no adverse effect. The weak link acoustically will be the duct between the two boxes. Make sure you have plenty of insulation around that - without crushing it of course. ScottSo the primary focus of my original question was, given the space constraints, how big of a deal is it if the inner wall of a silencer box is just a few (maybe 2" or 3") inches from the other wall ( which it is not attached to)?
Hey Scott, thanks for the advice. Well that was something I had considered but I wasn't a 100% sure on what did and didn't constitute a third leaf. I know that, for a two leaf room in room design, it has to be hermetically sealed for the system to work so I guess it stands to reason that, as long as the inner wall of the baffle box isn't sealed off, this wouldn't be of any concern. Stuart appeared to disagree with this notion, though it's possible he didn't understand my proposed design. Still, I was hoping to get some confirmation either way from one of mods here.I don't think it's going to create a 3 leaf system at all. The silencer box is not a sealed thing...it is in fact a somewhat direct part of the internal or external room (when you think about air flow etc.). So I'd say you can confidently build that with no adverse effect. The weak link acoustically will be the duct between the two boxes. Make sure you have plenty of insulation around that - without crushing it of course. ScottSo the primary focus of my original question was, given the space constraints, how big of a deal is it if the inner wall of a silencer box is just a few (maybe 2" or 3") inches from the other wall ( which it is not attached to)?
Actually, that isn't really correct. It needs to be sealed to get full isolation, yes, since even tiny cracks can let through a surprising amount of sound, but it does not need to be sealed in order to create an MSM system. You can have MSM between two large flat surfaces that are totally open around the edges. It will work, ... it just won't be as efficient as if it were fully sealed. Think of this: does a kick drum stop sounding like a kick drum if you cut a hole in one head to poke the mic through? Does it stop working as a drum if you cut a hole in the shell? Nope. In both cases the sound changes a bit, since you re-tuned things, but it still works as a resonant system.I know that, for a two leaf room in room design, it has to be hermetically sealed for the system to work
Ummm... if your silencer box is not sealed, then air will be leaking out of it! :ahh: It's probably not a good idea to have a leaky HVAC duct inside your wall cavity...it stands to reason that, as long as the inner wall of the baffle box isn't sealed off, this wouldn't be of any concern.
Right! That's the best way to do it.My plan had been to use a fan somewhere outside the vocal booth and connected to the output silencer on the outside wall to pull air through the vocal booth.
Speed isn't the issue: the fan needs to be able to move the correct volume of air (cubic feet per minute) against the static pressure imposed by the duct system. The cross sectional area of the duct will determine the speed of the airflow. Fan speed is irrelevant. It will turn as fast as it needs to in order to provide the flow rate.This will be a relatively low speed fan
Your wall is going to have to be EXTREMELY thick to fit in a full silencer box plus leave a 3" air gap! Something like about 16", if my estimates are correct....how big of a deal is it if the inner wall of a silencer box is just a few (maybe 2" or 3") inches from the other wall ( which it is not attached to)? If you take a look at Silencer Cross Section drawing that I attached, you can see that each box is attached to one and only one wall and that there is a gap (labelled as "Air Gap" on the drawing) between the box and the other wall. This is where my concern about a third leaf derives from. Would the close proximity of the box to the other wall present a major problem here or is it, as you say, only minor because the box comprises only a small portion of the total wall area?
There's a basic rule of thumb in HVAC; you need at least 5 to 10 times the effective diameter of the duct following a corner in order for the flow to become stable once again: Corners create turbulence, which is noise. You need a long piece of duct AFTER the final corner, leading up to the register. If not, the flow will still be turbulent where it hits the register vanes... lots of noise! - Stuart -which will step it up to double the cross sectional area (25.1 sq. in.) right at the point where air flow leaves the output silencer and exits through the outside wall.
That analogy make sense. However, based on your comments in your first reply, it sounds like a three leaf (or even four leaf) system won't really matter a whole lot in this case because of the relatively small fraction of the wall/ceiling that it will occupy. Is that correct?Actually, that isn't really correct. It needs to be sealed to get full isolation, yes, since even tiny cracks can let through a surprising amount of sound, but it does not need to be sealed in order to create an MSM system. You can have MSM between two large flat surfaces that are totally open around the edges. It will work, ... it just won't be as efficient as if it were fully sealed. Think of this: does a kick drum stop sounding like a kick drum if you cut a hole in one head to poke the mic through? Does it stop working as a drum if you cut a hole in the shell? Nope. In both cases the sound changes a bit, since you re-tuned things, but it still works as a resonant system.I know that, for a two leaf room in room design, it has to be hermetically sealed for the system to work
My silencer boxes WILL be sealed. I didn't mean that the boxes wouldn't be sealed. I was saying that the air pathway from outside the vocal booth to inside the vocal booth would be sealed against the air cavity between the walls but that air would be able to pass into/out of the vocal booth through the ducts and (obviously) there wouldn't be a seal between rooms. In other words, air could pass from outside the vocal booth to inside the vocal booth but the wall cavity itself would still be sealed from air inside and outside the booth so that a sealed MAM system could be maintained. If you haven't taken a look yet, go look at my reply to your first post and you will see a couple of diagrams I attached that illustrate what I'm planning on doing. That may help eliminate any confusion that may be occurring in this discussion.Ummm... if your silencer box is not sealed, then air will be leaking out of it! :ahh: It's probably not a good idea to have a leaky HVAC duct inside your wall cavity...
What is the origin of this 3" air gap requirement? I understand that there needs to be an air gap but why 3"? Is that some kind of basic minimum air gap? Also, do you mean a 3" air gap on each side of the box (for a total of 6" of air gap) or on only one side? This is an important distinction, as there will be ONLY ONE air gap in my design. One side of the box will be against the inside of one wall (basically part of the wall) with an air gap between the other side of the box and the inside of the other wall. If a single 3" gap is needed, that leaves me 9" to fit a box in the 12" (5.5" + 1" + 5.5") gap between the outer and inner walls. If a smaller gap (maybe 2" or 1.5"?) is fine, then I could have an even deeper box. Keep in mind that I'm building my walls with 2x6's on 24" centers so I've got wide bays to work with and can get away with making boxes that aren't that deep but can be very wide and still provide the necessary cross sectional area. I'll be using a fan in its own external silencer box at a different location than the vocal booth and this will be pulling air through a 4" flex duct (maybe 10 feet long) which will be attached to a 4"X6" rectangular transition at the exit point of the outside wall of the vocal booth (where the exit point of the silencer box is). By my calculations, I should have plenty of room, assuming that I continue the 4" interior depth of the box and make it as wide as needed to give me the necessary cross sectional area. A 4" interior depth with two layers of 5/8" MDF and 1" duct liner would give me a 8.5" (1.25" + 1" + 4" + 1" + 1.25") deep box and a 3.5" air gap between it and the other wall. If I squeezed another inch of interior depth in there, I'd still have a 2.5" air gap. As far as width goes, I have 22.5" (24" - 0.75" - 0.75") to work with between the studs. If I need 25.1 sq. in. of cross sectional area (double the 12.6 sq. in. cross sectional area of a 4" flex duct) inside each box and I have 4" of depth, that means that I only need 6.25" (25.1 / 4 = 6.25) of interior width. A 6.25" interior width with two layers of 5/8" MDF and 1" duct liner would give me a 10.75" (1.25" + 1" + 6.25" + 1" + 1.25") width and, if I doubled that (21.5") to allow space for the necessary turns inside the box, I'd still have 1" to spare.Your wall is going to have to be EXTREMELY thick to fit in a full silencer box plus leave a 3" air gap! Something like about 16", if my estimates are correct....
The "long piece of duct" after the last turn would defeat the purpose of the silencer box in the first place. I'm not understanding your recommendation here. I've seen many recommendations on this site (and elsewhere) to put the silencer box right at the entry/exit point to a room to effectively eliminate any flanking pathway for sound. Again, if you haven't taken a look at the diagrams I attached, please do so. I think there may be some confusion here that could be cleared up by looking at my attached diagrams. Now what I could do is to "extend" the ductwork leaving or entering the vocal booth by running an additional length of flex duct inside the vocal booth and outside the booth in the adjacent live room and soffit that in. Is that more or less what you meant?There's a basic rule of thumb in HVAC; you need at least 5 to 10 times the effective diameter of the duct following a corner in order for the flow to become stable once again: Corners create turbulence, which is noise. You need a long piece of duct AFTER the final corner, leading up to the register. If not, the flow will still be turbulent where it hits the register vanes... lots of noise!
Right. There will be some loss, of course, so do try to compensate with more mass in those areas if you can.it sounds like a three leaf (or even four leaf) system won't really matter a whole lot in this case because of the relatively small fraction of the wall/ceiling that it will occupy. Is that correct?
Correct! That's the right way to do it. It looks like I didn't' get what you were trying to say.In other words, air could pass from outside the vocal booth to inside the vocal booth but the wall cavity itself would still be sealed from air inside and outside the booth so that a sealed MAM system could be maintained.
It came from what you said in your previous post: "how big of a deal is it if the inner wall of a silencer box is just a few (maybe 2" or 3") inches from the other wall "What is the origin of this 3" air gap requirement?
The recommended minimum is actually 4"for an MSM wall, but since this is a silencer box inside the cavity itself, that does not fill a complete stud bay, and has plenty of mass on it, it's probably safe to go with 3".I understand that there needs to be an air gap but why 3"? Is that some kind of basic minimum air gap?
If the box sits in the middle of the cavity, then yes. If it is up against one of the leaves, then no.Also, do you mean a 3" air gap on each side of the box
Why? That does not make sense. The purpose of the silencer box is to allow air to pass through the wall penetration while preventing sound from doing so. How does putting a long duct after the box to get to the register, interfere with that?The "long piece of duct" after the last turn would defeat the purpose of the silencer box in the first place.
Yes. That's exactly where it needs to go: right at the point where the duct passes through the leaf. That's the ONLY place you can have a silencer box. But there's no reason at all to not run a duct from the end of the silencer box to the actual register. I just completed a room for a customer in Australia where the supply ducts run from the silencer box, which had to be on the front wall, down the full length of the control room and across half of the back wall, to get to the registers. I don't understand why you think that would be a problem, or somehow prevent the silence box from working. Why would that "defeat the purpose of the silencer box in the first place"? - Stuart -I've seen many recommendations on this site (and elsewhere) to put the silencer box right at the entry/exit point to a room to effectively eliminate any flanking pathway for sound.
Ok. I see what you mean now. You're talking about running additional duct work within the room itself, AFTER the entry/exit point at the wall/silencer interface. I previously didn't understand what you meant. This is actually exactly what I had intended to do. I think we're on the same page now. :DYes. That's exactly where it needs to go: right at the point where the duct passes through the leaf. That's the ONLY place you can have a silencer box. But there's no reason at all to not run a duct from the end of the silencer box to the actual register. I just completed a room for a customer in Australia where the supply ducts run from the silencer box, which had to be on the front wall, down the full length of the control room and across half of the back wall, to get to the registers. I don't understand why you think that would be a problem, or somehow prevent the silence box from working. Why would that "defeat the purpose of the silencer box in the first place"? - Stuart -
Right! :thu:Ok. I see what you mean now. You're talking about running additional duct work within the room itself, AFTER the entry/exit point at the wall/silencer interface.
:yahoo: Excellent! Now go build something already! :) I'm dying to see how it turns out, now that you have everything figured and calculated. And don't forget to take photos! There's a saying around here: "Photos, or it didn't happen". - Stuart -I think we're on the same page now
Thanks for all of your help. :D:yahoo: Excellent!
I would if somebody would answer my questions in my other thread. :wink:Now go build something already! :)
Crap. I forgot I had one other question. What do you recommend for connecting the inner box to the outer box inside the wall? I was thinking I'd use flex duct (4" or should I use 6"?) but I wonder if you have a better recommendation on how to do this?Now go build something already! :)
You could use flex duct, but what I often do is to make sure the two wood sleeves line up well, leave a gap between them, then use a canvas or rubber "sleeve" to cover that gap: it seals the air path, but keeps the decoupling.
- Stuart -
I may try that on one but the angles of the walls and available space for the other pair won't allow them to line up so flex duct is my only choice there. As long as flex duct provides sufficient decoupling? ThanksYou could use flex duct, but what I often do is to make sure the two wood sleeves line up well, leave a gap between them, then use a canvas or rubber "sleeve" to cover that gap: it seals the air path, but keeps the decoupling. - Stuart -
Flex duct is typically a wire spiral with some type of flexible jacket (PET, PTFE, plastic, etc.), and also insulation. All of that is flexible and resilient, even the wire: it is basically a spring. So it will decouple things fine. Just make sure you follow the normal precautions for flex duct: it must be stretched out to its full extension. You should never install it partly compressed, nor bend it tightly, nor kink it, etc. In other words, the interior must be completely round from one end to the other, fully stretched out. - Stuart -As long as flex duct provides sufficient decoupling?
So it’s been a while... I’ve been busy framing and our previous discussion gave me enough information to go ahead with that stage of construction. However, I’m now at the point in which I need to build the silencer boxes and I have some additional questions. At this point, I’m considering building the silencer boxes, as I previously discussed, with 25.1 sq. in. internal cross sectional area. I’m also considering using 4” diameter flex duct (12.57 sq. in.) to connect the outer box to the inner box (both pairs of boxes), as we also previously discussed. I have, however, had some time to think about all of this and have come up with some additional questions. They are as follows: 1. If air flow in the ductwork connecting the fan to the last silencer box (exit point on outside wall) on the vocal booth is below the desired air speed (300 fpm or whatever it needs to be), then is it still necessary to double the cross sectional area within the silencer boxes or can they just maintain the same cross sectional area as the ductwork? 2. Does the above scenario (question no. 1) make the air impedance mismatch created by the change (12.57 sq. in. to 25.1 sq. in.) in cross sectional area unnecessary? 3. If it remains necessary to double the cross sectional area of the silencer boxes (and I assume this also implies the necessity of an impedance mismatch), would a short (1 to 2 feet) piece of 4” diameter flex duct (12.57 sq. in.) between the inner and outer silencer boxes (each are 25.1 sq. in. cross sect. area) provide a sufficient impedance mismatch in the flow or is that location and length of the 4” flex duct not going to work to provide this? Being that the inner and outer box have to be somehow connected anyway, I figured that would be a good place to also introduce a narrower cross sectional area and resulting impedance mismatch since any noise created by the mismatch would be contained by a box on both sides. The alternative would be to use 6" flex duct but then you wouldn't get an impedance mismatch, if such a mismatch still remains necessary. If this idea won’t work, WHERE should I be trying to create the impedance mismatch(s)? 4. My proposed flow path is as follows: Free air in live room --> enters 25.1 sq. in. opening in outer wall entry box --> passes through 25.1 sq. in. cross sect. area within box --> leaves outer wall entry box --> passes through 4” flex duct (12.56 sq. in.) --> enters inner wall entry box --> passes through 25.1 sq. in. cross sect. area within box --> exits 25.1 sq. in. opening in inner wall entry box --> enters vocal booth --> exits vocal booth --> enters 25.1 sq. in. opening in inner wall exit box --> passes through 25.1 sq. in. cross sect. area within box --> leaves inner wall exit box --> passes through 4” flex duct (12.56 sq. in.) --> enters outer wall exit box --> passes through 25.1 sq. in. cross sect. area within box --> exits 28.27 sq. in. opening in outer wall exit box --> passes through 6” flex (or round metal) duct (28.27 sq. in.) --> enters 28.27 sq. in opening in machine room entry silencer box --> passes through 25.1 sq. in. cross sect. area (maybe bigger?) within box --> exits 28.27 sq. in. opening in machine room entry silencer box --> passes through 6" flex duct (28.27 sq. in.) --> passes through 6” inline fan At this point I've tentatively calculated that I will need about 30 to 45 cfm (not 100% sure about this yet?) coming into the vocal booth (for 1 to 2 people) to keep it well ventilated and cooled. Is there ANY problem with the above air flow path and/or its ability to achieve my stated cfm goals? 5. I’m separately building a machine room in the opposite corner of the room (HERE'S the thread about that). I’m thinking I’ll put the 6” inline fan discussed above in question no. 4 in this machine room rather than try to put it within its own dedicated silencer box next to the exit point of the vocal booth. As described above in my flow path description, this would obviously also require the 6” ductwork to travel from the vocal booth to and through a silencer box at the point where it enters the machine room and then on to the 6” inline fan inside the machine room. The path from the vocal booth to the machine room is a straight run (no bends) about 8 feet long from wall to wall. Can I get away with making the duct work between the vocal booth and machine room out of 6” flex duct or should I make it out of 6” round metal duct? FYI, this 8 foot section of duct work WILL be somewhat exposed (not behind a wall) and open to the live room (don't know if that matters?). 6. My plans are to bring the vocal booth air into the machine room in addition to a separate entry vent (through its own dedicated silencer box) to the machine room to pull in additional cold air directly from down near the floor in the adjacent live room. This second entry into the machine room will probably need to pull in a little more air than the relatively small amount that will be coming in from the vocal booth. The fan, ductwork and silencer box for the exit point of the machine room will be sized to accommodate the total volumetric input of the vocal booth air AND cold entry air, and exit near the ceiling into the adjacent live room. So this will be TWO smaller entry points and ONE larger exit point for the machine room. I’m still thinking through the various aspects of this machine room design so I haven’t developed it as far as the ventilation design for the vocal booth. That being said, I would also appreciate any input on the plan I’m proposing for the machine room cooling and how it ties into my vocal booth plans and vice versa?Flex duct is typically a wire spiral with some type of flexible jacket (PET, PTFE, plastic, etc.), and also insulation. All of that is flexible and resilient, even the wire: it is basically a spring. So it will decouple things fine. Just make sure you follow the normal precautions for flex duct: it must be stretched out to its full extension. You should never install it partly compressed, nor bend it tightly, nor kink it, etc. In other words, the interior must be completely round from one end to the other, fully stretched out. - Stuart -As long as flex duct provides sufficient decoupling?
The purpose of the change in cross sectional area is to create an "impedance mismatch" at that point. When a sound wave moves from an area with one impedance to an area with another impedance very suddenly, then part of the wave is reflected back up the way it came, and cancels itself out, partially. This is why, if you hold a tweeter driver out in the open air, without the waveguide attached to it, it sounds very thin, small, tinny, and weak, but if you put the waveguide on and mount it in the cabinet, it sounds much fuller, stronger, more powerful. Because the waveguide helps to match the impedance of the driver cone to the impedance of the air in the room. No waveguide = impedance mismatch = poor transfer of sound energy. In HVAC, that's what we want! We WANT for there to be poor transfer of sound energy through the silencer! So the sudden change in cross section is important. Very important. It isn't related to airflow at all: it is related to stopping the sound from getting through. Yes, it does man that the air must slow down or speed up, and also that the air pressure will change as well, but those are just side effects. We can also make good use of those side effects, to force the air to slow down. So basically it does not matter so much what speed the air is going in the duct. What matters is that it slows down to the correct speed inside the box, and does not speed up again. That's why I normally put the register on the end of a short wooden "sleeve" that is the same cross section as the interior of the silencer, or larger.1. If air flow in the ductwork connecting the fan to the last silencer box (exit point on outside wall) on the vocal booth is below the desired air speed (300 fpm or whatever it needs to be), then is it still necessary to double the cross sectional area within the silencer boxes or can they just maintain the same cross sectional area as the ductwork?
No. For the same reason as above: the impedance mismatch is an important part of how the silencer works.2. Does the above scenario (question no. 1) make the air impedance mismatch created by the change (12.57 sq. in. to 25.1 sq. in.) in cross sectional area unnecessary?
The mismatch does not create noise: it eliminates noise! There might by some air noise from the higher speed, but that will dissipate inside the silencer box. But the purpose of the mismatch is to kill low frequency sounds. It does not make more low frequency noise: it kills it.figured that would be a good place to also introduce a narrower cross sectional area and resulting impedance mismatch since any noise created by the mismatch would be contained by a box on both sides.
You need around 6 room changes per hour. Will 45 CFM give you that much? I'm really busy right now, and no time to do the math...At this point I've tentatively calculated that I will need about 30 to 45 cfm (not 100% sure about this yet?) coming into the vocal booth (for 1 to 2 people) to keep it well ventilated and cooled.
Will that room be inside the isolation shell of the studio, or outside it? If it is inside, then it will need silencer boxes,just like the booth. If it is outside it, then it does not need silencer boxes.5. I’m separately building a machine room in the opposite corner of the room
In general, building cods do not allow venting the air from one living space into another living space, so that might not be allowed by code. Of course, the machine room wont have people in it very much, so from that point of view what you are proposing is reasonable, but it still might not pass code. - Stuart -My plans are to bring the vocal booth air into the machine room
Duplicate post
Okay, that's what I've read before so I'm glad to get some validation on that. I will implement an impedance mismatch using 4" flex duct. I just need to make sure that 4" flex duct is capable of pulling the amount of cfm I require to provide fresh air/cooling for the vocal booth.The purpose of the change in cross sectional area is to create an "impedance mismatch" at that point. When a sound wave moves from an area with one impedance to an area with another impedance very suddenly, then part of the wave is reflected back up the way it came, and cancels itself out, partially. This is why, if you hold a tweeter driver out in the open air, without the waveguide attached to it, it sounds very thin, small, tinny, and weak, but if you put the waveguide on and mount it in the cabinet, it sounds much fuller, stronger, more powerful. Because the waveguide helps to match the impedance of the driver cone to the impedance of the air in the room. No waveguide = impedance mismatch = poor transfer of sound energy. In HVAC, that's what we want! We WANT for there to be poor transfer of sound energy through the silencer! So the sudden change in cross section is important. Very important. It isn't related to airflow at all: it is related to stopping the sound from getting through. Yes, it does man that the air must slow down or speed up, and also that the air pressure will change as well, but those are just side effects. We can also make good use of those side effects, to force the air to slow down. So basically it does not matter so much what speed the air is going in the duct. What matters is that it slows down to the correct speed inside the box, and does not speed up again. That's why I normally put the register on the end of a short wooden "sleeve" that is the same cross section as the interior of the silencer, or larger.
AgreedNo. For the same reason as above: the impedance mismatch is an important part of how the silencer works.
So my original question was about whether or not the location in between the two boxes would be the best place to put ductwork with a smaller cross sectional area than that in either box attached to it so as to create the necessary impedance mismatch. 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? That should help to block sounds from going into AND out of the vocal booth. Right?The mismatch does not create noise: it eliminates noise! There might by some air noise from the higher speed, but that will dissipate inside the silencer box. But the purpose of the mismatch is to kill low frequency sounds. It does not make more low frequency noise: it kills it.
The booth is only about 250 cubic feet so, at 6 room changes per hour, that works out to only needing 25 cfm of air. Were you basing your recommendation of 6 room changes per hour on just fresh air or also on cooling (assuming that the adjacent room is already cooled)? Do you think 25 cfm would be enough for one person in there? I had previously been considering 30 or even 45 cfm. Surely 30 or even 45 cfm should provide enough fresh air/cooling, right? Also, I haven't calculated the effects of friction loss due to using 4" flex duct but I would think that any friction loss would be minimal given the short length. There will only be about 2 feet of flex duct used in between each pair of boxes. What is the maximum amount of cfm that you could pull through a 2 foot long piece of 4" flex duct?You need around 6 room changes per hour. Will 45 CFM give you that much? I'm really busy right now, and no time to do the math...
It will be inside the isolation shell and I definitely had planned silencer boxes there as well.Will that room be inside the isolation shell of the studio, or outside it? If it is inside, then it will need silencer boxes,just like the booth. If it is outside it, then it does not need silencer boxes.
Correct!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?
For a small room like a vocal booth, that should be OK, yes.Surely 30 or even 45 cfm should provide enough fresh air/cooling, right?
Right. - Stuart -I would think that any friction loss would be minimal given the short length.
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.Correct!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?For a small room like a vocal booth, that should be OK, yes.Surely 30 or even 45 cfm should provide enough fresh air/cooling, right?Right. - Stuart -I would think that any friction loss would be minimal given the short length.
And, in this scenario, aren't you basically just swapping the locations where you speed up and slow down the air flow to create the impedance mismatches if you use 8" flex duct instead of 4" flex duct? It seems to me that this should work?