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How’s these HVAC baffles look?
Old 25th July 2019
  #1
How’s these HVAC baffles look?

Any advice to these baffles before I add the final layer of insulation and screw them closed? I’m cutting the 6” ducts on either end of them (I’ve made 4) and gluing in the acoustic foam over the insulation as the final step... unless anyone has any additional critique/suggestions? Any help is greatly appreciated. The final box is 48”x27.5”x10” with the diameter of the actual tunnel passage at exactly 6”. the sound hits perfect 90 degree sharp turns every corner, should work pretty well, right? I’ve also caulked every inch of every turn with locktite sealant. I even had to get building subcode permits (in addition to all the other permits for the studio) for these baffles! Inspectors said they never saw anything like it! I’m the only guy in town with a floated decoupled basement studio they said viva la GS

Thanks in advance
Attached Thumbnails
How’s these HVAC baffles look?-b2807106-529f-4574-9f7d-c4b5d040ca02.jpg  
Old 25th July 2019
  #2
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Quote:
and gluing in the acoustic foam over the insulation as the final step... unless anyone has any additional critique/suggestions?
I'm wondering why you didn't just use proper duct liner for that. I'm skeptical that acoustic foam will do the job properly, and stand well to the air flow up over time.


- Stuart -
Old 25th July 2019
  #3
Quote:
Originally Posted by Soundman2020 View Post
I'm wondering why you didn't just use proper duct liner for that. I'm skeptical that acoustic foam will do the job properly, and stand well to the air flow up over time.


- Stuart -
Good point. At $300 for 25 feet of duct liner I was hoping the acoustical absorption panels (at about 90% cheaper) would work. I’ve seen lots of guys use them on a budget but wanted to see what the community thinks of them as a solution, if duct liner is the only way to go I’ll do that. Thanks
Old 25th July 2019
  #4
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Quote:
Originally Posted by jml designs View Post
Good point. At $300 for 25 feet of duct liner I was hoping the acoustical absorption panels (at about 90% cheaper) would work. I’ve seen lots of guys use them on a budget but wanted to see what the community thinks of them as a solution, if duct liner is the only way to go I’ll do that. Thanks
The absorptive lining inside an HVAC silencer accomplishes several things, but one of the most important ones is to stand up to the continuous flow of air, day after day, year after year. Acoustic panels hung on the wall do not have to deal with that: the air around them is pretty much static. Moving air abrades things that are not designed to resist that abrasion. Acoustic foam, and house insulation are not designed to resist abrasion. Over time, the air can slowly wear down the surface. Maybe you get lucky and that doesn't happen in your case... but maybe it does. If the abrasions DOES happen then you have loose particles of porous absorption flying through your HVAC system and your room, getting into your gear, your furniture, and your lungs. If you do decide to stick with your current path, then I'd encourage you to put particle filters on all of your registers, to help in preventing those particles from getting into your room. However, adding those filters will also increase the total static pressure, so you should re-calculate everything to make sure your fans or AHU won't be overloaded.

Those same particles can also get caught up in the same porous absorption downstream from where they came loose, eventually clogging the pores that are a part of how the material works acoustically. Thus. over time, you could potentially get a reduction in acoustic performance.

These effects are small, yes, but over time they can build up enough to be a problem. You might not even notice them for months, years maybe, so do take that into account as well.

There's a reason why duct liner is so expensive: because it has to do things that ordinary porous absorption doesn't need to worry about.

- Stuart -
Old 26th July 2019
  #5
Gear Addict
 

What material is the foam? It looks like it might be Melamine.

Also, what is the air velocity in the duct? If it is low enough, I suggest abrasion would be negligible
Old 26th July 2019
  #6
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BIG BUDDHA's Avatar
Quote:
Originally Posted by jml designs View Post
Any advice to these baffles
they actually look very very similar to the ones that i made that feed air into my Drum room and the 2 rooms to left and right of it.

i put 50mm sound absorbing foam on all the inside surfaces, and my boxes are about 1200mm long. about 400mm high.

i found that the Hi Frequency sound was attenuated but the low/midd frequency from the Force feed air fan was still an issue.

my solution was to install a larger Fan (200mm) and speed control the fan down (to slow) and i put an extra diffuser over where the air enters the Room.

Buddha
Old 28th July 2019
  #7
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Well, you basically made a transmission line for certain frequencies... That isn't the style I would have chosen. You need to think about not just the frequencies traveling out, but what kind of resonations will occur within the device. Frequencies are pretty low that travel through piped ducting. You may have made them louder.

It's probably unrealistic to raise the frequencies above the hearing/recording level, so I'd think the the goal could be to move them down as far as possible to relatively innocuous low frequency, that won't resonate, and will be low flow exiting (but higher volume). Think of it like this, go from semi low frequency, high flow, high density (6") to lower frequency, low flow, low density but equivalent volume (of air, not sound). The higher volume at a lower intensity will be less audible.
Old 28th July 2019
  #8
Quote:
Originally Posted by Folsom View Post
Well, you basically made a transmission line for certain frequencies... That isn't the style I would have chosen. You need to think about not just the frequencies traveling out, but what kind of resonations will occur within the device. Frequencies are pretty low that travel through piped ducting. You may have made them louder.

It's probably unrealistic to raise the frequencies above the hearing/recording level, so I'd think the the goal could be to move them down as far as possible to relatively innocuous low frequency, that won't resonate, and will be low flow exiting (but higher volume). Think of it like this, go from semi low frequency, high flow, high density (6") to lower frequency, low flow, low density but equivalent volume (of air, not sound). The higher volume at a lower intensity will be less audible.
Wait, you think sound waves attempting to travel around four sharp 180 degree 3” thick corners will somehow increase their amplitude? How would this device INCREASE source volume...??

These boxes were built on authoritative CAD designs by some of the worlds most notable studio builders and mentioned in some reputable books on the subject and calculated cfm/6” ductwork etc. I was thinking the proposed absorption foam could be a feasible way to save a few hundred dollars but to think they would amplify the volume of the source passing through them seems, well... absurd, no?
Old 28th July 2019
  #9
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Well, the foam only works on higher frequencies because it's thin.

Please check out transmission line speakers. Scroll for a bit. All of them are designed to increase bass frequencies.

So yes, I think it's possible to amplify the frequencies that are resonate with the box & pipes. Everything has a resonance. Calculating them isn't easy. It's possible they're too low to matter. I just would have gone with a design that is something I know won't act like a transmission line. Check our car mufflers, note none of them do that design (but may not be what you want).
Old 28th July 2019
  #10
Gear Nut
 

Quote:
Originally Posted by jml designs View Post
Any advice to these baffles before I add the final layer of insulation and screw them closed?
Your baffles look fairly similar to the ones that I made for my studio but I decided to use proper duct liner despite its relatively expensive cost. I thought that doing it correctly is very important in this case becasue as the baffles are fixed in impossible to get to spaces (like the roof void in my case) and it would be a total nightmare to get them fixed if things were to go wrong or if they didn’t work properly. In addition to the particle issue that soundman was mentioning I was also concerned about bacterial growth (proper duct liner is anti-bacterial and bacteria in air sytems is very bad news) and also fire safety (proper ductliner is certified as fire safe).
Other than that I would say that my baffles are quite a lot more heavy duty - I think the idea is to build the boxes at the same mass as the walls, so at least 3 layers of plasterboard thickness (using equivalent mass of OSB) in my case. This makes the boxes very heavy - I chose to build mine in situ which worked very well.

Also I thought the idea of the bends was to at least double the cross section of the airflow, the change of impedance being important to help stop sound transmission. I may be wrong as the technicalities are way above my expertise but that is how I did mine and they seem to work well.

6” ducting sounds quite small to me but I guess you have made the correct measurements. I was very pleased I oversized mine because at almost the last minute the aircon fitters decided that it would be better to use 2x8” ducts for in and out on each box rather than the 1x8” I had planned for. Because I had oversized I didn't need to remake all the boxes, phew.
Old 29th July 2019
  #11
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Quote:
Well, you basically made a transmission line for certain frequencies...
Really? How do you figure that? What theory do you base your claim on? What do you mean be "transmission line"?

Quote:
Frequencies are pretty low that travel through piped ducting. You may have made them louder.
Really? And how would you calculate if that had happened or not? How would you figure the impedance mismatch into your calculations?

Quote:
It's probably unrealistic to raise the frequencies above the hearing/recording level,
Maybe you don't realize that this is an HVAC silencer box: it does not "raise" any frequencies! It attenuates all frequencies. The theory is quiet solid, and well proven in practice.

Quote:
so I'd think the the goal could be to move them down as far as possible to relatively innocuous low frequency, that won't resonate,
Please explain how you would accomplish that: How would you "move the frequencies down as far as possible"...

Quote:
Think of it like this, go from semi low frequency, high flow, high density (6") to lower frequency, low flow, low density but equivalent volume (of air, not sound). The higher volume at a lower intensity will be less audible
Maybe you should take a look at the equal loudness curves... I'm also interested in your theory that you can change the air volume inside an HVAC system...


Quote:
Wait, you think sound waves attempting to travel around four sharp 180 degree 3” thick corners will somehow increase their amplitude? How would this device INCREASE source volume...??
Exactly: That will not happen. You are perfectly correct.

Quote:
These boxes were built on authoritative CAD designs by some of the worlds most notable studio builders and mentioned in some reputable books on the subject and calculated cfm/6” ductwork etc.
And you have, indeed, designed and built them correctly. The only issue is the type of insulation, which won't have a huge effect on insertion loss anyway: it's more of a practical issue, with how the material stands up over time, and not so much of an acoustic issue. Ignore advice that isn't based on solid science.

Quote:
I was thinking the proposed absorption foam could be a feasible way to save a few hundred dollars but to think they would amplify the volume of the source passing through them seems, well... absurd, no?
Yes, it is absurd, exactly as you suspect.


Quote:
Well, the foam only works on higher frequencies because it's thin.
That's actually a myth... Thin foam is very much capable of absorbing low frequencies, under certain conditions. And those conditions are, in fact, met inside an HVAC silencer box.
Quote:
Please check out transmission line speakers. Scroll for a bit. A
Ummm.... and in what way are speaker related to what the OP is building here? Please don't tell me that you think he is building SPEAKERS!!!

Quote:
So yes, I think it's possible to amplify the frequencies that are resonate with the box & pipes.
Well, you might think that, but in reality it won't happen. I design and build silencers like these for my clients all the time, and the do not cause an increase in transmission through the silencer. Never once have we seen that, ... probably because acoustic theory predicts that it won't happen...

Quote:
Everything has a resonance.
Sure it does! But what does that have to do with the choice of insulation type for an HVAC silencer box?

Quote:
Calculating them isn't easy.
Actually, it's not that hard at all...

Quote:
I just would have gone with a design that is something I know won't act like a transmission line.
So you are saying that this very common, very popular, very frequently used design for HVAC silencer boxes is wrong, and does not work? What evidence to you have to support your claim? Please post it here... By the way, HVAC silencers are nothing like speakers... )

Quote:
Check our car mufflers, note none of them do that design
I'm trying to understand here: You are claiming that the design for an internal combustion engine muffler, is also valid for a recording studio HVAC silencer? Is that seriously what you are saying? If so, what would be the basis for your claim? I'd really love to see that...

(PS. Don't worry, jml designs, you have designed and built yours correctly. The only remaining issue is your choice of liner. You don't need to worry about any of the fantasy presented above: reality rules.)

- Stuart -
Old 29th July 2019
  #12
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Good. Now connect it with soft plastic ducts.
Old 29th July 2019
  #13
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DeadPoet's Avatar
Years ago I remember the late Eric Desart mentioning (on more than one message board and in direct conversation with him as well) that you don't need a labyrinth-styled baffle. Just straight look-through works as good for sound as long as you got around 40/60 air-to-absorption ratio. In terms of air flow a straight baffle works better because you don't force the air around corners (which creates some turbulence).

The baffles in my current studio are straight see-through and those are by far not my weakest link. Maybe someone with real knowledge could tell me if I just got lucky or that this is a real thing?



Herwig
Old 29th July 2019
  #14
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Hi again, I found this photo that I'd like to share.

https://hvac-hacks.com/wp-content/up...934df6123b.jpg

It looks similar, yes? But it's actually pretty different because the waves get interrupted, as opposed to flowing through a path. It's similar to a diffuser on a wall with varying height blocks. I'm not sure I'd say it's optimal, but were I to try and follow a design I'd be looking at something like this before making a path that presumes sound cannot chase a shape.

The size of your box cannot let air enter at a path that splits the approaching wave, and it will chase the shape. If your box's vanes were cut so they only overlapped a little, and the in/out were directly facing the middle of the overlaps, it'd be like the shown image. It'd still be bigger than maybe needed, but you could stuff the far ends of the the cavity in that situation with something absorptive.

I've seen a commercial option or two, that are nothing more than absorbing material surrounding the air path, in an oversized straight through duct. I'm sure they do something, but given that the sounds are running through ducting they are fairly directional at the point they reach the chamber... I suspect you're better off with something that has some sort of baffling inside - anyone used them to the otherwise?

But I'd also think about termination. Any flow of air exiting something has the potential to make turbulence and defract off of sharp shapes. I'd say in general that's part of what we hear, when we hear a vent. A sharp cutout thin-metal vent cap would not be the ideal. A damped exit is perfectly possible if size is sufficient not to impede the flow from the HVAC unit.
Old 29th July 2019
  #15
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Quote:
It looks similar, yes? But it's actually pretty different because the waves get interrupted, as opposed to flowing through a path.
Actually, that's not correct at all: It looks similar because it is identical in the principle of operation, but the one you show is actually a very poor implementation. Not at all suitable for a recording studio. To start with, there's no substantial change in the cross sectional area between the duct and the interior of the box, and thus there is no impedance mismatch at the two interfaces, which is a rather useful mechanism for studio HVAC silencers. Without that, the low frequency attenuation will not be very good. Next, in the image you show the baffles have no solid core and thus do not do what you claim they do: They do not interrupt the low frequency waves, and in fact low frequency waves would indeed "flow through" such a device, exactly the opposite of what you claim. Third, that device would have a rather high static pressure drop, as evidenced by the design. Not something you want to add to an HVAC system if you can avoid it! So it fails on at least three basic principles of how studio HVAC silencers work. I'm betting that was not used on a recording studio...

Quote:
It's similar to a diffuser on a wall with varying height blocks.
Actually, it is not similar in any way to a diffuser with blocks of varying height. Because "diffusers with blocks of varying height" are designed on the basis of number theory to have pseudo-random patterns that also guarantee smooth power response, where each well is carefully tuned to produce part of that effect. The well depth and width, as well as the width of the entire device, each play their roles in how a diffuser works. The device in the picture is nothing at all like an acoustic diffuser: it has a regularly repeating pattern, no wells, and no tuning. Properly designed diffusers work to diffuse sound waves without attenuating them, and do not allow air flow (they have to be sealed to be fully efficient). Properly designed HVAC silencers work to attenuate sound transmission as much as possible, while allowing air to flow. The exact opposite. The device in the picture does neither.

Quote:
The size of your box cannot let air enter at a path that splits the approaching wave, and it will chase the shape.
Why not? So you are saying that impedance mismatch at the interfaces is a BAD idea, when acousticians and HVAC designers, and ASHRAE all say the opposite? That silencers SHOULD be designed in such a way as to maximize acoustic impedance mismatch, in order to FORCE the wave to reflect back on itself? How some they say that splitting the wave is a good thing, but you say it should be avoided? Maybe you could explain your new theory on how sound transmission works in HVAC plenums, because it does not jibe well with current theory...

What do you mean by the sound "chasing the shape" of the box? Are you saying that sound waves are able to flow around corners? That the sound rays are malleable, and can bend to follow the shape of the box?

Quote:
If your box's vanes were cut so they only overlapped a little,
... then it wouldn't work very well, as there would be no solid, massive, rigid barriers in the path of the sound wave...

Quote:
It'd still be bigger than maybe needed,
You say that as though big silencer boxes are a BAD thing, but everyone else seems to agree that they are a GOOD thing. Why is that? What do you know that trey don't know?

Quote:
but you could stuff the far ends of the the cavity in that situation with something absorptive.
Say what? are you saying that the OP should stuff the duct transitions with absorptive materials?

Quote:
I've seen a commercial option or two, that are nothing more than absorbing material surrounding the air path, in an oversized straight through duct.
Because they are not meant to treat the typical sound levels and frequency range seen in a typical recording studio! The "commercial silencer" sold by many HVAC suppliers are meant to deal with FAN NOISE, not roaring bass guitars, screaming electric guitars, floor-rocking kick drums, and snares that have nearly enough energy to launch a rocket! You might want to do a bit of research on how silencers are designed and what they are designed for, and how the work. The blade pass frequency of a typical HVAC fan is constant: it doesn't change much (except briefly at start-up and shut-down). Therefore, like a car muffler, it can be designed to have optimal insertion loss (attenuation) for a specific frequency, or small frequency range. Recording studio HVAC silencers, on the other hand, must deal with the entire audio spectrum, from subsonic to ultrasonic, so they require an entirely different design.

Quote:
I'm sure they do something,
Yes they do: they do what they are designed to do: they attenuate fan noise, and air flow noise. And they do it very well, because they are tuned for that. But they do a pretty lousy job of attenuating Phil Collins or Peter Frampton or Amy Winehouse... Because the were never designed to do that. Loud rock music needs a rather different approach to attenuation, compared to an HVAC fan...

Quote:
but given that the sounds are running through ducting they are fairly directional at the point they reach the chamber...
Actually, low frequency sound isn't directional at all... And the typical ducting used in studio HVAC systems (flexduct) is not much of a waveguide for the lows: sound can arrive or leave at any angle, directly through the thin walls of the duct, right up to the point where the duct joins the silencer.

Quote:
I suspect you're better off with something that has some sort of baffling inside
But didn't you just show us what you consider to be a perfect design, that has NO baffling inside it? And didn't you just say that he should MINIMIZE his baffles, so they don't get in the way of the sound waves as they try to "flow" through the device? This is confusing: first you say that he should NOT have baffles, and now you say that he SHOULD have baffles?

Quote:
Any flow of air exiting something has the potential to make turbulence and defract off of sharp shapes.
What do you mean by the air flow being able to "defract"? I assume you mean "diffract", but even so, how can air flow "diffract"?

Quote:
I'd say in general that's part of what we hear, when we hear a vent.
... which is why the general recommendation for all studio HVAC systems is to limit the flow velocity at the register to speeds where the air flow noise is insignificant. Since you are presenting yourself as an expert here, maybe you can tell us what that velocity is?

Quote:
A damped exit is perfectly possible if size is sufficient not to impede the flow from the HVAC unit
So how should the OP go about "damping" his "exit"? What would be the solution to that? There's no evidence of any such "damping" in the image you linked to, of the supposedly perfect silencer. On the contrary, it seems like it would create a LOT of turbulence in the air flow (and probably also the "diffraction" you mention, since it has numerous sharp corners). I'm trying to understand why the device you linked to and lauded seems to violate the rules that you yourself said were necessary, while also violating the rules that everyone else says are necessary?

I'm also wondering why you have not yet answered all of the questions that I brought up about your earlier "recommendations"... Is that because you don't know the answers? Or is it because you are just trying to change the subject? The answers to those questions are fundamental to your suggestions for the OP to change his design, so you really should answer them.

Last edited by Soundman2020; 29th July 2019 at 04:55 PM..
Old 29th July 2019
  #16
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Quote:
Originally Posted by DeadPoet View Post
Years ago I remember the late Eric Desart mentioning (on more than one message board and in direct conversation with him as well) that you don't need a labyrinth-styled baffle. Just straight look-through works as good for sound as long as you got around 40/60 air-to-absorption ratio.
That really is interesting. Do you have a link to one of those conversations? I'd like to see exactly what Eric said there: what specifically he was taking about.

Quote:
In terms of air flow a straight baffle works better because you don't force the air around corners (which creates some turbulence).
If there's no baffles in it then I probably wouldn't call it a "baffle", just a silencer, but that's a semantics issue, not an acoustic one... And yes, you are right that putting baffles in the path of the air flow does, indeed, cause turbulence... which is one of the reasons why the interior of the box has to be so large (in terms of cross-sectional area), to keep the air flow velocity low enough that turbulence isn't too much of a problem, while also allowing sufficient air flow rate through the box. Also, there are guidelines about how much straight duct length you need downstream from anything that causes turbulence, in order for the flow to become laminar again (or rather, as close as possible to laminar). It's a good idea to leave at least that much distance between the end of your silencer box and the register it feeds, to minimize turbulence. But the key to dealing with this is to keep the flow velocity low while still allowing the correct flow rate. If your velocity is low enough through the silencer and through the rest of the duct, up to the register, then the noise generated by air movement itself, and by turbulence, will be minimal.

Quote:
The baffles in my current studio are straight see-through and those are by far not my weakest link. Maybe someone with real knowledge could tell me if I just got lucky or that this is a real thing?
Probably both! It's a real thing, for sure: you can indeed get some insertion loss from a straight duct. There are tables that show that, in many texts that deal with HVAC. ASHRAE is a good source for such information, for example. But it's also a question of how much isolation you need in your HVAC system, and what frequency you need it at. If you don't need high isolation, or only need it for a portion of the audio spectrum, then it might be good enough. You mentioned that your HVAC ducting is not your biggest isolation issue, which sort of implies that you DO have isolation issues elsewhere... thus, your HVAC is "good enough"! But when you solve those "other issues elsewhere", then you might notice that your HVAC is now the issue... because things got quieter in your room due to fixing he other issues, so now you can hear your HVAC issues for the first time: you couldn't hear them before, because the "other issues" were louder.... That might not happen, of course: perhaps you actually so have enough isolation, and that might be the case for any of many possible reasons. That's why it would be good to see what Eric actually said: all rooms are different, each has it's own special needs. Perhaps in your case there are other things involved in getting you good HVAC isolation. If you can find and share links to what Eric said, that would be great.

- Stuart -
Old 30th July 2019
  #17
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DeadPoet's Avatar
Quote:
Originally Posted by Soundman2020 View Post
That really is interesting. Do you have a link to one of those conversations? I'd like to see exactly what Eric said there: what specifically he was taking about.
I've tried searching. Only found a few short blurbs of himself at the (defunkt) homerecording.be forum. There's bits on the johnlsayers.com forum and I specifically remember the Paul Woodlock build thread years ago where he specifically mentioned "see-through" silencers. Even with all the absorptive material on one side of the silencer only, in order to maximize the absorption thickness.


Herwig
Old 30th July 2019
  #18
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Northward's Avatar
Using SONOFLEX (or equivalent) of the right size and length (easy to calculate based on manufacturer data) basically voids the need for any kind of HVAC baffle / muffler.

What you need to take in account is speed of air in the duct, static pressure and pressure loss etc.
Old 30th July 2019
  #19
Gear Addict
 

Quote:
Originally Posted by DeadPoet View Post
I've tried searching. Only found a few short blurbs of himself at the (defunkt) homerecording.be forum. There's bits on the johnlsayers.com forum and I specifically remember the Paul Woodlock build thread years ago where he specifically mentioned "see-through" silencers. Even with all the absorptive material on one side of the silencer only, in order to maximize the absorption thickness.


Herwig
http://www.homerecording.be/forum/t27409.htm

"Als je bedoeld dat dat geluid in zig-zag hoeft te gaan: HELEMAAL NIET.
Je zet een behoorlijk stromingsverlies op je demper (opvoerhoogte).

Ik weet dat hoe complexer hoe verleidelijker en "echter" het er uit ziet in audiogerelateerde fora.
Ik heb van mijn leven nog nooit zo'n demper gebouwd en heb dempers ontworpen voor miniventilatortjes tot monsterventilators (staalindustrie) van 6 tot 7 megawatt (geen schrijffout).

Mijn opgegeven relatie klopte wel (mijn hersens sloegen even kadul).

Je hebt meer dan genoeg lengte voor je demper zonder hier een complex pad in te werken. Hoe smaller de spleet, hoe beter je demping. Hoe dikker je wol hoe beter je demping.
Een smallere spleet betekent buiten betere demping (een plus) ook meer stromingsweerstand (een min). Ook hier is het belangrijk zoveel mogelijk bij het laag te geraken (zo dik mogelijke wol en zo smal mogelijke spleet)."

It is a discussion about using the gap between a double door as a duct for air exchange. The OP is asking if he has to make a labyrinth

"If you think the sound has to zigzag: NOT AT ALL. It leads to losses in the air current.
I know complex looking stuff is popular on audio related forums.
I never designed such a damper though I designed damers for mini vents to monstervents (steel industry) of 7 to 7 megaWatts (no writing error).

There is more than enough length for your damper without the need to apply a complex duct. The narrower the exhaust, the better the attenuation. The thicker the wool, the better the attenuation.
A narrow exhaust meant next to better attenuatien (good) alsso higher air current resistance (not good). It's important to influence low frequencies (thick wool and narrow exhaust).
Old 1st August 2019
  #20
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Quote:
Originally Posted by Northward View Post
Using SONOFLEX (or equivalent) of the right size and length (easy to calculate based on manufacturer data) basically voids the need for any kind of HVAC baffle / muffler.

What you need to take in account is speed of air in the duct, static pressure and pressure loss etc.
That's interesting...

And the equivalent is any insulated flex duct or is there something special about sonoflex?

Does this still need to be in a soffit of some sort? No impedance changes etc?
Old 1st August 2019
  #21
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Quote:
Originally Posted by jml designs View Post
Any advice to these baffles before I add the final layer of insulation and screw them closed? I’m cutting the 6” ducts on either end of them (I’ve made 4) and gluing in the acoustic foam over the insulation as the final step... unless anyone has any additional critique/suggestions? Any help is greatly appreciated. The final box is 48”x27.5”x10” with the diameter of the actual tunnel passage at exactly 6”. the sound hits perfect 90 degree sharp turns every corner, should work pretty well, right? I’ve also caulked every inch of every turn with locktite sealant. I even had to get building subcode permits (in addition to all the other permits for the studio) for these baffles! Inspectors said they never saw anything like it! I’m the only guy in town with a floated decoupled basement studio they said viva la GS

Thanks in advance
Is that lined with a closed cell foam insulation? Like EPS or Polyiso?
Old 2nd August 2019
  #22
Gear Addict
 

Quote:
Originally Posted by Northward View Post
Using SONOFLEX (or equivalent) of the right size and length (easy to calculate based on manufacturer data) basically voids the need for any kind of HVAC baffle / muffler.

What you need to take in account is speed of air in the duct, static pressure and pressure loss etc.
My understanding about "acoustic flexible ducts" is that they "work" because they let the sound breakout from the duct. However, there's not much to stop the sound breaking back in to the duct either.

Again, it is my understanding that for these to be effective,
  • the duct has to be a minimum length,
  • the inlet and outlet must be a minimum distance apart (that is, not just putting a long convoluted duct between inlet/outlet that are close together),
  • the duct has to be in a bulkhead that is appreciably bigger than the duct (say 4x the duct volume),
  • the bulkhead has to be lined with 50mm acoustic batts (say 703 or similar).
If the above conditions are met, then I have seen insertion loss equivalent to very expensive duct attenuators for a fraction of the price.
Old 2nd August 2019
  #23
Lives for gear
Quote:
Originally Posted by Sebg View Post
If the above conditions are met, then I have seen insertion loss equivalent to very expensive duct attenuators for a fraction of the price.
That's interesting...On my project, the mechanical engineer won't budge on taking out a provision that no more than 5 feet of insulated flex duct be used at any point. Lucky for me, my actual inspector doesn't care as long as the system is balanced to the required ventilation/fresh air specs.

My hvac guy, the mechanical engineer and inspector all agree that the downside to flex is you ultimately will only know the static pressure after it's installed. So for these types of systems would you all upsize the equipment and then use inline dampers? In the US there is a big pushback against insulated flex duct it seems after years of lazy installers tossing it in homes to save time/labor. But I know it also makes the actual duct noise quieter as well...

By bulkhead do you mean a soffit/box that is closed off (drywall etc)? Or it could it be a superchunk style bass trap with the flex duct in it?
Old 2nd August 2019
  #24
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Quote:
Originally Posted by RyanC View Post
That's interesting...

And the equivalent is any insulated flex duct or is there something special about sonoflex?

Does this still need to be in a soffit of some sort? No impedance changes etc?
Any micro-perforated or fabric lined (inner bit) Acoustic duct does the job 100% as long as you use enough length.

And... Pay attention to pressure, velocity etc. It's not useful to make 's' turns or anything like this.

Knowing your heat load and using the manufacturer data and HVAC system data you can get really accurate results.

If you put a precise measurement system in the room, you will see the noise floor rise a wee bit when it turns on, but nothing humanly audible. To give you an example, I've had clients get annoyed by a halogen filament noise with HVAC on.

Basically still eerily silent in the room with HVAC on.

On the outside of the shell, duct are just suspended, sometimes fitted in an Inox duct for looks or ease of installation.

Inside the room, it depends on ceiling height.

In rooms with height constraints we sometimes can't have the duct travel over the shell roof, so it comes in where there are no membranes (very few options there) and travels in soffits in ceiling, or within the lower flow treatment layers.

In spaces with more height over the shell they dive down at ideal location, also where there are no membranes.

In both cases they are connected to "open sides boxes" hidden in the ceiling that use the surrounding flow treatment layers as mufflers and decompression. Which kills the edge friction noise and slows down the air a bit more before it passes through the grilles.

See photos before and afters finishing. Look at rear corners.

These save so much budget... And space.
Attached Thumbnails
How’s these HVAC baffles look?-img_20190702_205055.jpg   How’s these HVAC baffles look?-img_20190722_190658_741.jpg   How’s these HVAC baffles look?-img_20190722_191330_281.jpg  
Old 7th August 2019
  #25
Op here... read everything word for word up till now and while I appreciate the comments, Im still a bit unsure on what material to line these baffles with. I bought both the acoustical absorption panels as well as the recommended duct liner (masterflow duct wrap is what I picked up) and have chosen to go with the duct liner... unless anyone has another suggestion. Thoughts?

Also, a friend of mine is a contractor here in Manhattan and his company has built a dozen or so studios and says he uses these in-line 110v 200cfm fans: one to push the air through the baffles and one to pull it out the other end (based on 6” duct width) to compensate for air velocity loss from the handler. He said he can hold them in his hand running and put them to his ear and they’re dead silent. Might be a bit unorthodox but his logic is sound and his results are impressive. My air flow is already pretty low coming down from two stories above so I think this could be a good idea. Anyone recommend this addition to my plan? He said these will work perfect with the in-line fans. Thoughts?

Thanks in advance
Attached Thumbnails
How’s these HVAC baffles look?-image_8505_0.jpg   How’s these HVAC baffles look?-image_6394_0.jpg  
Old 7th August 2019
  #26
Lives for gear
 

The fans shouldn't be a problem in terms of noise, since they will be on the "far" side of the silencers, with regard to the room. The silencer boxes are not really there for fan noise anyway: they are there for "studio" noise! To stop your insanely loud sounds from getting out to annoy the neighbors, and to stop the outside sounds from getting into your mics. If the boxes do a good job in that area, they'll be spectacular for fan noise.

Also, the noise of the fan motor itself and the the noise of the passing blades isn't really the air noise problem you should be worried about: turbulent air flow is a bigger issue. If the flow is close to laminar throughout the entire system, then there won't be any turbulence, and thus very little air noise (assuming you calculated your flow rates and flow velocities correctly, when you designed the system...). Of course, in the real world, there will always be turbulence at many points along he way, and true laminar flow is just a dream... but as long as you stick to the guidelines for minimizing turbulence, and keep the velocity as low as possible while still maintaining the rate, you should be OK.

- Stuart -
Old 7th August 2019
  #27
Lives for gear
As far as I know duct wrap goes on the outside of the duct. Duct liner is different and goes on the inside.

I found some made of ultratouch type insulation that is faced with a a material that is sound transparent, but contains the fibers. It made me wonder,. Why not just use ultratouch or fiberglass and cover it with acoustic fabric?

Curious if anyone has any thoughts on that...
Old 8th August 2019
  #28
Here’s the duct liner I ended up using. I’m going to put a 200 CFM in-line fan to push the air through since the velocity is a bit low. According to my calculation that should fill these rooms and give proper iso. Any other thoughts before I put these boxes in the wall ? Thanks again
Attached Thumbnails
How’s these HVAC baffles look?-image_6340_0.jpg  
Old 8th August 2019
  #29
Lives for gear
 

That's not duct liner...
Old 9th August 2019
  #30
Lives for gear
Duct liner is generally faced with something different. You can also tell when you found it because it's pretty expensive-

https://www.grainger.com/product/1VD...g!310977719892!

This is the one that I found. It's still pretty pricey, I got 700sf (200 of 1" and 500 of 2") for about the same as that one from Grainger but including freight.

https://www.acousticalsurfaces.com/q...uiet_liner.htm

UT has a high GFR, which should be a good thing in this application.

I do wonder about regular ultratouch, or safe'n'sound etc and just facing it with fire rated fabric like this-

https://www.acoustimac.com/fr-dmd-acoustic-fabric/
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