Sono Flex Duct: Snake Oil or the Real Deal?

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Sono Flex Duct: Snake Oil or the Real Deal?

#1

Postby SoWhat » Tue, 2020-Jul-28, 09:25

Greetings all,

I ran across a product called Sono Flex duct. Is this a viable product for studio builds, if the general warnings about flex duct are adhered to (avoiding sharp turns, etc)?

It seems that the biggest benefit would be smaller silencers, but I'm just guessing, of course.

Looking forward to finding out more.

All the best,

Paul



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#2

Postby Soundman2020 » Tue, 2020-Jul-28, 12:12

It's a typical, normal insulated flex duct, so from that point of view it will do the job of moving air from point "A" to point "B". That said, the "acoustic properties" tables are somewhat curious! There's no indication regarding HOW they made those measurements, but I'm finding it a little hard to swallow that an inch of pink fluffy with 12 micron thick metalized plastic layer, can produce 33 dB TL at 125 Hz, along a piece just 10 feet long (3m) and a 6" (16mm) in diameter. Apparently, they did not use any of the ASTM test methods for that... and they did not cite WHICH test method they used. That is suspicious.

Is this meant to be an insertion loss number? I VERY much doubt that! Way too high to be believable. Is it meant to be TL through the walls (from inside the duct to outside? Maybe, but still very high, and it does not make sense because the number varies by length of the duct, which implies it is insertion loss?? :shock: :roll: .

As a point of comparison, K-Flex 1" gray Duct Liner, which is pretty good stuff, claims 1 dB insertion loss at 125 Hz, and 12 dB insertion loss through the material.... far more believable. They also state the coefficient of absorption at 125 Hz is 0.06. Also very believable. OK, so this is not the same product, so I'm comparing apples to oranges here, but it is indicative of typical lined duct.

Thought experiment: if that Sonoflex number is supposed to be insertion loss, and you played bass heavy music from a full-range speaker at 80 dBC into one end of a straight piece of that duct just 3m long, then the sound level at the other end would be just 47 dBC... which is very quiet..... Hmmmm....

So, the numbers should be taken with a pinch of salt, since they are not explained and the test method is not identified, but the product itself is fine. In reality, it probably performs pretty much the same as other similar flex-duct products.

- Stuart -



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Sono Flex Duct: Snake Oil or the Real Deal?

#3

Postby SoWhat » Tue, 2020-Jul-28, 12:49

Greetings Stuart,

Thanks for the quick reply.

"If it seems too good to be true..."

As I noted in my post, if it worked as advertised, we'd all be making much smaller silencers.

Thought experiment: if that Sonoflex number is supposed to be insertion loss, and you played bass heavy music from a full-range speaker at 80 dBC into one end of a straight piece of that duct just 3m long, then the sound level at the other end would be just 47 dBC... which is very quiet.....


I'd love to try that very thing. Spinal Tap's "Big Bottom" would be a perfect track for the test (Three basses and drums). I'm being perfectly serious here.

All the best,

Paul



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Sono Flex Duct: Snake Oil or the Real Deal?

#4

Postby Soundman2020 » Tue, 2020-Jul-28, 13:00

SoWhat wrote:Source of the post I'd love to try that very thing. Spinal Tap's "Big Bottom" would be a perfect track for the test (Three basses and drums). I'm being perfectly serious here.
You could try it, for sure! To test it, you'd need to pass that duct through a wall between two rooms where there is already more than 33 dB TL... Something like 40 dB would be OK. Thus, something more substantial than a normal house wall.

- Stuart -



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Sono Flex Duct: Snake Oil or the Real Deal?

#5

Postby psb_87 » Mon, 2021-May-31, 04:17

I've spoken to Philip Newell (as well as various other acousticians) about this and yes, Stuart is right, acoustic flex duct in of itself is not good at all at attenuating LF, though it is pretty decent for MF and HF attenuation. But the thing is, (and this is a MASSIVE oversimplification) not much LF energy can actually get through a relatively small opening, such as a duct. This is due to a large impedance mismatch - the large body of air of the room and the small hole of the duct. As the LF wave hits the hole the LF struggles to squeeze through it and so much energy is lost, in fact, a portion of it is reflected back into the room, and any lf the other side of the hole will be greatly attenuated. If an acoustic flex duct is connected to the hole then the LF transmission is basically negligible. And any LF that does make it through the duct will then be reflected back in to the duct as it reaches the end - this is end reflection loss.

A way to visualise it is what happens with ear buds - when the ear buds are coupled to your ear canal the small driver becomes very efficient since it is roughly the same size as your ear canal and more or less air sealed to it. This means you can hear full low frequency glorious bass - but once you remove them, it completely disappears - in fact, assuming they are turned up quite loud, only the very HF remain audible. A similar phenomena happens with LF in a room with a hole in it, and the attenuation improves with distance.

Now, that's not to say that you shouldn't worry about holes, or air tightness - you still want to have a completely air tight enclosure and seal and gaps and crack, make sure the doors are sealed properly etc as HF and MF will leak through more easily than LF. And also, if you have many holes or many gaps, especially in close proximity, then LF "sees" the sum of them. But a couple of holes (fresh air in/out, for example) coupled to acoustic flex duct which has a suitable length will usually be all that is needed for attenuating sound in/out of a room. Where very high isolation is required then your best bet is to make silencer boxes. But, you may be surprised, or even shocked to know that many professional studios do not use silencer boxes for their HVAC, and simply use well positioned and properly installed acoustic flex duct, sometimes with an inline silencer to attenuate fan noise, and that's it.

I'm sure there will be those that will disagree with me, that's fine. I hope it has been useful to the rest of you

Paul



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Sono Flex Duct: Snake Oil or the Real Deal?

#6

Postby bert stoltenborg » Mon, 2021-May-31, 15:38

Dear Paul, you are not very well informed or educated on this. Stuart seems to be lost so I think people interested in these matters should seek an other forum.



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Sono Flex Duct: Snake Oil or the Real Deal?

#7

Postby psb_87 » Tue, 2021-Jun-01, 06:10

bert stoltenborg wrote:Source of the post Dear Paul, you are not very well informed or educated on this. Stuart seems to be lost so I think people interested in these matters should seek an other forum.


Dear Bert, since you are so well informed and educated on this, why don't you inform and educate us?



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#8

Postby Soundman2020 » Sat, 2021-Jun-26, 20:46

psb_87 wrote:But the thing is, (and this is a MASSIVE oversimplification) not much LF energy can actually get through a relatively small opening, such as a duct. This is due to a large impedance mismatch - the large body of air of the room and the small hole of the duct.
That's a common misconception, actually. Yes, there is an impedance mismatch, and therefore there is some loss, but the effect is nowhere near as big as some people imagine. It's only a few dB at best. Also, it is not related to the volume of air in the duct and the volume of air in the room, but rather to the cross-sectional area of the duct vs. the effective cross-sectional area of the room surface where the duct penetrates.

Here's a graph from Engineering Acoustics that shows the real effect:
HVAC-Insertion-loss-from-cross-section-change--Engineering-Acoustics.png
So for example, if you have a 10" duct going through a wall that is 8 feet high and 8 feet long, that means you have a bit more than 2 square feet going into 64 square feet, which is a ratio of 32 to 1. According to the graph, that would give you about 9 dB insertion loss.... interesting, but not exactly spectacular! Assuming that your duct is coming from the tracking room, where some gorilla band is thrashing the drums, bass, keyboards, and screaming electric guitar at 120 dB, and that you were getting mathematically perfect attenuation on BOTH ends of the duct, there would be an 18 dB drop (9 dB on each end). So George the Gorilla Group would still be smashing out 102 dB on the other send of the duct...

If you are interested in the actual equation, it goes like this:

dL = 10 log( ( 1 + A1 / A2 )2 / 4 (A1 / A2) )

where

dL = noise attenuation (dB)

A1 = inlet area (m2)

A2 = outlet area (m2)

There's another massive caveat to the above graph and equation: it is the best-case scenario! And it assumes a sudden, sharp change in cross-sectional area, with the duct hole in the exact center of the wall, both vertically and horizontally. Of course in real-life rooms, the duct is never in the middle of the wall, so you can only calculate the area to the closest intersecting surface (ceiling/floor/other wall), and the transition is never sharp and sudden: it pretty much always goes through a register of some sort. So the actual insertion loss is considerably less.

For a real-world scenario, assume that the duct comes in one foot below the ceiling and one foot away from an intersecting wall, so you can only consider a cross-section of about 10 square feet wall area vs. 2 square feet for the duct, so the actual ratio is about 5:1.... which would give you a 3 dB drop (being very generous!). Add a register into that, and even if it has really low-loss vanes and 70% open area, your actual losses are down to just 1 or 2 dB.... with luck.

This is why silencer boxes are designed with at least two sudden changes in cross section of at least 200% to 400% or so, as part of the many, many tricks that go on inside, and that gets you a couple of extra dB insertion loss. That's all. In a well-designed silencer box, there's also things like right-angle turns, sudden changes in airflow, tuning, baffles, porous absorption, mass, and a few other nifty bits. All of that, in total, can get you very decent insertion loss. A pair of boxes (one on each leaf, coupled with flex duct), can indeed get you a total TL greater than the TL of the MSM wall itself: commonly of the order of 50 dB, or more where needed.

If an acoustic flex duct is connected to the hole then the LF transmission is basically negligible.
Well, no, I would not agree with that. Just listening to such a duct coming through the wall of a drum booth will probably convince you that the losses are considerably lower than you imagine, and plenty of sound still gets through.

and the attenuation improves with distance.
Yes, at the normal rate of 3 dB loss for ever distance doubling, assuming we are talking about open air (not restricted by boundaries, such as walls and a ceiling).
Now, that's not to say that you shouldn't worry about holes, or air tightness - you still want to have a completely air tight enclosure and seal and gaps and crack, make sure the doors are sealed properly etc as HF and MF will leak through more easily than LF.
You seem to be contradicting yourself here: On the one hand you are saying that when sound goes through a small opening into a large volume of air, there are huge losses due to impedance mismatch, but then you are saying that when sound goes through the small gap under a door, there are NOT any large losses due to impedance mismatch, and the sound will still propagate into the room. This is confusing: Which side of the argument are you taking?

You are correct that small gaps can transmit a lot of sound. Here's a graph that illustrates that effect:
loss-through-tiny-cracks-and-reduction-effect-of-small-gaps-on-TL.jpg
Unfortunately the quality isn't very clear, but I can't find the original right now to re-scan it. The X axis is the theoretical TL through a wall with no gaps, and the Y axis shows the actual TL. The curves show the degradation for various "open area" percentages. The top curve is for 0.01% open area, which works out to a hole the size of a pencil tip in a wall 8 feet high and 10 feet long. So, with a tiny hole that size, the isolation of a wall designed for 50 dB TL would actually be only 40 dB. That's a major loss in isolation. If the gap were 0.03%, the wall would only isolate at about 32 dB. Which is similar to what an ordinary house wall would get. a 0.03% open area gap would be something like a 1/16" gap under a door.... So you are right about small gaps allowing through a lot of sound, but that does seem to contradict your claim that sound going through small gaps into large rooms is greatly attenuated...

And also, if you have many holes or many gaps, especially in close proximity, then LF "sees" the sum of them.
Actually, that isn't true either. You should probably look into perforated panel theory to get a better idea of how that really works. It's a lot more complex than I can go into here in a few paragraphs, but the way that a perf panel attenuates sound depends on many factors, including the sizes of the holes, the thickness of the panel, the shape of the holes, the pattern of the holes, the open area percentage of the holes vs. the panel size, and a few other things. Its not simple at all. The overall effect is something similar to a comb filter, usually. There's an interesting paper on that, by Jaouen and Bécot, titled "Acoustical characterization of perforated facings", where they propose a method for predicting the results, under some limited circumstances. There's an even better paper titled "FURTHER INVESTIGATIONS OF PARTITIONS HAVING BUILT-IN ACOUSTIC TREATMENT" from the BBC, from back in 1995 IIRC, that has both theory and experiment in it, with some rather interesting results. There's also the more recent paper by Kingan and Pearse, titled "SOUND ABSORPTION OF POROUS MATERIAL IN COMBINATION WITH PERFORATED FACINGS", but that's more about acoustic treatment than TL in walls. Still, there's some interesting stuff in there. They examined the effect of the hole size, hole pattern and open area ratio, and how they interact. Their conclusions also apply to perforated walls, to a certain extent.

But a couple of holes (fresh air in/out, for example) coupled to acoustic flex duct which has a suitable length will usually be all that is needed for attenuating sound in/out of a room.
Maybe you could provide some theory and equations to support your claims? For example: how long does the duct have to be to reduce the sound of drums by, say, 50 dB? And what role does diameter play? Does it make a difference if the flexduct is made with Mylar, foil, or something else? Does the type and thickness of the insulation have an effect? Does it matter if the flexduct is completely stretched out to the full length, vs. being only partially stretched? There's a lot of variables to consider here, so I'm hoping that your equations take all of those into account. I'd be very interested to know what combination of factors I would need in order to get 50 dB TL through a tracking room wall, which is a typical example. Maybe you could show how to calculate that?

But, you may be surprised, or even shocked to know that many professional studios do not use silencer boxes for their HVAC, and simply use well positioned and properly installed acoustic flex duct.
Could you provide a list of those studios, with photos of their actual duct installations, so we can see how they did it? And of course, actual test results, showing the real insertion loss they obtained in each case. That would be interesting, and very useful for us studio designers. It seems like we could greatly simplify HVAC systems, if only we had the data that you seem to have. I'm sure our clients would be pleased with the reduction in costs, too. Just poking a piece of flex duct through the wall is an awful lot cheaper than building silencer boxes! I'm intrigued...

- Stuart -



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#9

Postby Starlight » Sun, 2021-Jun-27, 01:37

Thomas Jouanjean of Northward Acoustics uses Sonoflex. Thomas is never one to give away his methods but there are a few clues in his posts in the gearspace topic How’s these HVAC baffles look?



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#10

Postby psb_87 » Sun, 2021-Jul-11, 20:07

Stuart,

It is not simply a case of putting a hole in the wall and shoving some flex duct through it, the location of the duct, how it terminates in the room, how it passes through the assembly are also part of it.

In regards to sound escaping through the cracks of doors etc i did not contradict myself - you generalised my answer to make it seem that way. I said that HF and MF will escape through small gaps and holes, but LF not so much - not ALL sound. But the flex duct is actually pretty good at absorbing HF and MF.

My reply was a paraphrase of my communications with Philip Newell, if you like I can paste his exact words? But with all due respect he has just a pinch more experience than you and has built hundreds of successful studios all over the world. Eric Desart also used the same method for hvac. John Brandt also. And as Starlight pointed out, Northward studios also do it like this. In fact, in the link Starlight linked to, Thomas describes in much better detail than I ever could about the why and how it works with formulas and even provides pictures. There are pictures all over gearslutz of his builds using this method - no silencer boxes in sight. I noticed that you even participated in the same thread but had nothing to say in response to Thomas when he described this exact same method?



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#11

Postby Soundman2020 » Tue, 2021-Jul-13, 23:05

It is not simply a case of putting a hole in the wall and shoving some flex duct through it, the location of the duct, how it terminates in the room, how it passes through the assembly are also part of it
If you read my post carefully, you would notice that I did, in fact, discuss that, in order to point out the errors in your own post. Why did you not also mention those rather important points in your original post?
What you actually said was: "This is due to a large impedance mismatch - the large body of air of the room and the small hole of the duct", but with no qualifiers. That sort of implies that you were, indeed, talking about the "mismatch" being due to "the small hole of the duct" shoved through the wall into the "large body of air in the room", with no mention of duct location or termination. You should probably also address your error in suggesting that impedance mismatch is related to the "large body of air in the room", when this is simply not true: Rather, it is related to the difference in cross sectional area. The volume of air in the room is irrelevant: the impedance mismatch would be the same regardless of if the volume of he air body in the room were 100 m3 or 10,000 m3, all other factors being equal. It's the ratio of cross sectional areas that matter, not the ratio of air volumes, as you suggested.

psb_87 wrote:Source of the post Dear Bert, since you are so well informed and educated on this, why don't you inform and educate us?
You might want to check into Bert's background and experience. He is, in fact is very well informed, and very well educated in the field of acoustics. He's rather well regarded in acoustic circles, too... Somewhat different from Phillip Newell... My opinion of Mr. Newell is rather well known, and shared by many other acousticians and studio designers (including Bert, if I recall correctly). I have mentioned before that Mr. Newell's famous book takes pride of place in my studio.... propping up an old keyboard that I sometimes use, since it happens to be just the right thickness to raise the keyboard to a comfortable height. That's about all it is good for... A rather expensive keyboard stand, in terms of what I paid for it, but the actual value is reflected in its current use. (I did try using it as a doorstop, but I kept tripping over it.)
Eric Desert is very well respected around here, and his death a couple of years ago was a sad loss to the world of acoustics and studio design. I do miss the conversations we had, and wish he were still around. I'm sure Bert does too, since he and Eric were close friends.
Now, since you are calling into question our experience with studio design and construction, and our knowledge of acoustics, I should probably return the favor and ask you to present a summary of your own experience in this area. Perhaps you could point to a list of studios you have designed and built?

- Stuart -



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#12

Postby psb_87 » Wed, 2021-Jul-14, 05:18

Stuart,

I think there has been a big misinterpretation here. I am only interested in learning and furthering my knowledge, and if i can - helping others as well. I am not claiming to be on the same level as yourself, bert or eric etc. I have no hostility towards you or your excellent forum, I'll be the first to admit that I am not as experienced at all with these things compared with other designers such as yourself. That is why I am trying to get a better understanding of how this works.

The advice I gave regarding the acoustic flex duct was based on that of other, much more experienced designers than myself. Due to my lack of experience and understanding I undoubtedly do not explain things well, and I am fine with being corrected, I explained in my own words, based on my understanding of how it worked but I fully accept that my interpretation may not be quite right. But, I am not the source of this information I am simply regurgitating and it may come out messy and wrong. That is where guys such as yourself can step in to put it right - but if I had not stepped in to this topic then people would still be under the impression that flex duct, without silencer boxes, is not a perfectly acceptable option.

I am also not claiming to know myself how to properly implement the use of flex duct as a replacement for baffle boxes, I just know that it is possible and that it relies on a certain procedure in order to not trash isolation. This is my quest, to find out the steps involved in the installation process to guarantee success. I have found some clues along the way, but need more details.

So thank you for correcting me on my confusion regarding cross sectional area and room volume, as well as all your other points. That is incredibly useful, and now I know.

I also agree with what you said here:
"Could you provide a list of those studios, with photos of their actual duct installations, so we can see how they did it? And of course, actual test results, showing the real insertion loss they obtained in each case. That would be interesting, and very useful for us studio designers. It seems like we could greatly simplify HVAC systems, if only we had the data that you seem to have. I'm sure our clients would be pleased with the reduction in costs, too. Just poking a piece of flex duct through the wall is an awful lot cheaper than building silencer boxes! I'm intrigued..."

THAT is the exact reason why this is so important. The fact remains, very well respected designers use it (Newell, Northward, Desart etc) and it is their primary solution that they use in their designs, it is not a compromised solution for special cases.

And yes, it greatly simplifies the problem of providing ventilation in our air tight highly isolated studios. - That is what is so great about this.

Yes I am very aware of Bert's background and experience, I have enjoyed his input on many threads and forums over the years, and my reply to his post was not sarcasm, I was hoping he would step in and educate us on procedure of using flex duct effectively. Here is a thread on this very forum where he mentions it without really explaining how it works, but just that it is a better way:

viewtopic.php?f=5&t=848&p=3066#p3066

Here is one of his quotes from that thread, referring to baffle boxes vs flex duct:

"I don't say they don't work at all. (baffle boxes)
All I say is that flex duct if dimensioned properly works as well and is even better for small spaces.
In a Gearslutz thread on baffle boxes this was brougth up by a Belgian studio owner who was adviced by Desart, and it was confirmed by Northwards. Stuart was surprised. :shock: :lol: . I see if I can retrieve it and place a link.
*edit: https://www.gearslutz.com/board/studio- ... -look.html"

And another one:
"What other info do you want? A (eventually) soffited Sonodec needs to fullfill the same conditions as a silencer box (without the irrelevant bends). That's all that is to it.
Make your silly baffle boxes if you want but don't confuse other people :-)."

So, perhaps if you don't mind you can explain in detail the correct way of using flexduct successfully in order to provide adequate ventilation without trashing isolation? I would love that and I'm sure many others would as well. It is okay if you do not know how, we can find ways to find out together, but do not tell us that it doesn't work when it clearly does.



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#13

Postby gullfo » Wed, 2021-Jul-14, 10:48

the Galaxy studio in BE - had silencers the size of cars on the shell of the big room, and most of the others were the size of refrigerators (the ultimate goal according to Eric was to get the speed down to 1m/sec). you're not getting sufficient room exchange in large rooms without big ducts @ low flow velocities.

in the case of flex ducts entering (or leaving a room) it's important to note several factors: duct velocity, duct generated noise, machine noise carried in ducts, opening size (also important in the flow velocity equation), and the termination in the room (typically a large damped plenum for directing the air through the room - usually involving a significant size change (impedance changes), and indirect path from the wall penetration).

so you can use this approach as well as silencers - it depends on the environment you're working in, if you have a lot of potential noise in the air space between the mass layers, you may need a baffle box or commercial silencer to ensure you're reaching the attenuation expected. and you should probably always use the damped plenum approach to distribute the air as well as leverage the indirect path and velocity shifts for ensuring quiet in the room.

as an example for Red Bull NYC - we used commercial duct silencers just before the plenum which was about 8x the duct volume (the plenum was about 10' wide 15' long x 2' high) going into the control room over the 112 ch SSL console. the silencers were used to suppress the machine noises, then about 6' of flex duct through the walls (using 4 16" flex ducts).



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#14

Postby endorka » Wed, 2021-Jul-14, 11:37

Cheers Glenn. It seems Rod Gervais has similar experiences, he wrote this on another forum;

Rod Gervais wrote:If noise is not an issue - and you can size the ductwork so that you get the right airflow directly - then there would be no reason to use a baffle.......

I have worked on projects where this was the case...... for example - Power Station New England (as well as the old Power Station studios in NYC) doesn't/don't have baffles for the ducting of the HVAC systems.

It simply wasn't required......

We have huge spaces outside of the rooms there (meaning above them) that are extremely well isolated from the outside world - and that (also) due to the volume of the space (above the studio) and the points where duct enters the spaces below, we didn't have issues with crosstalk between the rooms.

As such we sized the ductwork for the flows we wanted and didn't use baffles.

This is (of course) generally not the case for smaller studios - or even for larger studios where we don't have the option of constructing a fully isolated outer shell to house everything within.



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#15

Postby psb_87 » Wed, 2021-Jul-14, 12:19

gullfo wrote:Source of the post the Galaxy studio in BE - had silencers the size of cars on the shell of the big room, and most of the others were the size of refrigerators (the ultimate goal according to Eric was to get the speed down to 1m/sec). you're not getting sufficient room exchange in large rooms without big ducts @ low flow velocities.

in the case of flex ducts entering (or leaving a room) it's important to note several factors: duct velocity, duct generated noise, machine noise carried in ducts, opening size (also important in the flow velocity equation), and the termination in the room (typically a large damped plenum for directing the air through the room - usually involving a significant size change (impedance changes), and indirect path from the wall penetration).

so you can use this approach as well as silencers - it depends on the environment you're working in, if you have a lot of potential noise in the air space between the mass layers, you may need a baffle box or commercial silencer to ensure you're reaching the attenuation expected. and you should probably always use the damped plenum approach to distribute the air as well as leverage the indirect path and velocity shifts for ensuring quiet in the room.

as an example for Red Bull NYC - we used commercial duct silencers just before the plenum which was about 8x the duct volume (the plenum was about 10' wide 15' long x 2' high) going into the control room over the 112 ch SSL console. the silencers were used to suppress the machine noises, then about 6' of flex duct through the walls (using 4 16" flex ducts).


Hey Glenn,

Nice to see you here. Thanks so much for this. Were those large silencers you mentioned at Galaxy baffled?

This quote would seem to suggest not:
"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)."

Do you have either a drawing, image or description of these plenums you mentioned? "typically a large damped plenum for directing the air through the room - usually involving a significant size change (impedance changes), and indirect path from the wall penetration). "

I am finding it difficult to picture exactly what you mean here but would love to know more; Duct sizing, CFM, fan noise etc aspects you can leave out, but I am interested in how to maintain the isolation of the inner and outer shells themselves in regards to sound from the control room, bands in the tracking room and the outside world:

1. how the flex duct penetrates the isolation shell of a double leaf system - how is it sealed to the wall(s)? is there an elbow(s) in the duct?
2. what are the plenums constructed of? Same surface density as the shell? are they connected to the shell in the same way that baffle boxes are? Does it cover the entire length of duct inside the room? What does it look like etc
3. Why do I consistently not see any such plenums and the flex duct is simply buried in fibre treatment instead? (Seems like an easier option)

Any help is gratefully received




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