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 Post subject: Baffle box construction
PostPosted: Sun Feb 10, 2019 4:32 am 
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Location: London, UK
I've been looking through lots of HVAC baffle builds and most appear to be boxes with acoustic foam.

However, I just watched a YouTube video of a studio build and the guy snaked an insulated acoustic flexible ducting through a baffle box, and then stuffed fluffy insulation in the gaps.

https://youtu.be/_aONYq_8vnQ

(see around 15m 15sec)

What do you think of this approach compared to having acoustic foam on all inside surfaces, as I haven't seen any others where the ducting continues through the box.


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PostPosted: Sun Feb 10, 2019 7:42 am 
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What do you think of this approach compared to having acoustic foam on all inside surfaces, as I haven't seen any others where the ducting continues through the box.

Youtube is filled with hilariously incorrect studio builds.

First off, silencer boxes work off of a few principles that his completely ignore. Here are some points:
- low end is attenuated through the gross impedance changes achieved by immediate cross sectional changes. The larger the change, the better. His design uses a conical transition which allows for an impedance match and eliminates this very important characteristic of a good silencer. Look at the muffler on your car, do you see it gradually getting larger? Nope.
- Next, he used insulated, not lined flex duct. If he had used the correct stuff (lined), here are some lab results:
Attachment:
Lined Flexible Duct Insertion Loss Chart.png

Flex duct is famous for introducing gross amounts of static pressure and ASHRAE recommends no longer than 14' in a system. So, if this guy actually uses the correct number of silencers in his studio, he is guaranteed to go over that. To achieve any gross impedance change, he'd need very large flex duct, and as you can see in the chart, the bigger that duct, the less insertion loss is achieved.
- Proper duct liner should be used in silencer boxes. I highly recommend Owens Corning QuietR Rotary. This guy used fibreglass insulation that is way too light to be good for anything acoustically speaking. The fact that he didn't know to use the correct insulation alone shows that he doesn't really know what he's doing.
- His boxes are made out of material that doesn't have enough surface density. The surface density of this box must match his walls.
- He is using flex duct to penetrate the walls. That right there will trash his isolation as the sound will blow right through it. The penetrating material (we call it sleeves), must match the surface density of the wall/box.
- The sharp corners in lined ducts that are greater than 60 degrees are great at attenuating higher frequencies. This guy used round duct. On the forum, we suggest rectangular. Here's why:
Attachment:
Insertion Loss of Square Elbows without Turning Vanes.png

Attachment:
Insertion Loss of Unlined and Lined Square Elbows with Turning Vanes.png

Attachment:
Insertion Loss of Round Elbows.png


Lastly, watching the end of the video after the guy hoisted up his silencer box (obviously very heavy) with a pulley that was attached to a friggin box handle!, I noticed that his outer leaf has no sheathing whatsoever. In other words, this guy is a joke.

Greg


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PostPosted: Sun Feb 10, 2019 7:51 am 
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I watched part of his studio build video number 1 from just a few months ago and he "talked to some sound proofing companies that told him his 8 foot ceiling would be very bad for something I can't remember" -- then the video had text added over it that said "room modes".

This shows how much this fella knows. I wish he would have joined this forum and built his studio correctly and THEN posted his videos on YouTube. Instead, he is labelling his videos as "Building A Professional Recording Studio" and who knows how many people are going to follow his instructions. It's sad really.

Greg

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PostPosted: Mon Feb 11, 2019 2:40 am 
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Great reply, thanks Greg.

There seems to be some difficulty in getting anything remotely like Owens Corning QuietR in the UK. In fact, there's several threads scattered around various forums with UK guys struggling over this issue.

Your recommendation appears to be quite cheap in the US. I found only one product in the UK and its incredibly expensive.

Does anyone have any guidance as to a suitable UK product?


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PostPosted: Mon Feb 11, 2019 9:30 am 
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Don't go to big box stores as they won't have it. Find a commercial HVAC supply distributor. They'll have it. Once you find a place, be sure to post where the place is and the product you end up using so other people in your area don't have to go through a grueling duct liner hunt too!

Hint: If you can't easily find the distributor, call some small local HVAC companies and ask them as they buy their supplies from these commercial distributors :wink:

Greg

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PostPosted: Mon Feb 11, 2019 2:16 pm 
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Youtube is filled with hilariously incorrect studio builds.
:thu: Oh yeah! Some of them built dangerously, and a few probably illegal, too.

- Stuart -

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PostPosted: Mon Feb 11, 2019 7:22 pm 
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What do you guys think of this?

https://www.ductstore.co.uk/acatalog/Acoustic-foam.html
https://www.ductstore.co.uk/acatalog/info-AOF6.html


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PostPosted: Tue Feb 12, 2019 2:43 am 
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What you're looking for is something that is designed to safely be inside of ducts. The product you linked seems to be appropriate in that respect. However, there is no acoustic performance data posted. And for our silencer box needs, the product has to have great absorption coefficients. Here is the product I recommend people use in Canada -- Owens Corning QuietR Rotary Duct Liner. Here is some info I screen shot off of it's data sheet:
Attachment:
QuietR Absorption Coefficients.png

Greg


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PostPosted: Wed Feb 13, 2019 11:17 pm 
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After hours of searching, this seems reasonably priced. I really appreciate casting your expert eye over this product and see if its performance data makes it suitable? Thanks so much guys.

http://www.advancedacoustics-uk.com/Stu ... Foam-Panel

Attachment:
CO1-small.jpg


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PostPosted: Thu Feb 14, 2019 12:22 am 
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I also found a similar Class 0 acoustic foam which is suitable for ducting and asked for the performance data.

Attachment:
Screenshot_20190213-141534-576x1024.png


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PostPosted: Thu Feb 14, 2019 7:45 am 
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After hours of searching, this seems reasonably priced. I really appreciate casting your expert eye over this product and see if its performance data makes it suitable? Thanks so much guys.

That SRL product seems pretty good!

Quote:
I also found a similar Class 0 acoustic foam which is suitable for ducting and asked for the performance data.

That AcoustaFoam product is horrible.

Greg

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