Baffle box / Math question

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chdoom
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Baffle box / Math question

#1

Postby chdoom » Mon, 2020-Jul-13, 14:07

Hi everyone,

I would like to know if I get this air velocity things right.

My vocal booth will be 8x10x7 = 560cubic feet.

I multiply this 560 * 6 (6 change per hour) = 3 360 cubic feet an hour

3 360 cubic feet / 60 = 56 CFM

0.40 * 56 CFM = 22.4 CFM Fresh Air

22.4/200 FPM = 0.112 Square feet = 16.128 square inches = 3*6 register would do right?


There is 2 things I dont get...

1- With that info how do I determine my baffle box size?

2- I'm cooling/heating my house with a central unit...So the air from my house circulate from room to room / floor to floor . Do I need to add ERV/HRV Unit ?

Any help would be greatly appreciated :)
Dom



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Soundman2020
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Baffle box / Math question

#2

Postby Soundman2020 » Tue, 2020-Jul-14, 14:35

I'm assuming you are going with at ductless mini-split unit on the wall of the booth? If so, then your math looks good so far. 22.4 FPM is the "make-up" fresh air that you are sucking on from outside, so that's what would be coming in through the register just above the mini-split. (If I got that wrong and you are using a ducted mini-split, then things are different....)

So, what you have now is the size that the REGISTERS need to be: the grill things that let air into the room, and take it back out again. That's the purpose of that part of the math: to figure out the register size, such that the air velocity will be low enough. The reason being: if the air is moving too fast through the register, it creates noise! Keep it slow, and it will be silent.

The next step is to figure out duct size. The ducts can be smaller than the register size: there's no need to restrict the air flow velocity in the ducts to be as slow as through the register. It an be two of three time faster, if you want. So the cross-sectional area of the duct can be 1/2 to 1/3 of the area of the register. Assuming you want round duct, divide the area by pi to get the diameter. So, 17 in2 / pi = 5.4. To get the SAME velocity as the register, the duct would need to be 5.4" diameter. But the duct can be smaller (see above), so a 4" duct would be fine. I wouldn't go smaller than that.

Next step: The silencer box should have at least one sudden transition in cross section, and the difference should be as big as you can make it, but at least twice the area (or half the area). Assuming 4" round duct, the cross-sectional area of that is about 12 in2, so you'd want to have an internal cross-section of at least 24 in2, so 6x4" minimum, but 36in2 would be better, so 6x6": However, perfectly square sections are not such a good idea, so go up another step: make it 5x7, or even 6x7. Don't forget to allow for the fact that the interior surface of your silencer box should be covered with 1" thickness of proper duct liner, which means the internal size of the box itself it needs to be 2" bigger: thus, if you were planning on 6x7 open area, you would need 8x9 internal box size.

2- I'm cooling/heating my house with a central unit...So the air from my house circulate from room to room / floor to floor . Do I need to add ERV/HRV Unit ?
Ahhh! So you won't have any mini-split in the room itself? You will just be bringing in a branch duct from the main HVAC duct for the house, and taking out another branch duct to the main return duct? Great! That makes your job easier.

Do I need to add ERV/HRV Unit ?
No. If you did need one, that would be for the entire house, not just for your room. The house HVAC system would have the HRV on it, if there was a need for one. Your booth just takes a feed from the main house system, and returns the stale air into the main house system as well. You are relying on the main house system to do all of the conditioning for you, so you don't need to do anything additional. Worst case: you might need a small fan in one of the ducts, but even that probably isn't necessary. As long as the house system has enough capacity to handle the extra ducts and silencers, you wouldn't need that.

- Stuart -



chdoom
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Baffle box / Math question

#3

Postby chdoom » Wed, 2020-Jul-15, 17:54

Hi Stuart!

Thank you very much for your answer. No I don't have mini split in my room. So it clarify one thing :)

When you are saying that I need 8x9 internal box size are you refering to the complete box or only the green section in this picture?
[img]
Baffle_Plan_Green.PNG
[/img]

Thank you very much for your help.

Dom
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Baffle_Plan_Green.PNG
Baffle_Plan



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Soundman2020
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Baffle box / Math question

#4

Postby Soundman2020 » Wed, 2020-Jul-15, 19:11

chdoom wrote:Source of the post When you are saying that I need 8x9 internal box size are you refering to the complete box or only the green section in this picture?
Sorry, I should have clarified that better: I'm just talking about the cross-section of the air path through the box. The view you posted is the top view, or "plan view" of the silencer, seen from above: I'm talking about the "elevation view", seen from the front or back of the box. In other words, that image only shows the X and Y directions, but the Z directions is not seen there, and that's what I'm talking about.

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Baffle box / Math question

#5

Postby chdoom » Wed, 2020-Jul-15, 21:54

Great! Thank you very much Stuart! So my inlet will be let say 4" to connect my 4" round duct, but will fall into an 8x9 air path right?

Dom



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Baffle box / Math question

#6

Postby Soundman2020 » Sat, 2020-Jul-18, 22:53

chdoom wrote:Source of the post So my inlet will be let say 4" to connect my 4" round duct, but will fall into an 8x9 air path right?

Right! Which is an increase from about 13 in2 to about 56 in2. That's an increase of about 430%. :thu:

- Stuart -



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Baffle box / Math question

#7

Postby chdoom » Sun, 2020-Jul-19, 12:26

Great!

Thanks Stuart

Dom



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fossaree
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Baffle box / Math question

#8

Postby fossaree » Mon, 2023-Mar-20, 19:07

Hi
Soundman2020 wrote:Source of the post
chdoom wrote:Source of the post So my inlet will be let say 4" to connect my 4" round duct, but will fall into an 8x9 air path right?

Right! Which is an increase from about 13 in2 to about 56 in2. That's an increase of about 430%. :thu:

- Stuart -

chdoom wrote:Source of the post Great!

Thanks Stuart

Dom

Hi Stuart! Hi Dom! My first post in the Forum - Thanks so much.

I am trying to come up with my own design for a vocal booth that I am building. The dimensions (imperial) are
8' x 7' x 9' (HxWxD). The current duct that is coming to that room is 8". I've read everything I could find in the forum about the concept, so it's safe to say I am familiarized and just need to get down to actually building it. I purchase the wood and insulation already (https://www.grainger.com/product/SOUND- ... n-Wd-1VDN4)
Control room is already figured out - mini split ductless etc. SO just vocal booth advice is needed at this time.

Thank you very much!

(Stuart, I am from South America originally)
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Screen Shot 2023-03-20 at 2.55.46 PM.png
IMG_6268.jpg


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Baffle box / Math question

#9

Postby gullfo » Tue, 2023-Mar-21, 10:17

if the control room is temperature controlled (and you should also be providing fresh air as well) then you could make up a couple of small fan-silencer boxes to connect the two rooms and simply exchange the air in the vocal booth with the cool/heated air from the control room.
very low noise fans are a must. and a apir of silencer boxes on the vocal booth side. one supply one return



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Baffle box / Math question

#10

Postby fossaree » Tue, 2023-Mar-21, 14:46

duplicate
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Baffle_Plan_Green.png


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Baffle box / Math question

#11

Postby fossaree » Tue, 2023-Mar-21, 15:01

gullfo wrote:Source of the post if the control room is temperature controlled (and you should also be providing fresh air as well) then you could make up a couple of small fan-silencer boxes to connect the two rooms and simply exchange the air in the vocal booth with the cool/heated air from the control room.
very low noise fans are a must. and a apir of silencer boxes on the vocal booth side. one supply one return

Thanks a lot Gullfo, that's a great idea! However I still need to send heat/cool air to the vocal booth, and for that one I have decided to use the central air since it was easy to get a duct going into the vocal booth. What I am asking for help, is how to figure the Gregwor plan (x,y and as Stuart has mentioned the 'z') so I can build one. Thanks again!
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Baffle_Plan_Green.png


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gullfo
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Baffle box / Math question

#12

Postby gullfo » Tue, 2023-Mar-21, 17:00

so - 4" round duct = ~12-1/2" area, so if you made the path through the silencer at 6" x 6" then with 1" duct liner, it's 4" x 4" = 16" area. so a slight volume increase (good). if the dividers are 7/12 the width of the unit 8/12 = units of 0.666" so 7/12th + 1" duct liner = 5.625" so internal is 10-5/8" + 2" of duct liner + 1-1/2" plywood = 14-1/8" outside wide. 4x 4" path + 8x 1" duct liner + 5x 3/4" ply = 28-3/4" long.



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Baffle box / Math question

#13

Postby gullfo » Tue, 2023-Mar-21, 17:27

one way to build it would be to use 1" ductboard and layout the pattern and cut into a few sections and fold and tape it. then add the plywood into the divider section and around the outside. i might even add a layer of drywall around it as well.
Attachments
example 4x4 silencer ductboard pattern0003.jpg
example 4x4 silencer ductboard pattern0003.jpg (35.9 KiB) Viewed 10078 times
example 4x4 silencer ductboard pattern0003.jpg
example 4x4 silencer ductboard pattern0003.jpg (35.9 KiB) Viewed 10078 times
example 4x4 silencer ductboard pattern0002.jpg
example 4x4 silencer ductboard pattern0001.jpg



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Baffle box / Math question

#14

Postby fossaree » Wed, 2023-Mar-22, 05:54

gullfo wrote:Source of the post one way to build it would be to use 1" ductboard and layout the pattern and cut into a few sections and fold and tape it. then add the plywood into the divider section and around the outside. i might even add a layer of drywall around it as well.

Glenn, you’re a hero - that’s exactly what I was looking for. Thank you for this so much detailed plan as well as for your thoughtful explanation!


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Baffle box / Math question

#15

Postby ezramevisto » Fri, 2024-Mar-22, 17:16

Hi Stuart, bit late to the party.. apologies :D

I'm doing my due diligence of exploring baffle box science as best as I could only to a practical extent, till i stumble upon this thread. After trying to plug the numbers in my excel, this has been bothering me for quite sometime.. please pardon me in advance..

Soundman2020 wrote:Assuming you want round duct, divide the area by pi to get the diameter. So, 17 in2 / pi = 5.4. To get the SAME velocity as the register, the duct would need to be 5.4" diameter. But the duct can be smaller (see above), so a 4" duct would be fine.


I'm not a math monk, but I initially thought we can't get a diameter from a square shape area.. in fact I thought the diameter is just either the diagonal length of a square, or just from side to side width of a perfect square.. unless I'm missing something? I tried reversing the formula to get the area, if 17 in2 / pi = 5.4", then given the radius (half of diameter) is 2.7", then Pi * 2.7^2 = 22.9 in2 = 0.159 square feet multiply by 200fpm so 0.159 * 200 = 31.8 CFM..... :?:

I tried using this formula for 260cfm need, and that formula gave me a 60 inch diameter (1.5 meters) for a round duct... assuming I wanted the same speed as the register, so ok I tried halving it since it has no restriction, 75cm... still too big... tried 1/3 = 50cm or 20 inch diameter round duct... still... big... :roll:

Like I said, not a math god, but it's either I'm missing something about the equation, or I'm f*cked trying to create space.... :D

Hopefully you're still around to enlighten noobs like me :idea:

Ezra




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