Air Return Plenum for Dust Collector Closet

[Michael Zerance]

I built a soundproof equipment closet to house my compressors, phase converters, a/c, and dust collector. I'm in the process of building the return air plenum. I have a 30x14 filter grill on the closet side and an identical 30x14 filter grill on the shop side at the same elevation which I will be connecting with 30x14 ducts made from 1-1/2" fiberglass duct board.

There will be about 24 linear feet of duct comprising of a 90° elbow at each grill and (2) 90° elbows to comprise a 180° bend. I was originally planning on making 3-piece elbows to increase efficiency but now I'm wondering if the extra turbulence with hard 90°s would be better for soundproofing. Which type of elbow would be better to use for soundproofing?

workshop return air plenum v1.jpg workshop return air plenum v2.jpg


In addition, I'm considering expanding the width of the last long section of duct so that a pair of baffles could be added but I'm concerned about restricting the opening which ends up being about 20x14 at each baffle. Thoughts?

workshop return air plenum baffles.jpg

[George Yetka]

John Mansville Linacoustic® RC is pretty standard for soundproofing duct commercially

[George Yetka]

What is the room that it passes through? Could you line the room? Then locate the grill into the compresser room low.

[Michael Zerance]

John Mansville Linacoustic® RC is pretty standard for soundproofing duct commercially

I already have the material. It's John Mansville but it is the Micro-Aire type not the Linacoustic. It seems they have similar sound absorption properties.

[Michael Zerance]

What is the room that it passes through? Could you line the room? Then locate the grill into the compresser room low.

The area that the duct passes through is dead space above the spray booth. The grill in the compressor room was located high to exhaust the excessive heat from the equipment in the room and the grill locations have already been framed around and cut out. At this point in the project, the grills needs to be located where they are shown in the pictures. The only flexibility I have is the shape, size, and configuration of the duct run.

[Jim Becker]

I suspect that the rounded version will be slightly quieter as it will provide for a smoother air pathway. This is very low pressure, especially if you have it sized well above the DC's outlet size as it should be, but unless there's a big money reason to do the square corners, I'd go with the rounded.

[Aaron Inami]

I don't think you will see any difference in sound reduction between your square corners and round corners, since you are using 1-1/2" fiberglass duct board which is basically sound absorbing panels. In my opinion, it comes down to which is easier to fabricate. The 30x14 duct size is already significantly higher than the 8" duct from the dust collector, so you should not have any pressure restriction issues (like Jim stated). The two internal baffles would create more air restriction than a square corner would anyways. I'm not sure that you really need those two internal baffles.

That being said, I would start placing fiberglass sound absorbing panels on the walls inside your compressor/dust-collector closet. This will reduce the amount of mid and high frequency noise that is generated and amplified by a small room and transmitted through the walls. This would make a LOT more difference than choosing between square/round duct corners.

[Jim Becker]

I missed those two internal baffles that Aaron just mentioned. I wouldn't bother with them. The whole idea here is a non-direct pathway back to the work area and the bends in the duct work cover that. Given the material being used, the length of the pathway and the multiple direction changes, I'd not bother with the extra baffles.

[George Yetka]

Is this made of metal or wood?

[Michael Zerance]

Is this made of metal or wood?

1-1/2" John Mansville Micro-Aire fiberglass duct board

[Michael Zerance]

I don't think you will see any difference in sound reduction between your square corners and round corners, since you are using 1-1/2" fiberglass duct board which is basically sound absorbing panels. In my opinion, it comes down to which is easier to fabricate. The 30x14 duct size is already significantly higher than the 8" duct from the dust collector, so you should not have any pressure restriction issues (like Jim stated). The two internal baffles would create more air restriction than a square corner would anyways. I'm not sure that you really need those two internal baffles.

That being said, I would start placing fiberglass sound absorbing panels on the walls inside your compressor/dust-collector closet. This will reduce the amount of mid and high frequency noise that is generated and amplified by a small room and transmitted through the walls. This would make a LOT more difference than choosing between square/round duct corners.

Thanks for your input; I may use the duct board offcuts and secure them to the walls and ceiling of the closet around the duct inlet.

[Michael Zerance]

I missed those two internal baffles that Aaron just mentioned. I wouldn't bother with them. The whole idea here is a non-direct pathway back to the work area and the bends in the duct work cover that. Given the material being used, the length of the pathway and the multiple direction changes, I'd not bother with the extra baffles.

Thanks for your thoughts on this, Jim.

[George Yetka]

1-1/2" John Mansville Micro-Aire fiberglass duct board

Interesting I havent come across this before.

[Michael Zerance]

The duct with rounded corners and without baffles would be the most efficient at moving air with minimal turbulence and low velocity but is that the best in terms of sound reduction? (that's a legit question even though it reads kind of rhetorical) My intuition says that the duct with square corners and baffles will create a lot of turbulence and restriction forcing the sound waves to bounce around a lot more and allowing them to be better absorbed by the duct board walls. I understand that it would increase the sound of the air in the ducts. However, my main goal is to block/absorb the equipment noise in the closet from coming through the ducts and I feel that the air noise generated would not be a great concern considering that the air entering the ports in the tools in the shop would be more noisy than from the return duct anyway. Thoughts?

[Jim Becker]

IMHO, yes, it's perfectly fine for sound reduction because the whole purpose of the construction is to eliminate "direct" transmission of sound. You have more than 360º of bend with a reverse. It's also "lengthy" compared to most folks' setups. I'll go so far as to say, this is probably one of the best situations I've seen...not everybody can do the reverse bend before coming all the way around. Do keep in mind that the softer material will absorb a large amount of the higher ("screaming") frequencies, too. If the room where your DC and compressor are located has some reasonable sound insulation treatment, you're going to be in a very good place.