HVAC silencer boxes & ducted design

[ZSXI]

Thanks, this helps a lot. Yes, the diagram you added is what I meant by mirror image.

I agree with what you said about HVAC techs. I've had zero luck finding any who have a clue about what I'm trying to achieve. I even tried a couple MEs and a guy who said he was an acoustic consultant. One of the MEs wanted to charge $17k for a very simple system in a 350sq foot room but didn't provide a lot of detail on what it entailed.

I'll reply further on my design thread, which hasn't been updated in a while. Thanks again!

For the record, I also watched the series of YouTube videos as described above (TrueSound Studios) and had my doubts about his silencers. He did a bit of testing which seemed to indicate it was working better than nothing, but I still had some doubts. I think he got bits of free advice from guys like John Brandt throughout his build and pieced together a design from that. I scratched my head here and there at some of his decisions, but, if in the end he is able to produce mixes that translate well while not having the cops called by his neighbors, I'd say he did alright.

[MariusP]

Gregwor –

About the HVAC Ducts and Silencers…

I am planning a 4000 sq ft build in Texas. Three control rooms – one cutting room. If I am doing individual Mini-Splits in each room – I do not need Ducts and Silencers for that – correct?

But I do need Ducts and Silencers for the fresh air (ERV) unit – right?

And… do I need two silencers for the ERV - one for the duct of the fresh air coming in and one for the stale air going out?

The dimensions of the control rooms are 20 feet by 23 feet by 12.5 feet height.

And it seems to me that the ERV would not be moving as much air as a typical conventional AC system – so perhaps those ducts could be smaller…

Marius Perron
San Antonio

[Gregwor]

If I am doing individual Mini-Splits in each room – I do not need Ducts and Silencers for that – correct?
But I do need Ducts and Silencers for the fresh air (ERV) unit – right?

Correct. You only need the duct work and silencers to bring fresh air in and dump stale air out.

And… do I need two silencers for the ERV - one for the duct of the fresh air coming in and one for the stale air going out?

Ideally you want one silencer box for each wall penetration. So, if you have a traditional 1 stud wall, you would only need 1 silencer box for supply, and 1 for return. If your room is a true room in a room, that means you have 2 walls that your penetrating. So, you would need 1 silencer for your outer leaf supply, 1 for your inner leaf supply and then 1 for your inner leaf return and 1 for your outer leaf return.

Some people who are very limited for space and still have a room in a room will use a single silencer for both walls but personally I don't think 1 box is enough unless it is absolutely massive and does not couple the two leaves together.

And it seems to me that the ERV would not be moving as much air as a typical conventional AC system – so perhaps those ducts could be smaller…

Yes. The fresh air you're bringing in is only about 30% of the volume of air compared to a ducted forced air system.

Greg

[Bryanf87]

Hopefully this helps some people with designing basic single path silencer boxes.

Gregwor's Silencer Box.png

Greg

Firstly, thank you so much for this wonderful info and this mock up. This is exactly the kind of thing I am researching right now for my studio. Would you say it would be a good or bad idea to have a smaller channel width (X) if you have the available ceiling height to make the box taller? Is there a minimum value for X to give optimal air flow with sufficient sound dampening? I don't have a ton of space where I would be installing one of these but for my application I'm thinking Y = 3'-0" making the inside channel width 1'-2 1/2" and using that dimension for height making the overall height of the box 1'-5 1/2". This makes for a pretty large box, if I could save on space by going with a smaller value for Y and X , that would be fantastic.

[Paulus87]

Hopefully this helps some people with designing basic single path silencer boxes.

Gregwor's Silencer Box.png

Greg

Firstly, thank you so much for this wonderful info and this mock up. This is exactly the kind of thing I am researching right now for my studio. Would you say it would be a good or bad idea to have a smaller channel width (X) if you have the available ceiling height to make the box taller? Is there a minimum value for X to give optimal air flow with sufficient sound dampening? I don't have a ton of space where I would be installing one of these but for my application I'm thinking Y = 3'-0" making the inside channel width 1'-2 1/2" and using that dimension for height making the overall height of the box 1'-5 1/2". This makes for a pretty large box, if I could save on space by going with a smaller value for Y and X , that would be fantastic.

You need to have the CSA of the inside of the box ATLEAST double that of your fresh air inlet duct CSA.

So, work out what size of the inlet duct, work out its CSA, then double it. That will be the minimum size you can use for your boxes.

Paul

[Bryanf87]

Thanks for the additional info, Paul.

Since I will be running a main trunk of probably 12 inch dia insulated flex duct, the measurements I figured up on my previous post should be fine if I branch off with 6 inch flex duct to feed the silencer box since the channel width will be over 1 foot. Obviously, the smaller I go with the branch feeding the silencer box, the more air flow I will have exiting the box. I'm wondering if 6 inches is going to be too small and if I should try to go larger for the inlet? I've never built a silencer box before so I'm not sure at how effective it is at slowing air flow and reducing noise at the outlet.

[Paulus87]

Thanks for the additional info, Paul.

Since I will be running a main trunk of probably 12 inch dia insulated flex duct, the measurements I figured up on my previous post should be fine if I branch off with 6 inch flex duct to feed the silencer box since the channel width will be over 1 foot. Obviously, the smaller I go with the branch feeding the silencer box, the more air flow I will have exiting the box. I'm wondering if 6 inches is going to be too small and if I should try to go larger for the inlet? I've never built a silencer box before so I'm not sure at how effective it is at slowing air flow and reducing noise at the outlet.

You need to work out how much CFM you need, then size your inlet duct to suit. If you are branching off from a trunk then divide the CFM by the amount of branches, then size the branches according to that number.

Paul

[anodivirta]

I'm strugling to find duct liner locally, what are the downsides of using thin plastic film on top of some normal (mineral wool/fiberglass etc) insulation in silencer box instead of real duct liner?

[gullfo]

duct board is more effective, but if it's not available and shipping is too costly, then using the semi-rigid insulation and lining it with plastic will work.

[gearjunk1e]

Hi everyone

Hope this is "on topic" enough to sit here (I have posted elsewhere on my project http://johnlsayers.com/phpBB2/viewtopic.php?f=1&t=22503)

At this point I have a burning question on silencer design/sizing.

I see that where people are using ductless mini splits, they reduce their fresh air supply volume requirements by 70% in calculating CFM. This results in smaller silencer/iso/baffle boxes . While the penny has yet to drop for me on why that is the case given I thought that multi-splits (aside from heating/cooling) just recirculate air - right at the moment I only need to know one thing -

THAT is, if I'm going with a ducted mini split - which is what I'd like to do - does it just mean I have to take the "30%" out of the CFM equation?

I have a feeling that when the penny does drop here I'm going to feel embarrassed, but for now I just need to get a line on silencer/iso/baffle box sizing for my situation.

Any help - or a link to a post I've missed - that makes this clear would be appreciated!

Thanks team

Andrew

[gullfo]

your fresh air requirements don't change regardless of the approach used. the mini-split reduces the amount of cfm used for the temperature control. if you have little issue with humidity and no need for full filtering, then the mini-split will work as most do not manage humidity (like an AC unit, then can condense moisture but not add it) or have HEPA filtration/UV etc options.

you do still need fresh air and depending on the energy levels of people in the room, you need 25-35% of your air volume replaced 6-10x per hour. so if you have 1000ft3, then 8x = 8000ft3. 25% of that is 2000ft3 per hour of fresh air. figure a 50cfm ERV unit will meet that need @ 50fpm velocity with a 1ft2 area duct.

[gearjunk1e]

Brilliant...thanks Glenn!

[Paulus87]

your fresh air requirements don't change regardless of the approach used. the mini-split reduces the amount of cfm used for the temperature control. if you have little issue with humidity and no need for full filtering, then the mini-split will work as most do not manage humidity (like an AC unit, then can condense moisture but not add it) or have HEPA filtration/UV etc options.

you do still need fresh air and depending on the energy levels of people in the room, you need 25-35% of your air volume replaced 6-10x per hour. so if you have 1000ft3, then 8x = 8000ft3. 25% of that is 2000ft3 per hour of fresh air. figure a 50cfm ERV unit will meet that need @ 50fpm velocity with a 1ft2 area duct.

I've always calculated the amount of air based on the total number of people that will use the space rather than the volume of the space. At least 15cfm per person, or in the UK the standard is 10L/sec per person.

Here's a quote from Rod Gervais:

"I have stated before and will state again...... calculations based on air exchanges per hour just do not make sense.....

Tke a room 10 x 14 with an 8' ceiling - based on your calculations that room would have 6 exchanges oer hour for a total of X cubic feeet of air per hour (the actual number is not relevant)

Now - if we decide to make the ceiling 18' high - we suddenly need more than twice as much air?

Why?

We won't fit one more person in the room than we could before - and they are the ony thing we need to worry about dying if they don't get enough air...........

Fresh air supply should be provided based on the maximum number of people who will be in the room........ and then that number should be multiplied by a factor you feel is reasonable for the purpose.

As an example of this - the Internation Mechanical Code sets a minimum cfm per person in a music room of 15 cfm.

They have different rates for dfferent occupancies - but this is what they set for music rooms."

More on the topic here:

https://www.gearslutz.com/board/studio- ... -hour.html

[gullfo]

thanks! agreed a minimum per person can be useful when you know the number or using ASHRAE /ICCSAFE minimum numbers for computation.
reference: https://codes.iccsafe.org/content/IMC20 ... entilation

[gearjunk1e]

Thanks guys

I must say that "per person" makes sense given that the fresh air supply is all about keeping people awake and ultimately alive.

I'd had seen Rod's calculation basis (and that quote) in my study of his book but the room volume approach seemed to dominate posts I was seeing - so I just launched in to that as a starting point.

Andrew