Question on silencers

[RickLee]

Before I start building my silencer boxes I wanted to post a drawing to make sure I'm in the neighborhood.
First, this is a continuation of a build from a couple of years ago. http://www.johnlsayers.com/phpBB2/viewt ... =1&t=21073
That music room cottage is basically finished and is moderately quiet with simple single wall double drywall and decoupled flooring. Works well for what it is.

Since then I've built the shell of a room in room design right next to it. This room will have a small CR and iso booth when finished. I don't have a number on how much db reduction I need but I'm following Rod's book and the further advice from reading this forum to make sure it's a balanced approach.
I'm ready to put in the vented mini split system which I already have. The indoor handler will be mounted from the ceiling of my shop with the duct lines going out to a "utility room" and entering the studio from there. The utility room is an insulated space dedicated to the studio. The outdoor unit is denoted by the rectangle marked "outdoor".

Studio HVAC08262019.pdf

I have four basic questions:
Are the silencer boxes correct? (denoted on the drawing by the squares with X's) That is, two in/out boxes coming into the studio, two boxes for the iso booth, two boxes for the CR and two boxes leaving the studio going to the music room cottage?

Does it make sense to introduce the fresh air in/out at the handler? That is, fresh air in on the side of the handler as designed and a return air takeoff to the outside to relieve the pressure.

I'm planning on putting a low cfm inline fan for the fresh air. Should it be on the intake side or the exhaust side? I wish there was more info available from the manufacturer. All they say is that there's a fresh air duct on the side...

On the boxes entering and exiting the building is it best to keep the velocities the same in and out of those boxes?

From what I can figure, this setup will meet the air exchange per hour neccessary. The calculations get over my head quickly and I'm especially not too sure about how the different lengths of duct will affect the temperature differential in each room. I've had a mini split in my shop for several years which works well and I've based my calculations off of it.
I'll be building the boxes per Gregwor's diagram. The ducts and boxes will be in between the leafs in the ceiling. I'll be using a combination of flex duct and insulated sheet metal duct.
Since the music room won't be as quiet as the studio part I won't be using silencer boxes for it. However, the ducts will be insulated and contained within their own enclosure so they should be fairly quiet.

Thanks. This forum has been very helpful.

Just noticed on the drawing the silencers going into the music room are drawn against the wrong wall. They will actually be mounted between leafs against the outer wall.

[RickLee]

Ok, reading through some more and running more calculations I found this which really helps a lot. Thanks.

Frank--HVAC-overview.jpg

The biggest question I have is the stale air. Where did you takeoff on the return for the vent running to the roof? I'm trying to figure out if I will need an inline fan or not and/or rather if I should have a separate room for the air handler and treat it like in Rod's book with a vent at the bottom and top for temperature differential. Not sure how to calculate it. The other calculations are pretty straight forward.

[Soundman2020]

Looks like I missed your first post here! Sorry!

OK, some answers:

Are the silencer boxes correct? (denoted on the drawing by the squares with X's) That is, two in/out boxes coming into the studio, two boxes for the iso booth, two boxes for the CR and two boxes leaving the studio going to the music room cottage?

Sort of... I think! I'm not sure if I'm understanding your diagram, but basically what you need if you are looking for high isolation is one silencer box at each point where a duct goes through a leaf. You already re-posted the image from the small corner control room thread, so you can see how I did it there: that's the normal way I do it when I need high isolation.

Does it make sense to introduce the fresh air in/out at the handler? That is, fresh air in on the side of the handler as designed and a return air takeoff to the outside to relieve the pressure.

Do both in the return duct, just before it reaches the AHU. Take off the stale air several feet upstream from the point where you feed in the fresh air.

I'm planning on putting a low cfm inline fan for the fresh air. Should it be on the intake side or the exhaust side?

You can do it either way. I often do it on the stale air exhaust duct, to make sure I'm drawing out the right amount of air and dumping it overboard. The AHU should provide enough "suction" (pressure drop) to pull in the fresh air, assuming that the inlet point is fairly close to the AHU, and the the duct isn't too long.

On the boxes entering and exiting the building is it best to keep the velocities the same in and out of those boxes?

Definitely! Actually, it's not so much the velocities, as the flow rate: you want to make sure you have the same flow rate at both points. In other words, whatever volume of stale air you are dumping overboard, you need to bring in the same amount of fresh air. That should happen automatically, if you design the system properly, since it is mainly a closed loop through the rooms with just one entry point (fresh air intake) and one exit point (stale air outlet).

The biggest question I have is the stale air. Where did you takeoff on the return for the vent running to the roof

Ahhh! Trade secret! Not really... in this case, the easiest way to do this was to dump some air from the HVAC system into the bathroom and entry corridor, then take the stale air out through the bathroom roof. There's a vent in the bathroom door to ensure that there's always flow from the corridor, even with the door closed. In this case, that was pretty much the only way to do it, as there was not enough clear duct run into the AHU, to allow for both stale air dump and fresh air intake; they would have been too close, so the stale air fan would have been sucking some air backwards through the plenum from the fresh air inlet point, and that's not good. So I came up with this alternate route to get stale air out of the building, through the bathroom. In fact, it's not even really "stale" air: it's treated air from the "supply" side of the AHU, into the corridor and bathroom. It's only "stale" in the sense that it passed through those two rooms, and comes from the same recirculating air that is going around the main loop anyway, so in effect it's the same thing... just through a different path. Sometimes you have to get creative, when space is tight...

- Stuart -

[RickLee]

Thank you, this is very helpful. I think I've run through the various threads to know enough to run the calculations now. I have the vented mini-split and all the basic material I should need. In a few days I'll try to update my original thread from about 3 years ago. Basically it's going to be a small practice space with (rarely) up to 3 people in the spaces at once.