Another HVAC duct silencer question...

[Studio45]

Hi everyone,

I'm stuck on a problem and can't seem to find what I need to move forward. I need your help with making a few decisions.

I've searched and read through many different threads on HVAC and silencer boxes but still not clear on what I should be doing.

I'm trying to figure out the best way to proceed with isolating my supply ducts going to my individual rooms. (see picture attached showing what I'm trying to resolve)

To give a bit of context I just installed a brand new air-handler and got my main trunk installed as well. No take offs have been installed yet as I wanted to wait until I finished my room layout first.

My plan is to have/install 3 x 6" supply take offs from the main trunk. 1 going into my control room, 1 for the live room and 1 for the vocal booth. From what I've read a must is to have 1 silencer for each take off correct? What I was thinking of doing is for each take off, run insulated flex-duct (6") from the take offs to the silencers and from the silencer run flex-duct to the register.

Hopefully this is a step in the right direction that i'd like you to confirm. I don't want to do all this work and realize I did it all wrong.

My last question is about silencer options and designs. I've found and see many online that can be bought and have seen many DIY options for boxes that i could build. I'd like to think I'm pretty handy (since I've built my entire home myself) so I'd like to build my own silencers. That said, I've seen many "box" designs made out of wood and lined with either foam, duct liner or some other type of insulation.

While I would be okay with building such boxes...I've found another option that I'm considering but haven't seen/read much feedback about it so I'm hoping you guys can chime in and tell me what your thoughts are on this DIY option?

here's what I found... http://billpentz.com/Woodworking/Cyclon ... liveau.cfm

I really like the construction of this silencer and would prefer to build this type instead of the typical wood box type, but only if I'm going to get as good noise isolation from it. In other words if you guys think I'd be wasting my time and money on it I'll just build wood boxes and be done with that.

The only difference I see in the two designs is that the wood box style has baffles in them versus the round version having no baffles at all.

Any thoughts?

[Soundman2020]

I'm trying to figure out the best way to proceed with isolating my supply ducts going to my individual rooms. (see picture attached showing what I'm trying to resolve)

Your picture seems to show a situation that is very much not recommended, and probably a violation of code in many places. It appears to show an HVAC duct that is take the used, stale, exhaust air out of one room and supplying it to the next room. Nope. Not allowed.

To give a bit of context I just installed a brand new air-handler and got my main trunk installed as well. No take offs have been installed yet as I wanted to wait until I finished my room layout first.

Ummmm... I'm trying to understand this: you already installed the HVAC system, but you have not yet completed the design? How did that happen???

My plan is to have/install 3 x 6" supply take offs from the main trunk.

Have you done the math to make sure that 18 square inches of cross section is sufficient to provide the volume of air flow you need (flow rate), at a low enough speed (flow velocity)? Will that keep the speed below 300 FPS?

1 going into my control room, 1 for the live room and 1 for the vocal booth.

Why do you want the same size duct for each room? Are all three rooms the same size? That's the only time it would be correct to have the same size duct supplying each one... If they rooms have different dimensions, then each duct needs to be sized accordingly.

From what I've read a must is to have 1 silencer for each take off correct?

Actually, one for each wall penetration, and even more correctly, one for each leaf penetration. Every time a duct passes through a "leaf" of a wall, you need a silencer box, as a general rule. The silencer box must be located at the point of penetration.

What I was thinking of doing is for each take off, run insulated flex-duct (6") from the take offs to the silencers and from the silencer run flex-duct to the register.

That can work, yes, assuming that the silencer is at the penetration, and is of the correct size to do the job. But running a 3x6" take off into a 6" duct might not be a good idea on the return side. That would be forcing a 28 square inch cross section into an 18 square inch cross section...

I'd like to think I'm pretty handy (since I've built my entire home myself) so I'd like to build my own silencers.

Yup! They are pretty easy to build, actually: It's the design that is a lot harder. So many factors to consider. But once the design is complete and validated, you can build each box in a few hours, if you are hand with tools and have the right ones.

That said, I've seen many "box" designs made out of wood and lined with either foam, duct liner or some other type of insulation.

Forget the "foam" and "other insulation": Use only proper duct liner. It is expensive, yes, but think of it this way: every breath you take inside your room is air that passed through your ducts. Do you want to be breathing air that passed over surfaces that were specifically designed and manufactured for breathable air? Or do you want to breathe air that passed over "any old cheap thing that I found to save a few dollars"...

I've found another option that I'm considering but haven't seen/read much feedback

Well, if you happen to be isolating only the sound of power tools in a carpentry workshop, then that thing might provide a bit of isolation. But if you wanted to isolating full-range music from a typical contemporary rock band, then probably not...

There's also no indication of the insertion loss or frequency spectrum covered by that device, so I'd skip that and go with a tried and true design that is know to work for studio sounds.

The only difference I see in the two designs is that the wood box style has baffles in them versus the round version having no baffles at all.

The wood box design also has sudden large changes in cross sectional area, which creates major impedance mismatches and is responsible for much of the reduction in sounds, especially low frequency sounds, and the wood box one has a surface density of at least 10 kg/m2, hopefully over 20 kg/m2, as compared to the thin, light-weight, flimsy sheet metal, and uses "any old piece of foam that I found to save a few bucks". Not a problem for a dust collection system on a power tool, since nobody will be breathing that air, but a potentially big problem for a studio HVAC system...

- Stuart -