I am building a basement home theater and am trying to do all that I can for soundproofing (double wally, DD + GG, RSIC clips on walls and ceiling, good door, etc.).

However, on one wall I have run into the scenario in the attached picture. The inside of the wall will be double drywall with green glue, but the outside wall is right next to the main HVAC plenum for thou house. The plenum is above a hallway with 2x4's below if for a dropped drywall ceiling.

What would be recommended for the outside of the wall? I would imagine that DD + GG on the portion below the hallway ceiling wouldn't be worth it because sound would just be coming out the top portion an into the HVAC/joist space to then permeate the rest of the house. Would it be worth it to try to fit in an 8" piece of drywall above the hallway ceiling between the wall and the HVAC? Should I just forget about the outside wall and put a 3rd layer of drywall on the inside instead?

Kevin,

you have noise coming fromt he theater, but this has to pass through double drywall with GG before it can assault the HVAC plenum?

I sketched a couple things onto your pic, is that what's on your mind?

Brian

Those kinds of joists always give me a scare. If there's load in the wrong place, they fail.
Did an engineer tell you you could put that RSIC DC-04 clip there?

Perhaps a Mason WIC (www.mason-uk.co.uk/pages/wic.pdf) instead (sideways to the existing wall studs (they are existing right?), instead of up to the overhead joists)

Thanks for the replies, guys. Brian, you are correct that I am concerned about the flanking path that you drew with the blue line (and an additional one I added in the attached picture. The ceiling of the HT will also have DD+GG on RSIC-01 clips (at least, that is the plan).

I should mention that my primary concern is not sound from the HT getting into the hallway, but sound form the HT going to the first floor (and the HVAC, which seems like a nice path to the first floor).

Bob, I'm not exactly sure where your coming from on the joists and clips. Open web trusses are quite commonly used in the building industry (the same design is in most roofs I've seen), and most builders I've talked to preferred engineered joists to dimensional lumber. Some prefer I-joists, but the open web design makes it much easier to run utilities without worrying that the subs will cut somewhere they shouldn't. I've heard isolated cases where people complained of long-term sagging, but never failure due to load. They are designed to the same load capacities of any other flooring type.

As for the DC-04 clips, the only engineer that told me I could put them there is myself. The PAC-Intl literature recommends this type of installation, as do the various vendors of the clips. First, the wall is non-load bearing. Second, if the whole wall was suspended from the clips rather than being attached to the concrete floor, the clips would be supporting approximately 80-lbs each. The pullout rating of the wood screws holding the clip to the wall is around 320 pounds. The shear strength of screws is lower than a nail, but a wall with it's center of mass 2" away from the screws isn't going to exert much lateral force on the screw.

What is your concern?

Kevin_Wadsworth:

Open web trusses are quite commonly used in the building industry (the same design is in most roofs I've seen), and most builders I've talked to preferred engineered joists to dimensional lumber. Some prefer I-joists, but the open web design makes it much easier to run utilities without worrying that the subs will cut somewhere they shouldn't. I've heard isolated cases where people complained of long-term sagging, but never failure due to load. They are designed to the same load capacities of any other flooring type.
All true.

But that kind of joist is engineered to have load distributed across the top, and to have support at the ends, and to bow inbetween. If you put support in the middle of that kind of joist (unlike a 2x10) you can cause it to fail.

How do I remember that you ask? Guess.

If you do RSIC and DD with GG on the ceiling, I wouldn't worry too much about the hallway. Like the attached pic...