Ventilation plenum boxes inside the wall cavity?

Thank you all for your commitment and hard work on this forum. It has been a magical mystery tour. I operate the Amherst Red Barn Music School in Amherst, Mass and I am building a soundproof classroom/rehearsal/recording room in the basement of the historic Dickinson grain building. Fifteen years ago, the architect/owner jacked it up and put in a double drained basement. We have log joists with the bark still on, just to give you an idea. Overhead is a wooden office building separated by a six-inch steel reinforced poured concrete slab. Concrete is not soundproof and there is a massage therapist above who has made my life difficult. Hence the serious effort to make it bullet-proof -- or dagger-proof, as the case may be. The walls and ceiling are all framed. It will be double-stud, double-sheetrock, fiber glass, RC on the ceiling. The electrician came today, hooray! Waiting for the framing inspection. Then the ventilation. The ventilation. After months of agonizing, here's where I am. Eventually, I hope to expand again, tripling the studio. So I can't put in the full ventilation system yet, I would have too much capacity for too little space. For now, I just need to provide for the new room. I already have a space across the hall, 800 sq ft, with literally a ton of airconditioning, heating, dehumidification and plenty of fresh air to draw on. Based on what I've read, it should be enough. Max occupancy is 10. The architect-owner has made it clear that all HVAC decisions must be approved by him though the HVAC tech. But everything inside the studio walls is up to me -- plenum boxes, soffit-ducts, grill sizes, the fun stuff. The problem is space. The only place to put soundproof plenum boxes is in the 13 in deep wall cavity. The plan is to build 12 in boxes of 7/8 osb measuring 96 in x 62 in lined with 1 in duct board. A "neck" will be added on each duct so the boxes protrude through the walls on each side, sealed. The diagrams show 8 in feeds, that could anything up to 12 in. See below. Any comments/direction appreciated. Thanks again. Kevin

kevicoll wrote:

The walls and ceiling are all framed. It will be double-stud, double-sheetrock, fiber glass, RC on the ceiling. The electrician came today, hooray! Waiting for the framing inspection. Then the ventilation. The ventilation. After months of agonizing, here's where I am.

If I understand you correctly - the ceiling is an independent frame from the existing structure? Is this correct? If the answer is "YES" then do not install resilient channel on the ceiling - not only is it redundant - it is counterproductive in as it will not isolate as well at low frequencies as the frame itself.

The only place to put soundproof plenum boxes is in the 13 in deep wall cavity. The plan is to build 12 in boxes of 7/8 osb measuring 96 in x 62 in lined with 1 in duct board. A "neck" was added on each exit duct so the boxes protrude through the walls on each side, sealed. The diagrams show 8 in feeds, that could anything up to 12 in. See below.

Kevin, I would head more like this:

Thanks, Rod. I took your design and came up with the “folded horn plenum” (a nod to Paul W. Klipsch), see below. I graded the duct sizes, could have done all 18 in., but I went for a little horn. QWYATT has an article that supports this, somewhat: "It is the conversion of velocity pressure (VP) to static pressure (SP) through a DIFFUSER. Gently sloping walls create a gradual cross sectional area change, which allows smooth air flow deceleration and high efficiency static regain. The small dip in total pressure (TP) shows remaining frictional pressure drop and residual non-ideal effects." (attached below) The radii of the curves will vary according to the size of the smaller duct, i.e. a 12 going to 14 will have a 12 in. radius. Curved baffles will be made of a duct board sandwich, cut and faced with foil. I worried about the air speed entering and exiting the plenum, made up a spreadsheet that lets me play with the dimensions (including round ducts) until I get something I can live with, picture and download below. At 162 CFM, I get 4 air changes an hour and an air speed of 119 fpm into a 14 in. sq. soffit. The secret is in the variable-speed in-line duct booster fan. I can always turn up the fan. Or down. Re the ceiling pocket, in this case, the joists are attached to the beams. Based on your comments, that would be: RC - Yes -- the ceiling frame is not independent. I posted pictures in a new topic, Ceiling Pocket - Fire Code vs Soundproofing. It is interesting because the unsprinklered ceiling pocket is an exception to the code that must be approved by the inspector: http://www.johnlsayers.com/phpBB2/viewt ... 3149#73149 Thanks for the design and input. Cheers, Kevin Later-- Alas! The size of the space I have to work in shrank to 50 x 80. The beauty of this design is it is scalable and I was able achieve the same air flow over a shorter distance. It took me a while to grok Rod's suggestion: by adjusting the side of that little box up and down, the air flow (size of the 'folded horn') may be kept continuous while making the plenum box longer or shorter. The passages still progress stepwise: 14", 16", 18" 20", 22" and 24". Eventually, I'll post a step-by-step. Here it is, with curved baffles: