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Elite Shackster , HTS Moderator Emeritus
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Just FYI, I have an unsealed theater with 2 live vents and no return (the return is sucked up the stairs to the main floor and then back down to the air handler -- very poor design for the basement HVAC). I can barely hear the air handler running through the vent in the theater, especially with a movie playing. Sadly the AC compressor is right outside and I CAN hear that in the summer.

I mention this so you don't worry too much. Priority should be the heating and cooling.

That being said, the flexible duct will attenuate quite a bit of the noise on the supply, and I can't see why having a dead vent return would be bad. The trick to HVAC isolation is not having a clean path in or out for the sound. Both the dead vent and flexible ducting help accomplish this. I don't believe it should matter which one is supply and which is return.

Hopefully bpape can chime in with more advice. He's definitely our resident expert on soundproofing.
 

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Elite Shackster , HTS Moderator Emeritus
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2,216 Posts
The flexible duct usually has a lining of fiberglass on the outside.

That being said, the biggest attenuator of not having a "clean" path is impedance changes in the flow path. A meandering pipe with no insulation could still conduct the sound just fine. But a curved pipe going into a larger ante-chamber, and then another curved pipe exiting causes a "shock" to the sound wave and limits the transmission. That's really what I meant by no direct path.
 

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Elite Shackster , HTS Moderator Emeritus
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2,216 Posts
The combination of curves and fiberglass help absorb the sound. But a long straight fiberglass lined pipe will attenuate sound as well. Glasspack mufflers in cars work on this principal. The sound wave is often pictured as a sine wave or as a straight line compression wave, but while the wave does move forward, the air wants to expand in all directions (including sideways) and if there's fiberglass there, it will take out some of the energy.

But yeah, curves, with the lined ducts will be your best bet. Sadly with that joist space limitation, I don't know what is available, but I thought I saw flexible ducts smaller than that at Home Depot.
 
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