Air Noise

That is, the "whooshing" noise from the exhaust air exiting the room. I have a 4" hole in the ceiling that exhausts air out between two floor joists. That is fine, the filtered air gets blown out into a basement/crawlspace area. The fan noise is virtually undetectable but the sound of the air getting blown out is noticeable. The exhaust duct blows upward, hits the floorboards above and then gets pushed out between the joists.

A compounding problem is that when cutting the hole for the exhaust, I found that I could not cut a perfect circle without cutting into a floor joist. I decided to get as close as I could, and fill the gaps with expanding foam.

Seems to me that the least amount of sharp corners is best. I was thinking of getting my dremel and rounding out the hole where the duct exits the room. Also getting some sheet metal or plastic and making some cove moulding around where the air exits so there are no 90 degree angles.

For a quick fix, I was going to lightly stuff some insulation or foam in the area between the joists where the air exits. Im not thrilled about this because I dont really want anything restricting the airflow.

Its not a huge security threat. I spent a lot of time making the whole operation undetectable and this is the one giveaway.

Any suggestions?

I use insulated flex duct in my setup to cut down on air noise. I'm not sure how much room you have to work with, but you could try attaching a section of that to your exaust. The insulation should cover most if not all the sound of the air and if you use a long enough piece, by the time the air makes it to the the other end it will have slowed down enough to the point it makes no noise. Hope this was helpful.
Happy Growing.

sounds like you want a stackhead boot to make the transition. and then fill the void w/ loose insulation. maybe one of these?
http://www.bestbuyheatingandairconditioning.com/Merchant2/merchant.mvc?Session_ID=c81acb5e368fb28fa71134b5524cd826&Store_Code=BB&Screen=PROD&Category_Code=duct-stackhead_boot&Product_Code=RIS10X490

try a duct muffler, or i'm sure you could make up something yourself if you're the handy type.. in combination with the fan speed controller and the duct muffler, i'd say the noise was reduced by a good 80% or so..