Originally Posted by ozziegt
I assumed you used this article
as your starting point? Where do you think it fell short?
Yeap, in fact, I think I referenced that very article near the beginning of this thread. Looking at it again, I think it "falls short" in this paragraph:
A typical room will need a 6´´ supply and comparable return. These ducts are a large cross section for sound to freely escape, regardless of the direction of airflow.
Because a single 6" supply and a single 6" return will each have too much FPM to be quiet while accommodating the ACH (air change per hour) needs for a 15'x21' room. See http://www.bravoav.com/articles/ht_hvac.pdf
ACH = (CFM*60)/volume, thus CFM=(ACH*volume)/60. From the article, 6 ACH is recommended. Thus, CFM = (6*volume)/60 = volume/10. My room's volume is ~300 sq ft., so I need 300 CFM. 340 CFM is actually more than I need (6.8 ACH), but that was the next fan-size up gave me...
Remember, we want FPM to be close to 250, to be quiet. FPM = CFM / area (sq-ft). So, area = CFM/FPM = 340/240 = 1.42 sq ft. Note that two 12" ducts = 1.57 sq ft (room to spare), whereas two 8" ducts is only 0.7 sq ft. (not enough!)
BTW, I didn't know any of this until *after* hearing the air-turbulence (Starship Enterprise). I naively thought that a single 6" would be fine since the fan itself is 6" and, to a lesser extent, because Ted's article suggested that 6" was "typical". Oh well, life is full of learning experiences.