i don't see anything particularly wrong in my comment, especially since I have dB measurement included. Who is to say that Sp20 with fans at different speed would not produce waves that are out of phase with each other, but I digress.
Fan noise is 99+% a random mix of many frequencies that cannot be neutralized by anti-noise as there is not sync or phase.
your db rating were from very different distances afaik.
running sp20 or s5 with Scythe ultra kaze also does not make too much sense for me as that fan is 3pin and not pwm.
next point is the hashrate: downclocked to 900 Gh/s you can make eiter s5 or sp20 down to 40db with 0.3A PC Fans an let them run in a living room.
but at 1200+ GH they got to hot and I doubt you get it under 46db
a little physics:
db rating is not easy to understand.
example: 1 sp20 = 55db ; 2 sp20 = 58db
doubling (independent) noise sources are always +3db regardless of the distance.
if you double the distance your db gets down by 6 db. if you check that with db meter you'll not always agree, bit this is most up to reflections in smaller rooms.
your db ratings were from 1, 4 and 10 feet. in theory 10 ft. db should be roughly 20db less than 1 ft.
and the "human ear" is recognizing doubled noise in 10db steps but this depends a bit on the frequencies. It's probably a bit less than 10db.
