3db = twice the power
twice as loud = 10 x the power
two equeal fans at the same distance from ear would be 3 db and twice as lound would equeal 10 fans
And this is kinda getting off topic
The fans that come with the Swiftek Q-puwer case are real quite are just 25mm thick and can run at 5v or7v. I can't find my link to thier (fans) homepage or I would post it. "Mechatronics" is the brand name. But you can probably do no wrong with a Panaflow.