AFAIK , silenx fans always underrate their noise level. The actual noise is far higher than the rated dba.

Currently the best 120mm review at SPRC is NOCTUA NF-S12 SERIES Case Fan
http://www.silentpcreview.com/article695-page4.html

but it is for those who wants absolute silent

U can always mod it or ask for help from AllNGap bro

different fan comes with different RPM

normal fan we use usually runs at 12v ,except u got a fan controller or u mod it ofcourse

I not really sure, some fan company uses different technology like the Noctua fan which can provide higher cubic feet per minute (CFM) with a lower rpm...I think its very hard to say which fan is better

The review shown in SPCR is conducted with noise as the main concern ,it doesnt mean the Noctua is the best fan but it provide the highest noise/performance ratio according to them ,which IMHO is very important these days, people don't focus on raw performance anymore. More and more factors like noise, temp, power consumption etc etc are important in a pc as well

My personal experience : I have used Thermaltake Volcano 7 Max speed for over 1/2 year and I noe what its like to use a super noisy pc , if that time I can choose, ill choose to lower my fan speed by sacrificing the speed of my proc but what to do, fan controller rosak