[M] Noctua NF-A14 industrialPPC-24V-2000 IP67 PWM Fans Review
 

The NF-A4 industrialPPC 140mm fans are very well built and can be used for both industrial and PC chassis applications; the 24V versions are ideal as case fans because they produce little to no noise at half the voltage they receive inside a computer chassis, but if you are also planning on replacing the fans on the water cooling radiators, you have to get the 12V models. The 12V NF-A4 industrialPPC 140mm PWM fans will be ran at higher speeds in order to cool the radiator fins properly, while keeping the noise levels as low as possible thanks to the integrated technologies.

http://www.madshrimps.be/articles/article/1000858/