Noctua NF-S12 & NF-R8 Fans
 

The U.L.N.A (Ultra-Low-Noise Adaptor) is a power cable that has a
resister in line with the fan, cutting the amount of power that the fans
uses, which slows it down. If you use those, there is not a way to speed
up the fan without powering it down and removing it.

http://www.gideontech.com/content/articles/350/1