Trying to get your PC to run fanless, but just can't do it? Don't want to run completely fanless?

Well, one solution is to undervolt your fans. 

There are a number of acceptable methods to undervolt your DC fans. Suprisingly, you can get quite a bit of granularity of control with ATX power supplies.

See, another word for "voltage" is "potential difference". In other words, it is the difference between two potentials. A 12VDC fan is usually connected to 12V and 0V, which is a potential difference of 12V. By connection something that uses up energy, that potential is used to create work (spinning the fan). So, any difference can be used to power a fan.

One could connect the positive lead to 112VDC and the negative lead to 100VDC and would end up with a fan that spins at the same speed as one connected to 12VDC and GND/0VDC. But be careful with the electricity in the former situation :)

So, to get 7V, we could connect the +ve lead to 12VDC and the -ve lead to 5VDC, since the potential difference would be 12-5=7VDC. This will not cause issues with a normal power supply, and is relatively easy to wire.

Another possibility is 10VDC, which can be done by connecting the +ve lead to 5VDC and the -ve lead to -5VDC (which does exist as a wire on ATX power supplies). I wouldn't recommend doing this for larger fans, or a large number of fans, as there isn't usually much power on the -5VDC line.

Finally, some fans will run at 5VDC. Some may not. I currently do have a 120mm fan (which runs extremely loud at 12VDC) running at 5VDC. Running a really large fan at a low voltage has the advantage of being able to spin quietly at a low RPM, but still push a lot of air.