How does fan speed work?

[Frankenstyle]

I tried searching, but nothing turns up with the search variable fan, that I could find that would answer my question.

Im planning on getting a Thermaltake Ultra-120, and I know I would need to buy a 120mm fan. I was searching around on newegg.com (I find it the best, at least to pick parts), and I see three different kinds of fans. They have some that say they have a range such as 35-95 CFM. There also some that will have a few settings such as 20, 40, 60 CFM. Then there are the ones that just have one rating, ex: 45 CFM.

I want to know if the ones with just one rating can be controlled by your motherboard or a fan controller, or if they are truly just one speed. I also wanted to know if the fans with the three numbers can be controlled or if those ratings are the only settings they will run at.

My dilemma is that I want a fan that can perform when I need it too, but I don't want something that will sound like a tornado all the time. I know you are saying, just pick one of the variable speed fans, but from what I've seen, the selection is slim and the quality isn't up to par with the one speed fans. Any thoughts are useful. Thanks for the help!

 

[DeathWalking]

I'm currently not awake enough to answer your question on a recommendation of a fan, but I can answer your (semi-)technical question.
All fans can have their speed adjusted. However, the ease of that adjustment will vary. If the fan uses a 3-pin motherboard header, then you can adjust the speed through your BIOS, or by using an in-OS program, assuming there is one that is provided with your board. If the fan uses a molex connector, however, you will need to use an adaptor to allow it to be plugged into your motherboard. I actually saw something cool the other day, a fan that had a built-in rheostat, so you could adjust the fan speed dynamically, even while running. It was pretty cool.

 

[Frankenstyle]

Could a fan speed controller be used. The one I'm looking at has 3-pin connectors, so if I use a molex adapter that would still work. Im trying to give me the control over the speed, and I'll monitor the temps rather than the motherboard.

 

[Mugz]

Zalman has a nice range of fan controllers, from single-fan units to multiple-fan units. These can work perfectly off a molex-3pin adapter.

You'll find that Thermaltake and Coolermaster (and just about every other computer cooling company) will also have some nice units for the same purpose.

I don't know if any of them will display the RPM of the fan, since I've never used any of these myself.

What I do is rip dead PSUs apart and remove their thermal fan controllers. Removing the attached thermal sensor and replacing it with a ~10K variable resistor yields excellent results. It just doesn't look as pretty, though...