...with one of these...
http://www.radioshack.com/product/index ... d=12680845
Radio Shack has been offering an SPL meter for years. I have the older analog one and the company has the early digital version.
You do need to be careful with fans. You need to measure the noise without getting the actual airflow in the mix. You may perceive it as a nice cooling breeze, but the a sound meter may interpret it as an explosion-level sound event. "A" curve SPL is designed to minimize this effect. It takes into account your ear characteristics.
And no, you can't get Audacity and a microphone to do this. Too many variables and unknowns. You can, however, do a relative measurement. Do a measurement and Don't Change Anything and do a second measurement. Given you do everything right and don't overload or hit the noise floor, that
difference should be close -- in the "C" or flat weighing, not the "A" weighing that's written into law.
Koz