SPCR

Is there a formula or a general rule of thumb for getting a total dBA from multiple noise producing parts? I figure 3 fans at 15dBA together wouldn't be 45dBA .

Because it is a logarithmic scale, doubling of sound power actually adds a constant to the original dB level. See http://www.sengpielaudio.com/calculator-levelchange.htm. However it can even be the case that adding two sound sources gives lower noise than either one by itself, as happens with noise cancellation equipment, so it is not trivial.

To put it simply, under the presumed usual conditions, two identical noise sources would produce a sound that was approximately 3dB louder than just one. So two of your 15dBA fans would be about 18dBA. Add a third and it would be about 20dBA; a fourth, about 21dBA.

I use this calculator once in awhile:
http://sphere.sourceforge.net/flik/misc/db.html

Forgot to check the notify box. Thanks Reachable and Lawrence Lee, that's exactly what I was looking for.

and if you really want to know and not guess you'll have to spend a few hundred dollars on a sound meter. see viewtopic.php?t=47290&postdays=0&postorder=asc&start=0 for pricing.