Post
by MikeC » Mon Sep 16, 2002 10:15 pm
For my own systems (including the ref P3 ~12-15 dBA system), I only use an intake fan if I have more than 1 HDD and can't keep the drive temps below 50C. Then I position them in a rack on the bottom front of the case, just behind an intake fan that I decouple mount and run as slowly as possible. The HDDs are suspended in elastic of course, and sometimes, side-sinked. The system cooling does benefit somewhat, but mostly it's for the drives, which usually drop ~5C or more just from this minute amount of airflow.
There's nothing wrong with having only exhaust fans. The air that is blown out has to be replaced -- it naturally gets sucked in through whatever lowest resistance openings there are in the case. Fan theory says that 2 identical fans in push-pull mode through an unobstructed (low impedance) space don't create any more airflow than one. If there is some impedance, then 2 push-pull fans, because they create higher pressure, may help keep the airflow up. But if you're looking for maximum airflow, you really want to keep all the airflow in one direction -- either in or out -- because this nearly doubles the CFM for each additional identical fan (if there is no or low impedance, anyway). With or without impedance, 2 fans blowing in one direction almost always provides more airflow than 2 in push-pull mode.
If you want to control the inflow and the path of the cool air through the case, block all openings other than the intake vents, which should be as smooth-edged and unimpeded as possible. With very low airflow, the former doesn't matter much. YOu could try blowing some colored smoke (even -- horrors -- cigarette smoke! in a pinch) to see how the air flows through the case, if you have a transparent side case to view it all.
Another thing to note is that while any impedance (objects that obstruct) within say an inch of either side of the fan blades causes turbulence noise, the intake side is MUCH more affected. Just try it with your hands. Putting your palm with outstreached fingers anywhere less than 1 inch of the intake side of the fan dramatically increases turbulence noise. Doing the same on the other side only just barely affects it, basically it's not significant till you get within about 1/4" inch.
What does this mean?
Well the Alpha HS fan recommended position is to have it sucking in through the shroud, coupled right on top of the HS. This is a guaranteed way of maximizing turbulence noise. If you simply flip it over, you should notice a ~2-3 dB improvement in noise. Even better is to mount it on a 1" funnel on the back panel, blowing out, with a tube or duct fitted tightly around the fan & the HS. (The funnel is unnecessary if the back panel vent is completely unobstructed.) The duct I have used is flexible vinyl duct, which is basically a coil of stiff wire with a vinyl skin on it that accordians in length when pulled. Used to exhaust bathroom fans, available in 4-5-6-7-8" diameters, totally easy to work with, just a fews bucks for many many feet. Best when stretched to minimize wrinkles that can cause turbulence.
Sorry for the l-o-o-o-o-ng post. Got carried away...
Last edited by
MikeC on Fri Sep 20, 2002 12:27 pm, edited 1 time in total.
I can only find some aluminium ducts , grrrr!
