Found the world's quietest fan!
A few weeks ago I was going thru this ordeal trying to squeeze my new Emo XPA-5 and Denon 3312ci into a 1970's era stereo cabinet. My prior Denon 790 worked pretty well in there, though I had removed the back panel to allow passive air flow. But the new equipment generates a lot more heat, and the amp itself was just a few inches too large to even fit inside the cabinet. So I ended up setting the Emo on top of the cabinet and moved the cable box to the inside.
The new Denon however continued to run hot even though it is not driving any of the five main speakers. So I went on a search for a cooling fan quiet enough that it wold not interfere with quiet music and movies. Even the small cooling fan in my Argosy HDD-equipped media player is noticeably annoying during quiet passages. I looked at a lot of fans on NewEgg but most were 20db or louder. I did not want to mess with speed controls etc. I work near Microcenter so I checked out their website. Their inventory was categorized only by brand. Something drew me to Enermax. I've used some of their power supplies in the past and they always had unusually innovative cooling solutions. So they had this one fan listed at only 11db, so I picked one up last week.
The fan is the 120cm Enermax T.B.Silence... http://www.enermaxusa.com/fan_uctb12.php
Click on the specifications tab to see how it stacks up. It has an interesting batwing blade design and at only 900 rpm is indeed very quiet.
Fortunately it draws only 1.8 watts, which I calculated - dusting off technical formulas I haven't used in decades - that it could be powered by the second 150MA trigger output on the AVR (one is used by the Emo). Additionally I added an 18 ohm resistor to slow the fan by 10% and also lower the current draw, just in case the specs were off by a little (the Denon is rated at 150ma and the fan draws 150ma, and I know from experience that there should be a little slack designed in).
Originally I intended to mount the fan using rubber grommets, which fortunately I have a small supply in my tool case, but that didn't work out as I could not force the grommet into the hole, even after drilling out the holes several sizes larger. So instead just mounted the fan directly to the back corner of the wood cabinet, using three screws. Everything worked perfectly first time I fired it up. I stapled a sheet of construction paper to the back of the cabinet to block air being drawn in directly adjacent to the fan and force airflow across the AVR.
In this photo the AVR is on so therefore the fan is on, though it appears to be still. The whole thing is a kluge but it works very well. After only one week I've even stopped constantly checking to make sure the fan is on... yes, it is that quiet!