I am in the same situation as yourself. I have a CyberPower OL1000RTXL2U which is an online pure sine wave output UPS. For my purposes a line interactive will not work since I am using the UPS to convert from the output of a step down transformer which provides me 120V@50 Hz to the US standard of 120V@60 Hz. The fan that comes with the CyberPower is made by NSTECH and is a model PSAD18025BH (Data sheet here: http://www.nstech.wtools.com.tw/PDF/ns26_PS8025.pdf
). If you look at the data sheet it is horrendously loud and in real life it is loud as well. For it's normal intended purpose, which is to sit in a data center rack, this noise level is acceptable, but sitting in front of my home theater, it's unacceptable. I opened up the UPS and it's a simple 3 wire fan with the third wire being the RD (rotor lock) signal.
I've been looking around for something that can push roughly the same air but at a much lower noise level. I really like the Be Quiet Silent Wings 2 fan (26 CFM @ 14.5 dbA) but that is much lower than the 69.2 CFM rating on the NSTECH.
I think I can get away with just 40-45 CFM since I don't plan on running the UPS past 50% load.
Anyone have any suggestions on higher CFM fans that still provide a low noise rating (<= 24 dBA?