As has been said elsewhere here in far too many threads, from the s7 on up all Antminers require fans that deliver high-static pressure and yes that means LOUD ones. CFM is only a small part of the airflow spec and normal 'high CFM' fans only deliver spec'd airflow against zero back-pressure. Put them on a s7/s9 etc and airflow drops like a stone very quickly leading to an overheated miner.
I know this is an old thread, but found it looking for info on the fans. Do you know how it works specifically for the S9? Which fan needs to be faster - the front or the rear one?
I noticed the different fan speeds, so I assume the reason is to create either negative or positive pressure inside the miner, I'm just not sure which one. Also not sure if they are controlling the fans separately to create the different RPMs or together, and let the different RPM ratings of the front and back fans take care of the pressure difference.
Any insight into this would be appreciated.
As for front/rear speeds, even Bitmain can't seem to make up their mind. As a power systems designer, personally I'd put the faster fan on the intake side to provide higher pressure airflow in to overcome resistance from the turbulence all the little heat sinks make. What might change is the noise.