Success! Lowered noise to
60 db! Now it is not audible anymore when the door is closed. It is a comfortable level even when I'm sitting beside the rig, just feels like in a server hall, not industry warehouse with lots of machinery. The room's ambient noise is 33 db before running SP20
After 15 minutes of running, with a front temp of 25c degree, I get 60c degree for back temp, and none of the ASICs reached minimal watching threshold of 85c degree
The test method learned from Biffa:
The fan speed regulator is running at 5V, but with such a large load, it gets very hot. I hang it up at the exhaust to cool it, but will try to make the adapter to utilize the PWM socket on board later
Hero of the day: Delta AFC1212DE.
This fan consumes 3A at its maximum output, so should be as powerful as the stock fan. However, it has 7 blades instead of 5 on the stock fan, so the noise performance is much better. In principle any 7-blades 120x38mm delta fan could dramatically reduce the noise