Those are great advice, however, none of them actually answers his question, regardless of using a different quieter fan, he would still want to set a fixed a static speed on them, OP a simple search would have answered your question, I wrote a complete yet simple guide on how to achieve fixed fan speed on almost all Antminer models


Using the same tricks to show the fan settings, you get to see the LPM option on S9k and S9e, I have not tested it, but it does show, please follow the same guide to show the hidden features.