If you want it even quieter, try attaching some ducting to the muffler, or drape something over the end that is pushed by the exhaust., etc. Adding a small amount of resistance or a bend further muffles the turbulence and noise coming out of the muffler. At least with my fan (Can-Fan Pro 6-inch), it seems this turbine-type fan runs quieter (noticeable inside the tent) with some resistance or bends in the outflow. With my first grow, I simply taped a pair of long underwear on the muffler output (with the air coming out the pant legs).