My DC is very quiet - almost inaudible up to about 20hz - then the noise increases dramatically above 40hz - to quite the din at 60hz. I configured a trimpot provided by the mfg of the VFD to reduce the rpm and leave it on as an ambient air filter at 20hz since there is inevitably dust created by using tools with poor collection (biscuit joiner for instance). I have yet to test the air to see how effectively it works as an ambient filter ... not actually sure how to do that testing, though I vaguely remember reading a Bill Pentz article on the process. I'm also not sure what the impact is to the equipment of running at a lower speed.

Must say, I am shocked at the low noise readings of your dust collection, that is truly impressive! My readings posted here.