So I've cleaned up my office's server room, and now there's empty space to speak of, as well as less heat than there used to be. Still, though, the servers make a pretty loud droning noise (my guess is the center of the spectrum is around 1.5kHz), and it's pervasive enough that people sitting near the door can tell when someone walks down the hall because the noise from the server room changes. I'd like to make it quieter. The obvious solution, of course, is to close the door; the only problem with that is that the servers still make too much heat to close the door, and my company is too cheap to buy an A/C unit. Personally, I'd like to make it quieter without having to resort to A/C anyway, because the servers don't really need to be freezing-cold, and the less power consumed, the better. I was considering putting up some anechoic wall foam blocks, but of course they charge an arm and a leg for stuff like that; something like $160 for 12 square feet of the stuff, which is too rich for my blood. (Anything I do, I'll have to pay for, because there's no justifiable business reason to do anything about the situation.) What other methods could I employ to make things quieter? I'd also thought about getting a special door installed that has a ventilation grate in the bottom half, so that the door could be closed and cool air could still go into the server room and push hot air out through the ceiling vents, but I'll be damned if I can find such a thing.