||12-18-2009 02:39 PM
I've done a lot of reading on sound isolation (being a bagpipe player).
Most of what you'll read about sound mitigation is based on not knowing the frequencies of the sounds you want to block. There are different strategies you can use for different frequencies. Do you have a high pitched sound, a low pitched sound, or both. I'm guessing it's probably both. You can find out easily if you can make a recording and open it up in Audacity -- open source audio editing software and look at the frequency analysis window.
Somewhere on the internet you can find really cool graphs that show how much of each frequency is absorbed by all sorts of different methods.
Anyway, high frequency sound can be stopped with mass. If you put up more sheetrock, less noise will go through. It's pretty much linear -- if you hang a layer of sheetrock on the room side of the studs it will stop a lot of high frequency noise. Putting batts of insulation will help some, but not a lot. Putting acoustic ceiling tiles (the 2x4 suspended ceiling kind) will help a lot. So will 30# felt.
I'm guessing the room doesn't have a door. If you can put one up, and if it's a tight fitting solid door, then it will stop a lot of noise. But you have a conflict here, because your furnace needs make-up air. I can't help with that You can probably make some kind of baffled tunnel for the air to flow through. The baffles will work to muffle the sound while allowing the air to flow through.
It could be that a lot of the sound you hear in the basement is high frequency noise. If that's the case, then putting carpet (or an actual acoustic treatment) on the walls of the utility room will prevent the high freqency noise from bouncing around so much.
You mention sound going from the downstairs to the upstairs. If it's low frequency sound (the rumble of the motor) then you can put drywall on the ceiling and use special sound-isolation clips. You attach these metal clips to the studs, and then hang the drywall from the clips. That will help to stop the vibrations from going straight through the drywall, into the studs and the then radiating out from the floor. Put some insulation there too.
Unfortunately, sound travels through air, and your furnace moves air. One of the biggest issues in really isolating a room is that sound will move through the ducts and get from one room to another that way. Not much you can do about that.
A lot of the literature you read is also about trying to totally stop the sound. What I ended up doing with my practice room was to design it to greatly reduce the sound in the higher frequencies. You can try a similar approach -- figure out exactly what sounds are most irritating and try to reduce them.
Graphic Standards for architects (in your local library reference section) will show you how much sound can be absorbed with various wall and ceiling strategies.