Completely horrible,I have no idea.It will sound completely different.
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I recently built a dedicated room in a basement which had block walls and a concrete floor similar to yours. One of the walls had existing openings over the blocks between the joists. At the start of the project I carefully measured the frequency response of the room and measured again at various points along the way.
Despite using ideal room proportions, an insulated and suspended ceiling, and insulation and drywall over the sidewalls, the fact the room was now fully enclosed and airtight caused significant low frequency modal peaks that were not present at the start. The low frequency energy was reflecting off the hard cement surfaces and simply had no place to go to be disipated. This energy was previously escaping from the ceiling corners between the joist.
Thankfully, there was a solution which required trapping this energy with corner traps and constructing a hemholtz resonator behind the rear wall. But even though this solution worked very well, the measurements were not quite a good as where I started and extra effort was necessary.
Also, your proposed room will be very close to square, which will exasperate the modes even further. If there is any way to vary your room length/width/height proportions to a greater extent (see Cardas web site) and/or leave openings over the walls between the joists, I would highly recommend it. Even then, plan on plenty of corner trapping.
You might take a look at my system to better understand what I am describing. Making your room succede is possible, it just takes some careful planning and execution.
I've done a basement media room, and aside for some of the comments already made, attention needs to be paid to isolating the sound that the furnace will make. Not only the furnace itself but also the duct work. It is very difficult to get the noise floor down, especially if the furnace is old and the blower unit is not properly mounted in the housing. Depending on the unit, it may pay to look into replacing a replacement blower.
Thanks for the input everyone. I think what I'll do is wall in three sides instead of all four. That should trap enough heat to keep me more comfortable while working but keep it from being a little box of a room. Honestly, it is a pretty noisy area with the furnace and everything so there is only so much that can be done. I will have to replace the furnace in the next couple of years and will hopefully get one with a quieter blower.
Bentx2, I believe that if you leave a foot of space above the side and rear walls, and cover only the inside of the walls with drywall, the bass will move right through and not magnify the modes in your nearly square room. You can stuff insulation between the studs and still have an enclosed room for heat purposes and some reduction of the furnace noise. If noise reduction is needed to the upstairs, you can insulate the ceiling and add two layers of drywall for mass. Good luck and let us know how it turned out.