Tuning a room

[laptoppop]

I'm getting a new room and I'll be tuning it from scratch.

While I'd like to do something more robust, it looks like its going to have to be a multi-purpose room, shared with the family, with both tracking and mixing in the same room. The room is 21 feet by 15 feet 7 inches.

I'm wanting to attenuate the sound going in and coming out. The walls already have a layer of standard wallboard on them, so I'm thinking that I'll hang plywood on top of that, and another layer of wallboard on that. IIRC, Joe Egan did that, and it sure made sense. I'm thinking the added mass and dissimilar layers should give me some nice reduction. I'm planning on a second door, so the entry way to the room is a two door unit. I'm thinking I'll put rigid insulation on the side of the second door to absorb the vibrations without making a nice little resonator.

Does this make sense so far? How much different would 1/4" plywood be versus 3/8" or 1/2"?

For the floor, it will probably be a wood laminate.

The ceiling currently is a drop down ceiling -- about 9 feet high. I'm thinking about opening it up to the attic. Its a peaked roof, about 20 feet high. The center of the peak is offset by about 4 feet from the center of the room.

The room itself has some other fun issues. There is the entry door that I would be doubling on one of the smaller walls. On the opposite wall is a 3'x4' window. This will be a double paned wood window with dividers. On one of the long walls is a sliding glass door to the backyard. On the other long wall are three doors to bathroom, closet, and utility room respectively.

Just to get started, I plugged numbers into a Sabin-Mode spreadsheet I found at studiotips.com. If I understand correctly, I'd like to get the RT60 to be evenly about .5 seconds across the various frequency bands. From the spreadsheet, it looks like I've got to lower the rt60 significantly, especially in the 125 Hz, and 250 Hz range. I also expect reflection problems from the parallel walls.

Here's my plan right now -- put the extra covering on the walls, move in the furniture, etc., then rent a Terrasonde meter and find out what the RT60 really is. I haven't used one before, but it looks like I can measure the RT60 and frequency response, etc. directly, so I can plan better how to proceed. Does this sound right?

I'm hoping that I can tame the room -- not to perfection, but good enough that it is enjoyable to play and mix in. It will be interesting to plan out where I should put what in terms of absorbers/diffusers/whatever.

Please -- any thoughts about the best way to proceed? Any gotchas I should watch out for?

Thanks,
-lee-