mike - Thanks for the suggestions! I'm definitely considering a "room in a room" solution at the moment.
For AC, I have found wall-hanging units that go as low as 22db.
That space is probably too small for a vocal booth--at least one that is comfortable to be in, since once I treat it with acoustic panels you'd be brushing up against them all the time.
Update:
Photos of the current progress (now that the missing wall has been added.
Looking from the hallway at the entrance of the studio:
Standing inside the studio in front of the balcony door, looking at the entrance door:
Standing in front of the storage space opening, looking down the length of the studio:
Standing at the other end of the studio, looking at the storage opening space:
Close-up of the wall that was just added (yes, it's pitifully thin. I'll be adding a second wall to it):
I've started building a 3D mock-up in Sketchup. It's still not done yet though (missing details like the support beams, inner isolation walls/floor/ceiling, isolation plugs/doors for the glass window/doors...etc), but this is the direction I'm taking:
So now I have some new questions:
6) What is the best way to add soundproofing to the existing construction (which is all concrete)? Is adding another layer of concrete wall the best way to go? Or should I do the wooden frames with glassfiber panels between them thing? When doing either, should I be leaving an air space of a couple of inches from the original walls?
7) For the floor, I have seen conflicting views. In Jeff Cooper's book, he has diagrams of wood floating floor, but in Rod's book, he says that it's a bad idea to do floating wood floors and strongly discourages it. Who is right?
For my situation, my studio is on the 2nd floor of our apartment (it's a 2-story loft apartment in a high rise. We have other units below and above us. We're technically the 7th and 8th floor. My wife isn't concerned about any sounds I make upstairs while she's downstairs. She's never complained about my music in the 5+ years we've been married. So, I'm not as concerend about the floor isolation in terms of bothering others, and my wife is prett quiet in general, so I can't imagine her making enough noise downstairs to travel up from the ceiling into the studio. Or am I wrong? Should I bother with soundproofing for the floor, or just add a wooden floor on top of the concrete and call it the day?
8 ) All those support beams you see on the ceiling and at some places in the walls--how do people generally treat those? Just ignore them? Treat them like any other corner and put bass traps around them? Since I need to build an inner layer of soundproofing walls/ceiling/floor anyway, maybe I should just make the soundproofing walls flush with the beams so they no longer protrude?
9) Should I just build a soundproofing door for the storage space and treat the room as if that storage space does not exist, or I should turn that storage space into a giant bass trap? If I do, would it throw off the symmetry of the room?
10) For isolation and absorbtion, as I understand it, you can use cheaper material as long as it has similar density to the often mentioned glassfiber stuff (703, 705). So what are these cheaper materials? It also seems that rigid glassfiber panels are preferred because they hold their shape better when making soundproofing walls and acoustic panels. Is that the only reason?