Ok I'll start with telling you some info about some of the studios I've worked in. Some may still be in business, some maybe not.
My 2 fave studios are Manta Sound...and Sounds Interchange, both in Toronto, both massive. They also both have multiple studios like 3 sessions at a time.
Studio 1 at Interchange is to die for....a massive room, that can hold Toronto Symphony Orch one day, and a Rush session the next. When I was there it was all done ceiling to floor with beautiful hardwood.
The Control booth is like Starship Enterprize, and about 4 couches to relax, one session I did there in the 80's was $325/hour
Manta is just as big and cool and has a video arcade for when someone is not needed. These guys also had lots of wood.
Wood and carpet are good for Studios, but theye are not soundproofers...a buddy of mine has a completely carpeted, floor, walls, ceilings....it still bleeds to his home upstairs.
Cherry Beach Studio in Toronto had a major vibration problem due to the TTC Streetcars, so they built a Room within a room on a rubber floor, also they had a neat sound baffler, flip over panels on the walls...one side carpet, other side mirror, they could set the room for sound, and reflect sound with the mirrors.
NorthHumberland Music Studio, is in a Barn....very nice also, not much sound proofing needed, it's in the middle of a farm property.
What I have learned about sounproofing:
1) Treble is weak, Bass is indestructable (soundproofers fight bass)
2) Bass loves corners, get rid of the corners in your studio
3) Best soundpoofing is a room-within'-a-room, with an air gap in between the walls, rubber floor
there are also lots of great soundproofing materials, but most are wickedly expensive.
More later