Basement Iso Room

Hi, I like to thank everyone for all the great info on this forum. Im in the process of getting ready to build an iso room in my basement. The basement is in a old building built in 1868. Heres my details. My main studio space is on the second floor. More of a control room.There is one tenant below me. The space is used for song writing and editing. I will never do drums or anything loud here on the second floor. I have a raw basement that I am looking to utilize for my amps. Because of my situation I am going to build a iso room in the basement. The only thing down there will be the cabinets and mics. I will run speakers cable and mic lines to this room. The speaker lines and mic lines will be on a wallpanel. Power will be run to the room as well The space I am going to isolate is 22 feet long 42 inches wide and 7 feet high. The rest of the basement is for storage. The floor is cement. There is a one foot brick wall that seperates this space from the rest of the basement.There are three openings in this wall. I plain to close off two of the openings and have a door built in the last one. Heres what Im thinking. The floor above this space is directly on the joists. So between the joists that are 24 on center I an going to put in two layers of 5/8 drywall between the floor ( ceiling ) joists caulk the seams then insulated with R-19.. Next will be the room within this space. I decided there really is no need to float the floor. The walls will be 2x4 16 on center 1/2 inch off the brick. Insulate with R-19 and double drywall with 5/8. After the top of it is framed I will and more R-19 between the studs then double drywall the ceiling as well. Green Glue will be used between the layers of drywall as well as caulking the seams. Im wondering if I am on the right track and if I am leaving anything major out of this plain. The one thing I wan to get right if the space below the flooring above. Is the double drywall enough ? Thanks for any help !

is the width only 42 inches wide? also, is it your apartment above this space or the other tenant?

Hi yes its only 42 inches wide.The widest cabinet that willbe down there is 30 inches. The space above is the other tenant Thanks