One reason for my suggestion of "hire a professional" BEFORE building the room is that there are differing schools of thought on the importance of room dimension ratios. I am quite familiar with golden ratios and their theoretical desirability, but in a particular application their use may not matter as much as maximizing room size, and/or they may not be practical for whatever reason... and if not, then what ratios do make the most sense?
Also, in a purpose-built dedicated audio room non-parallel walls are often appropriate, and if such is the case for a big dual-use room like this, might as well get those angles right to begin with.
Finally, construction techniques and materials choice make a significant difference. For instance, sound isolation to achieve a low noise floor will involve proper attention to the air conditioning duct work, as well as to any other openings to the room - including the wall sockets. And low-frequency damping behavior can be built into the walls for far less money than it would cost to add it after the room has been built. And so forth.
Duke
Also, in a purpose-built dedicated audio room non-parallel walls are often appropriate, and if such is the case for a big dual-use room like this, might as well get those angles right to begin with.
Finally, construction techniques and materials choice make a significant difference. For instance, sound isolation to achieve a low noise floor will involve proper attention to the air conditioning duct work, as well as to any other openings to the room - including the wall sockets. And low-frequency damping behavior can be built into the walls for far less money than it would cost to add it after the room has been built. And so forth.
Duke