I accidentally bought a house with an outstanding audio room. My dealer says it is one of the best two he has ever heard. He has been in the high end business for over 20 years and installs many megabuck systems in custom rooms. It is about as irregular as possible. You can get some ideas from my photos.
Broadly it is a big Z with the audio located at the bottom of the Z. Broadly speaking the Z is about 45’ by 40’. There is a bar, hallway, and more than one nook in just the bottom leg of the Z. A bay window and fireplace offset to the left at the top of the Z. There is hardly a 6 foot wall segment uninterrupted anywhere other than behind the speakers. Even the ceiling is more than one level. The speaker wall and left and right sides are underground… completely dropping the noise floor to mid 20 db. Cement slab under carpeting.
There is no special wall material… or fancy / expensive audio construction. My audio treatments are heavy thickly woven wool wall hangings (oriental hand woven carpets) also, there are bookshelves and record racks I put up without thought to location only convenience.
It is also located downstairs and on the opposite side of the house from the major bedrooms. My partner is disabled and sleeps during the day. I can crank up the volume and she will never hear it.
All this serendipity, made for one in hundreds the perfect audio room..
So, I think if I had thought about building an audio room a couple decades ago, I would have researched the perfect three dimensions. I would not do that now. I would look at the whole house… the whole floor and think of a very irregular open area away from the rest of the house… below ground with halls and irregular walls everywhere to define an open audio area with lots of space and irregular short wall segments to deaden and prevent reinforcement.