Listening Room Dimensions... I'm building one and need your help

Another "what @audiokinesis said" OR, if you do it alone, here are the ideal height - width / length relations to minimize modes. If H is for height and X & Y the two walls of a rectangular room, then H=1 
to X=1.9 to Y=1.4/1.3 is best
X=1.5, Y=2.1 / 2.2 is second best

The above are all good suggestions.  I did similar research when building my dedicated audio room. (You can check out my virtual system)  I must say, when I first listened to my room before any treatments were done, I recall a distinct disappointment relative to a small random audio room in my prior house.  

Just last night I was thinking how pleased I was with the transformation of this room.  The change was due to room treatments and tinkering with placement issues.  And I'm not quite done.  Still have some diffraction additions I will make.  

So ideal ratios are a good place to start, but like a fine musical instrument the room will need to be tuned for sure.

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

There are obviously many, many issues involved in constructing a good-sounding room, and room dimensions is only one. As long as you don’t design a room that’s nearly square then there are probably other factors to address that may be even more consequential to your ultimate result. The best advice I can give is to educate yourself on at least the basics and principles of constructing a good dedicated listening room. Knowing just the basics will put you in a MUCH better position to make critical decisions even if you end up working with an expert. I went through several books, etc. in trying to get a handle on this myself, and BY FAR the best thing I did was read Premium Home Theater: Design and Construction by Earl Geddes (no longer in print, but you can download it at gedlee.com). Don’t let the title fool you — this book is as relevant to building an audio-only room as it is to HT as the design principles are largely the same. More than any other source I found, this book explains both the theory and practice of designing and constructing a good-sounding room in a very approachable way so even a layman like myself can easily understand and comprehend them. And it also gives specific recommendations on the actual products to buy to complete a project, which I found most helpful. Before I read the book I didn’t even know what a resilient channel was, but now I’m confident I could instruct any contractor how to build a very good-sounding room (and I know nothing about construction). Even if I wasn’t building a listening room I’d find Earl’s book to be an interesting read. After all, I think most of us would agree that the room is one of — if not the — most influential components in our sound systems, so why not get a little smarter about it? Hope this helps, and best of luck with your project.