Some suggestions to consider:
1/ Think through and identify what your goals are and what the limitations are. (i.e. whether aesthetics is important, or you don't care and just want to get the acoustics done correctly, how much money you are wanting to spend)
2/ Cheapest option: get some ideas from others here as well as contact some of the vendors/manufacturers that sells room treatments. They will give you some quick advice for free and all you need to do is buy the stuff and apply to your room. (If you want, e-mail me, and I will give you the list of all these vendors that I have compile together for my own project)
2/ Hire an acoustic engineer to do an actual design of the room for you. Rives is OK. They have one generic solution and they try to apply that to everyone with some minor tweaks. So if you don't like it, you are stuck with that. But they are the cheapest. I just hire a real acoustic engineer for about the same price or not much more and I think I am getting better results. But all you are going to get is recommendations of where to apply what types of off-the-shelve or custom-made acoustic panels.
3/ If aesthetics is important, and you can't stand staring at acoustic panels, you might want to do a combination of the following:
a/ Spend some money and find an interior designer that can work with the acoustic engineer to come up with some solutions that will look nice (than just staring at acoustic panels), or
b/ if you are design savvy, get some of these acoustic books and understand some fundamental acoustic principles. Dictate the (aesthetic) design of the room and challenge the acoustic engineer to come up with some aesthetically pleasing alternatives (than the acoustic panels).
I am currently doing #3 after spending some time interviewing Rives, some established hi-end shops, some acoustic treatment manufacturers, and acoustic engineers (who design symphony halls and recording studios). My background is in architecture, so I know enough about design to be dangerous. But I am sure the acoustic engineer curses my name every night, because I am always asking him to look at these ideas I come up with as room treatments (as they blend in with the rest of the design better rather than the ugly acoustic treatments that sticks out like a sore thumb).
In most of the books mentioned above, you will get some ideas of various theories of room dimensions as well as key points on room construction and basic ideas on room acoustics/treatment.
One concern I have off the top of my mind is the fact that this room will be on top of the garage. The room will be very "bouncy" and it may affect your bass response. A friend of mine has the same issue, and he's spent tons of money on Rives to try to resolve the issue and so far without any good results. I told him to park his cars outside and put some posts underneath the floor to stiffen the floor. But his boss (wife) would not go for that. Ha! Ha!
Send me an e-mail if you want to discuss further. I have all these books at home, so I can do some quick search for you and provide you with some "unprofessional" advice based on my own experience and the knowledge I have gained. If nothing else, I think it would be important to organize your thoughts first to be able to provide enough information to get the best out of the vendors or design consultants.
FrankC
