I don't understand how I can get the mic placed into the same position within millimeters each time I test.
Just measure from fixed positions in the room. For example, from the floor upwards, from the face of the studs on one wall inwards, from the face of the glass in a window inwards. Etc. Or mark the exact position on the floor with tape and marker pen, then position your mic stand with a plum bob so the mic is exactly above that spot, an exact height above it. Or mark a spot on the ceiling in the same way, and hang a plum bob from that of the exact length you need to get the mic at the right spot: Lots of different methods.
Can I start with some corner bass traps already installed, or do I have to remove what I have started to get back to a completely empty room?
Best to start with a completely empty room, so that you have a "base-line" measurement against which you can check all future measurements. It is very useful like that to be able to see what is working, how well it is working, and what still needs to be done.
I am enjoying the thread. Thanks for all of the detail. I will also be building a garage soon in a humid area (Memphis) and this has been most helpful.
It has been a while since my last post. I still need to get an audio interface and run the Room EQ Wizard software properly in the room to get the data for the treatments. I feel like I am dragging my feet on this. After spending about $52k on this project, I am having a hard time pulling the trigger on yet another expense even though it is small in comparison. That is my only excuse besides being busy. I will get it done though soon enough.
I thought I would go back and document the missing sequence of events here and then come back to the inside room acoustics issue.
My wife really wanted the project wrapped up for a crawfish boil party we were having, and the band needed a place to practice for some upcoming gigs so the finishing up (as close as possible) of the project was very quick and I did not get sufficient photos of everything like I was planning. The contractor finished up his work, I finished the sound baffle for the ERV vents, threw up some treatments inside to calm the acoustics down, got the bathroom functional, loaded in some equipment, and built a flagstone patio.
The room is performing wonderfully as far as isolation goes. I finally got around to taking some measurements with the sound level meter. Anywhere near the waterfall outside measures around 66 dB ambient level of the waterfall noise. I cranked up some speakers inside the room to 100dB with some music and did not even register anything above ambient noise level, nor could I hear anything. Around the back side the pool pump noise masks anything coming from the room. I had someone play my drums as hard as they could and I could barely hear the kick drum standing outside the window. I play my drums at 10:00 at night and nobody knows it. Not my wife sleeping in the house, or any of my neighbors. I need to be a little more scientific than that and have some measurements taken while the band is playing, but you get the idea. Everyone is impressed with the isolation this room provides. I am very pleased.
The fantech energy recovery ventilator and sound baffle inside the garage. The space I made for the air path through the baffle is a little less than what is recommended in this forum for quieting the air flow once I got the duct board inside. However, the fantech unit I specified has a low and high setting. On the low setting, it is very quiet but probably less air flow than what is recommended. We rarely stay in the room for long periods without the door being opened for some reason, so the fresh air on the low setting seems fine.
Thanks Stuart for the praise, encouragement, and help. You can certainly move it to the completed studios, but I will probably never say it is "finished". I still need to do the REW measurements, get the treatments correct, build the stage boxes for the wire I pulled through the conduit. Just fiddling around with it now. But yes, it is being used for sure.
I finally got the audio interface (focusrite scarlette 18i8), set everything up, emptied the room except for 2 1/2 corner bass traps already built, calibrated the equipment, and took some measurements using Room EQ Wizard software, RadioShack digital sound level meter, interface, and one powered speaker. First, here is the layout and some photos of the (almost) empty room.
While calibrating the sound card, I could not get the clipping error to go away whenever I set the input and output levels close to the same, no matter what I tried. I set the output level about 6 dB below the input level and the headroom worked out and I was able to take measurements. Does this soundcard measurement look right?
Here is the waterfall plot from the measurement of the (almost) empty room. I can only guess that the issues in the low frequencies mean I will need more than 3 corner traps and should add some at the wall to ceiling corners as well and the issues in the higher frequencies may be due to a parallel reflective ceiling and floor. I will try adding ceiling clouds next with a hard back surface and hung at a 12 degree angle to see if that makes a difference. I am just guessing wildly here.
I went ahead and put the 8 broadband absorber panels back up on the walls and re-tested as well. That did not seem to make much of a difference at all.
No correction necessary! Spot on. Thanks for adding that! I completely missed out on mentioning it... 
Way cool. And some really nice workmanship there too.