The ubiquitous Vocal booth
:roll:
Do you really need it?
Perhaps better to have one good sounding room? What kind of music are you doing?
As far as shooting the sound down the 10.5 ft direction, the 21' space is desirable because you need the space behind the monitor position to catch the reflections created by the angled walls (RFZ -Reflection Free Zone). This improves the monitoring by putting the direct sound to your ears and keeping rear wall reflections from bouncing back to you and causing phase cancellations at the mix position. (see first drawing)
I don't know how small you can go -- maybe John has a few rules of thumb, but you do have the garage ceiling to absorb/redirect frequencies as well, and maybe you don't need a rear couch listening area. (see 2nd drawing)
Best,
Dave
Image not preserved: Billins2.gif
Image not preserved: Billins.gif