if you're building from scratch here, you might as well do it the right way. If music, you'll do well with the coffered 9' ceiling. I would also NOT do exactly 14' x 21'. You will get much much better sonics if you add 6%-7% difference between multiple dimmensions. (i.e, add 6% to either the 14' dimmension, or 6% to the 21', but not both).
Also, a HUGE benefit of your coffered ceiling is that you can put a massive bass absorber in the ceiling/attic above! In a room of that size - if it's a issolated, sealed in space- you can be assured that you will need to do something with all that excess bass energy. Building a bass trap in the attic space above -accessed from within the inner spaces of the coffering (spaces covered with acoustic transparent fabric- transparent to space above) is an ideal way to balance out the RT time in the room, and make for a supper fast, dynamic, solid bass response from the system. The norm is a slow, fat, overly warm bass sound from sytems, with too much bass overhang covering the sound - especially in smaller acoustic spaces such as this. (yes, 21'x14' is still a small acoustic space).
Regardless, keep the hard dimmensions of the room at multiples that are at least 5%,6%,7% different than each other dimmension, to keep bass modes from piling up and compounding in the room. Results will be a better foundation.
There is of course tons more to the acoustics of your space, building techniques, etc. But I hope this coveres the issues you mentioned.
Also, a HUGE benefit of your coffered ceiling is that you can put a massive bass absorber in the ceiling/attic above! In a room of that size - if it's a issolated, sealed in space- you can be assured that you will need to do something with all that excess bass energy. Building a bass trap in the attic space above -accessed from within the inner spaces of the coffering (spaces covered with acoustic transparent fabric- transparent to space above) is an ideal way to balance out the RT time in the room, and make for a supper fast, dynamic, solid bass response from the system. The norm is a slow, fat, overly warm bass sound from sytems, with too much bass overhang covering the sound - especially in smaller acoustic spaces such as this. (yes, 21'x14' is still a small acoustic space).
Regardless, keep the hard dimmensions of the room at multiples that are at least 5%,6%,7% different than each other dimmension, to keep bass modes from piling up and compounding in the room. Results will be a better foundation.
There is of course tons more to the acoustics of your space, building techniques, etc. But I hope this coveres the issues you mentioned.