In that size room, you're probably looking for a decay time (pure HT) of around .25 seconds in the mids. Below that it could go up to maybe .4, above that, maybe .20 Those are RT60 times so address it accordingly. You can find absorbant coefficients for materials at
www.bobgolds.com. In addition you need to account for the coefficients of:
- wood doors
- single or double drywall walls
- carpet (and if it's over wood or concrete
- glass for any windows
- type and number of seats
- number of people
If you model your room first with the basics (walls, doors, furniture, people), it will become painfully clear what needs to be addressed. Add in an acoustic ceiling instead of a drywall ceiling and see what happens - try to get the curve to fit then without having the high frequencies too dead before you get the bottom end under control.
As for the clap test, sorry, but that's not really something to hang your hat on. First of all, it's using a source at the same place where the listener is - which doesn't happen. Also, it's upper mid and high frequency only and tells you nothing about vocal range and bottom end control - nor does it tell you if the room is TOO dead in the highs because the bulk of the frequencies are where our hearing is very efficient and it's a very short, dynamic sound - very much unlike most of what you'll be listening to in a home theater.
Sorry - it's one of those logic things. All dogs are mammals but not all mammals are dogs. If the room is right, the clap won't give you an echo - but - just because you don't get an echo, doesn't mean the room is right.
Seriously, I can put curtains on every surface of a room, ceiling included and you'll pass the clap test - but it will sound like do-do when watching a movie. It's WAY too dead in the highs and has basically zero addressing the bottom except chairs, people, and drywall wall cavities.
Do your homework. Figure out what your target range should be, look at what the basic structure and furnishings will give you, then adjust and treat from there.
Bryan