If the shortest dimension of your room is 16 feet (or more) it's best if you can place your speakers (centered, of course) on the long wall. This allows two things to happen:
1. You can pull the speakers (faces) out 3 feet from the wall behind them (mandatory with planar/bipolar speakers), and you can sit (with your ears) 2 feet from the wall behind you. This allows roughly 11 feet between you (your ears) and the (face of) the speakers -- which is enough distance to put you beyond the "nearfield" listening configuration (AKA, the "speakers-as-headphones" layout ;--)
2. The short (end) walls, now being relatively farther away from you and also farther away from the outside edge(s) of the 2 speakers, than in the "short wall" setup, the path of secondary (sidewall) reflections (speaker-to-sidewall-to-you) becomes so much longer than the path of first arrival sound (speaker-to-you) that even without any sidewall absorption, the soundstage is not compromised, and the volume of the room is fully utilized.
So in a normally furnished room (carpet, maybe drapes, a typical upholstered furniture grouping -- coffee table, end tables, bookshelves (or drapes) behind the listening position if possible, you shouldn't need any "room treatments" except possibly bass traps, if the room develops standing waves -- and with cathedral ceilings, that will be unlikely.
As for those cathedral ceilings, mostly they won't be a problem. Yes, they reflect stuff, but, like with the sidewalls being far away, the secondary reflection path length is too long for your brain to combine it (timewise) TOGETHER with the first arrival sound (which when THAT happens is what screws up the audio hologram ;--) Low flat ceilings are a problem, not high angled ones. And if you are using panel speakers, or line arrays, like Pipedreams, or tall ribbons like Dali's, you won't have problems even in a low ceilinged space because those speaker types have almost no vertical dispersion anyway.
1. You can pull the speakers (faces) out 3 feet from the wall behind them (mandatory with planar/bipolar speakers), and you can sit (with your ears) 2 feet from the wall behind you. This allows roughly 11 feet between you (your ears) and the (face of) the speakers -- which is enough distance to put you beyond the "nearfield" listening configuration (AKA, the "speakers-as-headphones" layout ;--)
2. The short (end) walls, now being relatively farther away from you and also farther away from the outside edge(s) of the 2 speakers, than in the "short wall" setup, the path of secondary (sidewall) reflections (speaker-to-sidewall-to-you) becomes so much longer than the path of first arrival sound (speaker-to-you) that even without any sidewall absorption, the soundstage is not compromised, and the volume of the room is fully utilized.
So in a normally furnished room (carpet, maybe drapes, a typical upholstered furniture grouping -- coffee table, end tables, bookshelves (or drapes) behind the listening position if possible, you shouldn't need any "room treatments" except possibly bass traps, if the room develops standing waves -- and with cathedral ceilings, that will be unlikely.
As for those cathedral ceilings, mostly they won't be a problem. Yes, they reflect stuff, but, like with the sidewalls being far away, the secondary reflection path length is too long for your brain to combine it (timewise) TOGETHER with the first arrival sound (which when THAT happens is what screws up the audio hologram ;--) Low flat ceilings are a problem, not high angled ones. And if you are using panel speakers, or line arrays, like Pipedreams, or tall ribbons like Dali's, you won't have problems even in a low ceilinged space because those speaker types have almost no vertical dispersion anyway.