Dedicated listening room 25' L X 9' H and ? W

What would be the best width for a room that is 25' long with 9' high ceilings? It is also said that non-parallel walls is the way to go. Would a slow taper of one foot in width difference be enough from front to back? (Ex. front wall 17' and back wall 16'.) Would rounded corners also help? Thanks for any help that you can give.
Talk to Rives. They are the best!
We can definitely help. As to the width, you are in the ballpark. It takes a little time to determine the exact width. I do like non-parallel walls, but you have to consider the whole room for this to work. You also have to consider that the largest problem in room modes will be from floor to ceiling as this is the largest surface area of two parallel surfaces. Splaying the walls actually doesn't get rid of room modes, but it does reduce the Q factor (sharpness of peak) and does make modal distribution better (in most cases--there are exceptions of course). As to your question about corners--no rounded corners is generally not a good idea, but building frequency dependent traps into the corners (based on the room response) is a good idea. Feel free to give us a call or look on our website to get a better idea of what we offer.
15.45 feet wide for a 25 foot deep room.
Use the Golden Ratio which is 0.618 = (sqrt(5)+1)/2
This is also the ratio used in the Cardas speaker placement formula.
This is a truly remarkable number with numerous websites dedicated to it:
any dimension greater than 13' up to say 17' 6" would be a good start.

Reason, if you double H=9 you get 18 and this would create too many like room nodes canceling each other.