Never heard this, I have heard the three sides not divisable between them or by a number. something like 15 x 21 x 7, where 3 or 7 would be a common or 12 x 16 x 7 where 4 would be the common
axiom_man,
You are supposed to avoid having any of the dimensions divisible by the same number if at all possible. Since it sounds like you will be picking the dimensions then you can avoid having them all divisible by 3 (15.5 is nearly divisible by 3). I would just change them all slightly to avoid this issue, which can cause acoustic problems.
I read this in a good article somewhere but don't remember exactly where. It's just something that has been ingrained in my memory for a while now. The best I could find in a quick google search was this link:
http://www.gcmstudio.com/acoustics/acoustics.html . About a third of the way down it mentions the following.
"Redundant room dimensions or evenly divisible room dimension ratios will compound standing waves so this is why room shape is almost as important as room size"
So, like I said since you are building the room and are free to choose the dimensions then just try to add or subtract one foot to each dimension so that they are not all divisible by the same number. Then you will have essentially the same size room, but less problems with some frequencies being boosted or missing in certain locations in the room.
