What would be a correct ceiling angle to be efficient ?

The title say it all but i would also add : is the 15degres rule apply here too. Or just about anything out of parrallel is OK. Thanks Eric Arbour OrangeNumerik Montreal

Anything over 6 degrees will help with flutter echo; for RFZ without absorption you need geometry. Typically more around 30 degrees for ceiling over the mix position, but this depends on dimensions between speakers and your head, ceiling height, etc - it needs to be drawn out to scale to see where reflections will go, and angled enough to redirect them AWAY from your head... Steve