Question

what happened if cinema hall wall, roof have smooth ceiling? ​

Answer 1

what sound waves wont bounce

Answer 2

Flat and smooth surfaces of ceilings in cinema halls tend to reflect sound that results in echo and creates a problem for the audiences.

Therefore, the ceilings are made with a curved surface so that sounds can be controlled, prevent them from bouncing around and to help the audiences hear better without any disturbance.