Question

why and how is reverberation removed in an auditorium or in a big hall

Answer 1

high absorbent materials like fiberboard and other material are used so that the repetation of the sound is absorbed
#shimmers

Answer 2

the reverberation is commonly and highly seen in the big halls or rooms and even auditorium because there the reflection of sound is taken place in less than the 0.1 seconds its just totally opposite to the echo system.

the ways followed to remove the reverberation in an auditorium are :
1. the roof and the walls of the auditorium will be covered with the sound absorbent materials like fiber board, draperies.
2. the materials in the auditorium also will made with some selective materials so that the reverberation process will not take place.