In an auditorium or a cinema hall, the roof and walls are covered with draperies or compressed fibreboard. Why?

Hint: In an auditorium hall, the sound from the stage undergoes reflection multiple times by hitting the walls and the roof. As a result, there will be reverberation and echo formation, which may be a disadvantage. Therefore, the walls and roof must be covered by a material that absorbs sound waves.

Complete step-by-step answer:
Sound is a wave that is produced due the vibrations of air molecules. Since sound is wave, it can travel through different mediums.
When a wave passes from one medium to another medium, it is partially absorbed, partially reflected back at a specific angle and the rest transmitted through the medium. This rate of absorption, reflection and transmission depends on the material (medium) of the body on which the wave is incident.
There exist materials that are good reflectors of sound and there also exit materials that are good absorbers of sound.
When we talk about a concert hall, the sound waves from the stage must reach the all the audience sitting in front. If so may happen that the sound may reflect multiple times and reverberation may occur. When the sound reflects and comes, we hear the same sound twice or multiple times. These repeated sounds are called echoes. Hence, there will be echoes produced in the hall due to multiple reflections.
This may have a disadvantage as the audience will not be able hear the exact sound clearly. Therefore, to reduce the echoes or reverberation of sound that walls and roof are covered with a material that is a good absorbent of sound that will absorb the unnecessary sounds.
Draperies and compressed fibreboard are good absorbent of sound waves. Hence, In an auditorium or a cinema hall, the roof and walls are covered with draperies or compressed fibreboard.

Note: One may think that if a reflection of sound produces an echo then why do not we hear echoes everywhere. This is because humans can sense an echo only if the sound comes back to us in a time interval of more than 0.1 second. When the obstacles are closer, the sound returns in less than 0.1 second.