The arched ceiling and walls of large halls often reflect the sound waves. These reflected sound waves interfere with the words of the speaker. This problem is solved by hanging curtains, putting up screens or by using soundboards. A soundboard is often a concave rigid surface.
The speaker is located at the focus of the sound board, placed behind the speaker. The soundboard renders the reflected sound waves parallel. This enables the sound to reach large distance. The sound board prevents the spreading out of the sound waves in a different direction.