A concave board is placed behind the speakers of a large hall is known as a sound board. This is used so that voice can be heard to every person sitting in that hall at a considerable distance. The sound board works as follow:- the speakers are mde to stand on the focus of the concave board. The concave sound board reflects the sound back to the audience (meaning it donot allow sound to go in different direction). This helps in uniform distribution of sound in the hall and even the person sitting last in the hall can hear the sound clearly. It can be seen from the diagram