Question

The shape of the soundboard kept behind the speaker on the stage of a big hall is
A. concave
B. convex
C. plain
D. uneven

Answer 1

Hint: The concave soundboards cover an effectively higher area. Once the speakers are placed at the focus of this soundboard, the reflected sound waves travel towards the audience in a straight line for effectively longer distances. This does not happen when the speaker is solely present. This event of distribution of sound waves is absent in case of other options.

Complete step-by-step solution:
Here we need to understand that by the soundboard, they mean the sound reflector. It is in concave shape since only concave objects allow for the wave to travel for a long distance.
These soundboards are placed such that the sound reaches each and every one of the audience. That is, the speaker is kept at the focus of the soundboard. As shown in the image

In the figure, Speaker is placed at focal point F.
These sound waves travel to the soundboard from the loudspeakers. These incident sound waves get reflected and deliver the sound to the audience present all over the auditorium.
Thus, concave reflectors enable us to gather the sound from the speaker and direct it as an outward beam.
So, the sound is distributed equally towards the audience, unlike when the speaker is only present.
This also delays the reflection of the sound waves back to the speaker.
In the case of plain soundboards, this effect is not seen.
Therefore, the correct option is A.

Note: You must understand that the concave soundboard is only distributing the sound waves evenly. It is not amplifying the waves. Only concave reflectors have the property of generating a beam of light from a single focal point. This is also helpful in illumination through light waves. Here we are using the word reflectors, which generalizes the case for both light and sound.