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sounding board. The sound waves, on striking the sounding board, get reflected
parallel to the principal axis and spread the sound evenly in the hall. In big halls or
auditoriums sound is absorbed by walls, ceiling, seats, etc. So, a curved board
(sound board) is placed behind the speaker. Then the voice of speaker suffers
multiple reflections to increase its loudness so that his speech can be heard easily
by audiences.

Echo
(i)we stand in one corner of a big empty hall and shout; we will hear the word again.
It is heard because the sound gets reflected from the walls of the empty hall. This
repetition of sound is called echo.
(ii)The phenomenon due to which repetition of sound is heard after reflection from a
distant object (such as a high building or a hillock), after the original sound from a
given source dies off, is called an echo.
(iii)It has been found that the sensation of any sound persists for 1/10 second in our
ear, after the exciting sound dies off. This time is called persistence of audibility. If
the repeating sound comes before this 1/10th of a second that is before the original
sound dies off, then we are not able to hear it. But if it comes after 1/10th of second
then we are able to hear echo.

Let’s say a person stands at one corner and shouts the word ‘hello’. This sound
travels a distance let’s say ‘x’, suffers reflection through a wall and returns the same
distance ‘x’ towards the speaker. Then the time taken to hear an echo (that includes
the travelling time towards wall and reflecting time towards speaker) should be
minimum 0.1sec. Then,

Total distance travelled by sound =x +x=2x
Time interval=0.1sec