A student stands facing a high school building on a calm day and claps once. He hears a distinct repetition of the sound after a short interval.
(i) Define an echo and state the minimum time interval after which it can be heard distinctly by the human ear.
(ii) If the speed of sound in air is taken as 340m/s, calculate the minimum distance that the building must be from the student so that the echo is heard clearly.
(iii) Explain briefly why a wall that is closer than this minimum distance does not produce a separate echo but leads to reverberation of the original sound.