When can we distinctly hear the echo of a sharp sound?

cbse
sound

#1

When can we distinctly hear the echo of a sharp sound? Why cannot we hear an echo in a small hall?


#2

As we know that, to listen echo for a sound wave whose speed in air is 344 m${{s}^{-1}}$ and persistance of sound is 0.1 s, a minimum distance of 17.2 m between observer and reflecting surface is required. As the condition of minimum distance between the source and reflecting surface is not satisfied, so in a small hall we do not hear echoes.