Dear Experts,

 The minimum distance to hear a echo is 17.2m, but if we go in a empty hall, why we can hear the echo although the distance of the hall is 17.2m?

Hi! Chinmay
 
A reverberation is perceived when the reflected sound wave reaches our ear in less than 0.1second after the original sound wave. Since the original sound wave is still held in our memory, there is no time delay between the perception of the reflected sound wave and the original sound wave. The two sound waves tend to combine as one very prolonged sound wave. In case of rooms which are shorter than 17.2m, you are hearing this reverberation, It is not a proper echo.
Your question seems to be ambiguous, I believe you were asking about a situation when the reflecting distance is 17.2 m.

Hope it is clear to you.

Cheers!!

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