The two conditions are: It distance between the reflector and the source should be 17 m And the 2nd condition should be that the reflector should not absorb the sound become it will not produce any echo
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The conditions are :-

 1. The minimum dist. between source and reflecting surface must be 17.2 m. 

2. The tym interval between the orignal and the reflected sound must be 0.1 sec.

3. T
he reflecting object must be bigger then the wavelength of the sound.