If the speed of sound in air is 344 m/s at $$22^\circ$$ C, and the minimum time interval between sound and echo is 0.1 s for a distinct echo, what is the distance the sound should travel from source to reflecting surface and then back to ear?
 total distance covered by the sound from the point of generation to the reflecting surface and back should be at least $$ (344 m/s) \times 0.1 s $$ = 34.4 m. Thus, for hearing distinct echoes, the minimum distance of the obstacle from the source of sound must be half of this distance, that is, 17.2 m.
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