Question

what is distinct Eco so that the reflecting surface must be situated at the minimum distance from the listener to hearing distinct Eco calculate this distance at room temperature

Answer 1

Echoes are reflected off walls or hard surfaces like mountains. 
When dealing with audible frequencies, the human ear cannot distinguish an echo from the original sound if the delay is less than 1/10 of a second. Thus, since the velocity of sound is approximately 343 m/s at a normal room temperature of about 20°C, the reflecting object must be more than 16.2 m from the sound source at this temperature for an echo to be heard by a person at the source. 
Sound travels approximately 343 meters/s (1100 ft/s). If a sound produces an echo in 2 seconds, the object producing the echo would be half that distance away (the sound takes half the time to get to the object and half the time to return). The distance for an object with a 2-second echo return would be 1 sec X 343 meters/s or 343 meters (1100 ft). 
In most situations with human hearing, echoes are about one-half second or about half this distance, since sounds grow fainter with distance. In nature, canyon walls or rock cliffs facing water are the most common natural settings for hearing echoes.