If the echo (the reflected sound) reaches your ear less than about 0.1 second after the original sound, your brain doesn't separate them, and you're not aware of the echo even though it's there.
If the echo comes from, say, a wall, 0.1 second means you'd have to be about 17 meters away from the wall. If you're closer than that, then the echo reaches you in less than 0.1 second and you're not aware of it.
A. 30 meters . . . No. You hear that echo easily
B. you're standing within range of both sounds . . . No. You hear that echo easily, if you're at least 17 meters from the wall.
C. less than 0.1 second later . . . That's it. The echo is there but your brain doesn't know it.
