The minimum distance from a reflecting surface needed to hear a CLEAR echo (with speed of sound ≈ 340 m/s) is about:

For two sounds to be heard separately, the time gap between them must be at least 0.1 s. Sound travels 340 m/s × 0.1 s = 34 m in this time — but this is the TO-AND-FRO distance. So the reflector must be at least 34/2 = 17 m away. Closer than this and the original sound and its echo blur together.