About 500m would be optimal.
Imagine how loud you'd need to shout to be heard 500m away.
Now imagine how loud you'd have to shout and how sensitive your ear would have to be to hear a reflection of your shout after a round-trip of 1km.
Also think of how wide the "beam" of your shout would be at 500m range, and all the objects it could bounce off along the way.