Well, there are actually three conditions for echo. i'll give you the three of them, choose any two for yourself.
 The conditions for echo to be heard are :
 1) The distance b/w the source and the reflecting body should be atleast 17m
 2) The wavelength of the particles should be more than the reflecting body.
 3) The particles should have enough energy to be cover the entire distance , the path from source to reflecting body and return back,

Bats cannot see , but still they catch their prey. It's just an application of echo . They send high frequency ultrasonic waves which are reflected by the obstacle(here: their prey) and they receive the reflected waves and come to know about the position of the prey.